MICROSCOPY RESEARCH AND TECHNIQUE 20~177-186(1992)
UIt rastructure of the Neurohypophysis B.W. SCHEITHAUER, E. HORVATH, AND K. KOVACS Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, Minnesota 55905 (B.W.S.); Department of Pathology, St. Michaels Hospital, University of Toronto, Toronto, Ontario, Canada M5B 1 W 8 (EH., K.K.)
KEY WORDS
Anatomy, Neurosecretion, Pituitary
ABSTRACT This review summarizes our current knowledge of the ultrastructure of the human neurohypophysis and includes comments on its anatomy, physiology, and embryology. The neurohypophysis represents a unique tissue having neural and endocrine characteristics and possessing ultrastructural features distinct from those of conventional endocrine organs such as the anterior pituitary, thyroid, pancreatic islets, etc. In contrast to these glands, the neurohypophysis is composed of the processes of mature neurons. As such, it is not capable of synthesizing hormones but only of their storage and release. Neurosecretion is one of the most exciting areas of neuroendocrinology and, although spectacular progress has been achieved in elucidating the process, a number of aspects are incompletely understood. Recent evidence indicates that the magnocellular nuclei of the hypothalemus, the anatomic origin and functional basis of the neurohypophysis, produce not only vasopressin and oxytocin, the so-called “neurohypophyseal hormones,” but a number of other biologically active peptides as well. The physiologic function of these substances is largely unknown but they may be of profound importance in endocrine homeostasis. Based on these novel findings, the role of the neurohypophysis in endocrine regulation has to be re-evaluated.
INTRODUCTION Anatomy Much has been written regarding the ultrastructure of the human adenohypophysis (Horvath and Kovacs, 1988). In contrast, relatively little is available on the neurohypophysis. A short review of its anatomy and physiology is necessary in order to draw structurefunction correlations. The neurohypophysis per se is composed of the infundibulum, a conical downgrowth of the tuber cinereum, a cylindrical portion termed the pituitary stalk, and the bulbous posterior or neural lobe of the pituitary gland. These structures consist predominantly of nonmyelinated nerve fiber tracts originating in the hypothalamus. The neurohypophysis represents only one portion of a more complex neurosecretory unit which begins with the paired supraoptic and paraventricular nuclei. The magnocellular neurons of these hypothalamic nuclei are engaged in the production of vasopressin and oxytocin, substances erroneously referred to as “posterior lobe” hormones. The supraoptico- and paraventriculohypophyseal fiber tracts emanate from these nuclei and, in combination, comprise the majority of the hypothalamohypophyseal tract, which in turn makes up the bulk of the pituitary stalk. The supraoptic and paraventricular nuclei overlie the proximal optic tracts and underlie the subependyma1 zone of the anterolateral third ventricle, respectively. Topographically, they are relatively defined and highly vascular and consist of 30,000-50,000 neurons of large (20-35 nm) size, hence the term “magnocellular nuclei.” Their cell bodies possess eccentric nuclei, prominent nucleoli, and abundant cytoplasm containing well-developed Nissl substance. The distribution of hormone-producing neurons between the two nuclear
0 1992 WILEY-LISS, INC.
groups is distinctive (Zimmerman and Defendini, 19771, the oxytocin and vasopressin cells being most numerous in the supraoptic and paraventricular nuclei, respectively. Despite their biochemical similarity, the hormones are manufactured in separate cells (Perlow et al., 1982). Axons of the supraoptic and paraventricular nuclei often demonstrate swellings along their paths (Herring bodies) and typically end in terminal expansions; both contain neurosecretory materials. Whereas the former likely represent sites of hormone storage, axon terminals abut perivascular spaces in the neural lobe and are the site of neurosecretion. Both oxytocin and vasopressin stain with aldehyde fuchsin, aldehyde thionin, Gomori’s chromalum hematoxylin, and performic acid-Alcian blue, as well as with pseudoisocyanine on fluorescence microscopy. These methods, which stain primarily the cystine-rich neurophysins, carrier proteins of oxytocin and vasopressin, have been replaced by more specific immunohistochemical methods. Nucleated components of the posterior pituitary consist primarily of pituicytes, supportive cells of glial nature found throughout the neurohypophysis, and of capillaries. The processes of pituicytes end near perivascular spaces, particularly about axon terminations; it is no surprise, therefore, that pituicytes play a role in the regulation of hormone transfer to the vasculature. Neurosecretion, the process of hormone manufacture, transfer, and release, is incompletely understood.
Received August 9, 1990; accepted in revised form September 7, 1990. Address reprint requests to B.W. Scheithauer, M.D., Department of Laboratory Medicine and Pathology, Mayo Clinic, 200 First Street S.W., Rochester, Minnesota 55905.
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Despite their decidedly endocrine function, neurosecretory cells of the hypothalamus as a whole retain the fine structure and electrophysiologic properties of neurons. For instance, their function is modulated by stimulatory and inhibitory synapses and they are capable of reacting to neurotransmitter substances. The transit of hormones from their site of synthesis in nerve cell bodies involves the process of axoplasmic flow. Hormones are stored in both axonal expansions and terminations prior to release into the circulation. Release of hormones from axon terminations into the circulation from the nerve terminal takes place in the posterior or neural lobe. A minority of vasopressin and oxytocin-containing fibers from the paraventricular nucleus project to sites other than the neural lobe. These include the limbic cortex, the vagal complex of the brain stem, and autonomic cells of the spinal cord. Functions subserved by such pathways include memory, emotion, and the maintenance of cardiovascular as well as cerebrovascular tone. Their physiology appears to be distinct from that of the hypothalamohypophyseal tract in exhibiting a circadian rhythm, one independent of factors controlling pituitary vasopressin and oxytocin secretion (Dierickx and Vandesande, 1979).
Physiology Oxytocin and vasopressin, structurally similar noiiapeptides, have molecular weights of approximately 1,000. Each is separately synthesized as a prohormone within cells of the supraoptic and paraventricular nuclei. They may have originated from a common gene since the hormones differ in structure by only two amino acid residues. Processing and cleavage of the prohormones takes place within secretory granules during transport. The release of hormone involves calcium-dependent exocytosis. Secretion of the hormones is separately controlled. The prohormones, propressorphysin and prooxyphysin, are glycoproteins of 20 kd molecular weight, the structures of which include the cysteine-rich neurophysins. The latter are distinct; neurophysin I accompanies oxytocin whereas the type I1 variant is secreted with vasopressin. Although oxytocin and vasopressin are clearly the principal hormones of magnocellular neurons, others of unknown physiologic significance have been identified within these cells including dynorphin, an opioid peptide (Watson et al., 19821, corticotropin-releasing factor (Ohtani et al., 1988), glucagon, cholecystokinin, and angiotensin 11. The neurohypophysis also contains neurotransmitters and hormones, the functions of which remain to be explained; these include somatostatin, thyrotropin-releasing hormone (TRH), substance P, gonadotropin-releasing hormone (GnRH), dopamine, serotonin, histamine, beta melanocyte-stimulating hormone (beta-MSH) (Pelletier, 1980), and corticotropin-releasing hormone (Ohtani et al., 1988). Although vasopressin functions as a neurotransmitter a t various locations in the central nervous system (CNS), its principal physiologic role is as an antidiuretic hormone. Its principal regulators include plasma osmolality and intravascular volume. Like vaso-
pressin, oxytocin is widely distributed in the CNS where it presumably functions as a neurotransmitter (Eiden and Brownstein, 19811,but its principal function relates t o labor (uterine contraction) and lactation (contraction of myoepithelial cells of the breast).
