Electron microscopic studies on the adenohypophysis of the White-Crowned sparrow,Zonotrichia leucophrys gambelii

Summary The pars distalis of the adenohypophysis of normal (78), thyroideotomized (6), adrenalectomized (6), and castrated (14) White-crowned Sparrows were observed with the electron microscope. Six types of glandular cells were identified and the ultrastructural characteristics of each have been described. To each has been assigned tentatively an endocrine function.STH cells are characterized by the presence of large, dense secretory granules ranging from 220–280 m, a poorly developed endoplasmic reticulum, and a fragmented Golgi apparatus; they occur only in the caudal lobe. They show no remarkable changes after adrenalectomy, castration, and thyroidectomy.Prolactin cells, whose identity is suggested by their responses to photostimulation and surgical experiments, are characterized by large, polymorphic, dense secretory granules; they have been found mainly in the cephalic lobe.ACTH cells, whose function is confirmed by their cytological responses to adrenalectomy, have a peculiar type of secretory granule (220 m) with high and low phases of electron density. They occur exclusively in the cephalic lobe and are transformed, after adrenalectomy to large, vacuolated adrenalectomy cells.TSH cells are so designated by their response to thyroidectomy. After thyroidectomy, they lose their specific fine secretory granules and are transformed into large, vacuolated thyroidectomy cells. We have found TSH cells and thyroidectomy cells only in the cephalic lobe.Two types considered to be gonadotropic cells from their responses to gonadectomy, occur in both the cephalic and caudal lobes. One of them contains spherical, dense secretory granules (180–220 m), prominent rough endoplasmic reticulum and a well developed Golgi apparatus; the other type contains dense secretory granules of variable size (150–350 m), a less extensively developed Golgi apparatus, and sac-like endoplasmic reticulum. Both types of gonadotropic cells show extreme enlargement and vacuolization after castration. However, they retain differences in appearance in the structure of cytoplasmic organelles and vacuolization.Dedicated to Professor Dr. H. Grau in honor of his 70th birthday.The investigation reported herein was supported by a research grant (HE 07240 NEUA) from the National Institutes of Health to Professor Vitums, by a research grant (5R01 NB 06187) from the National Institutes of Health to Professor Farner, and by a scientific research grant (No. 91049) from the Ministry of Education of Japan to Professor Mikami. The authors wish to thank Professor James R. King for his assistance in obtaining and maintaining the birds, and for his helpful advice concerning the experiments.     

Stereological study of gonadotropes in the frog,Rana pipiens,after GnRH stimulation in vitro

Summary Previous physiological results have indicated the existence of two releasable pools of gonadotropins in amphibian pituitaries: an acute releasable pool that appears independent of protein synthesis, and a storage pool involved in chronic release that depends on protein synthesis. To elucidate the ultrastructural localization of these pools and the morphological changes induced in gonadotrope cells after treatment with gonadotropin-releasing hormone, we carried out a morphometric study of immuno-identified gonadotrope cells using an in vitro superfusion system. Treatment with gonadotropin-releasing hormone induced a degranulation of small (110–255 nm) and medium (236–360 nm) secretory granules as well as hypertrophy of the endoplasmic reticulum and Golgi complex. Simultaneous incubation with gonadotropin-releasing hormone and cycloheximide inhibited the release of secretory granules although the endoplasmic reticulum and Golgi complex were hypertrophied. These morphological results strongly suggest: (1) that gonadotropin-releasing hormone induces degranulation and hypertrophy of the biosynthetic machinery in gonadotrope cells; and (2) that the activation of the endoplasmic reticulum and Golgi complex by stimulation with gonadotropin-releasing hormone is independent of protein synthesis, while the release of secretory granules is protein synthesis-dependent. In addition, the second or storage pool of gonadotropin is associated mainly with the small and medium secretory granules.     

