Arch. histol. jap., Vol. 40, No. 4 (1977) p. 305-320 Department of Internal Medicine (Prof. M. MATSUOKA) andDepartment of Anatomy (Prof. T. FUJITA), Niigata University School of Medicine, Niigata, Japan Megakaryocyte

and

Platelet

Microscope Toshio

Formation: Study

in Mouse

IHZUMI, Akira

A Scanning

Electron

Spleen

HATTORI, Masayoshi and Masaki MUTO

SANADA

Received April 20, 1977

Summary. Mouse spleen was examined by scanning electron microscopy after perfusion fixation and cryofracture. Megakaryocytes are recognized usually in the splenic cord near the sinus and postsinal venules but rarely in their lumen. They are usually fixed to the reticulum cells and sometimes both cells are connected to each other by cytoplasmic interdigitations. Immature megakaryocytes are round but mature ones are irregular in shape with many cytoplasmic protrusions. The cell surface reveals small pores and many microprocesses of two types: Type I microprocesses are blebs known to be involved in platelet liberation, while the significance of type II or villous microprocesses remains unknown. Surface pores represent the orifices of the platelet demarcation membrane system (DMS) in the cytoplasm. On the fracture surfaces of the cell, the DMS is recognized as a network of furrows and its development is confirmed to be initiated by invagination of the plasma membrane. Individual platelets formed by developing DMS become seined spheroid in shape. Platelet release occurs in four different modes: 1) After the DMS has divided individual platelets in the intermediate zone of megakaryocyte, it invades the ectoplasm encircling small surface areas. Some ectoplasmic pieces thus formed are liberated from the cell like a cork being withdrawn from a bottle. Through these ectoplasmic defects the interior platelets are released either in masses or in beaded ribbons. Individual platelets liberated in the splenic cord migrate into the sinus lumen through the intracytoplasmic fenestration of the endothelium. 2) Megakaryocytes project larger pseudopods into the sinus lumen, which become fragmented into platelets. 3) Megakaryocytes themselves enter the circulation and release platelets in remote organs such as the lung. 4) The megakaryocytes project small ectoplasmic blebs (type I microprocesses) into the sinus lumen and then detach them. By this process, as well as by the fragmentation of the ectoplasm above mentioned, megakaryocytes may produce platelets lacking in organelles. Since WRIGHT(1914) described that the megakaryocyte is the mother cell of blood platelets, many investigations have been made on this cell by means of light microscopy, phase contrast cinematography, transmission electron microscopy (TEM) and recently by means of scanning electron microscopy (SEM). In these studies, the areas of inquiry have been how and where the megakaryocyte releases the platelets and, since YAMADA(1957) found the platelet demarcation membrane system (DMS), how the system is formed in the cell. Various observations on the origin of the DMS have been made: YAMADA(1957) reported that the system was formed by the coalescence of special vesicles which were distinctly different from the elements of smooth endoplasmic reticulum. Some authors regarded the DMS as a derivative (SCHULTZ, 1966) or a specialization (HAN and BAKER,1964) of endoplasmic reticulum, whereas 305