ULTRASTRUCTURE The salient ultrastructural features of the normal human neurohypophysis are illustrated in Figures 11 0
1x2.
Embryologic Studies The development of the human fetal neurohypophysis takes place between 7.5 and 19 weeks of gestation, a time of rapid central nervous system development. Increase in neural lobe growth, for instance, is maximal in the third and fourth months (Daikoku, 1958). Aldehyde fuchsin staining of “so-called”posterior lobe hormones is demonstrable within supraoptic nuclei and the infundibular process at 14 weeks (Rinne et al., 1962). Hormone activity, as assessed by radioimmunoassay methods, is detectable in the neural lobe at the 10 week stage (Skowsky and Fisher, 1973). Much of our knowledge is based upon the excellent work of Okado and Yokota (1980) as well as Ikeda et al. (1988). The former specifically studied the ultrastructure of the developing posterior lobe, particularly the process of secretory granule and vesicle formation as well as the establishment of neurovascular relationships. These authors found that granules not only appeared in waves at 7.5-8.5 weeks and at 15.5-19 weeks, but they underwent a process of maturation, increasing in size and reaching adult dimensions a t 19 weeks. The uniform and high cellularity of the 7.5 week neural lobe, attributed to the presence of numerous pituicytes, was found to change a t 11 weeks, a t which time pericapillary zones become axon-rich and relatively devoid of pituicytes. Further dispersion of these unique glial cells by other constituents was observed between 15.5 and 19 weeks. Axoglial synapselike contacts, perhaps transient structures, were more numerous, fully formed, and extensive in young (7.5 week) fetuses than a t 19 weeks; in addition, a variety of junctional types were noted between pituicytes in fetuses older than ll weeks (Okado and Yokota, 1982). At 17.5-19 weeks, perivascular spaces, functionally specialized components of the posterior lobe, became well formed and extended in a stellate configuration into surrounding neural tissue. Compared to the situation in experimental animals, neurohemal contacts were earlier to develop in humans. Thus, the appearance of mature secretory granules coincides nicely with vascular maturation. According to Okado and Yokota (1980), unmyelinated axons in the posterior lobe contain microtubules and occasional clear vesicles at 7.5 weeks. Granular vesicles, on the other hand, are best visualized 1 week thereafter. With time thick-walled vessels become replaced by thin fenestrated ones with well-formed basement membranes. Early on, the developing perivascular spaces are best visualized in the periphery of the gland and lack ramifications into the surrounding parenchyma. The space is lined by two basal lamina, one
ULTRASTRUCTURE OF ‘i’HENEURCHYPOPHYSIS
perivascular and the other, the abluminal membrane, delimiting neurd tissue. In the interval between 11 and 19 weeks a number of changes were noted, e.g., a) pituicyte processes were seen to develop, b) axon profiles of all types increased in number, as did secretory granules per axon, c) fenestrated vessels with thin walls and wide perivascular spaces increased at the expense of unfenestrated forms, and d) perivascular processes more often contained granulated vesicles than a t earlier stages of development. Lastly, the uniinodal peak of granule size noted at earlier stages was replaced by a bimodal peak a t 19 weeks. With progressive development, various axon types were noted including ones containing granular vesicles (43 percent), small clear vesicles (40 percent), and both (20 percent). Given the preponderance of vasopressin and oxytocin production in the supraoptic and periventricular nuclei, respectively, the bimodal distribution was thought to be a reflection of differing hormone composition, i.e., large vasopressin granules and small oxytocin-contailing forms. This interpreation is supported by the obs xvation that at 14 weeks neurosecretory material representing vasomessin is found in the supraoptic n,icieus, whweas tl le periventricular nucleus contains no se?”ef;ion uitil later stages (Rinne et al., 1962). Throughout their stuuy, Okado and Yokota (1980) fouid nc evidence of granule exocytosis, even at 19 weeks. Furthermore, they drew no firm conclusion regarding the mesence G r morphology of cholinergic or aminer g i c fibers in t l daeloping ~ posterior lobe.
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acting to facilitate or retard hormone release (Hatton, 1988). Pituicytes The celldlar elements of thi- pars nervosa, first described by Bucy (1930), are termed “pituicytes.” Despite conjecture regarding their nature, it was Lederis (1965) and subsequently Br:rgland and Torack (1969) whu postulated and confirmed their astrocytic derivation. This ifi’isrpretation is supported by the observation that pituicytes are capable of Rose:lthal fiber formation arid demonstrate imnlunoreactivity for glial fibrillary acidic protein (GFAP). Critical ultrastructural ;tL.-liesdivided these speci :lized cells into several types (Fujita et al., 1961: Kurosumi e l al., 1961). The most detailed and compiete de miption of pituicytes was t?a t of Takei et al. (1980). Based on these studies, five principal forms of pituicytes are recognized:
1. Major pituicytns are astrocj-te-like cells of irregular contoui which psscss long processes. Their .rregular nuclei often \arbor various types of nuclear inclusions, most contmonly spheridia. Pituicytes have hynodense cytoplasm rich in Golgi memljranes but with relatively little rough endoplasmic reticulum. The cvtoplasm, particularly that of processes, contains intermediate filaments (glial filaments). As noted above, rare Rosenthal fibers ma1 be observed. In addition to microfilaments, occasional microtubules, lysosomes, Vasculature and iirlid droplets may be present. Major pituicytes h+previovsly noted, the vasculature of the posterior are frequently indented by nearby axons in such a way Situitary i s cnlque. Like several sites in the central as to suggest tha‘ the latter are eiBsheathed, pituicyte nerwiis system including the area postrema and hypo- c: toplasm serving as an embedment. Occasional intertha!amus, tbe posterior lube lacks a blood brain bar- cellular desmosomes have been seen between piturier. Tis ultrastrxtcral features have been studied in icytes. Unlike ths sitdation in fetal glands, contact bedetaii by Seyima et al. (1980b), a work that forms the tween ucons and pituicytes is unaccompanied by the basis of cur present u r der fanding. formation of spdcialized junctions to suggest intercelV‘iryipg greatly in 1;mension the vascular compart- lular communication. The capacity of pituicyte proment consists of fenestrated vessels with accompany- cesses to cover a5:ons and omasionally other pituicyte ing pericytts scliroundeci by a basal lamina as well as processes by the tormation of F9iral lamellae suggested a pel ivasculw- q>ac?.in turn surrounded by an ablu- to 3re”uss et al. (1975) that they serve a physiologic minal b s d lamina. The space not only surrounds ves- function rather tha? simply acting as a stroma. Thtt sels l u , extends in mdtiple dirctions to penetrate this was the case has been shown in physiologic experwidely between -ou3 of parenchymal cells. The ablu- i m e r A both in ~ i v (datton o 1988) and in vitro (Perlminal 1 aserient membraQe has been found to possess mutter et al., 1984). It appears that, under basal condiscontinuities which nat o :ly permit contact between ditions. Pituicytes not only enshroud axms and their the extracel-ul ar and verivascular space, but are occa- termhations but interpose their processes between sionally filled by a>-o?al ‘,erminations.Despite this in- these endings and the abluminal