The distribution and form of the endoplasmic reticulum during spermatogenesis in the crayfish,Cambaroides japonicus

Summary The form and differentiation of the endoplasmic reticulum has been studied in the developing sperm of the crayfish, Cambaroides japonicus. Throughout development a relationship between the nuclear envelope and cytoplasmic portion of the endoplasmic reticulum has been shown to exist. Furthermore, large contributions of material from the nuclear envelope to extranuclear cytoplasmic systems has been noted in the development of early spermatids and nearly mature sperm.A sequential predominance of several types of endoplasmic reticulum has been described in the differentiating sperm. An agranular vesicular reticulum is the most common in the early stages although annulate lamellar stacks and rough surfaced stacks are scattered randomly throughout the cytoplasm. Blebs of the nuclear envelope appear to contribute rough surfaced reticulum to the cytoplasmic system in the early spermatid. A fusion of vesicular elements results in the formation of the dense filamentous reticulum which is typical of the nearly mature sperm. Densely packed lamellae develop on the nuclear envelope in the maturing sperm and are connected to both the nuclear envelope and filamentous endoplasmic reticulum. The possible relationships of these lamellar groups to mitochondria or Golgi is discussed.Supported in part by Grant No B-2314 of the National Institute of Neurological Diseases and Blindness, U.S. Public Health Service.Predoctoral Research Fellow of the National Institute of Neurological Diseases and Blindness, U.S. Public Health Service.     

Multiple rough endoplasmic cisternae in “C” cells

Summary Multiple rough endoplasmic cisternae were found in C cells of the adult rat, at interphase. They are considered to be normal constituents of C cells. Their morphological relation to rough endoplasmic reticulum and their close proximity to mitochondria, Golgi dictyosomes and secretory granules suggest that they may have a role in the secretory activity of this endocrine cell.     

Glycoprotein gonadotropin in the plasma and its cellular origin in the adenohypophysis of sham-operated and ovariectomized rainbow trout,salmo gairdneri

Summary Among the cells of the pituitary generally believed to produce glycoprotein gonadotropin (GTH) five forms were distinguished, based on the amount and the diameter of granules and globules and the appearance of the rough endoplasmic reticulum. In sham-operated trout so-called globular cells predominated, whereas after ovariectomy these were replaced by so-called cisternal cells, suggesting that both belong to one GTH-cell type. In addition, ovariectomy caused a strong increase in plasma GTH-levels. This indicates that the transition from globular to cisternal cells is accompanied by extrusion of GTH, and thus points to a storage of GTH in the granules and globules. It is argued that one of the five forms has the morphological characteristics of thyrotropic cells and may not produce glycoprotein GTH.The authors are indebted to Mr. L.W. van Veenendaal for preparing the illustrations and the photographic layout     

Manipulation of β-glucuronidase for use as a reporter in vacuolar targeting studies

It has been documented that when furnished with an endomembrane signal sequence for the endoplasmic reticulum, -glucuronidase (GUS) is N-glycosylated, resulting in the nearly complete loss of enzymatic activity. To enable use of -glucuronidase as a reporter protein in secretory and vacuolar targeting studies, one of the two putative N-linked glycosylation sites within the GUS gene was altered by site-directed mutagenesis. The second N-linked glycosylation site was not altered because sequence analysis of nucleotide sequences around the second putative glycosylation site revealed that the published sequence was incorrect, and that no such site existed.     