306

T. IHZUMI et al.:

others thought that its formation was initiated by invagination of the plasma membrane (BEHNKE, 1968; MACPHERSON, 1972). YAMADA(1957) proposed that the slits of the DMS in the mature magakaryocytes extended by their coalescence and subdivided small cytoplasmic areas (platelet zones), each of which developed into platelets. Although YAMADAdid not mention the possible mechanism of the release of platelets thus separated, later researchers suggested various modes of platelet release. Some observed that whole megakaryocytes were simultaneously fragmented into platelets (ALBRECHT,1957; KOSAKI, 1974). Others described that individual platelets were liberated one after another from the pseudopods of the megakaryocyte (WEISS, 1970; KEYSERLINGK and ALBRECHT, 1968; BEHNKE, 1969). Intrasinal occurence of platelet release has been commonly accepted, but BEHNKE (1969) considered that both intra- and extrasinal platelet liberation was possible. The extrasinal platelet release has remained to be confirmed and it is also obscure as to how platelets born in the splenic cord could migrate into the sinus lumen. TANAKA(1972) described in his textbook that ectoplasmic bleb-like projections lacking in organelles (type I microprocesses in the present paper) are released as a kind of platelet into sinus lumen, although DE BRYUN(1964) had regarded them as a type of apocrine secretion. In accordance with the view of TANAKA,we sometimes encounter platelets lacking organelles in the peripheral blood, although their significance is unknown. We observed megakaryocytes by SEM in various stages of maturation in the cryofractured mouse spleen in order to clarify the above problems. This material was chosen because the mouse is known to have numerous megakaryocytes in the spleen. Materials and Methods Twenty ICR mice, weghing about 20g, were used in this study. Under anesthesia by ethyl-ether inhalation, the abdominal and pleural cavities were opened to expose the heart and spleen. A polyethylene tube was inserted into the left ventricle of the heart; and perfusion was made through the tube first with 200ml of saline to wash the vascular bed and then with 100ml of 1% glutaraldehyde (in 0.1M phosphate buffer, pH 7.4) to fix the tissue. The portal vein was cut down to drain the blood and perfusate. The perfused and fixed spleen was taken out and immediately cut into small pieces about 1mm3. These specimens were kept in the same fixative for 2hrs, and then transferred to a solution of 2% glycine, 2% glutamate and 2% sucrose and a 2% aqueous solution of tannic acid and finally to 2% osmium tetroxide according to the method of MURAKAMI (1974). The specimens were dehydrated by acetone, transferred to isoamylacetate, and freeze-cracked in liquid nitrogen by the method of TOKUNAGAet al. (1974). The cracked specimens were dried by the CO2 critical point method, evaporated with gold palladium, and observed in a Hitachi HFS-2 field emission SEM with an accelerating voltage of 10kV. Small specimens of the perfusion-fixed spleen were post-fixed in OsO4 and simultaneously processed for TEM according to the conventional method. The thin sections were doubly stained with uranium acetate and Sato's lead staining (SATO, 1968) and examined in a Hitachi HU 125-DS electron microscope.

Megakaryocyte and Platelet Formation

307

Observations General structure

of megakaryocyte

The megakaryocytes, easily identified by their unusually large size, were frequently found in the splenic cord but rarely in the sinus and venule lumen. In the splenic cord, many megakaryocytes were located near the sinus and venule wall; some actually attached to their wall. Their gross contour was generally spherical or ovoid (Fig. 1) and occasionally irregular (Fig. 2). The diameter of the megakaryocytes

ranged

from

parenchymal size

19

tissue

should

be

to

42μ.

during

between

If

the

fixation 20

and

44μ.

shrinkage and

rate(-64%

drying

is taken

Megakaryocytes

by into were

TOSHIMA,

1976)

consideration, usually

the

of

the

actual

surrounded

and

partly tightly attached by reticulum cells. When the cell body of the megakaryocyte was encircled by reticulum cell processes, the former was sometimes constricted into a dumb-bell shape (Fig. 2). A close association was recognized between the interdigitated portions of both types of cells (Fig. 4). The cell surface was smooth or slightly uneven, showing some pores and microprocesses (Fig. 1, 2, 5). The latter were divided into two types: Type I, rounded ones

Fig. 1. Spherical megakaryocyte with its exposed nucleus (N). The platelet demarcation membrane system (*) shows mainly tubular structures in the cytoplasm. Microprocesses (thin

arrows)

and

pores

(thick

arrow)

are

seen

on

the

cell

surface

(**).

×9,400

308

T. IHZUMI et al.:

Fig. 2. Mature megakaryocyte. Pores have developed at multiple sites on the cell surface (**). Well developed DMS is seen in the fractured portion of the cell (*). The cell is constricted

by

reticulum

fibers

(arrow).

