Morphological study of the fetal vasculature of the human term placenta: scanning electron microscopy of corrosion casts.

Microvascular corrosion casts of the fetal vessels of the normal human placenta were studied using the scanning electron microscope. All available evidence suggests that when prepared under standard conditions the resultant casts accurately replicate the original luminal morphology of the vessels, and are comparable between placentae. In conjunction with stereo-pair photography this technique allows for the rapid and unequivocal interpretation of the three-dimensional configuration of villous vasculature. Previously described structures such as dilated capillary loops and perivascular capillary networks can be viewed in new and dramatic perspective. It is hoped the technique may thus be of assistance to physiologists in the interpretation of experimental data concerning placental transport. Further work is in progress to apply the technique to the investigation of possible changes in fetal vasculature occurring in abnormal pregnancies.

Development of the adrenal cortex in the fetal foal: an ultrastructural study.

The adrenal cortex from twelve fetal foals (gestational ages from 61 to 300 days) was examined by light and electron microscopy. Adrenal glands from three newborn foals were also examined by light microscopy. Between 61 and 100 days of gestation the adrenal cortex became organised into two distinct regions, the zona glomerulosa and zona fasciculata, which grew steadily in thickness until the 300th day. Between 300 days and birth there was a dramatic increase in the width of the zona fasciculata. From 200 days a narrow band of compact cells marked the cortico-medullary border. Though these cells were ultrastructurally indistinguishable from those of the overlying zona fasciculata it seems possible that they may later give rise to the zona reticularis of the adult gland. The cells of the zona glomerulosa were columnar in shape and were tightly packed together into cell clusters. They contained smooth and rough endoplasmic reticulum and mitochondria with lamellar cristae. The ultrastructural characteristics of the cells of the zona glomerulosa changed little throughout the course of gestation. In the zona fasciculata the predominant cell type was angular in profile, and its cytoplasm contained smooth and rough endoplasmic reticulum and mitochondria with lamellar cristae. The second and larger cell type was situated nearer the cortico-medullary border. It contained an abundance of smooth endoplasmic reticulum, mitochondria with lamellar and/or vesicular cristae, and large perinuclear Golgi complexes. These cells, which appeared to be derived from the smaller more angular cells of the zona fasciculata, showed signs of a high level of metabolic and synthetic activity. As gestation progressed the number of functionally differentiated cells increased slowly in a radial direction towards the zona glomerulosa. Indirect evidence from two newborn foals suggests a rapid increase in the functional differentiation of zona fasciculata cells between 300 days and birth.

Sheep trophoblast in monolayer cell culture.

Methods are described for the preparation and maintenance of ovine trophoblast in monolayer cell culture, and for the preparation of cultured cells for ultrastructural examination. Monolayers grown from dispersed and explanted primary cultures were maintained in consecutive subcultures for periods of 12 to 16 weeks. Towards the end of the first week of incubation high concentrations as the cultured developed. In the second week of incubation small binucleate cells were seen in cultures prepared for electron microscopy. By the 14th day larger binucleate cells were visible at the outer edges of the developing monolayer, and by the third to fourth week 25 to 40 per cent of the cells in culture were in binucleate form. Mature binucleate cells seeded in primary dispersed cultures did not survive beyond the 14th day. The result suggest that (a) the binucleate cells which first appear in monolayers of cultured trophoblast during the second week of incubation are formed by nuclear division of uninucleate cells and not from mature binucleate cells seeded in primary dispersed cultures; and (b) there may be a correlation between the numbers of binucleate cells present in the monolayer and the rate of production of ovine placental lactogen. Monolayer cultures of sheep trophoblast may well provide a useful and relatively inexpensive experimental system for the study of binucleate cells and the mechanisms of placental hormone production.

Ultrastructural observations on the placenta of the alpaca (Lama pacos).

The alpaca is one of the four South American species of the family Camelidae. Its placenta, like that of other camelids, is diffuse and epitheliochorial in type. The chorionic epithelium is thrown into unbranched villi or folds which are closely apposed to corresponding undulations of the uterine epithelium, and the fetal-maternal interface consists of an intricate interdigitation of fetal and maternal microvilli. In late gestation both chorionic and uterine epithelia are deeply indented by placental capillaries, so that the minimum intercapillary distance across the diffusion pathway may be as little as 2 microns. This distance appears to be less than that found in the epitheliochorial placenta of any other species of domestic ungulate in late gestation: it may be one of several adaptations to pregnancy at high altitude. Delivery of the fetal membranes occurs some 45 minutes after the birth of the fetus. The placenta is non-deciduate.

Ultrastructural changes in the placenta of the ewe after long-term intravascular infusion of 2-bromo-alpha-ergocryptine (CB154) into mother or fetus.