basement membrane, tima& neurovascalar relationship, no contacts have effectively acting as barrier, to secretion. Alternabeen demoist *atedbetween endothelium and neurose- tively, in situations of hormone demand, e.g., dehydracretory cells. The phwiologic significance of the extra- tion or lactation, pituicytes facilitate secretion; their cellular space is uncatain; however, it is likely that it processes retract, bar terminstions, m d permit coneither represents a “pool” of hormone or, in its incom- tact between axons as well as axons and abluminal plete form, facilitates the regulation of neurosecretion basal lamina. (Livingston and Wiiks, 1976). In addition to the per2. Dark pituicytes are electron dense and demonicytes about the microvasculature, the perivascular strate an increased n r clear cytoplasmic ratio, but othspace contains small numbers of macrophages, fibro- erwise resemble major pituicytes. blasts, and occasional mast cells. Depending upon os3. Oncocytic pituicytes, like “ d a r k variants, probamolality, pituicytes either remove themselves from or bly represent a form of major pituicytes. I’ltrastructurabut the abluminal basement membrane; hence they ally, they possess im increased number of mitochonplay a physiologic role with respect to the vasculature, dria, often abnormal in morphology, to the deficit of
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other organelles. Mitochondria1 crystalloids may be seen. 4. Granular pituicytes contain abundant lysosomes, some resembling simple lipofuscin, others myelinoid bodies. Lipid droplets may also be noted. These cells are thought to represent yet another form of major pituicyte, perhaps one subjected to marked stimulation (Davis and Morris, 1973). Not surprisingly, their content of acid phosphatase may be significant (Whitaker et al., 1970). It has been suggested that granular pituicytes form by way of micropinocytosis-mediated phagocytosis (DeRobertis et al., 1975; Theodosis, 1979) resulting from transfer of secretory material from nearby axonal endings, some of which may show a distinct paucity of secretory material (Krsulovic and Bruckner, 1969). Not only may phagocytosis be ongoing under normal physiologic conditions, but experimental transection of the pituitary stalk also results in an increase in phagocytosed neurosecretory materials within pituicytes (Sterba and Bruckner, 1967). A similar process may occur in humans. Influx of phagocytes from the bloodstream into the posterior pituitary has been suggested as an alternate mechanism by which granular cells accumulate (Takei et al., 1980). Lastly, granular pituicytes may represent the cellular constituent of granular cell tumors or “choristomas” of the neurohypophysis (Liwnicz et al., 1984). These benign lesions are identical in composition to the granular cell clusters or “tumorlets” commonly found in the posterior lobe and pituitary stalk (Costello, 1936). 5. Ependymal pituicytes are uncommon. Like ependymal cells lining the ventricular system, they possess cilia and microvilli, particularly when clustered, engage in the formation of extracellular lumina, and exhibit distinct ribbon-like junctional complexes. In all other ways, their cytoplasm resembles that of major pituicytes. Ependymal variants are particularly frequent in lower vertebrates wherein they represent the principal form of pituicytes (Rodriguez, 1971).In mammals they are less frequent, their place being taken by cells of astrocytic nature (Holmes and Ball, 1974).
Neurosecretory Axons and Their Terminations As with pituicytes, the contributions of Lederis (19651, Bergland and Torack (19691, and more recently of Takei et al. (1980) and Seyama et al. (1980a1, constitute the only in-depth studies of the neurosecretory processes as they occur in the normal human neurohypophysis. Although a vast literature is based on experimental animal studies, our discussion will be limited to mammalian, in particular human, morphology. Oxytocin and vasopressin are produced within neurons of the supraoptic and paraventricular nuclei and are transferred to the posterior pituitary as electron dense secretory granules by a process of axoplasmic flow. Biochemical modifications are known to occur during transit, but the mechanism of hormone secretion is poorly understood. For the purposes of discussion, the principal zones of metabolic activity within the posterior pituitary are a) dilations or “swellings” occurring in the course of axon transit (Herring bodies), and b) axonal terminations from which hormone
transfer to the vasculature takes place. Variations in the ultrastructural appearance of axon dilations are thought to be a manifestation of different physiologic states. That such is the case has been demonstrated in lower animals (Lullman-Rauch, 1976; Rufener, 1974). As previously noted, questions remain regarding the mechanism by which hormones are released into the circulation. Whether the process involves actual extrusion of secretory granules (exocytosis), or transfer of released hormones across intact membranes at cell terminations, a process known as “molecular dispersion” (Krisch et al., 1972),remains unsettled. Despite careful research, Seyama et al. (1980a) were unable to identify the presence of exocytoses. The axons of the supraopticohypophyseal and paraventriculohypophyseal tracts are unmyelinated, measure 0.5-1.0 micron in diameter, and contain both microtubules and intermediate filaments (neurofilaments). Secretory granules are present in greatest concentration within axonal swellings which measure 150 microns in diameter . In light microscopic terms, such swellings correspond to “Herring bodies” (Herring, 1908).Vasopressin and oxytocin-containing fibers are, according to the classification of Seyama et al. (1980a), considered type A fibers. They contain 100300 nm secretory granules which vary in electron density as well as in the extent to which their limiting membranes are discernible. Type A fiber terminations within the posterior lobe contain not only electron dense secretory granules, but microvesicles, mitochondria, lysosomes, and tubuloreticular structures. The degree to which these various structures are represented in type A dilations is the basis of their morphologic classification (Seyama et al., 1980a). Type I Dilatations. Type I dilatations, rich in neurosecretory granules, likely represent stored neurosecretory materials, whereas the content of other dilatations probably reflects varying states of hormone metabolism. Though autoradiographic studies (Heap et al., 1975; Pickering et al., 1974) have shown labeled hormones t o appear within axon terminations prior to their accumulation in swellings (Herring bodies), there is, nonetheless, a distinct tendency for granules to accumulate in swellings. In the study of Morris (1976) swellings contained the majority (55 percent) of granules whereas terminations and nondilatated portions of axons contained only 31 and 13 percent, respectively. Type III Dilatations. Attempts have been made to correlate the morphology of axonal swellings with physiologic states. Dehydration, for instance, serves to stimulate vasopression secretion and has been found to be associated with increased numbers of lysosome-rich (type 111)dilatations (Krisch et al., 1972; Whitaker and LaBella, 1972). Their increase has also been noted when the need for neurohypophyseal hormones suddenly decreases (Kodoma and Fujita, 1975) or when axon degeneration follows either stalk transection (Dellmann et al., 1974) or destruction of paraventricular nuclei (Zambrano and DeRobertis, 1968). These observations support the contention of Seyama et al. who suggested that type I11 dilatations are a manifestion of degradation of unused neurosecretory granules.