×3,200

Fig. 3. High magnification of a part of Figure 2, showing both fracture surface (*) and cell surface (**). A pore is connected with the demarcation membrane (thin arrow). A microprocess is found in a pore and some others stand the margin of the pores (thick arrow). The mage-like interior of the megakaryocyte is seen through a large complex pore (paired arrows). ×12,300

Megakaryocyte with

or

without

processes

a thin

correspond

processes

were

neck; to

villous

their

the

maximum

blebs

in

and Platelet

Formation

thickness

hitherto

was

described

appearance,

about

in

1.6μ

309

long

about

TEM

1.6μ.

These

studies.

and

0.3μ

Type thick

microII

(Fig.

micro5).

No

clear description seems to have been previously made of this type. The frequency and the distribution of these microprocesses differed from cell to cell; their distribution was sometimes localized to a limited section on the surface. Some megakaryocytes possessed both types of processes, while others only one type (Fig. 2). The

pores

appeared

generally

round

(0.04-0.6μ

in

diameter)

but

occasionally

ir-

regular, elongate or slit-like; a complex form consisting of a few smaller pores was sometimes encountered. When the pores were large enough, a maze-like internal structure of the megakaryocyte could be recognized through them. Occasionally single villous microprocesses (type II) stood on the margin of the pores; a few processes were also noted projecting from the pores (Fig. 3). Viewing up

to

20μ

Some

were

the length)

fracture were

tubular,

surface, observed

showing

numerous to

be

round

either

holes

furrows

and

scattered

or

when

crossly

slits grouped

cut

(about

(0.2-0.3μ in 0.2μ

width

the

and

cytoplasm. in

diameter)

(Fig. 1, 2, 3, 5). On the basis of their distribution and dimension, this intracellular space system was considered to correspond to the DMS found in thin sections. In appropriate fracture surfaces which included the cavities of the surface pores, the plasma membrane was confirmed to continue into the demarcation membrane; the pores were the orifices of DMS (Fig. 3). On a favorable fracture surface, a characteristically lobulated nucleus was found in the central part of the cell (Fig. 1, 5). Its surface was smooth, showing occasional nuclear pores. Near the nucleus, Golgi apparatus could be recognized as lamellar slits which

were

narrower

(0.07μ)

than

the

demarcation

slits

above

mentioned.

Apparent

Golgi vesicles often contained small granules. Larger granules (future platelet granules) and their sacs were also found in the cytoplasm as spherical bodies or cavities, 0.2μ

in

diameter

(Fig.

Fig. 4. Fracture surface of the plasma membrane seen at the cell periphery

3, 5).

of immature megakaryocyte (M) and reticulum cell (R). Invagination (single arrow) and tubular elements (paired arrows) of the DMS are of the megakaryocyte. Note cytoplasmic interdigitation between the megakaryocyte

and

reticulum

cell.

×6,200

310

T. IHZUMI et al.:

The TEM observation by BEHNKE(1968) and the present study indicate that the DMS first appears at the cell surface as plasma membrane invagination in immature megakaryocytes. The DMS then develops into a large network in the intermediate zone. This network later spreads to occupy almost the entire cytoplasm just prior to platelet release. The amount and distribution of the DMS in the cytoplasm as observed by SEM of fractured specimens, therefore, indicate the maturation degree of the cells. The SEM findings in immature and mature megakaryocytes thus identified are described below. Immature

megakaryocytes

Immature megakaryocytes were generally spherical in gross contour (Fig. 1). Both types of surface microprocesses were found. The villous ones (type II) were relatively few but they were occasionally densely gathered in a limited area. The pores

were

generally

round

(0.1-0.4μ

in

diameter)

and

often

gathered

in

groups.