The fine structural appearance of the placenta of the ewe has been examined following long-term infusion of CB154 into either the fetus or the pregnant ewe. Binucleate cells which usually contain aggregations of spherical membrane-bound electron-dense inclusions, are a characteristic component of the chorionic epithelium of the sheep. Following CB154 infusion into either the fetus or ewe at 111 to 137 days of gestation, binucleate cells were partially or completely depleted of the droplets which are present in binucleate cells of control animals at a similar gestational age. No obvious changes in the maternal epithelial syncytium were observed after CB154 administration. Infusion of CB154 into the fetus alone was also followed by degenerative changes in some binucleate cells which ranged from condensation of nuclei to complete cell fragmentation. Either a direct or an indirect action of CB154 on binucleate cells is suggested by these observations. Hypoprolactinaemia followed CB154 infusion in all treated animals; its possible influence on binucleate cell activity is discussed.

On the presence of a ductus venosus in the fetal pig in late gestation.

Evidence is presented to show that there is a functional bypass in the liver of the fetal piglet between the umbilical vein and the posterior vena cava. Injections of labelled microspheres (14 micrometer) into the umbilical vein in six fetuses in late gestation resulted in the appearance of radioactivity in the arterial blood and throughout the tissues of all piglets. About 60% of the umbilical venous blood bypassed the liver whereas in a fetal foal, injection in a similar manner, no evidence for a shunt was found. Radiographic studies confirmed the presence of a large vascular connection, equivalent to the ductus venosus, between the umbilical vein and posterior vena cava in the fetal piglet.

Scanning electron microscope studies of the endometrium of the cyclic mare.

Endometrial biopsies obtained from mares at different stages of the oestrous cycle, during anoestrus and in various abnormal conditions were examined with the scanning electron microscope. Preliminary observations suggest that the patterns of secretory and ciliary activity in the uterine epithelium are similar to those observed by electron microscopical techniques in laboratory and other large domestic animals. The response of the epithelial cells to hormonal variations and infections is compared with that of the endometrium as seen with the light microscope.

Ultrastructural studies of the equine uterus and placenta following parturition.

Post-partum placentae and uterine biopsy samples from mares after normal and abnormal foalings are described. After normal delivery there is little damage to fetal or maternal tissues. The villous epitheliochorial palcenta separates cleanly at the maternal-fetal interface and the afterbirth consists almost exclusively of fetal tissue. Uterine involution is well advanced by the 3rd and 4th days post partum and the changes are usually complete by the oestrus 7--10 days after parturition. Placental separation and involution of the uterus appear to proceed normally in malpresented foals and in otherwise viable foals with musculoskeletal defects. In aborted, stillborn or dysmature foals there are obvious signs of damage to both fetal and maternal tissues. It is generally accepted that the growth and development of the fetus is dependent upon a placenta of adequate functional capabilities. The observations suggest that the placenta is similarly dependent upon its association with a normal healthy fetus.

Ultrastructural changes in the placenta of the ewe after fetal pituitary stalk section.

Binucleate cells are a normal component of the ovine chorionic epithelium, but are usually separated from the fetal-maternal interface by a thin layer of cytoplasm derived from the principal or uni-nucleate cells of the trophoblast. They are distinguished not only by two distinct and separate nuclei, but also by conspicuous membrane-bound cytoplasmic inclusions in the form of haloed droplets. After fetal pituitary stalk section binucleate cells move up to and participate in the formation of the fetal-maternal interface; furthermore they extend clear blunt-ended pseudopodia into the maternal epithelial syncytium. These activities do not appear to be supppressed by fetal infusion of cortisol or ACTH. The apparent motility of binucleate cells, together with the presence of haloed droplets within the maternal epithelial syncytium, suggests that after fetal pituitary stalk section binucleate cells invade the uterine syncytium, lose their limiting membranes and discharge their contents into the syncytial cytoplasm. Large molecules such as ovine placental lactogen may be transported from fetal to maternal tissues by this mechanism.

Studies on the equine placenta. III. Ultrastructure of the uterine glands and the overlying trophoblast.

Ultrastructural studies of the uterine glands at intervals during pregnancy in the mare show that secretory activity continues after formation of the placental exchange units. The nature of the glandular secretion appeared initially to be proteinaceous, but cellular debris was also present during the last third of gestation. These secretions were absorbed by the trophoblast overlying the mouths of the glands. The fate secretions and their significance for the fetus and placenta are unknown.

Studies on the equine placenta II. Ultrastructure of the placental barrier.