LH-producing cells in the ovine pituitary

Summary An immunocytochemical technique using the peroxidase-antiperoxidase complex (PAP) was applied to identify and characterize the LH-secreting cells in the ovine pituitary at the ultrastructural level. These cells, round or oval in shape, possessing flattened cisternae of the rough endoplasmic reticulum, contain one class of secretory granules (mean diameter 250 nm) and large dense bodies (600 to 800 nm in diameter). LH molecules and the two subunits LH and LH were localized on the secretory granules and on the small granules near the Golgi complex. The large dense bodies, the cisternae of the endoplasmic reticulum and the saccules of the Golgi complex showed no reaction product.Abbreviations used in this Article O-LH ovine luteinizing hormone - b-LH bovine luteinizing hormone - p-LH porcine luteinizing hormone - p-LH porcine LH subunit - p-LH porcine LH subunit - O-FSH ovine follicle stimulating hormone - b-TSH bovine thyrotropic hormone - A-b LH antiserum to bovine LH - A-pLH antiserum to porcine LH subunit - A-pLH antiserum to porcine LH subunit     

Prolactin secretory bypath exposed in cultured lactotrophs

In the present report, the prolactin secretory pathways were re-examined in cultured lactotrophs submitted to various experimental conditions of stimulation, inhibition and/or alteration of the intracellular flow of the synthesis and release of prolactin.Primary cultures of rat pituitary cells stimulated with thyrotropin-releasing hormone, or inhibited with either cycloheximide or dopamine in the presence or absence of 0.1µg/ml brefeldin A, were used. The radioimmunoassay quantification of released and intracellular prolactin was correlated with ultrastructural and immunocytochemical studies.Brefeldin A diminished significantly the secretion and the intracellular content of prolactin 4h after application, while morphological effects were seen starting from 30min. The drug did not modify the response to thyrotropin-releasing hormone (120% increment). The simultaneous incubation of brefeldin A with cycloheximide or dopamine diminished the released prolactin concomitant with a lower (cycloheximide) or greater (dopamine) hormonal intracellular prolactin content with respect to brefeldin A. The combined treatment cycloheximide–dopamine inhibited prolactin secretion. The ultrastructural and immunocytochemical features of lactotrophs supported these radioimmunoassay data.These results revealed that prolactin release in vitro in the presence or not of brefeldin A is dependent on either: the neo-synthesized hormone that can be inhibited by cycloheximide, and the hormone stored in granules, the exocytosis of which was blocked by dopamine, indicates the contribution of both constitutive and regulated pathways in the secretory process. The brefeldin A blockade of the intracellular transport also disclosed morphological evidence of an alternative pathway of prolactin secretion through vesicles originated in the rough endoplasmic reticulum bypassing the Golgi complex.     

Effect of postganglionic sympathectomy on the ultrastructure of the rat parotid gland

Summary Following two weeks of superior cervical ganglionectomy, the parotid glands of adult rats were removed and studied by electron microscopy. Sympathectomy induced striking alterations of acini, resulting in a heterogeneous population of acinar cells, but it had no obvious effect on the duct system. Most of the altered cells could be classified on a cytological basis as dark cells or light cells. Dark cells predominated and contained more secretory granules, less granular endoplasmic reticulum, fewer Golgi membranes, and smaller lumina and intercellular canaliculi than normal acinar cells. The synthesis and extrusion of secretory products appeared to be minimal in these cells. Light cells possessed ultrastructural features, such as dilated cisternae of granular endoplasmic reticulum and prominent Golgi membranes, which were opposite to those of dark cells and indicative of a high degree of secretory activity.The heterogeneous population of cells following sympathectomy indicates that the sympathetic nervous system may play an important role in regulating the secretory synchrony of acinar cells.Supported by U.S.P.H.S. Grant DE 02110.     

On the fine structure of corpuscles of Stannius of the eel,Anguilla japonica

Summary The Stannius corpuscle of the eel (Anguilla japonica) consists of numerous ovoid or polymorphic lobules separated by loose connective tissue containing blood capillaries. Each lobule is composed of a number of columnar secretory cells, containing numerous secretory granules, and arranged more or less radially. Each secretory granule, spherical, 0.5–1.0 in diameter, and osmiophilic, is surrounded by a limiting membrane derived from the Golgi membrane. The well developed rough-surfaced endoplasmic reticulum is closely associated with the Golgi field. Some Golgi elements might also be intimately associated with the outer nuclear membrane. Numerous glycogen particles are distributed throughout the cytoplasm. The secretory granules which tend to accumulate near the basal part of the cell might be released into the loose connective tissue. From these facts, the corpuscles of Stannius are considered to be protein-secretory endocrine glands without any similarity to the adrenal gland.     