The DMS in this stage was mainly represented by tubular channels which occurred only near the plasma membrane (Fig. 4); this was confirmed also by the present observation of thin sections by TEM. Mature megakaryocytes Mature megakaryocytes tended to be more irregular in shape than the imma-

Fig. 5. Mature megakaryocyte surface (**). Two types of the

in the splenic cord, showing cell surface (*) and fracture surface microprocesses are observed: Type I micropocesses

(blebs) are indicated by thick arrows, while type II or villous microprocesses by thin arrows. In the cell center, platelet zones (P) encircled by well developed DMS and some cytoplasmic granules are discernible. This cell attaches to the sinus wall, projecting its bleds into the sinus

lumen

(S).

N

nucleus.

×6,200

Megakaryocyte

and Platelet

Formation

311

Fig. 6. Megakaryocyte projecting its blebs into the sinus lumen (S), some of which have been detached

Fig.

7.

from

Fracture

the

cell.

surface zones,

which

They

may

of mature are

seen

be

platelets

without

organelles

megakaryocyte.

Well

as

(arrows).

spheroid

bodies

(arrow).

developed

×14,000

DMS forms

×11,900

platelet

312

T. IHZUMI et al.:

Fig.

8.

Irregular

shaped,

platelet

(arrows),

which

releasing may

Fig. 9. Part of megakaryocyte surface. into platelets zones without organelles,

be

megakaryocyte detached

as

with

platelets.

many

tear-drop

processes

×4,500

Small surface areas are divided by channels of DMS some of which may detach from the megakaryocyte. ×25,000

Megakaryocyte

ture

ones

rence on

the

surface

of

due to the constriction large

protrusions.

surface; pores

although were

increased

and Platelet

of the bodies Many

they

by reticular

processes

were in

of both

densely

number

Formation

and

cell processes types

gathered size,

313

were

in some

measuring

up

and

generally 0.8μ

occur-

scattered

portions. to

the The

in

cell

diameter.

They were generally round in shape. Occasionally, they accumulated at multiple sites on the cell surface. Complex pores now appeared at this stage. The spaces of the DMS became numerous at the intermediate zone of the cytoplasm, forming a complicated network of slits with a sponge-like appearance. On the fracture surface they appeared mostly saccular or irregular in shape (Fig. 3. 5). The cytoplasmic areas encircled by them (so called platelet zones) were not always discoid but often rounded

in

shape.

processes (Fig. 7).

They

measured

Some mature

about

1.6μ

in

megakaryocytes

diameter

and

often

possessed

were observed migrating

spiny

into the

Fig. 10. Fracture suface of a platelet releasing megakaryocyte. Numerous platelet zones (P) are seen in the intermediate cell portion. The ectoplasm (*) is discontinuous at two sites (arrows), from where the platelet zones are exposed and ejected to the extracellular space. N

nucleus.

×5,600

314

T. IHZUMI et al.:

lumen of the sinus and the postsinal venule, i.e., the pulp vein portion close to the sinus through their walls; a few cells had invaded the sinus lumen. Platelet

releasing

megakaryocytes

The megakaryocytes under platelet liberation were irregular in shape: their surface was characteristically wavy and intricate with many cytoplasmic protrusions (Fig. 8, 10). The previously round pores now have elongated into irregular spaces. Finally they encircled small surface areas or platelet zones in a manner similar to those of the DMS in the interior of the cytoplasm (Fig. 9). The DMS then invaded the ectoplasm and the platelet zones were sometimes elevated as small cytoplasmic protrusions (Fig. 10.). Some of them detached from the megakaryocyte like a cork being withdrawn from a bottle. Besides, bleb-like microprocesses occured here and there on the cell surface. Some of the blebs became larger and developed into a teardrop shape which was attached to the megakaryocyte only by a thin neck (Fig. 8). In the lumen of the sinus and of the postsinal venule, two types of protrusions of megakaryocytes were found: the smaller type showed shapes like pearl beads or

Fig.

