Formation of secretion granules in the Golgi apparatus of plasma cells in the rat

The three-dimensional structure of the components of the Golgi apparatus was analyzed in plasma cells of rat duodenum. The spheroidal juxtanuclear Golgi apparatus was formed by a continuous ribbonlike structure composed of the following stacked elements. On the cis-face of the Golgi stack, there was a tubular membranous network referred to as the cis-element and/or a slightly dilated saccule perforated with small pores. The two or three subjacent saccules, which showed few pores, were slightly dilated and contained a fluffy granulofilamentous material. They were also perforated in register by cavities or wells containing 80-nm vesicles. The next one or two underlying elements were fenestrated saccules showing flattened portions as well as distended portions containing a homogeneous material denser than that seen in the overlying saccules. The last two or three elements of the stack showed a partially separated or "peeling off" configuration. These last elements consisted of prosecretory granules attached to flattened, empty-looking saccules showing buds at their surface; detached, more-or-less fenestrated, flattened saccules; and shrivelled residual trans-tubular networks. In the trans-region of the stack, in addition to numerous small vesicles, short membranous tubules, detached prosecretory granules, and denser fully formed secretion granules were also seen. These images were interpreted to indicate that secretory material present in the trans-saccules flows toward the dilated portions which become prosecretory granules. The trans-most elements seemingly peel off the stack to yield prosecretory granules and fragmenting trans-tubular networks.     

Tridimensional architecture of the Golgi apparatus and its components in mucous cells of Brunner's glands of the mouse

The three-dimensional structure of the Golgi apparatus and its components has been analyzed in thin and thick sections of mucous cells of mouse Brunner's glands by using low- and high-voltage electron microscopes and a stereoscopic approach. In thick sections of glands impregnated with osmium or treated to detect nicotinamide adenine dinucleotide phosphatase (NADPase) or thiamine pyrophosphatase (TPPase) activity, the Golgi apparatus appeared, at low magnification, as a continuous network located in the supranuclear region. At higher magnifications and in thin sections of tissue postfixed with reduced osmium and stained with lead citrate or treated to demonstrate phosphatase activity, the following components were observed: on the cis-face of the Golgi stacks, an osmiophilic tubular network referred to as the cis-element; a cis-saccular-compartment composed of a distended porous saccule slightly reactive for NADPase and three or four underlying NADPase-positive, flattened, poorly fenestrated saccules; a trans-saccular-compartment consisting of four to six TPPase-positive saccules or sacculo-tubular elements, prosecretory granules, and "peeling off" trans-tubular networks. The saccules of the cis-compartment were often perforated by large pores in register. The cavities thus formed in the stacks were called wells and were pan-shaped with a mouth directed toward the cis-face of the stacks and a bottom closed by TPPase-positive saccules. The wells always contained 80-nm vesicles. The saccules of the trans-compartment were involved in the formation of secretory granules according to the following proposed sequence of transformation. The secretion product appeared initially as a granular material evenly distributed throughout a slightly distended, poorly fenestrated saccule. These saccules appeared to transform into fenestrated elements with irregular pores and with parts of them taking on the appearance of a tubular network; they were thus referred to as sacculotubular elements. The secretory material initially distributed throughout these elements accumulated in nodular dilatations randomly distributed along the tubular portions of the elements. The dilatations, considered as prosecretory granules, increased in size as they drained the secretory material from the rest of the sacculotubular elements. Such prosecretory granules, large and irregular in shape, "peeled off" from the stacks of saccules with residual saccular or tubular structures still attached to them, some of the latter forming trans-tubular networks. The prosecretory granules detached from such membranous residues, condensed, and finally transformed into spherical secretion granules.     

The cytomorphology of goblet cells of the fetal intestine. Studies of the large intestine of cattle (Bos primigenius taurus)

