SEM_05_Organogenesis. Germ layers derivatives_PARTE2.docx

Endoderm derivatives The early endoderm organogenesis is simpler than that of either the mesoderm or ectoderm. After gastrulation, the endoderm is a single-layered epithelium that encloses the nutrients within the yolk sac. When the body is folded, the midline endoderm is incorporated inside of the...

Endoderm derivatives The early endoderm organogenesis is simpler than that of either the mesoderm or ectoderm. After gastrulation, the endoderm is a single-layered epithelium that encloses the nutrients within the yolk sac. When the body is folded, the midline endoderm is incorporated inside of the embryo whereas the lower part of the endoderm is pinched off outside the embryo. Thus, the endoderm situated under the notochord gives rise to the primitive gut which keeps connected with the yolk sac by a narrow stalk located inside of the umbilical cord. https://sway.office.com/u0iLxHnPhRK5Hl03#content=0fB4wp9Bfmwk2C Primitive gut 19 - Endoderm derivatives The primitive gut is derived from the dorsal endoderm adjacent to the notochord which is incorporated into the body during the folding of the embryo. The endodermal epithelium of the gut tube is wrapped by visceral (splanchnic) mesoderm which will give rise to the intestinal vasculature and a variety of connective tissues. One of them is the mesentery that suspends the intestinal tube within the body cavity. Although the embryonic gut extends continuously from the head to the tail, three major parts can be distinguished for description purposes: foregut, midgut, and hindgut (including the cloaca). The midgut communicates with the yolk sac by a connecting stalk called vitelline duct located inside of the umbilical cord. This duct is initially wide but, as development proceeds, become long and narrow. The endoderm of the primitive gut will give rise to the epithelial lining of the gastrointestinal and respiratory tracts. Also, the endodermal epithelium of the primitive gut is the origin of a wide range of organs derived from the pharynx and caudal parts of the urogenital system. In any case, the visceral mesoderm associated with the endodermal epithelium also contributes with the intestinal blood vessels and connective tissues. https://sway.office.com/u0iLxHnPhRK5Hl03#content=vPJq3GNiUUWBZK Yolk sac 20 - Formation of the gut The primary yolk sac is an extra-embryonic membrane that encloses the vitellus. In all animals, the yolk sac is the first foetal membrane to be formed. It starts to develop early, after the hypoblast formation, but only is it completed after gastrulation, when it becomes a bi-layered membrane with hypoblast-derived endoderm on the inside and extra-embryonic visceral mesoderm on the outside. The yolk sac cavity stores the vitellus or yolk, that is the main source of nutrition for non-placental embryos until hatching. In birds, as the embryo develops, the size of the yolk sac decreases, and its cavity shrinks. Eventually, it is gradually drawn into the body cavity for use by the chicken for the first two to three days after hatching while the chicken learns what to eat/drink and where to find it. In mammals, the development and fate of the yolk sac vary greatly depending on the type of the placenta. In some mammals, the yolk sac supports the embryo during the first stage of gestation. As a rule, this involves the attachment of vitelline blood vessels to the chorion, forming a choriovitelline placenta. This happens in horses and dogs, in which the yolk sac forms a functional choriovitelline placenta which persists until it is displaced by the expansion of the definite chorioallantoic placenta. In other domestic mammals, such as ruminants and pigs, there is a quick development of the chorioallantoic placenta and the yolk sac is never attached to the chorion. In those cases, the yolk sac floats as a vesicle within the coelom from the earlier stages of pregnancy (first trimester in humans) but it rapidly degenerates into the umbilical cord, where it becomes a vestigial vesicle attached to the midgut. Even in species where the yolk sac is no longer utilised for the nourishment of the embryo, it continues playing a critical role in embryonic development by serving a variety of vital functions: the first site for blood cell and vessel formation, endocrine, metabolic and immunological functions and contributing to the development of the foetal gastrointestinal and reproductive systems. https://sway.office.com/u0iLxHnPhRK5Hl03#content=GAtIMFVR6m5uws Allantois 21 - Formation of the yolk sac The allantois is a late extra-embryonic membrane that starts to develop inside of the embryo and grows toward the exterior through the umbilical cord. The primary function of the sac-shaped allantois was to collect liquid waste from the embryo, as well as to exchange gases used by the embryo. Originally, in reptiles and birds, the allantois acted as a reservoir for waste products. Also, for exchanging gases, the wall of the allantois became webbed with blood vessels (allantoic vessels), which, over evolution, gave rise to the placental circulation in mammals. As a result of the formation of the placenta, the role of the allantois as a waste container became subsidiary and the size of the allantoic sac was reduced depending on the efficiency of the placenta, even disappearing as such an extra-embryonic cavity in the humans. The allantois arises from inside of the embryo as a tubular diverticulum of the posterior part of the primitive gut (hindgut). The diverticulum is lined by endoderm and covered by visceral mesoderm, which carries the allantoic vessels. The allantoic diverticulum grows toward the umbilical cord, passes through it and stretches outside the embryo where it becomes the allantoic sac. The allantoic sac remains connected to the intra-embryonic part by an allantoic duct or urachus that runs within the umbilical cord. Soon, the allantois sac becomes filled with the allantoid fluid which volume increases during gestation. The intra-embryonic part of the allantois, from the hindgut to the umbilicus, will contribute to the formation of the urinary bladder, the permanent container for the waste products after birth. The extra-embryonic part of the allantois make posible the formation of the mammalian placenta. https://sway.office.com/u0iLxHnPhRK5Hl03#content=aB7POKZlBm08Mf 22 - Formation of the allantois In birds, the expansion of the allantois surrounds the amnion and the yolk sac; its outer wall becomes fused with the chorion forming the allantochorion, which lies immediately inside the shell membrane. The allantoic membrane carries along the allantoic blood vessels which are of the utmost importance in connection with the respiration and nutrition of the chicken. Oxygen is taken from, and carbon dioxide is given up to the atmosphere through the egg-shell, while nutritive materials of the albumen are absorbed by the allantoic circulation. In mammals, the allantois also expands in the extra-embryonic coelom in all directions, and like in birds, it joins the chorion too, to give rise to the mammalian allantochorion which constitutes the foundation of the placenta. In horses and dogs, the chorioamniotic raphe or mesoamnion is temporary; as a result, the amnion quickly becomes free inside of the embryonic sac. In these species, the allantois expands dorsally interposing itself between the amnios and the chorion while the yolk sac forms a temporary choriovitelline placenta. Eventually, the allantois also expands ventrally, to form the definitive chorioallantoic placenta. https://sway.office.com/u0iLxHnPhRK5Hl03#content=GMx2GgfTrBvopP 23 - Expansion of the allantois in horses and dogs In ruminants and pigs, the chorioamniotic raphe or mesoamnion persists throughout gestation. In these species, the allantois does not expand dorsally where the amnion remains attached to the chorion by a permanent mesoamnion. Contrary, the allantois expands ventrally to quickly form the chorioallantoic placenta while the yolk sac shrinks towards the umbilicus. https://sway.office.com/u0iLxHnPhRK5Hl03#content=E1sgVFCo9ZmnNT 24 - Expansion of the allantois in ruminants and pigs