11.

karyocyte

Beaded in

the

(thin arrows) venule

lumen.

and bunched The

yare

(thick arrows) believed

to

be

cytoplasmic separated

protrusions into

platelets.

of the mega×2,200

Megakaryocyte

and Platelet

Formation

315

Fig. 12. A platelet (arrow) passing the sinus wall (S) through an intacellular fenestration of the endothelium. Two platelets are further shown: one at the outer surface of the sinus wall and

the

other

in

the

sinus

lumen.

×18,000

bunches of grapes similar to the protrusions of the platelet zones in the splenic cord (Fig. 11), and the larger type had pseudopods possessing round (type I) and villous (type II) microprocesses as well as pores. Near these pseudopods some single platelets were scattered. By TEM as well as SEM, the blebs lacking in organelles were confirmed to be projected into the sinus lumen through fenestrations of the endothelium; and some of them most probably were being detached from the megakaryocytes (Fig. 5, 6). Platelet

migration

into the sinus lumen

A number of separate platelets were found in the splenic cord and also on the outer surface of the sinus wall. Several platelets were in the sinus wall and some of them were confirmed to be located not between but in the endothelial cells. Furthermore, some platelets were observed on the inner surface of the sinus wall (Fig. 12). Thus it seems most probable that the platelets born in the cordal tissue may migrate through the intracytoplasmic fenestrations of the endothelial cells. Residual

megakaryocytes

Residual megakaryocytes were recognized as smaller cells with a strikingly scanty cytoplasm having nuclei similar in dimension to the mature megakaryocytes. The surface was characteristically undulated in a reticular pattern. This structure presumably represented the residual DMS.

316

T. IHZUMI et al.

Discussion Demarcation

membrane

system

In his TEM study on mouse spleen megakaryocytes, YAMADA(1957) first observed that the earliest manifestation of DMS was the appearance of vesicles in the intermediate zone of the cytoplasm, that those vesicles developed to a maze-like structure by coalescence, and that finally the DMS of mature megakaryocytes separated numerous platelet zones throughout the intermediate portion of the cytoplasm. As for the origin of the DMS, YAMADA(1957) considered that it was a distinct element differing from the smooth endoplasmic reticulum. Later, DE BRYUNin guinea pig bone marrow and HAN and BAKER(1964) in rat bone marrow reported that the DMS continued to the smooth endoplasmic reticulum. BEHNKE(1968), by TEM, demonstrated that both the DMS and the plasma membrane were similary demonstrated by horse radish peroxidase, thorotrast and ruthenium red. From this observation and, from the fact that a continuity of the DMS to the plasma membrane has been repeatedly reported in mature megakaryocytes (YAMADA,1957; HAN and BAKER,1964; KEYSERLINGK, 1968;BEHNKE,1968), BEHNKE (1968) concluded that the origin of the DMS was plasma membrane, i.e., its formation initiated by an invagination of plasma membrane. In his TEM study using rat bone marrow, MACPHERSON (1971) observed different stages of initial invagination of plasma membrane in very immature cells. By SEM examination of fractured cells, we demonstrated the continuity of the surface pores to DMS in both immature and mature cells as shown in Figures 4 and 3. Furthermore, some villous microprocesses were found to stand on the margin of pores or even in pores as shown in Figure 3. These microprocesses can be reasonably considered to have been pulled into the pores by the invagination of plasma membrane. On the basis of these findings, the present observation supports the conclusion of BEHNKE (1968)and MACPHERSON (1972) onthe origin of the DMS. Concerning the site of the invagination, two observations were reported. BEHNKE (1968) showed that it occurred at multiple sites of the cell membrane, whereas MACPHERSON(1972) hypothesized that it initiated first at a point on a pseudopod and that later it occurred at mulitiple sites. This initiation of the invagination at one particular site was not confirmed in our study. This discrepancy may have resulted from the fact that the megakaryocytes in our observation were probably more mature than those observed by MACPHERSON (1972).Our observation demonstrated that the plasma membrane invagination occurred at several points in relatively immature cells. The invagination later developed at more numerous sites throughout the surface, and they became intricately infolded into the intermediate zone of the cytoplasm. Platelet

zones and platelet

release

As viewed in SEM, the DMS separated cytoplasmic portions and subdivided them progressively until numerous small platelet zones of spined spheroid contour were formed (Fig. 5, 7, 10). These SEM findings confirm the previous observations obtained in thin sections by various investigators. YAMADA(1957) postulated that DMS and organelles, such as granules and mitochondria in the intermedriate zone invaded the outer zone (ectoplasm) and that the DMS was inserted into the cell membrane at the last stage of maturation. Our SEM images showed that DMS invaded the ecto-