ULTRASTRUCTURE OF THE NEUROHYPOPHYSIS
Fig. 1. Low-power view of normal human neurohypophysis, depicting capillaries, axonal processes most of which are agranular, and prominent granular pituicytes (arrows). x 4,700.
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Fig. 2. Fine structural details of granular pituicytes are shown. x 8,150.
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Fig. 3. Major pituicyte. Four hxonal processns, including one containing a large mych-figure (1-41,traversing through the cytoplasm. Note nuclear spheridium (arrow). X 12,800.
Fig. 4. Ependymal pituicytes joined by junctional complexes (arrowheads). Note cilia projecting into the small lumen formed by the pituicytes. The nucleus harbors spheridia (arrows). x 14,000.
Fig. 5. The area shown comprises mostly granule-rich type I dilatations, two of which contain glycogen alpha- and beta-particles (a,b, respectively). x 14,750.
Fig. 7. Type I dilatation, containing a large cluster of microvesicles (arrow). x 28,900.
Fig. 6. Micrograph depicting a few type I fibers and what appears to be a large type V dilatation (arrow) containing tubular structures
Fig. 8. The large axonal dilatation occupying a sizeable area of the photograph cannot be clearly typed. It contains low-density neurosecretory granules having the appearance of vesicles, scattered composite lysosomes, as well as clusters of glycogen beta-particles (arrows). x 8,700.
but entirely devoid of neurosecretory granules. x 28,000.
Figs. 9 and 10. The figures document large Herring bodies. The one of Figure 9 represents a type I1 dilatation containing numerous mitochondria and a large number of neurosecretory granules displaying low electron opacity. x 7,850. Figure 10 shows a type I11 axonal dilatation harboring numerous lysosomes. X 7,850.
Fig. 11. Capillary and vicinity. Note extension of the perivascular space (“sinusoid”)into surrounding neural tissue (asterisks). X 6,200. Fig. 12. Neurohypophyseal capillary. Note the luminal and abluminal basement membranes and the delicate fenestrated endothelium (arrows). x 15,400.
ULTRASTRUCTURE OF T H E NEUROHYPOPHYSIS
Type N Dilatations. Given the demonstration by Barier and Lederis (1966) that content of stored hormone within the posterior pituitary need not change despite diminution or disappearance of neurosecretory granules, Seyama et al. (1980a) suggested that type IV dilatations may represent “readily releasable pools” of hormone. That being the case, the presence of “empty” neurosecretory granules and perhaps vesicles suggests that hormone release is preceded by a process of molecular dispersion (Vollrath, 1974) rather than by exocytosis. Alternatively, the possibility that vesicles, particularly those clustered into “synaptoid clusters, represent recapture of granule membranes, following their fusion with the plasmalemma and resultant hormone discharge, cannot be discounted. It should be noted, however, Seyama et al. (1980a) found no direct evidence of exocytosis. Type ZZ, V, and VI Dilatations. The physiologic correlates of type 11, V, and VI dilatations remain to be elucidated. Type VI dilatations, characterized by an increase of microvesicles, may be a consequence of increased release of neurohypophyseal hormones (Dreifuss et al., 1975; Santolaya et al., 1972). The possibility that they represent an increase in acetylcholine-containing synaptic vesicles similar to those in the central nervous system (Dehbertis, 1962) is considered unlikely. Type B Fibers In contrast to type A fibers, those of type B are far less numerous and contain secretory granules of smaller size (50-100 nm). In addition to their smaller dimensions, secretory granules of type B fibers are separated from their enveloping membrane by a definite electron-lucent halo. Their ultrastructural features are less complex and clustering of 30-40 micron microvesicles in “synaptoid structures is far less prominent (Seyama et al., 1980a).Microvesicles are more uniform in configuration than in type A fibers, but show greater variation in diameter (30-50 nm). Seyama et al. (1980a) undertook a morphologic comparison of fiber types and likened those of type B to fibers occurring in the median eminence and infundibulum of animals (Duffy and Menefee, 1965; Monroe, 1967) as well as of man (Bergland and Torack, 1969). Their findings support the concept that type B fibers are aminergic rather than associated with oxytocin and vasopressin release. The function of such fibers in the posterior pituitary is unknown. Two principal forms of dilatations are described in type B fibers, some rich in granules and others in which microvesicles are particularly numerous.
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structural development of the neural lobe in the human fetus. Am. J. Anat., 159:261-273. Okado, N., and Yokota, N. (1982) Axoglial synaptoid contact in the neural lobe of the human fetus. Anat. Rec., 202:117-124. Pelletier, G. (1980) Immunohistochemical localization of somatostatin. Prog. Histochem. Cytochem., 121-41. Perlmutter, L.S., Hatton, G.I., and Tweedle, C.D. (1984) Plasticity in the in vitro neurohypophysis: Effects of osmotic changes on pituicytes. Neuroscience, 12:503-5 11. Perlow, M.J., Reppert, S.M., Altrnan, H.A., Fisher, D.A., Self, S.M., and Robinson, A.G. (1982) Oxytocin, vasopressin, and estrogenstimulated neurophysin: Patterns of concentration in cerebrospinal fluid. Science, 216:1416-1418. Pickering, B.T., Jones, C.W., and Burford, G.D. (1974) Biochemical aspects of the hypothalamoneurohypophysial neuron. In: Neurosecretion-The Final Neuroneodocrine Pathway. F. Knowles and L. Vollrath, eds. Springer-Verlag, Berlin, Heidelberg, New York, pp. 72-85. Rinne, U.K., Kovalo, E., and Talanti, S. (1962) Maturation of human hypothalamic neurosecretion , Biol . Neonate, 4 351-364. Rodriguez, E.N. (1971)The comparative morphology of neural lobes of species with different neurohypophyseal hormones. In: Subcellular Organization and Function in Endocrine Tissues. H.Heller and K. Endocrinol. No. 19. Cambridge University Lederis, eds. Mem. SOC. Press, London, pp. 263-292. Rufener, C. (1974) Autophagy of secretory granules in the rat neurohypophysis. Neuroendocrinology, 13:314-320. Santolaya, R.C., Bridges, T.E., and Lederis, K. (1972) Elementary granules, small vesicles and exocytosis in the rat neurohypophysis after acute hemorrhage. Z. Zellforsch., 125277-288. Seyama, S., Pearl, G.S., and Takei, Y.(1980a) Ultrastructural study of the human neurohypophysis, I. Neurosecretory axons and their dilations in the pars nervosa. Cell Tissue Res., 205:253-271. Seyama, S., Pearl,'G.S., and Takei, Y. (1980b) Ultrastructural study