In the region of the base of the intestinal crypts undifferentiated goblet cells display a configuration and constellation of organelles and membrane structures that are indicative of their importance for function. These images at this stage of development deliver a scenario of the mechanism of secretory granule production: aggregates of protein vesicles from the "transitional elements" (PALADE) of the granular endoplasmic reticulum are, so to speak, rolled up on the trans side of the Golgi apparatus by inversion of peripheral membrane segments of the innermost Golgi lamellae, thereby forming corpuscles. The origin of the capsulated vacuoles, which contain vesicles as single elements or as conglomerates, is well established. Their capsule consists of a trilaminar external and external and internal membrane; between them lies condensed material of the Golgi apparatus. In the opinion of the present author, the development of the ensheathed vacuoles represents a basic, more general mechanism. In contrast, the further steps of synthesis, for the formation of secretory granules, are more heterogeneous. Condensation of the vesicles and the inner capsular membrane results in the formation of a prosecretory granule, which in the basic element in the process of secretory granule production. The prosecretory granules develop singly or by fusion with other granules to give primary secretory granules. The complexity of this mechanism of secretory granule formation, however, becomes evident when considering the apposition of capsulated vacuoles and prosecretory--primary--secondary secretory granules, of prosecretory and primary secretory granules as well as prosecretory granules and secondary secretory granules. Generally, primary granules show a tendency to become secondary secretory granules or to fuse with them. During maturation of the goblet cells the secretory granules fuse to form larger mucous bodies in the theca by fusion of the laminae of the membranes; a final product, there is a homogeneous mucous mass devoid of membranes.     

SYNTHESIS, MIGRATION, AND RELEASE OF PRECURSOR COLLAGEN BY ODONTOBLASTS AS VISUALIZED BY RADIOAUTOGRAPHY AFTER [3H]PROLINE ADMINISTRATION

The elaboration of dentin collagen precursors by the odontoblasts in the incisor teeth of 30–40-g rats was investigated by electron microscopy, histochemistry, and radioautography after intravenous injection of tritium-labeled proline. At 2 min after injection, when the labeling of blood proline was high, radioactivity was restricted to the rough endoplasmic reticulum, indicating that it is the site of synthesis of the polypeptide precursors of collagen, the pro-alpha chains. At 10 min, when the labeling of blood proline had already declined, radioactivity was observed in spherical portions of Golgi saccules containing entangled threads, and, at 20 min, radioactivity appeared in cylindrical portions containing aggregates of parallel threads. The parallel threads measured 280–350 nm in length and stained with the low pH-phosphotungstic acid technique for carbohydrate and with the silver methenamine technique for aldehydes (as did extracellular collagen fibrils). The passage of label from spherical to cylindrical Golgi portions is associated with the reorganization of entangled into parallel threads, which is interpreted as the packing of procollagen molecules. Between 20 and 30 min, prosecretory and secretory granules respectively became labeled. These results indicate that the cylindrical portions of Golgi saccules transform into prosecretory and subsequently into secretory granules. Within these granules, the parallel threads, believed to be procollagen molecules, are transported to the odontoblast process. At 90 min and 4 h after injection, label was present in predentin, indicating that the labeled content of secretory granules had been released into predentin. This occurred by exocytosis as evidenced by the presence of secretory granules in fusion with the plasmalemma of the odontoblast process. It is proposed that pro-alpha chains give rise to procollagen molecules which assemble into parallel aggregates in the Golgi apparatus. Procollagen molecules are then transported within secretory granules to the odontoblast process and released by exocytosis. In predentin procollagen molecules would give rise to tropocollagen molecules, which would then polymerize into collagen fibrils.     

Effects of melatonin on the ultrastructure of the golden hamster parathyroid gland.

Ultrastructural changes of the parathyroid glands of melatonin-treated golden hamsters were studied. Many chief cells in the parathyroid glands after 1 hour of administration of melatonin contained poorly-developed Golgi complexes associated with a few prosecretory granules and numerous lipid droplets as compared with those of the control animals. The morphology of the parathyroid glands after 5 hours of administration resembled that of the control animals. Many chief cells in the parathyroid glands after 24 hours of administration had well-developed Golgi complexes and cisternae of the granular endoplasmic reticulum, numerous prosecretory granules, a few lipid droplets and many secretory granules in the peripheral cytoplasm as compared with those of the control animals. The ultrastructure of the parathyroid glands after 48 hours of administration was almost similar to that of the control animals. It is considered that melatonin affects the secretory activity of the parathyroid gland.     