Megakaryocyte and Platelet Formation

317

plasm in the platelet releasing megakaryocytes and at some portions the ectoplasm of the megakaryocytes was subdivided into small segments, i.e., "platelets" without organelles. Such development of DMS on the cell surface coincided with our observation that elongate and irregular surface pores coalesced into channels encircling small surface areas corresponding in size and shape to the platelet zones in the cytoplasm. The small surface areas were apparently ejected as shown in Figure 10. This process has not been previously reported. Through the site where the ectoplasm thus became lost, the megakaryocyte porjected its interior platelet zones into the sinus and venule lumen as beads of pearls or bunches of grapes (Fig. 11). Ribbonlike protrusions of megakaryocytes into the sinus lumen have been reported by MUTO (1974) and BECKER(1974) in their SEM observations of rat bone marrow, and by THIERY and BESSIS (1956) in cell culture. Platelets are believed to be liberated by segmentation of this type of process. On the other hand, the grape-like forms found in this study probably correspond to proplatelets (BECKER,1976) which will separate into platelets later. Another mode of platelet separation was that the megakaryocytes projected larger cytoplasmic pseudopods into the sinus or postsinal venule lumen; later, platelets detach from these pseudopods one after another. This mode in which the DMS does not seem to play a major role in platelet release has been described by BEHNKE(1969). The third mode was that the megakaryocytes themselves migrated into the sinus lumen. These megakaryocytes were usually mature but not so far as to liberate platelets. These cells are believed to enter the circulation and release platelets after settling in other organs, as KAUFMANN (1965) demonstrated platelet liberation from megakaroycytes in the lung. Concerning the blebs, their detachment from the megakaryocyte ectoplasm was found in the sinus lumen as shown in Figures 5 and 6. Previonsly, DE BRUYN(1964) observed a similar phenomemon in guinea pig bone marrow but regarded it as an apocrine secretion of megakaryocyte. TANAKA(1372) described it in his textbook as a mode of platelet release. On the other hand, BEHNKE(1969) could not find the blebs in rat bone marrow megakaryocytes fixed by perfusion and considered that the blebs represented a degenerative change produced by incomplete fixation. Our stuby using fixation conditions similar to those of BEHNKEsuggests that the blebs are not degenerative changes but proper cell structures. This descrepancy between BEHNKE's result and ours may possibly be attributed to the difference in species. After being detached, the blebs seem to circulate as a kind of platelet since platelets lacking in organelles are occasionally found in mouse vessels and in human peripheral blood (HATTORI,1967; TANAKA,1972). Their function in comparison is unknown. Platelet

with normal platelets

migration

The course and mechanism of platelet migration from the splenic cord into the sinus lumen has not been clear. In his TEM study in rat bone marrow, BEHNKE (1969) observed migrating platelets through intercellular spaces of the lining cells. By TEM and SEM OGAWA(1975) reported that platelets might pass through the intracellular fenestrations of the sinal endothelial cells. The present study in the mouse spleen clearly demonstrates that platelet migration occurs, at least for the major part, intracellularly (Fig. 12).

318

T. IHZUMI et al.:

Acknowledgment. of Scanning Electron

The authors express their sincere thanks to Mr. K. ADACHI (Laboratory Microscopy of our school) for his technical assistance.