of the human neurohypophysis. 111. Vascular and perivascular structuren. Cell Tissue Res., 206:291-302. Skowsky, R., and Fisher, D.A. (1973) Immunoreactive arginine vasopressin (AVP) and arginine vasotocin (AVT)in the fetal pituitary of man and sheep. Clin. Res., 21:205. Sterba, G., and Bruckner, G. (1967) Zur Funcktion der ependymalen glia in der Neurohypophyse. Z. Zellforsch., 81:457-473. Takei, Y., Seyama, S., Pearl, G.S., and Tindall, G.T. (1980) Ultrastructural study of the human neurohypophysis. 11. Cellular elements of neural parenchyma, the pituicytes. Cell Tibsue Rw,, 205: 273-287. 'I'heodosis, D.T. (1979) Endocytosis in glial cells (pituicytes) of the rat neurohypophysis demonstrated by incorporation of horseradish peroxidase. Neuroscience, 4417-525. Vollrath, L. (1974) New trends in vertebrate neurosecretion. In: Neurosecretion-The Final Neuroendocrine Pathway. F. Knovles and L. Vollrath, eds. Springer-Verlag, Berlin, Heidelberg, New York, pp. 276-284. Watson, S.J., Akil, H., Fischli, W., Goldstein, A,, Zimmerman, E., Nilaver, J., and Van Wimersma-Griedanus, T.B. (1982) Dynorphin and vasopressin: Common localization in magnocelldar neurons. Science, 216:85-87. Whitaker, S., and LaBella, F.S. (1972) Ultrastructural localization of acid phosphatase in the posterior pituitary of the dehydrated rat. Z. Zellforsch ., 125:1-1 5. Whitaker, S.,Labella, F.S., and Anwal, M. (1970) Electron microscopic histochemistry of lysosomes in neurosecretory nerve endings and pituicytes of rat posterior pituitary. Z. Zellforsch., 111:493~ n 4
Zambrano, D., and DeRobertis, E. (1968) The ultrastructural changes in the neurohypophysis after destruction of the paraventricular nuclei in normal and castrated rats. Z. Zellforsch., 88:496-510. Zimmerman, E.A., and Defendini, R. (1977) Hypothalamic pathways containing oxytocin, vasopressin and associated neurophysins. In: Neurohypophysis. A.M. Moses and L. Share, eds. S. Karrrer. Basel, _. . . pp. 22-29.
UIt rastructure of the Neurohypophysis B.W. SCHEITHAUER, E. HORVATH, AND K. KOVACS Department of Laboratory Medicine and Pathology, Mayo Clinic, Rochester, Minnesota 55905 (B.W.S.); Department of Pathology, St. Michaels Hospital, University of Toronto, Toronto, Ontario, Canada M5B 1 W 8 (EH., K.K.)
KEY WORDS
Anatomy, Neurosecretion, Pituitary
ABSTRACT This review summarizes our current knowledge of the ultrastructure of the human neurohypophysis and includes comments on its anatomy, physiology, and embryology. The neurohypophysis represents a unique tissue having neural and endocrine characteristics and possessing ultrastructural features distinct from those of conventional endocrine organs such as the anterior pituitary, thyroid, pancreatic islets, etc. In contrast to these glands, the neurohypophysis is composed of the processes of mature neurons. As such, it is not capable of synthesizing hormones but only of their storage and release. Neurosecretion is one of the most exciting areas of neuroendocrinology and, although spectacular progress has been achieved in elucidating the process, a number of aspects are incompletely understood. Recent evidence indicates that the magnocellular nuclei of the hypothalemus, the anatomic origin and functional basis of the neurohypophysis, produce not only vasopressin and oxytocin, the so-called “neurohypophyseal hormones,” but a number of other biologically active peptides as well. The physiologic function of these substances is largely unknown but they may be of profound importance in endocrine homeostasis. Based on these novel findings, the role of the neurohypophysis in endocrine regulation has to be re-evaluated.
INTRODUCTION Anatomy Much has been written regarding the ultrastructure of the human adenohypophysis (Horvath and Kovacs, 1988). In contrast, relatively little is available on the neurohypophysis. A short review of its anatomy and physiology is necessary in order to draw structurefunction correlations. The neurohypophysis per se is composed of the infundibulum, a conical downgrowth of the tuber cinereum, a cylindrical portion termed the pituitary stalk, and the bulbous posterior or neural lobe of the pituitary gland. These structures consist predominantly of nonmyelinated nerve fiber tracts originating in the hypothalamus. The neurohypophysis represents only one portion of a more complex neurosecretory unit which begins with the paired supraoptic and paraventricular nuclei. The magnocellular neurons of these hypothalamic nuclei are engaged in the production of vasopressin and oxytocin, substances erroneously referred to as “posterior lobe” hormones. The supraoptico- and paraventriculohypophyseal fiber tracts emanate from these nuclei and, in combination, comprise the majority of the hypothalamohypophyseal tract, which in turn makes up the bulk of the pituitary stalk. The supraoptic and paraventricular nuclei overlie the proximal optic tracts and underlie the subependyma1 zone of the anterolateral third ventricle, respectively. Topographically, they are relatively defined and highly vascular and consist of 30,000-50,000 neurons of large (20-35 nm) size, hence the term “magnocellular nuclei.” Their cell bodies possess eccentric nuclei, prominent nucleoli, and abundant cytoplasm containing well-developed Nissl substance. The distribution of hormone-producing neurons between the two nuclear
0 1992 WILEY-LISS, INC.
groups is distinctive (Zimmerman and Defendini, 19771, the oxytocin and vasopressin cells being most numerous in the supraoptic and paraventricular nuclei, respectively. Despite their biochemical similarity, the hormones are manufactured in separate cells (Perlow et al., 1982). Axons of the supraoptic and paraventricular nuclei often demonstrate swellings along their paths (Herring bodies) and typically end in terminal expansions; both contain neurosecretory materials. Whereas the former likely represent sites of hormone storage, axon terminals abut perivascular spaces in the neural lobe and are the site of neurosecretion. Both oxytocin and vasopressin stain with aldehyde fuchsin, aldehyde thionin, Gomori’s chromalum hematoxylin, and performic acid-Alcian blue, as well as with pseudoisocyanine on fluorescence microscopy. These methods, which stain primarily the cystine-rich neurophysins, carrier proteins of oxytocin and vasopressin, have been replaced by more specific immunohistochemical methods. Nucleated components of the posterior pituitary consist primarily of pituicytes, supportive cells of glial nature found throughout the neurohypophysis, and of capillaries. The processes of pituicytes end near perivascular spaces, particularly about axon terminations; it is no surprise, therefore, that pituicytes play a role in the regulation of hormone transfer to the vasculature. Neurosecretion, the process of hormone manufacture, transfer, and release, is incompletely understood.
Received August 9, 1990; accepted in revised form September 7, 1990. Address reprint requests to B.W. Scheithauer, M.D., Department of Laboratory Medicine and Pathology, Mayo Clinic, 200 First Street S.W., Rochester, Minnesota 55905.
