Embryology
Anthony R. Mundy, John M. Fitzpatrick, David E. Neal, Nicholas J. R. George in The Scientific Basis of Urology, 2010
On the fifth or sixth day after fertilization, the blastocyst implants into the endometrium of the uterine cavity in which the endometrium has been primed by progesterone secreted by the corpus luteum. Over the ensuing 10 days, two cavities develop within the spherical mass of rapidly proliferating embryonic cells. The embryonic disc, from which the early embryo is derived, forms in the three-layered interface between the amniotic cavity and definitive yolk sac. The amniotic surface of the trilaminar embryonic disc gives rise to the ectodermal tissues of the embryo, whereas the endodermal derivatives originate from the yolk sac–derived surface. The intraembryonic mesoderm is formed by the inpouring of cells on the amniotic surface of the disc via the primitive streak (Fig. 2). The layer of intraembryonic mesoderm created by the inpouring process soon subdivides into three components, comprising the paraxial mesoderm, intermediate mesoderm and lateral plate mesoderm. It is from the blocks of intermediate mesoderm on either side of the midline that much of the genitourinary tract is ultimately derived. The third and fourth weeks of gestation are dominated by the processes of segmentation and somite formation that characterize all vertebrate embryogenesis, which are closely regulated by expression of the Hox group of genes. At this stage, folding of the expanding embryonic disc imparts recognizable shape to the growing embryo.
Prenatal Development of the Facial Skeleton
D. Dixon Andrew, A.N. Hoyte David, Ronning Olli in Fundamentals of Craniofacial Growth, 2017
These two primary germ layers are soon joined by a third, the mesoderm, that comes from either side of the primitive streak at the caudal edge of the now roughly circular embryonic disc and lies between the other two layers. At about 2 weeks the primitive streak is recognized as a linear heaping-up of ectodermal cells that continue to divide at a rate comparable to other regions of the ectoderm. The production of mesodermal cells by this region is not so much the direct result of a proliferative process, but rather a rearrangement or migration of certain ectodermal cells that will assume other developmental responsibilities. This event has its parallel in the rapid change in shape and complex rearrangement of cells that characterize gastrulation in other vertebrates. The resulting trilaminar stage of development is complete by the end of the 3rd week after fertilization.
Applications of Placenta-Derived Cells in Veterinary Medicine
Ornella Parolini, Antonietta Silini in Placenta, 2016
In contrast, the folding modality that characterizes the differentiation of amnion in domestic animals occurs in the early stage of gastrulation when the embryo becomes a trilaminar structure (trilaminar embryonic disc). The two layers located at the edge of the embryonic disc form the lateral chorion-amniotic folds and gradually expand upward and fuse. The union of the trophectoderm and the extraembryonic mesoderm will form the outer layers of the embryonic part of the placenta, named the chorion. The site where the chorion-amniotic folds meet and fuse is known as the mesamnion. In the horse and carnivores, the mesamnion disappears during fetal development, leaving no connection between the amnion and chorion. As a result, foals, pups, and kittens are born covered by an intact amnion that can be easily collected during delivery. In contrast, in the pig and ruminants,the mesamnion persists; as a result, the amnion is torn during parturition and offspring are generally born without covering membranes.
A rare pseudo tumour in the extraembryonic coelom in first trimester of pregnancy: ultrasound and pathology
Published in Journal of Obstetrics and Gynaecology, 2019
Seiji Sumigama, Atsushi Enomoto, Satoshi Matsukawa, Takafumi Ushida, Kenji Imai, Tomoko Nakano, Tomomi Kotani, Fumitaka Kikkawa
In a review of normal early human development, there is a ‘primary yolk sac’ and ‘secondary yolk sac’ as embryological terms (Moore and Persaud 2011). At 4 weeks of gestation, there is an embryonic disc consisting of two-layered cells; embryonic epiblasts and hypoblasts. The primary yolk sac is formed from embryonic hypoblasts, and is lined with an exocoelomic (Heuser’s) membrane (cuboidal cells), outside of which is the extraembryonic mesoderm (spindle-shaped cells) (Moore and Persaud 2011; Sadler 2011). Several days later, additional cells are produced by the hypoblasts, migrate along the inside of primary yolk sac and form a new cavity, the secondary yolk sac (Sadler 2011). Until the end of 4 weeks, the primary yolk sac is pinched off and its residue is found in the extraembryonic coelom; however, it is rarely detected by ultrasonography. The ‘yolk sac’ usually detected by ultrasonography after 4 weeks is the ‘secondary yolk sac’ (Kurjak et al. 2008). In this case, the cystic mass was not observed at 4 weeks and was found at 5 weeks of gestation. The timing coincided with the formation of a residual primary yolk sac, and further, the pathological structure consistent with it. Thus, we speculated that the mass was a residual primary yolk sac that had become hyperplastic for some reason.
Application of amniotic membrane in reconstructive urology; the promising biomaterial worth further investigation
Published in Expert Opinion on Biological Therapy, 2019
Jan Adamowicz, Shane Van Breda, Dominik Tyloch, Marta Pokrywczynska, Tomasz Drewa
The mammalian embryo is enclosed in the fluid filed amniotic sac of the placenta, surrounded by the AM. In humans, 6–7 days after fertilization, AM starts to develop during blastocyst implantation in the endometrium [6]. Subsequently, the embryoblast (inner cell mass within the blastocyst) differentiates into a bilaminar disc composed of the hypoblast and epiblast. Eventually, amnioblasts derived from the epiblast invade the space between the trophoblast and the embryonic disc, migrating to the inner amniotic layer and gradually constitute the external lining of the amniotic cavity. The amniotic and chorionic fetal membranes separate the embryo from the endometrium. The amniochorionic membrane forms the outer limits of the sac that encloses the embryo, while the innermost layer of the sac is the AM [7].