The hypothalamo-hypophysial system of the frog Rana temporaria

Summary The median eminence (ME) of the adult frog, Rana temporaria, was studied by means of electron microscopy including quantitative electron-microscopic autoradiography. In frogs captured in May and June numerous peptidergic neurosecretory fibres extending via the internal zone to the pars nervosa display large swellings containing few granules, mitochondria, neurotubules and cisternae of the smooth endoplasmic reticulum. In addition, few secretory globules up to 1.5 m in diameter occur in these varicosities. In animals collected during the autumn period many of these neurosecretory swellings filled with neurosecretory granules and polymorphic inclusions resemble Herring bodies. Three types of granule-containing neurosecretory fibres were observed in the external zone (EZ) of the ME of adult R. temporaria. Peptidergic A₁- and A₂-type fibres are characterized by granules 150–220 nm and 100–160 nm in diameter, respectively. Monoaminergic fibres of type B with granules approximately 100 nm in diameter represent 50% of all neurosecretory elements in the EZ of the frog ME; 12% of the total number of granule-bearing axons in the EZ actively taking up radiolabelled 5-hydroxytryptophan are thought to be serotoninergic terminals. Neurosecretory terminals of all types and glial vascular endfeet establish direct contacts with the perivascular space of the primary portal capillaries. Some neurosecretory terminals are separated from the lumen of the third ventricle by a thin cytoplasmic lamella of tanycytes. The possible physiological significance of this structural pattern is discussed.     

Thyroidectomy cells and their response to thyrotrophin releasing hormone (TRH) in the rat

Summary Thyroidectomy cells of the rat pituitary gland were studied by the peroxidase-antibody labeling procedure and by electron microscopy. Secretory granules accumulated in these cells in response to a short-term treatment with thyroxine, and the cells were then reactive to the peroxidase-antibody labeling procedure. An intravenous injection of synthetic thyrotrophin releasing hormone (TRH) to thyroxine-treated, thyroidectomized rats provoked an acute and active extrusion of secretory granules from the thyroidectomy cells. The secretory granules in these cells were mostly haloed after primary fixation in osmium tetroxide. It is concluded that TRH causes thyroidectomy cells to release their secretory granules, and presumably TSH, by the usual process of exocytosis or granule extrusion.This study was supported by USPHS Grant AM 12583.     

Quantitative energy dispersive X-ray microanalysis of eight elements in pancreatic endocrine and exocrine cells after cryo-fixation

Quantitative X-ray microanalysis of 8 elements was performed on ultrathin, freeze-dried sections of islets and pancreas pieces from non-inbredob/ob-mice. Diffusion of elements was reduced to a minimum by rapidly freezing the tissue samples between nitrogen-cooled polished copper surfaces and avoiding the use of chemical fixatives and stains. The ultrastructural morphology was adequately maintained to allow measurements on secretory granules, mitochondria, cell nuclei, and cytoplasm free of these organelles. The distribution of the various elements between cellular compartments was similar in islet -cells and exocrine pancreas cells. However, the insulin secretory granules were outstanding in exhibiting the highest concentrations of zinc and calcium. In comparison with cytoplasm in the -cells, the insulin granules accumulated calcium 2-fold and zinc as much as 40-fold. As no correlation could be made for endoplasmic reticulum in the cytoplasmic measurements areas, the true accumulations above cytosol are likely to be even higher.     