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Despite their decidedly endocrine function, neurosecretory cells of the hypothalamus as a whole retain the fine structure and electrophysiologic properties of neurons. For instance, their function is modulated by stimulatory and inhibitory synapses and they are capable of reacting to neurotransmitter substances. The transit of hormones from their site of synthesis in nerve cell bodies involves the process of axoplasmic flow. Hormones are stored in both axonal expansions and terminations prior to release into the circulation. Release of hormones from axon terminations into the circulation from the nerve terminal takes place in the posterior or neural lobe. A minority of vasopressin and oxytocin-containing fibers from the paraventricular nucleus project to sites other than the neural lobe. These include the limbic cortex, the vagal complex of the brain stem, and autonomic cells of the spinal cord. Functions subserved by such pathways include memory, emotion, and the maintenance of cardiovascular as well as cerebrovascular tone. Their physiology appears to be distinct from that of the hypothalamohypophyseal tract in exhibiting a circadian rhythm, one independent of factors controlling pituitary vasopressin and oxytocin secretion (Dierickx and Vandesande, 1979).
Physiology Oxytocin and vasopressin, structurally similar noiiapeptides, have molecular weights of approximately 1,000. Each is separately synthesized as a prohormone within cells of the supraoptic and paraventricular nuclei. They may have originated from a common gene since the hormones differ in structure by only two amino acid residues. Processing and cleavage of the prohormones takes place within secretory granules during transport. The release of hormone involves calcium-dependent exocytosis. Secretion of the hormones is separately controlled. The prohormones, propressorphysin and prooxyphysin, are glycoproteins of 20 kd molecular weight, the structures of which include the cysteine-rich neurophysins. The latter are distinct; neurophysin I accompanies oxytocin whereas the type I1 variant is secreted with vasopressin. Although oxytocin and vasopressin are clearly the principal hormones of magnocellular neurons, others of unknown physiologic significance have been identified within these cells including dynorphin, an opioid peptide (Watson et al., 19821, corticotropin-releasing factor (Ohtani et al., 1988), glucagon, cholecystokinin, and angiotensin 11. The neurohypophysis also contains neurotransmitters and hormones, the functions of which remain to be explained; these include somatostatin, thyrotropin-releasing hormone (TRH), substance P, gonadotropin-releasing hormone (GnRH), dopamine, serotonin, histamine, beta melanocyte-stimulating hormone (beta-MSH) (Pelletier, 1980), and corticotropin-releasing hormone (Ohtani et al., 1988). Although vasopressin functions as a neurotransmitter a t various locations in the central nervous system (CNS), its principal physiologic role is as an antidiuretic hormone. Its principal regulators include plasma osmolality and intravascular volume. Like vaso-
pressin, oxytocin is widely distributed in the CNS where it presumably functions as a neurotransmitter (Eiden and Brownstein, 19811,but its principal function relates t o labor (uterine contraction) and lactation (contraction of myoepithelial cells of the breast).
ULTRASTRUCTURE The salient ultrastructural features of the normal human neurohypophysis are illustrated in Figures 11 0
1x2.
Embryologic Studies The development of the human fetal neurohypophysis takes place between 7.5 and 19 weeks of gestation, a time of rapid central nervous system development. Increase in neural lobe growth, for instance, is maximal in the third and fourth months (Daikoku, 1958). Aldehyde fuchsin staining of “so-called”posterior lobe hormones is demonstrable within supraoptic nuclei and the infundibular process at 14 weeks (Rinne et al., 1962). Hormone activity, as assessed by radioimmunoassay methods, is detectable in the neural lobe at the 10 week stage (Skowsky and Fisher, 1973). Much of our knowledge is based upon the excellent work of Okado and Yokota (1980) as well as Ikeda et al. (1988). The former specifically studied the ultrastructure of the developing posterior lobe, particularly the process of secretory granule and vesicle formation as well as the establishment of neurovascular relationships. These authors found that granules not only appeared in waves at 7.5-8.5 weeks and at 15.5-19 weeks, but they underwent a process of maturation, increasing in size and reaching adult dimensions a t 19 weeks. The uniform and high cellularity of the 7.5 week neural lobe, attributed to the presence of numerous pituicytes, was found to change a t 11 weeks, a t which time pericapillary zones become axon-rich and relatively devoid of pituicytes. Further dispersion of these unique glial cells by other constituents was observed between 15.5 and 19 weeks. Axoglial synapselike contacts, perhaps transient structures, were more numerous, fully formed, and extensive in young (7.5 week) fetuses than a t 19 weeks; in addition, a variety of junctional types were noted between pituicytes in fetuses older than ll weeks (Okado and Yokota, 1982). At 17.5-19 weeks, perivascular spaces, functionally specialized components of the posterior lobe, became well formed and extended in a stellate configuration into surrounding neural tissue. Compared to the situation in experimental animals, neurohemal contacts were earlier to develop in humans. Thus, the appearance of mature secretory granules coincides nicely with vascular maturation. According to Okado and Yokota (1980), unmyelinated axons in the posterior lobe contain microtubules and occasional clear vesicles at 7.5 weeks. Granular vesicles, on the other hand, are best visualized 1 week thereafter. With time thick-walled vessels become replaced by thin fenestrated ones with well-formed basement membranes. Early on, the developing perivascular spaces are best visualized in the periphery of the gland and lack ramifications into the surrounding parenchyma. The space is lined by two basal lamina, one
ULTRASTRUCTURE OF ‘i’HENEURCHYPOPHYSIS
perivascular and the other, the abluminal membrane, delimiting neurd tissue. In the interval between 11 and 19 weeks a number of changes were noted, e.g., a) pituicyte processes were seen to develop, b) axon profiles of all types increased in number, as did secretory granules per axon, c) fenestrated vessels with thin walls and wide perivascular spaces increased at the expense of unfenestrated forms, and d) perivascular processes more often contained granulated vesicles than a t earlier stages of development. Lastly, the uniinodal peak of granule size noted at earlier stages was replaced by a bimodal peak a t 19 weeks. With progressive development, various axon types were noted including ones containing granular vesicles (43 percent), small clear vesicles (40 percent), and both (20 percent). Given the preponderance of vasopressin and oxytocin production in the supraoptic and periventricular nuclei, respectively, the bimodal distribution was thought to be a reflection of differing hormone composition, i.e., large vasopressin granules and small oxytocin-contailing forms. This interpreation is supported by the obs xvation that at 14 weeks neurosecretory material representing vasomessin is found in the supraoptic n,icieus, whweas tl le periventricular nucleus contains no se?”ef;ion uitil later stages (Rinne et al., 1962). Throughout their stuuy, Okado and Yokota (1980) fouid nc evidence of granule exocytosis, even at 19 weeks. Furthermore, they drew no firm conclusion regarding the mesence G r morphology of cholinergic or aminer g i c fibers in t l daeloping ~ posterior lobe.
