The fine structure of the interhemal membrane of the rat chorioallantoic placenta during prolonged pregnancy

Placental Evolution within the Supraordinal Clades of Eutheria with the Perspective of Alternative Animal Models for Human Placentation

Here a survey of placental evolution is conducted. Placentation is a key factor for the evolution of placental mammals that had evolved an astonishing diversity. As a temporary organ that does not allow easy access, it is still not well understood. The lack of data also is a restriction for better understanding of placental development, structure, and function in the human. Animal models are essential, because experimental access to the human placenta is naturally restricted. However, there is not a single ideal model that is entirely similar to humans. It is particularly important to establish other models than the mouse, which is characterised by a short gestation period and poorly developed neonates that may provide insights only for early human pregnancy. In conclusion, current evolutionary studies have contributed essentially to providing a pool of experimental models for recent and future approaches that may also meet the requirements of a long gestation period and advanced developmental status of the newborn in the human. Suitability and limitations of taxa as alternative animal models are discussed. However, further investigations especially in wildlife taxa should be conducted in order to learn more about the full evolutionary plasticity of the placenta system.

Evolutionary transformations of chorioallantoic placental characters in rodentia with special reference to hystricognath species

The aim of this paper is to reconstruct the evolution of chorioallantoic placental characters in Rodentia. The analysis is based on pre-existing hypotheses of rodent relationships and the tracing of character evolution. Data on 64 rodent species of 49 genera are derived from the literature. New results refer to the hystricognath species Petromus typicus A. Smith, 1831 and Octodon degus (Molina, 1782). This comprehensive analysis confirms that the stem species pattern of Rodentia is characterised by a haemochorial placenta which is divided horizontally. Inside the placental labyrinth, fetal vessels and their trophoblastic external border build up a network through which the maternal blood flows. The trophoblastic tissue is one-layered, syncytial and possess a considerable surface extension. Within Rodentia, evolutionary transformations occurred on the macroscopic as well as the fine structural level. The results suggest that the stem species of Hystricognathi underwent transformations only on the macroscopic level, i.e., forming a ring-shaped arrangement of placental regions with centrally situated maternal arteries and the acquisition of a subplacenta. By contrast, in Muridae the chorioallantoic placenta shows derived features only in regard to the fine structure of the labyrinth, i.e. the interhaemal membrane is modified in composition, and the fetal capillary endothelium is fenestrated. Geomyoidea underwent transformations on both levels. Macroscopically, their placenta is modified into a hillock shape. Microscopically, the interhaemal membrane is formed by the cytotrophoblast. In addition to the mentioned transformations, some aspects of other fetal membrane differentiation in Rodentia are briefly discussed.

Transport of glucose across the blood-tissue barriers

In specialized parts of the body, free exchange of substances between blood and tissue cells is hindered by the presence of a barrier cell layer(s). Specialized milieu of the compartments provided by these "blood-tissue barriers" seems to be important for specific functions of the tissue cells guarded by the barriers. In blood-tissue barriers, such as the blood-brain barrier, blood-cerebrospinal fluid barrier, blood-nerve barrier, blood-retinal barrier, blood-aqueous barrier, blood-perilymph barrier, and placental barrier, endothelial or epithelial cells sealed by tight junctions, or a syncytial cell layer(s), serve as a structural basis of the barrier. A selective transport system localized in the cells of the barrier provides substances needed by the cells inside the barrier. GLUT1, an isoform of facilitated-diffusion glucose transporters, is abundant in cells of the barrier. GLUT1 is concentrated at the critical plasma membranes of cells of the barriers and thereby constitutes the major machinery for the transport of glucose across these barriers where transport occurs by a transcellular mechanism. In the barrier composed of double-epithelial layers, such as the epithelium of the ciliary body in the case of the blood-aqueous barrier, gap junctions appear to play an important role in addition to GLUT1 for the transfer of glucose across the barrier.

The structure of the tupaia placenta. II. Ultrastructure

Both placental discs of the tupaia placenta consist of three large mesometrial and numerous additional smaller antimesometrial lobules. The larger lobules can be subdivided into a central (maternal arterial inflow), intermediate (maternal capillaries, exchange area), and a peripheral zone (maternal veins). The ultrastructure of the intermediate lobular zone has been studied from day 21 until day 41 of pregnancy. The materno-fetal barrier is endotheliochorial in structure. The voluminous maternal endothelium is highly differentiated. The adjacent basement membrane, separating endothelium and trophoblast, is constantly interrupted by endothelial and trophoblastic extensions, forming intense interdigitations between both tissue layers. The trophoblast is a multinuclear continuous layer, syncytial or plasmodial in nature. Cytotrophoblast is missing in all stages studied so far. Towards term, the trophoblast differentiates into three distinct types, arranged beneath each other in a mosaic-type pattern: trophoblast with rough e.r. and trophoblast with smooth e.r. as dominating organelles, as well as thin trophoblastic lamellae, almost devoid of organelles. The connective tissue is composed of very compact, epithelium-like cells, whereas connective tissue fibres and interstitial space are virtually absent. Macrophages, undifferentiated fibroblasts and highly differentiated fibroblasts can be observed. There are structural transitions between connective tissue and neighbouring fetal capillary endothelium.

