Showing papers on "Epiblast published in 1988"

Formation and differentiation of extraembryonic mesoderm in the rhesus monkey.

Abstract: Differentiation of extraembryonic mesoderm in the rhesus monkey was studied from the epithelial penetration stage of implantation (stage 4) through the first week of postimplantation development (to stage 6). It was found that the first cells that appeared between the primitive endoderm (hypoblast) and trophoblast were separated from the latter by a basal lamina but appeared to be either loosely attached to the endoderm or to have been detached from it. Cells in this intermediate position differentiated cytologically into mesenchymal cells, which, by stage 5, had a distinctive intraendoplasmic reticulum marker. This differentiation occurred prior to the time at which the primitive streak could be recognized. By the time the primitive streak was readily discernible (stage 6), the extraembryonic mesoderm had already produced substantial extracellular matrix. The sequence of differentiation was repeated, with a 1- to 2-day lag, in the secondary implantation site. No evidence of a contribution from cytotrophoblast or primitive streak to the extraembryonic mesoderm was found. It is concluded that the origin of the first extraembryonic mesoderm in the rhesus monkey is probably a two-step process, with formation of a reticulum from primitive endoderm followed by differentiation in situ into mesenchymal cells. The first blood vessels formed also differentiated in situ from the extraembryonic mesenchymal cells. Primitive capillaries were identifiable as early as the 13th day of pregnancy.

Analysis of germ line development in the chick embryo using an anti-mouse EC cell antibody.

Abstract: We have found that EMA-1, a monoclonal antibody originally raised against mouse embryonal carcinoma (Nulli SCC1) cells (Hahnel & Eddy, 1982), also labels chick primordial germ cells (PGCs). We have used this antibody in immunohistological studies to follow the development of PGCs in the chick embryo from the time of their initial appearance beneath the epiblast, through their migratory phase and subsequent colonization of the germinal epithelium. During hypoblast formation, individual EMA-1-labelled cells appeared to separate from the basal surface of the epiblast and enter the blastocoel, coincident with the appearance of morphologically identifiable PGCs in this same area. EMA-1 continued to label germ cells until the initiation of gametogenesis in each sex; specifically, labelling was absent by 7–8 days of incubation in females and started to decrease at 11 days of incubation in males. There was a recurrence of the epitope on oogonia at 15 days of incubation, but not on spermatogonia during the remainder of development through hatching. These observations are consistent with an epiblast origin for the avian germ line, and are strikingly similar to those reported for the early mouse embryo using the same antibody (Hahnel & Eddy, 1986).

Changes in the expression of the carbohydrate epitope HNK-1 associated with mesoderm induction in the chick embryo

Abstract: We report that a monoclonal antibody, HNK-1, identifies specific regions and cell types during primitive streak formation in the chick blastoderm. Immunohistochemical studies show that the cells of the forming hypoblast are HNK-1 positive from the earliest time at which they can be identified. Some cells of the margin of the blastoderm are also positive. The mesoderm cells of the primitive streak stain strongly with the antibody from the time of their initial appearance. In the epiblast, some cells are positive and some negative at pre-primitive-streak stages, but as the primitive streak develops a gradient of staining intensity is seen within the upper layer, increasing towards the primitive streak. At later stages of development, the notochord and the mesenchyme of the headfold are positive, while the rest of the mesoderm (lateral plate) no longer expresses HNK-1 immunoreactivity. This antibody therefore reveals changes associated with mesodermal induction: before induction, it recognizes the ‘inducing’ tissue (the hypoblast) and reveals a mosaic pattern in the responding tissue (the epiblast); after primitive streak formation, the mesoderm of the primitive streak that results from the inductive interactions expresses the epitope strongly. Affinity purification of HNK-1-related proteins in various tissues was carried out, followed by SDS-PAGE to identify them. The hypoblast, mesoderm and epiblast of gastrulating chick embryos have some HNK-1-related proteins in common, while others are unique to specific tissues. Attempts have been made to identify these proteins using Western blots and antibodies known to recognize HNK-1-related molecules, but none of the antibodies used identify the bands unique to any of the tissues studied. We conclude that these proteins may be novel members of the HNK-1/L2 family, and that they may have a role in cell interactions during early development.

Horseshoe kidney: a new theory on its embryogenesis based on the study of a 16-mm human embryo.

Abstract: We studied a human embryo of 16 mm crown-rump (CR) length in excellent condition with a horseshoe kidney malformation. An exhaustive study of this specimen and a review of published material on the human embryo brings us to propose a new theory on the embryogenesis of this malformation. The most commonly accepted theory consists of a mechanical interpretation based on the relation between the metanephroi and the umbilical arteries during the development of the latter. Nevertheless, in those cases where renal parenchyma constitutes the isthmic region, we believe that these arise from nephrogenic cells that have migrated across the primitive streak in the final phase of gastrulation and thus arise from the posterior nephrogenic area of the epiblast.

