Placental permeability during early gestation in the hamster. Electron microscopic observations using horseradish peroxidase as a macromolecular tracer

Developmental and mechanical roles of Reichert's membrane in mouse embryos

Embryonic development and growth in placental mammals proceeds in utero with the support of exchanges of gases, nutrients and waste products between maternal tissues and offspring. Murine embryos are surrounded by several extraembryonic membranes, parietal and visceral yolk sacs, and amnion in the uterus. Notably, the parietal yolk sac is the most outer membrane, consists of three layers, trophoblasts and parietal endoderm (PaE) cells, and is separated by a thick basal lamina termed Reichert's membrane (RM). RM is composed of extracellular matrix (ECM) initially formed as the basement membrane of the trophectoderm of pre-implanted embryos and followed by the heavy deposition of ECM mainly produced in PaE cells of post-implanted embryos. In addition to the physiological roles of RM, such as gas and nutrient exchange, it also plays a crucial role in cushioning and dispersing intrauterine pressures exerted on embryos for normal egg-cylinder morphogenesis. Mechanistically, such intrauterine pressures generated by uterine smooth muscle contractions appear to be involved in the elongation of the egg-cylinder shape, along with primary axis formation, as an important biomechanical element in utero. This review focuses on our current views of the roles of RM in properly buffering intrauterine mechanical forces for mouse egg-cylinder morphogenesis. This article is part of the theme issue 'Extraembryonic tissues: exploring concepts, definitions and functions across the animal kingdom'.

A field of myocardial-endocardial NFAT signaling underlies heart valve morphogenesis

The delicate leaflets that make up vertebrate heart valves are essential for our moment-to-moment existence. Abnormalities of valve formation are the most common serious human congenital defect. Despite their importance, relatively little is known about valve development. We show that the initiation of heart valve morphogenesis in mice requires calcineurin/NFAT to repress VEGF expression in the myocardium underlying the site of prospective valve formation. This repression of VEGF at E9 is essential for endocardial cells to transform into mesenchymal cells. Later, at E11, a second wave of calcineurin/NFAT signaling is required in the endocardium, adjacent to the earlier myocardial site of NFAT action, to direct valvular elongation and refinement. Thus, NFAT signaling functions sequentially from myocardium to endocardium within a valvular morphogenetic field to initiate and perpetuate embryonic valve formation. This mechanism also operates in zebrafish, indicating a conserved role for calcineurin/NFAT signaling in vertebrate heart valve morphogenesis.

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.

Unusual aspects of inner cell mass formation, endoderm differentiation, Reichert's membrane development, and amniogenesis in the lesser bulldog bat, Noctilio albiventris

BACKGROUND AND METHODS

The early embryogenesis of the lesser bulldog bat, Noctilio albiventris (family Noctilionidae), was examined histologically in 59 pregnant females collected from a reproductively synchronized population in Colombia.

RESULTS

Early blastocysts of Noctilio are unusual in lacking a typical inner cell mass. Instead, cells inside of the trophoblast are dispersed for a period as a monolayer. A typical inner cell mass (ICM) only forms and becomes properly oriented after the initiation of implantation. Several features of Reichert's membrane in this species are also noteworthy: it develops between the ICM and trophoblast and between the parietal endoderm and trophoblast; it becomes linked to a meshwork of basal laminalike material that extends into the ICM; and it appears to be continuous, or fused, with prominent basal laminae that develop within the cytotrophoblastic villi that radiate throughout the preplacenta. Amniogenesis occurs by cavitation and converts the ICM into a hollow epiblastic vesicle. Gastrulation commences before this vesicle exhibits obvious differentiation into an embryonic shield and amniotic ectoderm.

CONCLUSIONS

Because development and proper orientation of a typical ICM in Noctilio occur after the initiation of implantation, these may involve the migration of cells on the interior of the blastocyst and/or an unusual method of early endoderm differentiation. The possibility exists that epiblast, endoderm, and cytotrophoblast may all contribute to the secretion of Reichert's membrane in this bat. Although the early embryogenesis of Noctilio exhibits many similarities to that in phyllostomid bats, substantial differences also exist between these closely related species.