Formation of secretion granules in the Golgi apparatus of pancreatic acinar cells of the rat

The three-dimensional structure of the Golgi apparatus and its components has been analyzed in sections of pancreatic acinar cells by using stereopairs of electron microscope photographs. Pancreatic tissue fixed in glutaraldehyde was postfixed in reduced osmium, and the sections were stained with lead citrate. Tissues were also treated to demonstrate phosphatase activity (i.e., nicotinamide adenine dinucleotide phosphatase, NADPase; thiamine pyrophosphatase, TPPase; cytidine monophosphatase, CMPase). The following stacked components were observed along the branching, anastomotic, continuous, ribbonlike Golgi apparatus. 1) On the cis-face of the Golgi stack there was a tubular membranous network known to be osmiophilic and referred to as the cis-osmiophilic tubular network or cis-element. 2) A first, poorly fenestrated saccule, unreactive for the phosphatases tested, was slightly distended in places and contained a fluffy granulofilamentous material. 3) The subjacent three or four saccules, reactive for NADPase and/or TPPase, showed dilated portions containing a granulofilamentous secretory material similar to that filling the rest of the saccule. They also showed nondilated portions perforated with large fenestrations, some of which were in register and formed wells containing 80-nm vesicles. The dilated portions of these saccules were present at random along the length of the saccules and were not located exclusively at their edges. 4) The remaining one or two elements of the stack, CMPase positive, showed dilated spheroidal portions or prosecretory granules containing a homogeneous secretory material and flattened fenestrated regions free of secretory material and having the appearance of networks of narrow membranous tubules. 5) Lastly on the trans-aspect of the stack there were detached prosecretory granules reactive for CMPase and surrounded by a corona of small vesicles, and smooth-surfaced spherical CMPase-negative granules having a denser content that were identified as fully formed secretion granules; there were also occasional free trans-tubular networks strongly reactive for CMPase that appeared to undergo fragmentation and numerous small vesicles free from acid-phosphatase activity. These various images were interpreted as indicating that prosecretory granules formed in relation to two or three fenestrated saccules on the trans-side of the stack. Such granules, following their detachment from the trans-face of the stack, their separation from trans-tubular networks, and condensation of their content, yielded mature secretion granules.     

Effects of hypergravity environment on the parathyroid gland of the isoproterenol-treated hamster

The ultrastructure of the parathyroid glands of hamsters subjected to 5 g environment after an intraperitoneal injection of isoproterenol was studied. In the isoproterenol-treated hamsters exposed to hypergravity environment, the Golgi complexes, cisternae of the granular endoplasmic reticulum and lipid droplets were significantly increased and secretory granules were significantly decreased compared with those of the control group. In addition, many chief cells contained numerous prosecretory granules in the Golgi areas and several secretory granules were situated close to the plasma membrane of the chief cells. These results suggest that the synthesis and release of parathyroid hormone may be markedly stimulated in the parathyroid glands of the isoproterenol-treated hamsters exposed to hypergravity environment.     

Effects of isoproterenol on the fine structure of the hamster parathyroid gland

Ultrastructural changes of the parathyroid glands of isoproterenol-treated golden hamsters were investigated. Many chief cells in the parathyroid glands after 5 and 10 minutes of administration of isoproterenol contain well-developed Golgi complexes and granular endoplasmic reticulum, numerous prosecretory granules, and many secretory granules in the peripheral cytoplasm as compared with the control animals. Many chief cells in the parathyroid glands after 1, 3, 6 and 12 hours of administration have poorly-developed Golgi complexes, granular endoplasmic reticulum, many secretory granules and numerous lipid droplets as compared with the control animals. The morphology of the parathyroid gland after 30 minutes and 24 hours of administration resembles that of the control animals. It is considered that isoproterenol affects the secretory activity of the parathyroid gland.     

Ultrastructure of the pineal gland of the eastern chipmunk (Tamias striatus)

The ultrastructure of the pineal gland of the wild-captured eastern chipmunk (Tamias striatus) was examined. A homogenous population of pinealocytes was the characteristic cellular element of the chipmunk pineal gland. Often, pinealocytes showed a folliclelike arrangement. Mitochondria, Golgi apparatus, granular endoplasmic reticulum, lysosomes, centrioles, dense-core vesicles, clear vesicles, glycogen particles, and microtubules were consistent components of the pinealocyte cytoplasm. The extraordinary ultrastructural feature of the chipmunk pinealocyte was the presence of extremely large numbers of “synaptic” ribbons. The number of “synaptic” ribbons in this species exceeded by a factor of five to 30 times that found in any species previously reported. In addition to pinealocytes, the pineal parenchyma contained glial cells (oligodendrocytes and fibrous astrocytes). Capillaries of the pineal gland of the chipmunk consisted of a fenestrated endothelium. Adrenergic nerve terminals were relatively sparse.     