Morphometric evaluation of subcellular changes induced by in vivo TRH treatment in the pituitary gland of Rana perezi: Effects on prolactin and thyrotropic cells

Summary The effects of synthetic thyrotropin-releasing hormone on pituitary prolactin and thyrotropic cells were investigated in adult male Rana perezi (formerly Rana ridibunda) frogs. Animals were given daily injections of synthetic thyrotropin-releasing hormone into the dorsal lymph sac. Prolactin and thyrotropic cells were identified by the colloidal-gold method, using anti-human prolactin and anti-human--thyrotropin hormone as primary antisera. The stereological parameters of the rough endoplasmic reticulum, Golgi complex, and secretory granules of prolactin and thyrotropic cells were evaluated by ultrastructural morphometry (point-counting method). Thyrotropin-releasing hormone caused cytological changes in both cell-types which were consistent with increased synthesis and release of both prolactin and thryrotropin. These changes were still significant after 48 h treatment in the case of thyrotropic cells, while in prolactin cells the thyrotropin-releasing hormone increased the number of secretory granules. After 6 days, the cells resembled essentially those used as controls. These results indicate that thyrotropin-releasing hormone stimulates the synthesis and release of prolactin and thyrotropin, and that the response of each cell type to this hypothalamic stimulus follows a different time-course.This work has been supported by grants no. 2184-83 and PB 86-0095 from the Comisión Interministerial para la Ciencia y Tecnología, Spain     

Effects of tunicamycin on the hypothalamo-neurohypophysial system of the rat

Summary Intracisternal injections of tunicamycin, an inhibitor of glycosylation, decreased the incorporation of [³⁵S]cysteine into the neurophysins in the rat neurohypophysis. Histochemical and immunocytochemical studies showed that there was no concomitant decrease in the amount of secretory product in the perikarya of the hypothalamo-neurohypophysial neurones. Indeed there was an increase, although this was not associated with neurosecretory granules as judged electron-microscopically. Tunicamycin led to the formation of socalled colloid droplets which were immunopositive and of which the ultrastructural correlates appeared to be product-filled dilatations of the rough endoplasmic reticulum. The observations are interpreted to suggest that glycosylation plays a rôle in the packaging of secretory material in the hypothalamo-neurohypophysial system.Supported by grants from the Medical Research Council and The Wellcome Trust     

Immunocytochemical and morphometrical study of thyrotropic cells of Rana ridibunda

Summary Indirect immunoflorescence and PAP techniques for light microscopy as well as the immunogold complex technique for electron microscopy were used to localize and identify thyrotropic (TSH) producing cells in the pars distalis of Rana ridibunda. A double immunostaining procedure was used to distinguish TSH cells from other glycoprotein hormone producing cells. Rabbit anti-human--TSH was used as the primary antiserum and revealed a basophil, PAS and alcian blue positive cell type in the ventro-central zone of the gland. Under the electron microscope, TSH cells show irregular morphology, polymorphic secretory granules with diameters ranging between 120 and 375 nm and poor development of the endoplasmic reticulum and Golgi complex; they are usually polarized towards capillaries. Ultrastructural morphometry (point-counting method) was used to evaluate stereological parameters of rough endoplasmic reticulum, Golgi complex, secretory granules and mitochondria.This work has been supported by grant 2184-83 from the Comisión Asesora (CAICYT) of Spain     

The ultrastructure of hemocytes inDactylopius confusus (Cockerell), and the role of granulocytes in the synthesis of cochineal dye