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acting to facilitate or retard hormone release (Hatton, 1988). Pituicytes The celldlar elements of thi- pars nervosa, first described by Bucy (1930), are termed “pituicytes.” Despite conjecture regarding their nature, it was Lederis (1965) and subsequently Br:rgland and Torack (1969) whu postulated and confirmed their astrocytic derivation. This ifi’isrpretation is supported by the observation that pituicytes are capable of Rose:lthal fiber formation arid demonstrate imnlunoreactivity for glial fibrillary acidic protein (GFAP). Critical ultrastructural ;tL.-liesdivided these speci :lized cells into several types (Fujita et al., 1961: Kurosumi e l al., 1961). The most detailed and compiete de miption of pituicytes was t?a t of Takei et al. (1980). Based on these studies, five principal forms of pituicytes are recognized:
1. Major pituicytns are astrocj-te-like cells of irregular contoui which psscss long processes. Their .rregular nuclei often \arbor various types of nuclear inclusions, most contmonly spheridia. Pituicytes have hynodense cytoplasm rich in Golgi memljranes but with relatively little rough endoplasmic reticulum. The cvtoplasm, particularly that of processes, contains intermediate filaments (glial filaments). As noted above, rare Rosenthal fibers ma1 be observed. In addition to microfilaments, occasional microtubules, lysosomes, Vasculature and iirlid droplets may be present. Major pituicytes h+previovsly noted, the vasculature of the posterior are frequently indented by nearby axons in such a way Situitary i s cnlque. Like several sites in the central as to suggest tha‘ the latter are eiBsheathed, pituicyte nerwiis system including the area postrema and hypo- c: toplasm serving as an embedment. Occasional intertha!amus, tbe posterior lube lacks a blood brain bar- cellular desmosomes have been seen between piturier. Tis ultrastrxtcral features have been studied in icytes. Unlike ths sitdation in fetal glands, contact bedetaii by Seyima et al. (1980b), a work that forms the tween ucons and pituicytes is unaccompanied by the basis of cur present u r der fanding. formation of spdcialized junctions to suggest intercelV‘iryipg greatly in 1;mension the vascular compart- lular communication. The capacity of pituicyte proment consists of fenestrated vessels with accompany- cesses to cover a5:ons and omasionally other pituicyte ing pericytts scliroundeci by a basal lamina as well as processes by the tormation of F9iral lamellae suggested a pel ivasculw- q>ac?.in turn surrounded by an ablu- to 3re”uss et al. (1975) that they serve a physiologic minal b s d lamina. The space not only surrounds ves- function rather tha? simply acting as a stroma. Thtt sels l u , extends in mdtiple dirctions to penetrate this was the case has been shown in physiologic experwidely between -ou3 of parenchymal cells. The ablu- i m e r A both in ~ i v (datton o 1988) and in vitro (Perlminal 1 aserient membraQe has been found to possess mutter et al., 1984). It appears that, under basal condiscontinuities which nat o :ly permit contact between ditions. Pituicytes not only enshroud axms and their the extracel-ul ar and verivascular space, but are occa- termhations but interpose their processes between sionally filled by a>-o?al ‘,erminations.Despite this in- these endings and the abluminal basement membrane, tima& neurovascalar relationship, no contacts have effectively acting as barrier, to secretion. Alternabeen demoist *atedbetween endothelium and neurose- tively, in situations of hormone demand, e.g., dehydracretory cells. The phwiologic significance of the extra- tion or lactation, pituicytes facilitate secretion; their cellular space is uncatain; however, it is likely that it processes retract, bar terminstions, m d permit coneither represents a “pool” of hormone or, in its incom- tact between axons as well as axons and abluminal plete form, facilitates the regulation of neurosecretion basal lamina. (Livingston and Wiiks, 1976). In addition to the per2. Dark pituicytes are electron dense and demonicytes about the microvasculature, the perivascular strate an increased n r clear cytoplasmic ratio, but othspace contains small numbers of macrophages, fibro- erwise resemble major pituicytes. blasts, and occasional mast cells. Depending upon os3. Oncocytic pituicytes, like “ d a r k variants, probamolality, pituicytes either remove themselves from or bly represent a form of major pituicytes. I’ltrastructurabut the abluminal basement membrane; hence they ally, they possess im increased number of mitochonplay a physiologic role with respect to the vasculature, dria, often abnormal in morphology, to the deficit of
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other organelles. Mitochondria1 crystalloids may be seen. 4. Granular pituicytes contain abundant lysosomes, some resembling simple lipofuscin, others myelinoid bodies. Lipid droplets may also be noted. These cells are thought to represent yet another form of major pituicyte, perhaps one subjected to marked stimulation (Davis and Morris, 1973). Not surprisingly, their content of acid phosphatase may be significant (Whitaker et al., 1970). It has been suggested that granular pituicytes form by way of micropinocytosis-mediated phagocytosis (DeRobertis et al., 1975; Theodosis, 1979) resulting from transfer of secretory material from nearby axonal endings, some of which may show a distinct paucity of secretory material (Krsulovic and Bruckner, 1969). Not only may phagocytosis be ongoing under normal physiologic conditions, but experimental transection of the pituitary stalk also results in an increase in phagocytosed neurosecretory materials within pituicytes (Sterba and Bruckner, 1967). A similar process may occur in humans. Influx of phagocytes from the bloodstream into the posterior pituitary has been suggested as an alternate mechanism by which granular cells accumulate (Takei et al., 1980). Lastly, granular pituicytes may represent the cellular constituent of granular cell tumors or “choristomas” of the neurohypophysis (Liwnicz et al., 1984). These benign lesions are identical in composition to the granular cell clusters or “tumorlets” commonly found in the posterior lobe and pituitary stalk (Costello, 1936). 5. Ependymal pituicytes are uncommon. Like ependymal cells lining the ventricular system, they possess cilia and microvilli, particularly when clustered, engage in the formation of extracellular lumina, and exhibit distinct ribbon-like junctional complexes. In all other ways, their cytoplasm resembles that of major pituicytes. Ependymal variants are particularly frequent in lower vertebrates wherein they represent the principal form of pituicytes (Rodriguez, 1971).In mammals they are less frequent, their place being taken by cells of astrocytic nature (Holmes and Ball, 1974).