Ultrastructural localization of calcitonin in control and stimulated thyroid C cells of the rat using protein A-gold immunocytochemical technique

Using anti-human calcitonin serum and a protein A - gold technique, calcitonin was localized at the ultrastructural level in control and calcium gluconate-stimulated thyroid C cells of the rat. In control rats calcitonin was detected within a majority of the secretory granules while in experimental animals it was demonstrated also within prosecretory granules present in Golgi apparatus.     

Corticotroph cells in primary cultures of rat adenohypophysis: A light and electron microscopic immunocytochemical study

Summary Monolayer cultures of trypsin-dispersed cells of the rat adenohypophysis were grown for 5 to 54 days. ACTH was localized by immunocytochemistry using an antiserum to synthetic ACTH^1–28 prepared in rabbit and sheep anti-goat immunoglobulin coupled with peroxidase. ACTH content of the culture medium was measured by radioimmunoassay.Corticotrophs were found in the cultures and ACTH in the medium at each cultivation time. The corticotrophs retained their essential morphological characteristics. Immunological staining was found in the secretory granules, some tubular or saccular structures, parts of the rough endoplasmic reticulum, and the cytoplasmic matrix. Immature secretory granules in the Golgi apparatus as well as some Golgi elements showed different degrees of immunoreactivity. In agreement with the high ACTH content of the culture medium the number, size and shape of the secretory granules, the active Golgi apparatus, the high amount of extragranular ACTH as well as pictures suggesting granule extrusion claim for a high ACTH synthesis and transport (and low ACTH storage) in the cultured corticotrophs.     

EFFECT OF DEHYDROASCORBIC ACID ON THE ISLETS OF LANGERHANS OF THE RAT PANCREAS

Rat pancreatic islets have been studied following successive daily administration of dehydroascrobic acid (DHA) and during the recovery phase following 3 daily injections. One injection of DHA produces degranulation of B cells seen in the light microscope as a loss of aldehyde fuchsin positivity. In the electron microscope the B cells appear to have secretory granules accumulated subjacent to the plasma membranes. Following 2 and 3 daily injections, B cells evidence alterations in the organization of the granular endoplasmic reticulum and mitochondria, and secretory granules are scant but when present are subjacent to the plasma membrane. After 5 to 7 days' recovery few secretory granules remain in B cell cytoplasm, but the cells have prominent granular ER and a Golgi apparatus with numerous prosecretory granules. The primary effect of DHA is an exaggerated secretory response of B cells, which is intensified with subsequent injections. Necrosis of B cells as produced by alloxan is not seen.     

Fine structure of the corpuscles of stannius in the toadfish.

The micro-anatomy of the corpuscles of Stannius of the toadfish, Opsanus tau, an aglomerular marine teleost, has been studied by light and electron microscopy. The corpuscles are composed of extensively anastomosed cords of epithelial cells which maintain intimate contact with blood capillaries. Most of the epithelial cells contain acidophilic granules which also show a positive reaction with the periodic acid-Schiff technique and aldehyde fuchsin. On the basis of fine structural criteria, three cell types can be recognized. The granular cells contain abundant quantities of granular endoplasmic reticulum, ribosomes, Golgi apparatus with prosecretory granules, coated vesicles, polymorphic mitochondria with lamellar cristae, filaments, microtubules, a cilium, a variety of lysosome-like dense bodies, glycogen particles, lipid droplets, secretory granules and intranuclear lipid-like inclusions. One variety of agranular cell (type I) is characterized by the total absence of secretory granules, but it contains large amounts of granular endoplasmic reticulum and ribosomes, conspicuous profiles of Golgi apparatus, coated vesicles and sometimes an abundance of glycogen. Another variety of agranular cell (type II) has poorly developed cytoplasmic organelles. The perivascular space between the capillary and parenchyma contains connective tissue cells and abundant nerve fibers. The different types of epithelial cells observed in the corpuscles of Stannius of this fish may represent functional stages of the secretory cycle in a single cell type.     