Giant and binucleate trophoblast cells of mammals

The cellular origin, structure, and function of trophoblastic giant cells (GC) and binucleate cells (BNC) are reviewed. Mammals in which these cells have received the greatest attention include rodents, rabbits, and humans (GCs), and ruminants and equids (BNCs). In almost all cases these cells arise from the cytotrophoblast. All are large cells and contain either two diploid nuclei (BNCs), multiple nuclei (human placental bed GCs), or single nuclei with amplified DNA content (rodent and rabbit GCs). Giant and binucleate cells typically exhibit the capacity for migration or invasion, although the degree of migratory activity varies between species. While most end up within, or at the interface with, endometrial tissue, in some instances the GCs or BNCs contribute directly to the interhemal membrane of the placenta. Hormone production is a property which most GC-BNC populations have in common. Lactogen or gonadotropin has been documented in almost all cells of this type examined to date, and in some animals they are also steroidogenic (e.g., rats and sheep). In spite of some common features, both structural and functional differences remain and it is suggested that use of terms such as mononuclear giant cells, multinucleate giant cells, and binucleate cells be continued rather than assuming that these cells are all members of a single trophoblastic subtype.

Trophoblast-decidual cell interactions and establishment of maternal blood circulation in the parietal yolk sac placenta of the rat

Implantation sites from rats were studied on days 6, 7, and 8 of pregnancy to determine the sequence of events in the formation of blood spaces in the trophoblast that is part of the parietal wall of the yolk sac placenta and to determine how trophoblast gains access to maternal blood. The maternal blood flowing through these spaces is the source of nutrients that reach the embryo via the visceral endoderm. Tissues were prepared for light microscopy, scanning electron microscopy, and transmission electron microscopy. Trophoblast blood spaces are derived from the lateral intercellular spaces of trophoblast cells and are present in a collapsed condition until day 8, when maternal vessels are tapped by trophoblast. These spaces then contain circulating maternal blood, and trophoblast cells reflect adaptations for metabolic exchange including thinning of trophoblast covering Reichert's membrane and the appearance of numerous fenestrations, with and without diaphragms, in the areas where trophoblast is attenuated. Between days 6 and 7 decidual cells appear to form a barrier between the maternal circulation and trophoblast. On day 7, however, decidual cell processes penetrate the residual uterine luminal epithelial basal lamina, and then the decidual cells that are juxtaposed to trophoblast undergo degradative changes that resemble apoptosis. There is condensation of cytoplasmic contents, fragmentation of the cells, and phagocytosis of the fragments by trophoblast. Some decidual cells are interposed between endothelial cells in the walls of maternal vessels as early as day 7. Trophoblast may gain access to the maternal vessels by replacing decidual cells or by direct imposition of trophoblast cell processes between endothelial cells.

Trophoblast invasion and alteration of mesometrial arteries in the pregnant hamster: light and electron microscopic observations

During gestation in the Syrian golden hamster, an extensive migration of trophoblast into the maternal uteroplacental arterial system takes place. The present light and electron microscopic study has focused on one facet of this phenomenon, namely the trophoblastic invasion and alteration of the mesometrial 'vascular knot' (a complex of anastamosing, small arterial channels carrying blood to the chorio-allantoic placenta). Beginning at about the 8th day of gestation, motile trophoblastic giant cells (TGC), originating from the primitive trophospongium (Träger), invade and migrate as a wave up the central maternal arteries of the decidua basalis. During the 12th and 13th days of gestation, they reach the mesometrium and successively settle in the lumina of the vascular knot arterial channels. Between this time and parturition (day 16) the arrested TGC sequentially remove and replace the arterial endothelium, breach the elastic lamina, and apparently destroy (by lytic activity) nearly all of the mural smooth muscle. Near term, the vascular knot arterial channels are composed almost entirely of TGC. The functional significance of these events, and, more generally, the phenomenon of trophoblastic invasion of maternal uterine arteries in mammalian species having haemochorial-type placentae, are still uncertain. These invasions may be requisite to adapting, structurally and physiologically, the maternal uterine arteries to support of the fetoplacental unit and/or to maintaining the immunologically privileged status of the latter.