巨 核球 と血 小 板 形 成 機 構-マ

ウ ス脾 臓 の 走 査 電子 顕 微 鏡 に よ る 観 察







雄, 服







好, 武

部 藤

晃 正



マ ウス脾 臓 を灌 流 固 定, 凍 結 割 断 の の ち, 巨核 球 に つ い て走 査 電 子 顕 微 鏡 に よ る観 察 を 行 な った. 多 くの 巨核 球 は脾 索 内 に存 在 し, 脾 洞 と細 静 脈 の壁 に近 接 し てい た. ま た 稀 に は 洞 内 と細 静 脈 内 に も認 め られ た. 巨核 球 は細 網 細 胞 に よ っ て囲 ま れ,

とき には 両 者 の 胞

体 の鋸 歯 状 結 合 もみ られ た. 巨核 球 の概 形 は, 未 熟 な も のは 円形, 成 熟 す るに 従 って 不 整 形 とな り, 細 胞 表 面 に は大 きな 突起 が み られ た. 巨核 球 の 細 胞 表 面 に は 小孔 と小 突起 が 認 め られ た. 小 突 起 に は2種 類 が 識 別 され, 一 つ は 血 小 板 放 出 に 関 与 す る と考 え られ る球 状 の 突 起 (第 一型) で, 他 は絨 毛 状 の細 長 い 突 起 (第 二 型) で あ るが, 後 者 の 意 義 は 明 らか に され な か った. 細 胞 表 面 の小 孔 は 細 胞 内の 血 小 板 分 離 膜 系 の入 口で あ る こ とが 明 らか に され た. 細 胞 の割 断 面 す な わ ち細 胞 内 に は,

血小 板 分離 膜 系 が 小 溝 とし て認 め られ,



の発 達 が形 質 膜 の細 胞 内へ の 陥 凹 に よ って 始 ま る こ とが確 認 され た. 発 達 し た血 小 板 分 離 膜 系 に よ って 囲 まれ た 血 小 板 小 野 は 金 米 糖 様 の形 を し てい た. 血 小 板 放 出 様 式 につ い ては 四つ の機 序 が 観 察 され た. 1) 血 小 板 分 離 膜 系 が 巨核 球 の 中 間層 に発 達 し, 個 々の 血 小 板小 野 を 形成 した の ち, この分 離 膜 系 が エ ク トプ ラ ズ ム にま で 及 び,

つ い に は細 胞 表 面 を細 区分 す る.

この 部 分 が び んか ら コル ク栓 を ぬ く よ うに して

細 胞 か ら離 れ た の ち, このエ ク トプ ラ ズ ムの 欠 損 部 よ り内部 の血 小 板 が 集 合 して あ るい は リボ ン状 に連 な って放 出 さ れ る. この さい脾 索 で放 出 され た血 小 板 は 洞 壁 の 内皮 細 胞 内 の 小 窓 を 通 って 洞 内 に 流 出 す る. 2) 巨核 球 の大 きな 細 胞 質 偽 足 が洞 内 に突 出 し, 後 に な っ て これ が 個 々 の血 小 板 に 分 離 す る. 3) 巨核 球 が 直 接 血 流 に入 り, 肺 な どで血 小 板 を放 出 す る. 4) 巨核 球 の細 胞 小 突 起 (第 一 型) が 洞 内 に 突 出,

これ が血 小 板 と して離 れ る. こ

の 場 合 は小 器官 の な い血 小 板 が 生 ず る と考 え られ た.

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飯泉 俊 雄 〒951新 潟市旭町1 新 潟大学医学部 第一内科学教室

Dr. Toshio IHZUMI First Department of Internal Medicine Niigata University School of Medicine Asahi-machi 1 Niigata, 951 Japan

Megakaryocyte and platelet formation: a scanning electron microscope study in mouse spleen.

Arch. histol. jap., Vol. 40, No. 4 (1977) p. 305-320 Department of Internal Medicine (Prof. M. MATSUOKA) andDepartment of Anatomy (Prof. T. FUJITA), N...
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