Clathrin-coated vesicular transport of secretory proteins during the formation of ACTH-containing secretory granules in AtT20 cells

We have studied by electron microscopy and immunocytochemistry the formation of secretory granules containing adrenocorticotropic hormone (ACTH) in murine pituitary cells of the AtT20 line. The first compartment in which condensed secretory protein appears is a complex reticular network at the extreme trans side of the Golgi stacks beyond the TPPase-positive cisternae. Condensed secretory protein accumulates in dilated regions of this trans Golgi network. Examination of en face and serial sections revealed that "condensing vacuoles" are in fact dilations of the trans Golgi network and not detached vacuoles. Only after presumptive secretory granules have reached an advanced stage of morphological maturation do they detach from the trans Golgi network. Frequently both the dilations of the trans Golgi network containing condensing secretory protein and the detached immature granules in the peri-Golgi region have surface coats which were identified as clathrin by immunocytochemistry. Moreover both are the site of budding (or fusion) of coated vesicles, some of which contain condensed secretory protein. The mature granules below the plasma membrane do not, however, have surface coats. Immunoperoxidase labeling with an antiserum specific for ACTH and its precursor polypeptide confirmed that many of the coated vesicles associated with the trans Golgi network contain ACTH. The involvement of the trans Golgi network and coated vesicles in the formation of secretory granules is discussed.     

THE CYTOLOGY OF THE NORMAL PARATHYROID GLANDS OF MAN AND VIRGINIA DEER : A Light and Electron Microscopic Study with Morphologic Evidence of Secretory Activity

The normal parathyroids of six humans and a Virginia deer were studied by light and electron microscopy. The parenchyma of the deer parathyroid is composed of uniform chief cells, which contained 100 to 400 mµ electron-opaque, membrane-limited granules, presumed to be secretory granules, in addition to the usual cytoplasmic organelles. Desmosomes are present between adjacent cells, and rare cilia are observed protruding from the chief cells into the intercellular space. The human parathyroids contain chief cells in two phases—active and inactive—as well as oxyphil cells. Active chief cells have a large Golgi apparatus, sparse glycogen, numerous secretory granules, and rare cilia. Inactive chief cells contain a small Golgi apparatus, abundant glycogen, and few secretory granules. Both forms have the usual cytoplasmic organelles and, between adjacent cells, desmosomes. Oxyphil cell cytoplasm is composed of tightly packed mitochondria and glycogen granules, with rare secretory granules. Cells with cytoplasmic characteristics intermediate between chief and oxyphil cells, possibly representing transitional cells, have been observed. Secretory granules of both man and deer are composed of 100 to 200 A particles and short rods, and the granules develop from prosecretory granules in the Golgi region of the cell. The human secretory granules are smaller and more variable in shape than those of the deer. The granules are iron and chrome alum hematoxylin-positive, argyrophilic, and aldehyde fuchsin-positive, permitting light microscopic identification. They are also found in the capillary endothelial cells of the parathyroid and in its surrounding connective tissue. The secretory granules of the parathyroid cells can thus be followed from their formation in the Golgi apparatus almost to their extrusion into the blood stream.     

Intracellular localization of basement membrane precursors in the endodermal cells of the rat parietal yolk sac. I. Ultrastructure and phosphatase activity of endodermal cells

The parietal layer of the rat yolk sac includes a 5 microliter thick sheet known as Reichert's membrane that exhibits properties of basement membranes. Its inner side is lined by a single layer of loosely distributed cells referred to as endodermal cells. Both Reichert's membrane and endodermal cells were examined at 13-14 days' gestation with emphasis on the ultrastructure of the Golgi apparatus, the identification of its component parts by specific phosphatase activities, and its possible role in the cells' secretory process. Reichert's membrane is composed of a series of stacked layers similar to basal laminae and composed of a network of fibrils with a diameter of 2-8 nm along which dots are located at irregular intervals. The endodermal cells contain the usual organelles, including interconnected rough endoplasmic reticulum (rER) cisternae and a prominent Golgi apparatus. With the help of phosphatase reactions, the stacks of Golgi saccules were divided into a) "phosphatase-free" saccules, the first ones on the cis or forming side, b) one or two "intermediate" saccules in the middle of the stacks, containing nicotinamide adenine dinucleotide phosphatase activity, c) one or two "last" saccules rich in thiamine pyrophosphatase activity on the trans or mature side, and d) continuing beyond the trans side, the GERL element displaying acid phosphatase activity. The latter is associated with profiles equally rich in acid phosphatase and tentatively considered to be prosecretory granules. Finally, the ectoplasm adjacent to Reichert's membrane displays large, acid phosphatase-containing structures tentatively considered to be secretory granules. Thus, the extensive rER network, the well-compartmentalized Golgi apparatus, and the presence of structures which may be prosecretory and secretory granules indicate that the endodermal cells are well-equipped for the secretion of the components of Reichert's membrane.     

