J. Anat. (1979), 129, 2, pp. 377-397
With 31 figures Printed in Great Britain
Morphological observations on the mesonephros in the postnatal opossum, Didelphis virginiana WILLIAM J. KRAUSE, J. HARRY CUTTS AND C. ROLAND LEESON Department of Anatomy, School of Medicine, University of Missouri, Columbia, Missouri
(Accepted 14 August 1978) INTRODUCTION
The North American opossum (Didelphis virginiana) has a gestation period of only 121 days. Immediately after birth the young crawl into their mother's pouch where each attaches to a nipple. Development continues within the protection of the pouch until just after weaning, which occurs approximately 80 days postnatum. The newborn opossum is immature in appearance apart from well developed fore-limbs: the hind-limbs are paddle-like, and show only the initial stages of digit formation. The epidermis consists of a Malpighian layer, a layer of incompletely cornified cells, and a periderm (Krause, Cutts & Leeson, 1978). Visceral structures such as the lung (Krause & Leeson, 1973, 1975; Krause, Cutts & Leeson, 1976a), liver (Cutts, Leeson & Krause, 1973; Krause, Cutts & Leeson, 1975), pancreas (King, Krause & Cutts, 1978) and alimentary canal (Krause, Cutts & Leeson, 1976b, c, 1977) are immature at birth, and undergo an extensive period of postnatal development and differentiation. The kidneys of the newborn opossum also are immature, and the mesonephros and metanephros may overlap functionally during the first week of postnatal life (Gersh, 1937). Very little information is available concerning the morphological changes that occur during the gradual regression and reabsorption of the mesonephros. The present study examines the changing structure of the mesonephros in the opossum during the first 22 days of postnatal life. MATERIALS AND METHODS
Sixty pouch young opossums (Didelphis virginiana) were divided into the following six groups according to their snout-rump lengths: 1P4 (newborn), 2-0, 2-25, 2 5, 3-5 and 4-5 cm. Age determinations are based on results presented by Cutts, Krause & Leeson, 1978. The animals were killed by decapitation and as quickly as possible tissues were removed and placed in Bouin's solution or in 10 % buffered neutral formalin for light microscopy. The tissues were processed routinely, embedded in paraffin, sectioned at about 6 ,um and stained with one of the following: haematoxylin and eosin, van Gieson, toluidine blue, and periodic acid-Schiff (PAS) before and after treatment with saliva. In addition, the entire urinary system from three animals of each group was fixed and sectioned serially. Gross weights of the mesonephroi were determined, in separate groups of animals, on a torsion balance. Additional blocks of tissue were fixed for 4 hours at 0 'C in 3.5 % glutaraldehyde 0021-8782/79/2828-6470 $02.00 C 1979 Anat. Soc. G.B. & L.
W. J. KRAUSE, J. H. CUTTS AND C. R. LEESON
I Fig. 1. An idealized sketch of a mesonephric nephron of the opossum. The renal corpuscle (A) is drained by a proximal tubule (B) which courses through the mesonephros but eventually loops back to lie adjacent to the parent renal corpuscle. At this point there is an abrupt transition to the distal mesonephric tubule (C). The distal tubule courses to lie just beneath the surface of the mesonephros and gradually merges into a collecting tubule (D). The collecting tubule drains into the mesonephric duct (E).
Fig. 2. The external features of portions of the urinary system viewed on the posterior abdominal wall of a newborn (1-4 cm) opossum. The mesonephros (me), mesonephric duct (arrows), Miillerian duct (Md), metanephros (mt) and urinary bladder (B) are clearly shown. The adrenal cortex (A) and developing gonad (G) also are shown. x 20. Fig. 3. A longitudinal section through the mesonephros of a newborn opossum. The renal corpuscles (arrows) lie in one or two strata along the ventromedial surface. The mesonephric duct (Md) is shown near the bottom of the micrograph. The stomach (S), spleen (SP) and a loop of a small intestine (SI) also are shown. Haematoxylin and eosin. x 40. Fig. 4. Adjacent mesonephric renal corpuscles are closely approximated. Glomeruli (G) and the parietal layer of Bowman's corpuscle (arrows) are evident. Newborn opossum. Epon 812. Toluidine blue. x 250. Fig. 5. The pyramidal-shaped cells of the proximal tubules (P) show numerous vacuoles, and stain more intensely with toluidine blue, than do cells comprising the distal tubules (d) of the mesonephros. Newborn opossum. Epon 812. Toluidine blue. x 250. Fig. 6. Collecting tubules (ct) from the mesonephros of a newborn opossum. The tubules show a heterogeneous population of light and dark (arrows) cells. Toluidine blue. x 400. Fig. 7. The mesonephric duct (md) of a newborn opossum is lined by a light staining columnar epithelium. Epon 812. Toluidine blue. x 250.
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