In early pregnancy the equine placenta consists of a simple apposition of fetal and maternal epithelia, but it becomes more complex with the formation of microcotyledons between 75 and 100 days of gestation. Although the placental barrier maintains an epitheliochorial arrangement throughout the course of pregnancy, a thinning of the maternal epithelium and a progressive indentation of the chorionic epithelium by fetal capillaries shortens the length of the diffusion pathway and reduces the amount of placental tissue between fetal and maternal bloodstreams. These structural modifications may reflect the changing requirements of the fetus for O2 and other metabolites as gestation proceeds. During the first 200 days of pregnancy there is evidence of intense pinocytotic activity by the cells of the trophoblast. From the 100th day of pregnancy there is a pronounced development of smooth endoplasmic reticulum, while rough endoplasmic reticulum and irregular, dense, membrane-bound bodies are a prominent feature of the paranuclear cytoplasm from Day 200. These changes suggest that the cells of the trophoblast become more highly involved in synthetic processes with increasing gestational age.

PIP: The ultrastructure of the equine placenta was observed on gestation Days 46, 61, 75, 98, 101, 150, 200, 250, and 300 in Welsh Mountain Ponys mares. In early pregnancy, up to 75 days, when the placenta is epitheliochorial, the trophoblast forms a wide band of columnar epithelium with large nuclei, pinocytotic vesicles, and a defined basal lamina beneath the fetal vessels. Maternal epithelium has branched microvilli, spherical nuclei, droplets of mucopolysaccharide, and a moderately thick capillary endothelium. From Days 100-200 the villi and crypts of the microcotyledons develop. The trophoblast shows larger pin ocytocic vesicles (.5 MCM), extensive smooth endoplasmic reticulum (ER), and the fetal capillaries indent into the trophoblast. Maternal epithelium becomes progressively thinner. In the last trimester the trophoblast becomes thinner with nuclei and mitochondria near the indented capillaries, and dense bodies apical to the nucleus. By this time the maternal epithelium is only 1/3 its original thickness, with few organelles except Golgi, mitochondria, and an irregular and sometimes pyknotic nucleus. The ultrastructure supports the idea that the trophoblast may be synthesizing steroids in the later stages of pregnancy.

Ultrastructural studies of the placenta of the ewe: phagocytosis of erythrocytes by the chorionic epithelium at the central depression of the cotyledon.

Extravasated maternal blood, which escapes from capillaries and larger blood vessels within the tips of the maternal septa, is responsible for the characteristic pigmentation of the central depression of the ovine cotyledon in the last third of pregnancy. The chorionic epithelium of this region is actively engaged in the uptake and subsequent breakdown of maternal erythrocytes, which may represent an important source of iron for the foetus during the period of maximum intra-uterine growth.

Development of pituitary and adrenal glands in the fetal horse.

The various cell types in the adult anterior pituitary may be distinguished at the ultrastructural level of the sizes of the secretory granules within the cytoplasm. In the fetal adenohypophysis, with the exception of prolactin-producing cells, all types may be identified after Day 100 of gestation. Morphological evidence suggests that they are producing and secreting trophic hormones. The three constituent layers of the adrenal cortex are also discernible by Day 100 of pregnancy. The cells of the zona glomerulosa have large numbers of darkly stained inclusions within the cytoplasm. The zona fasciculata shows changes consistent with the production and secretion of steroid hormones.

Ultrastructural development of the equine placenta.

Microcotyledons, which are a distinctive feature of the mature equine placenta, are fully formed by Day 150 of gestation. The fetal component of each microcotyledon is developed from several primary folds of trophoblast which become elaborately subdivided as gestation proceeds. These changes are reflected in the structure of the maternal crypts, which receive the fetal villi. Between Days 60 and 150 of gestation the maternal epithelium is greatly reduced in height. No such change occurs on the fetal side of the placenta, but between Days 100 and 250 a progressive indentation of the epithelium by fetal capillaries reduces the effective thickness of the placental barrier. The ultrastructural characteristics of fetal and maternal epithelia are described.

Anatomy of the placental barrier in the mare.

The study of the equine placenta, which began in Venice in 1598, has a long but discontinuous history. Early observations were purely morphological, but new techniques have stimulated a broader and more functional approach. Histological and ultrastructural observations at various stages of pregnancy have shown that the fetal side of the placenta comes to acquire certain features in common with the air-blood barrier of the mammalian lung. These changes may reflect the increasing O2 requirements of the fetus as gestation proceeds.

Ultrastructural changes in the placenta of the ewe following foetal infusion of cortisol.

The placental changes which followed continuous infusion of cortisol into the sheep foetus in the later stages of gestation were, like the hormonal changes, broadly similar to those of spontaneous parturition. There was, however, a premature separation of foetal and maternal tissues in certain areas of the placental cotyledons, and this separation appeared to protect the foetal epithelium from the degenerative changes which normally take place in the short space of time between the birth of the lamb and the delivery of the foetal membranes. The results suggest that an experimental model in which premature labour is induced by the administration of cortisol to the foetus is probably incomplete, and that additional factors almost certainly contribute to the cascade phenomenon of spontaneous parturition.