Ultrastructural localization of calcitonin in control and stimulated thyroid C cells of the rat using protein A — gold immunocytochemical technique

Summary Using anti-human calcitonin serum and a protein !-gold technique, calcitonin was localized at the ultrastructural level in control and calcium gluconate-stimulated thyroid C cells of the rat. In control rats calcitonin was dedected within a majority of the secretory granules while in experimental animals it was demonstrated also within prosecretory granules present in Golgi apparatus.The study was partially supported by a grant from the Committee of Cytobiology, Polish Academy of Sciences     

Intracellular pathway and kinetics of protein secretion in the coagulating gland of the mouse

The coagulating gland of male rodents is part of the prostatic complex. Various mechanisms of secretion have been postulated, in part because organelles commonly involved in the secretory process possess unusual features, such as extreme distension of the rough endoplasmic reticulum. In the present study, the pathway, kinetics, and mode of secretion in the coagulating gland of the mouse were studied by electron microscope autoradiography at intervals between 5 min and 8 h after administration of 3H-threonine. The percentage of grains associated with the rough endoplasmic reticulum was initially high and generally decreased throughout the experiment, while a pronounced rise in the proportion of grains associated with the Golgi apparatus and secretory granules was observed 6 h after injection of precursor. In addition, there was a smaller elevation in the percentage of grains over the Golgi apparatus and secretory granules between 1 and 4 h, and radioactive material first reached the lumen of the gland 4 h after injection of the precursor. Although the general pathway of intracellular transport of secretory protein resembles that in other cells, the results indicate that there are several unusual aspects to the secretory process in the coagulating gland. First, the rate of transport was markedly slower than in most other exocrine gland cells, since the bulk of the labeled protein did not reach the Golgi apparatus and secretory granules until 6 h after administration of precursor. This reflected prolonged retention of secretory products in the endoplasmic reticulum. Second, in addition to the major bolus of labeled material that traversed the cells at about 6 h, a smaller wave of radioactivity appeared to pass through the Golgi apparatus and secretory granules and reach the lumen earlier, within the first few hours after the injection. Finally, the primary mode of secretion in the coagulating gland appears to be merocrine because the secretory granules contained much labeled protein.     

Intracellular localization of basement membrane precursors in the endodermal cells of the rat parietal yolk sac. III. Immunostaining for laminin and its precursors

Reichert's membrane and the endodermal cells of the parietal yolk sac were examined for the presence of laminin antigenicity using anti-laminin antibodies and the peroxidase-antiperoxidase sequence. Immunostaining was observed through the full width of Reichert's membrane and within endodermal cells. In these cells immunostaining was observed in rough endoplasmic reticulum (rER) cisternae and Golgi apparatus. The Golgi staining could occur in any saccule, but predominated in components interpreted as the last saccule of the stack, the GERL element, and associated prosecretory granules. The secretory granules found in the ectoplasm were also immunostained. Finally, multivesicular bodies showed some staining. The immunostaining of Reichert's membrane indicates the presence of laminin itself, while that of rER cisternae and the Golgi apparatus is attributed to laminin precursors. Presumably the biosynthesis of laminin occurs along the usual protein pathway, that is, from rER through Golgi saccules and the GERL element to secretory granules, which release their content into Reichert's membrane. The laminin immunostaining of Reichert's membrane and endodermal cells is similar to that of type IV collagen. It is, therefore, likely that the two substances are processed and secreted simultaneously.     

Ultrastructure of the parathyroid gland of fetal and pregnant golden hamsters subjected to hypergravity environment.

The ultrastructure of the parathyroid glands of fetal and pregnant golden hamsters exposed to 5-gravity environment was studied. In the centrifuged fetal animals the Golgi complexes associated with some prosecretory granules were significantly increased compared with those of the control fetal animals, and several secretory granules were located in a peripheral position just beneath the plasma membrane. In the centrifuged pregnant animals the Golgi complexes associated with numerous prosecretory granules were significantly increased, cisternae of the granular endoplasmic reticulum appeared to be increased compared with those of the control pregnant animals, many secretory granules were located in the peripheral cytoplasm and several granules were situated close to the plasma membrane. These findings suggest that in the parathyroid glands of the fetal golden hamsters as well as the pregnant animals the synthesis and release of parathyroid hormone may be stimulated in response to a hypergravity environment.