Summary The ultrastructural study of free circulating hemocytes in the adult cochineal scale,Dactylopius confusus (Cockerell), demonstrated five cell types: prohemocytes, typical granulocytes (T-granulocytes), oenocytoids, plasmatocytes, and granulocytes with modified sub-cellular structure to perform a special synthetic and secretory function, which we refer to as modified granulocytes (M-granulocytes). Prohemocytes showed undifferentiated sub-cellular structure of the basic stem cell type (i.e., high cytoplasmic density with numerous ribosomes, centrally located large nucleus with a distinct nucleolus, and poorly developed endoplasmic reticulum). The commonly observed typical granulocytes (T-granulocytes) had several smooth endoplasmic reticulum (SER) with dilated cisternae and many SER-derived membrane bounded granules of different sizes and electron density. Oenocytoids were identified by the presence of many crystals, RER-originated fine secretory granules, and an eccentric nucleus. Plasmatocytes were easily characterized by their variable shapes and irregular outline with pseudopodia-like cytoplasmic extensions, possession of an elongated lobed nucleus, multivesicular bodies, RER-derived membrane bounded, electron-dense, lysosomelike vacuoles, well-developed SER cisternae, and numerous pinocytic and SER-originated vesicles of different sizes along the peripheral region. M-granulocytes comprised the largest proportion of hemocytes in all samples observed. M-granulocytes were distinguished not only by the presence of membrane bounded granules of different sizes and electron density, but by the possession of large nuclei with distinct nucleoli, many mitochondria, and a highly developed network of rough endoplasmic reticulum (RER). M-granulocytes had abundant, rosette-shaped, RER-derived chains of fine secretory granules, which accumulated in the cytoplasm and vacuoles, and were ultimately deposited into the hemolymph by exocytosis. These fine granules gave a positive result with periodic acid-Schiff (PAS) test. Based on RER-synthesized fine secretory granules (M-granulocytes), their ultimate deposition into hemolymph, the red pigmentation of hemolymph, positive PAS histochemical test of these granules, and the high population of these hemocytes, no such cell type has been described in previous studies in insects. The sub-cellular structure of the granulocyte in this insect has been modified to perform a special synthetic and secretory function (i.e., possibly the synthesis of the red pigment found in hemolymph, which has been the source of commercially important cochineal dye).Abbreviations EM electron microscope - ER endoplasmic reticulum - LM light microscopy - MVB multivesicular body - PAS periodic acid-Schiff - RER rough endoplasmic reticulum - SER smooth endoplasmic reticulum - SG secretory granules - TEM transmission electron microscopy - UA uranyl acetate     

Response of host embryonic chicks to grafts of additional adrenal glands

Summary The ultrastructure of the developing adrenal gland of the chick has been studied over the embryonic period 10 to 18 days.Cortical cells occur in double-rowed strands, are loosely attached in early development but more firmly attached later in development. Hypertrophy of mitochondria, endoplasmic reticulum, golgi body, lipid and vacuoles increases with increasing age. Two cell types (designated Dark and Light) were present at 17 days of incubation. Their significance is discussed.Medullary cells occur as single cells at 10 days but more usually in groups by 18 days. Catecholamine-containing granules are a prominent feature of the medullary cells, at all ages. Two cell types could be distinguished in the medulla at 17 days of incubation. These may represent adrenalin and noradrenalin-containing cells.Changes in the ultrastructure of host adrenal glands after exposure to an 18 day adrenal gland, grafted onto the host chorio-allantoic membrane at 8 days, were studied. The chief response within the cortex of the host involved retardation of organelle hypertrophy, so that 17 day hosts resembled 14 day controls. More light cells were seen in the host than in the control cells. The medullary tissue of the host was also retarded in development and the release of catecholamine-containing granules inhibited. The significance of these observations in relation to compensatory hypertrophy within the host is discussed.Supported by National Research Council of Canada Grant No. A 5056, and by a Dalhousie University Research Development Grant. HPH is a Post Doctoral Research Fellow, supported under Grant A 5056.     

