Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson
OUTLINE OF SIXTEENTH LECTURE: Feb 22, 2006, by Corey JohnsonExtraembryonic membranes
Many teleost fishes lay huge masses of eggs with a very protective covering. Amphibians, despite their venture onto land often must return to land to lay eggs. There are some who lay them in very moist soil. Some have adopted other strange forms of maternal investment... the marsupial frog carries it's tadpoles in a pouch. Reptiles and birds have the advantage of a shelled egg that conserves moisture, while allowing gas exchange to compensate for the reduced volume of their environment. Mammals have largely dispenses with shelled eggs and large yolks and utilize a huge maternal investment and a drastically reduced number of offspring. Without the benefit of an infinite capacity for diffusion as occurs in a pond or other body of water, embryos of amniotes must have a means to remove wastes, exchange respiratory gases, and obtain nutrients. Reptiles, birds and mammals have a set of 4 extraembryonic specializations called extraembryonic membranes. Amnion: the amnion is a protective membrane that surrounds the embryo forming a sac of fluid Yolk sac: the yolk sac in birds and reptiles is intimately associated with the yolk, and provides the embryo with nutrients. In mammals it is involved in the formation of blood vessels and may retain a role in nutrition Allantois: the allantois is an evagination of the hind gut that is involved in waste removal Chorion: the outermost layer of membranes that is principally involved in respiration in birds and many other functions in mammals
Chick development:Body folds: A series of folds undercut the developing embryo separating it from the underlying yolk in the chick. Prior to the production of these folds, embryonic and extraembryonic tissues are indistinguishable. That is the ectoderm, mesoderm and endoderm extend beyond the boundaries of the embryo, surrounding the yolk. The embryonic tissues that extend beyond the head are pulled ventrally, beneath the embryo. Then the sides, and finally the posterior of the extraembryonic regions are pulled beneath the embryo. This process separates the embryo from the extraembryonic tissues which previously were indistinguishable.The result of this process is that the embryo appears to raise up from the surrounding tissues, suddenly making the size and shape of the embryo distinct. Yolk Sac: The first extraembryonic membrane to form is the yolk sac. As the body folds undercut the embryo, the splanchnopleure (splanchnic mesoderm + endoderm) narrows substantially to form the yolk stalk: a connection between the gut and the yolk. The yolk sac forms as the splanchnopleure surrounds the yolk. Blood vessels form in the yolk sac. Cells of the splanchnic mesoderm coalesce forming small aggregates called blood islands. They join one another to form the vitelline arteries and veins, which lead into the embryo. Blood cells form in the lumen of the blood islands. Nutrients absorbed from the yolk are brought to the embryo through these blood vessels. The germ cells that will later populate the gonads develop in front of the head of the chicken, migrate laterally, and when the blood vessels form from the yolk sac they will ender the blood until they find the gonads! Strange. As the embryo grows larger the yolk is digested and the yolk sac becomes smaller, eventually being incorporated into the floor of the gut. Amnion and Chorion: These two membranes are derived from the extraembryonic somatopleure (somatic mesoderm + ectoderm). Like the splanchnopleure, the somatopleure grows around the yolk. The embryonic coelom is continued to the extrabembryonic regions as the extraembryonic coelom, which separates the somato- and splanchno-pleure layers. The outer somatopleure eventually encompasses a space external to the yolk sac. This outermost layer of somatopleure is the chorion. The region of the chorion/somatopleure closest to the embryo undergoes a folding around the embryo so that a smaller chamber forms immediately surrounding the embryo. This is the amnion. The tissues of the amnion pinch off from the chorion and the chorion fuses together over the amnion. Allantois: The Allantois forms as an evagination of the hindgut. This is consistent with its role in waste removal. The two layers of tissue are the same as the yolk sac: the splanchnic mesoderm and endoderm. Illustrations:
figure 1
Mammalian development:Mammals have the same four membranes, however they form them in slightly different ways. Different mammals form them differently too.In humans and many other mammals: The trophoblast cells of the blastocyst surround the inner cell mass (ICM). These cells later become known as the chorion. Just like the chick, the outermost layer is the chorion, however, its origin is completely different. When the ICM forms it flattens and becomes like a disc. After delamination which produces the epiblast and hypoblast, gastrulation begins. Around this time the cells of the perimeter of the epiblast and hypoblast proliferate and crawl along the inside surface of the trophoblast. The epiblast cells crawl in one direction and the hypoblast crawl in the opposite. When the cells have met at the poles of the blastocyst it looks something like two balloons smashed together, inside of a larger balloon. The smaller balloon made of epiblast is the amnion; the larger balloon formed of hypoblast is the yolk sac. The flat spot where the "balloons" are in contact is the embryo. As gastrulation ensues, mesoderm crawls out to line the membranes so that the final arrangement looks a lot like the chick. How many times have we seen structures formed by different mechanisms in different animals, only to end up looking the same!?!?!? The pig is different than the human, and the mouse is different from either human or pig. The allantois forms in most/all mammals much like the chicken. The allantois and chorion forma close association often referred to as the chorioallantoic membrane. This is where the embryonic circulation communicates most directly with the maternal circulation. They never mix blood cells but many things can cross the placenta such as small proteins, nutrients, wastes, gas. Bad things too like alcohol and other drugs and chemicals. By observing which of their extraembryonic membranes are shared between identical twins, you don't have to be Sherlock Holmes (or even Dr. Watson) to deduce which of these mechanisms of twinning has occurred in a given case. Sometimes identical twins don't share any of their extraembryonic membranes, so that their placentas are as separate as if the were non-identical twins. In other cases, the twins share the same chorion, but have separate amnions. In other cases, both twins have developed inside the same amnion. Whether they ever share the same yolk sac, I don't know, and wish someone could tell me where to find out. In a few kinds of mammals, the third type of identical twinning is normal. Armadillos always give birth to identical quadruplets, formed by 4 primitive streaks (4 Hensen's nodes, etc.).
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