Changes in the fine structure of rat visceral yolk-sac placenta during prolonged pregnancy

Visceral yolk-sac membranes were obtained surgically from viable rat fetuses which had been retained experimentally in utero in lactating animals for as long as 3 days beyond delivery of their littermates. They were examined with the electron microscope. The fine structure of the yolk-sac placental membrane (i.e., the tissues separating vitelline capillary lumina from the uterine cavity) remained unimpaired during the delay period. Progressive changes included a decrease in the height of the endodermal epithelium, but a thickening of the basement membrane underlying it by repeated replication of the subepithelial basal lamina and an increase in collagen fiber content in the reticular lamina. Transcytosis across the endothelium of the capillaries of the peripheral vitelline circulation remained unchanged throughout the delay period; endocytosis of materials, presumably maternal serum proteins, from the uterine cavity into the apices of the endodermal epithelium, however, decreased with time. At the oldest stage (26 days post-coitum) most endocytosed materials had been stored for hydrolysis in large subapical vacuoles which were identified as secondary lysosomes; and evidence of uptake from the uterine lumen was essentially absent. Accordingly, placental transport of protein by the visceral yolk sac of the rat can be regarded as a perinatal process. The action of an elaborately developed and well-preserved Golgi apparatus in both cellular mechanisms, viz., digestion of maternal proteins and protein transfer, was inferred.

Fetal lung development in surgically induced prolonged gestation

In our previous study we prolonged gestation in rats with daily subcutaneous injections of progesterone starting on gestation day 20 (P20). To determine whether exogenous progesterone affects the growth of post-term fetal lungs, we utilized surgical techniques to prolong gestation by retaining 4 living fetuses 1, 2, or 3 days beyond the date of spontaneous delivery of the rest of the litter. The retained fetuses were harvested by caesarean section on gestation days 21 through 25. In spite of an improved general appearance in the retained fetuses, there were many similarities between the retained and the post-term P20 fetuses such as: increase in fetal body, kidney and spleen weights; reduction in lung and liver weights; progressive lung atelectasis; loss of lung and liver glycogen; hypoglycemia; and reduction in placenta DNA content. The significant differences between the two were in the amount of lung disaturated phosphatidyl choline (DSPC) and lung DNA (cell number), expressed per lung or per body weight. In contrast to the P20 post-term fetuses, which exhibited a biphasic phenomenon in DSPC and DNA contents, the retained fetuses showed no change in lung DSPC and DNA contents after term. The P20 fetuses had larger adrenals, which increased with prolongation of gestation. The results suggest that exogenous progesterone does not affect the fetal lung growth post-term. It is speculated that the cellularity and the DSPC content of the post-term fetal lung is adversely affected by the degree of fetal distress.

The comparative fine structure of the interhemal membrane of chorioallantoic placentas from six genera of myomorph rodents

In order to extend the cytological information available on the structure of the interhemal membrane in cricetid rodents, in particular, and myomorph rodents, in general, we have examined the fine structure of the placental labyrinth in five genera of cricetid rodents (Lemmus, Dicrostonyx, Clethrionomys, Microtus and Peromyscus) and one genus of murid rodent (Acomys). Small pieces of labyrinth from near-term placentas were fixed in glutaraldehyde and osmium tetroxide and processed for electron microscopy. The interhemal membranes of all the species examined were hemotrichorial. The outermost layer of trophoblast, bordering the maternal blood spaces, was cellular and often contained some patent fenestrae, whereas the middle and linner trophoblastic layers were apparently syncytial. The outermost trophoblastic layer of all the species contained abundant granular endoplasmic reticulum. The intercellular space between the outer and middle layers was variable in width, primarily due to the complex folding of the surface of the middle layer. The surfaces of the middle and inner layers were closely apposed. The middle trophoblastic layer in several species contained filamentous "glomerular bodies" and the innermost layer, particularly of Peromyscus, contained smooth membranous whorls. A basal lamina separated the innermost trophoblastic layer from the fetal capillary endothelium. The endothelial cells in most of the species contained fenestrated regions. These observations are compared to those on other hemochorial placentas, and common features of cricetid (and myomorph) rodent placental find structure are emphasized.

Fine structure of the placental labyrinth in the rat at term and during prolonged gestation

The fine structure of the placental labyrinth in the rat was described at term (Day 22) and during prolonged gestation (Day 23, 24 and 25). Gestation was prolonged by dialy subcutaneous injection of progesterone to the mothers from Day 20 through Day 24 of pregnancy. The placental labyrinth at term consisted of maternal and foetal blood spaces separated by three layers of trophoblast and an endothelium. During prolonged gestation (Day 24 and 25 only) the layers of trophoblast exhibited discontinuity, loss of cytoplasmic elements and the presence of fibronoid material. In addition ot the morphological observations, it was found that foetal blood glucose levels on Day 24 and 25 were significantly lower than values at term. The results were discussed in reference to placental insufficiency and its effect on the foetus.