Light and electron microscopic investigations of six types of glandular cells of the bovine adenohypophysis

Summary Twelve bovine adenohypophyses were prepared for light and electron microscopy of the cell types of pars distalis. Correlation between the light and electron microscopy was effected by use of alternate thin and thick sections. Cytological changes in the experimental animals were used as criteria for the identification of six different types of secretory cells.Two types of acidophils, alpha and epsilon cells, are recognized in peripheral area of the pars distalis by light and electron microscopy. The alpha cells contain orangeophilic secretory granules of a maximum diameter of 400–450 m and correspond to ordinary acidophils (STH cells). The second type, epsilon cells, contains larger, fuchsinophilic granules of 600 to 900 m in diameter, increase in number and granulation after pregnancy and thyroidectomy, and are thought to be prolactin cells (LTH cells).Two types of amphophils, zeta and delta 1 cells, were found in the central area of the pars distalis. The zeta cells contain smaller numbers of amphophilic, cored granules (200 m maximum diameter) and based on the comparison with literature on other species of animals, are designated as ACTH cells. The delta 1 cells are round or oval and contain very dense, spherical granules (250–300 m) which are stained red or reddish purple with PAS, aldehyde thionin and PAS-methyl blue methods. They show extreme enlargement and bizarre cytoplasmic appearance after castration and are designated tentatively as LH gonadotrophs or LH cells.Two types of basophils, beta and delta 2 cells, were also identified by correlative light and electron microscopy. The beta cells are polygonal in outline, distributed exclusively in the zona tuberalis and contain large, less dense secretory granules (300–400 m) which are stained selectively with Gomori's aldehyde fuchsin. After thyroidectomy, they lose their secretory granules and are transformed into large, vacuolated thyroidectomy cells. They are therefore, identified as thyrotrophs or TSH cells. The delta 2 cells are round, oval or polygonal in shape and contain basophilic granules ranging from 220 to 300 m in diameter. They show extreme enlargement and vacuolization due to the dilation of endoplasmic reticulum, after castration, and are designated tentatively as FSH gonadotrophs or FSH cells.The investigation reported herein was supported by a Scientific Research Grant (No. 291049) from the Ministry of Education of Japan.     

Ultrastructure of human amnion and amniotic plaques of normal pregnancy

Summary The fine structure of amniotic and amniotic-plaque epithelia has been studied from normal term pregnancies. The columnar/cuboidal amniotic epithelial cells usually have apical or central nuclei, some free ribosomes, patches of granular endoplasmic reticulum, juxtanuclear Golgi complexes, rod-shaped mitochondria, lipid droplets and some glycogen granules. They have short, blunt microvilli which frequently branch and bathe in the amniotic fluid. The lateral plasma membranes enclose tortuous intercellular spaces which are always interrupted by variously folded processes and desmosomes. The epithelial cells rest on a basal lamina and exhibit highly folded basal processes. The amniotic epithelial cells are neither distinctly Golgi and fibrillar types nor light and dark in appearance.Amnion from near the umbilical cord contains many microscopic and several large plaques. Similar structures are not found on the reflected amnion. The microscopic plaques are whitish and translucent, whereas the large ones are opaque. The large plaques vary between 1–3 mm in diameter, and are over 15 cell layers thick. Each large plaque has a main central region and edges continuous with either the microscopic plaque or the simple amniotic epithelium. The main region shows four zones, namely, stratum basale, stratum spinosum, stratum granulosum and stratum corneum. Such zones are not distinct at the edges. The fine structure of basal cells compares with the amniotic epithelial cells, but the cells of spinosum and granulosum layers possess variable amounts of tonofibrils, keratohyalin granules, free ribosomes and other cytoplasmic organelles and inclusions. The corneum cells are keratinized and are frequently separated by intercellular spaces. They slough into the amniotic cavity singly or as a sheet, and contribute towards the composition of the amniotic fluid. The plaques are of amniotic origin, and are not formed by adhesion of either squamous cells or fetal skin cells (masses of keratinized squames). The present observations suggest that the occurrence of amniotic plaques is normal. The presence of plaques may not be necessarily associated with fetal abnormality. However, increase in numbers of plaques may be caused by conditions of fluid imbalance. The homology and significance of plaques in eutherian mammals have been discussed.This research was supported by USPHS Grant AM-11376 and NIH Grant 69-2136.