Neurosecretory Axons and Their Terminations As with pituicytes, the contributions of Lederis (19651, Bergland and Torack (19691, and more recently of Takei et al. (1980) and Seyama et al. (1980a1, constitute the only in-depth studies of the neurosecretory processes as they occur in the normal human neurohypophysis. Although a vast literature is based on experimental animal studies, our discussion will be limited to mammalian, in particular human, morphology. Oxytocin and vasopressin are produced within neurons of the supraoptic and paraventricular nuclei and are transferred to the posterior pituitary as electron dense secretory granules by a process of axoplasmic flow. Biochemical modifications are known to occur during transit, but the mechanism of hormone secretion is poorly understood. For the purposes of discussion, the principal zones of metabolic activity within the posterior pituitary are a) dilations or “swellings” occurring in the course of axon transit (Herring bodies), and b) axonal terminations from which hormone
transfer to the vasculature takes place. Variations in the ultrastructural appearance of axon dilations are thought to be a manifestation of different physiologic states. That such is the case has been demonstrated in lower animals (Lullman-Rauch, 1976; Rufener, 1974). As previously noted, questions remain regarding the mechanism by which hormones are released into the circulation. Whether the process involves actual extrusion of secretory granules (exocytosis), or transfer of released hormones across intact membranes at cell terminations, a process known as “molecular dispersion” (Krisch et al., 1972),remains unsettled. Despite careful research, Seyama et al. (1980a) were unable to identify the presence of exocytoses. The axons of the supraopticohypophyseal and paraventriculohypophyseal tracts are unmyelinated, measure 0.5-1.0 micron in diameter, and contain both microtubules and intermediate filaments (neurofilaments). Secretory granules are present in greatest concentration within axonal swellings which measure 150 microns in diameter . In light microscopic terms, such swellings correspond to “Herring bodies” (Herring, 1908).Vasopressin and oxytocin-containing fibers are, according to the classification of Seyama et al. (1980a), considered type A fibers. They contain 100300 nm secretory granules which vary in electron density as well as in the extent to which their limiting membranes are discernible. Type A fiber terminations within the posterior lobe contain not only electron dense secretory granules, but microvesicles, mitochondria, lysosomes, and tubuloreticular structures. The degree to which these various structures are represented in type A dilations is the basis of their morphologic classification (Seyama et al., 1980a). Type I Dilatations. Type I dilatations, rich in neurosecretory granules, likely represent stored neurosecretory materials, whereas the content of other dilatations probably reflects varying states of hormone metabolism. Though autoradiographic studies (Heap et al., 1975; Pickering et al., 1974) have shown labeled hormones t o appear within axon terminations prior to their accumulation in swellings (Herring bodies), there is, nonetheless, a distinct tendency for granules to accumulate in swellings. In the study of Morris (1976) swellings contained the majority (55 percent) of granules whereas terminations and nondilatated portions of axons contained only 31 and 13 percent, respectively. Type III Dilatations. Attempts have been made to correlate the morphology of axonal swellings with physiologic states. Dehydration, for instance, serves to stimulate vasopression secretion and has been found to be associated with increased numbers of lysosome-rich (type 111)dilatations (Krisch et al., 1972; Whitaker and LaBella, 1972). Their increase has also been noted when the need for neurohypophyseal hormones suddenly decreases (Kodoma and Fujita, 1975) or when axon degeneration follows either stalk transection (Dellmann et al., 1974) or destruction of paraventricular nuclei (Zambrano and DeRobertis, 1968). These observations support the contention of Seyama et al. who suggested that type I11 dilatations are a manifestion of degradation of unused neurosecretory granules.
ULTRASTRUCTURE OF THE NEUROHYPOPHYSIS
Fig. 1. Low-power view of normal human neurohypophysis, depicting capillaries, axonal processes most of which are agranular, and prominent granular pituicytes (arrows). x 4,700.
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Fig. 2. Fine structural details of granular pituicytes are shown. x 8,150.
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Fig. 3. Major pituicyte. Four hxonal processns, including one containing a large mych-figure (1-41,traversing through the cytoplasm. Note nuclear spheridium (arrow). X 12,800.
Fig. 4. Ependymal pituicytes joined by junctional complexes (arrowheads). Note cilia projecting into the small lumen formed by the pituicytes. The nucleus harbors spheridia (arrows). x 14,000.
Fig. 5. The area shown comprises mostly granule-rich type I dilatations, two of which contain glycogen alpha- and beta-particles (a,b, respectively). x 14,750.
Fig. 7. Type I dilatation, containing a large cluster of microvesicles (arrow). x 28,900.
Fig. 6. Micrograph depicting a few type I fibers and what appears to be a large type V dilatation (arrow) containing tubular structures
Fig. 8. The large axonal dilatation occupying a sizeable area of the photograph cannot be clearly typed. It contains low-density neurosecretory granules having the appearance of vesicles, scattered composite lysosomes, as well as clusters of glycogen beta-particles (arrows). x 8,700.
but entirely devoid of neurosecretory granules. x 28,000.
Figs. 9 and 10. The figures document large Herring bodies. The one of Figure 9 represents a type I1 dilatation containing numerous mitochondria and a large number of neurosecretory granules displaying low electron opacity. x 7,850. Figure 10 shows a type I11 axonal dilatation harboring numerous lysosomes. X 7,850.
Fig. 11. Capillary and vicinity. Note extension of the perivascular space (“sinusoid”)into surrounding neural tissue (asterisks). X 6,200. Fig. 12. Neurohypophyseal capillary. Note the luminal and abluminal basement membranes and the delicate fenestrated endothelium (arrows). x 15,400.
ULTRASTRUCTURE OF T H E NEUROHYPOPHYSIS
Type N Dilatations. Given the demonstration by Barier and Lederis (1966) that content of stored hormone within the posterior pituitary need not change despite diminution or disappearance of neurosecretory granules, Seyama et al. (1980a) suggested that type IV dilatations may represent “readily releasable pools” of hormone. That being the case, the presence of “empty” neurosecretory granules and perhaps vesicles suggests that hormone release is preceded by a process of molecular dispersion (Vollrath, 1974) rather than by exocytosis. Alternatively, the possibility that vesicles, particularly those clustered into “synaptoid clusters, represent recapture of granule membranes, following their fusion with the plasmalemma and resultant hormone discharge, cannot be discounted. It should be noted, however, Seyama et al. (1980a) found no direct evidence of exocytosis. Type ZZ, V, and VI Dilatations. The physiologic correlates of type 11, V, and VI dilatations remain to be elucidated. Type VI dilatations, characterized by an increase of microvesicles, may be a consequence of increased release of neurohypophyseal hormones (Dreifuss et al., 1975; Santolaya et al., 1972). The possibility that they represent an increase in acetylcholine-containing synaptic vesicles similar to those in the central nervous system (Dehbertis, 1962) is considered unlikely. Type B Fibers In contrast to type A fibers, those of type B are far less numerous and contain secretory granules of smaller size (50-100 nm). In addition to their smaller dimensions, secretory granules of type B fibers are separated from their enveloping membrane by a definite electron-lucent halo. Their ultrastructural features are less complex and clustering of 30-40 micron microvesicles in “synaptoid structures is far less prominent (Seyama et al., 1980a).Microvesicles are more uniform in configuration than in type A fibers, but show greater variation in diameter (30-50 nm). Seyama et al. (1980a) undertook a morphologic comparison of fiber types and likened those of type B to fibers occurring in the median eminence and infundibulum of animals (Duffy and Menefee, 1965; Monroe, 1967) as well as of man (Bergland and Torack, 1969). Their findings support the concept that type B fibers are aminergic rather than associated with oxytocin and vasopressin release. The function of such fibers in the posterior pituitary is unknown. Two principal forms of dilatations are described in type B fibers, some rich in granules and others in which microvesicles are particularly numerous.
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