Interference with gastrulation during the third week of pregnancy as a cause of some facial abnormalities and CNS defects.

Abstract: During the third week of pregnancy the human embryo undergoes a major developmental process, gastrulation, during which the two-layered embryo is converted into a three-layered embryo. At the same time, the upper epiblast layer is induced to form the neural plate. Evidence is presented which suggests that interference with this process by genetic, physical, or chemical agents can cause a range of CNS abnormalities and facial abnormalities, including those described as characteristic of the FAS.

Loss of polar trophoblast during differentiation of the blastocyst of the horse

Abstract: Twelve blastocysts, collected 7-12 days after ovulation (Day 0), were examined by light and electron microscopy to investigate the nature of the relationship of the polar trophoblast (Rauber's layer) to the inner cell mass. On Day 7, the polar trophoblast was intact and formed a flattened layer overlying the epiblast cells of the inner cell mass. As blastocysts enlarged to greater than 1 mm in diameter, small discontinuities appeared in the polar trophoblast, where epiblast cells intruded onto the surface. At this time, trophoblast cells adhered closely to adjacent and underlying epiblast cells, forming an irregular layer of cells capping the epiblast. With continued increase in blastocyst size, polar trophoblast cells became isolated but maintained their characteristic apical endocytic structures. By Days 10-12, the scattered trophoblast cells showed evidence of deterioration, and vacuoles containing cell debris were common within the epiblast. It is suggested that polar trophoblast cells become scattered, rather than withdrawing as a unit, because they become more adherent to subjacent epiblast cells than to adjacent trophoblast cells. It is further suggested that most of the isolated cells are eventually phagocytosed by epiblast cells.

Gastrulation in birds: a model system for the study of animal morphogenesis.

Abstract: Lewis Wolpert is alleged to have stated that 'it is not birth, marriage or death, but gastrulation which is truly the most important time in your life' (quoted in Slack 3o). Indeed, gastrulation is the period of early embryonic development in which the third germ layer, or mesoderm, arises as a distinct tissue. From the mesoderm will arise the skeleton, the muscle and many of the internal organs of the adult organism. The segmental pattern of somites that develops in the mesoderm also dictates the pattern of some structures that do not derive from the mesoderm, such as the peripheral nervous system. Bird embryos have been for some time the 'classical' vertebrate in which gastrulation has been studied. More recently, however, progress in avian gastrulation has lagged behind that made using other vertebrates, particularly because of two disadvantages: first, unlike amphibian embryos, the cells of bird embryos are small, which makes the study of cell lineage by injection of markers into individual cells very difficult. Second, unlike the mouse, very little is known about the genetics of birds. Nevertheless, avian embryos offer some attractive advantages for the study of gastrulation: unlike most other vertebrate embryos, they are flat, fairly transparent and considerably larger than either amphibian or mammalian embryos, they lend themselves to sophisticated microsurgery and can be obtained cheaply. The lack of genetic markers is overcome, to some extent, by the availability of the chick-quail chimaera technique introduced by Le Douarin 2z, which has been used extensively for fate-mapping. The process of gastrulation, like the rest of embryonic development, consists of a series of processes that fall into two major categories: cytodifferentiation and morphogenesis. The first concerns the divergence of cell fates from pluripotent progenitor cells to give rise to different cell types. The second refers to the processes that ensure that these different cell types are distributed correctly within the embryo and that the correct pattern is generated. It is commonly assumed that morphogenesis precedes and is required for the allocation of cell fates 3o. Thus, cell diversity is thought to result from geometry: the morphogenetic movements of gastrulation are believed to be required to bring certain tissues together in the embryo, and the interactions ('induction') between these tissues will influence the fate of the 'responding' or 'competent' cells. The mesoderm of amphibian embryos is assumed to arise in this way, as the result of the influence of the primitive endoderm on the ectoderm, as indeed is the mesoderm of the chick and other vertebrate embryos 3o. The nervous system also arises from an inductive interaction, between the mesoderm and the ectoderm, a discovery for which Hans Spemann received the Nobel prize for Physiology and Medicine in 1935 33 The process of gastrulation therefore represents a good model system in which the relationships between morphogenesis and cell diversification can be studied. In this paper we will survey the classical views of chick gastrulation, and we will present some new data which may throw some light on these complex processes. We will argue that the evidence in favour of geometry as the causative force for cell diversification is not as strong as is generally assumed, at least in birds.