Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson
NINETEENTH LECTURE- March 3, 2006, by Corey Johnson: A few words on cardiovascular development and... Endoderm Development:The special kind of epithelial cells that line all blood vessels are called endothelial cells. The narrowest blood vessels are called capillaries, and their walls are made of endothelial cells only (one cell layer thick; and with few or no smooth muscle cells of fibroblasts to provide mechanical reinforcement or control constriction. Arteries are made of layers of smooth muscle cells (and some fibroblasts) and type I collagen fibers wrapped around their capillary cells. Veins have fibroblasts and collagen wrapping the endothelial tube that lines where the blood cells are. Not only are artery walls much thicker than those of veins, another difference is that artery walls are strongly contractile in all but the largest arteries (like the aorta). Notice that the blood pressure in capillaries is often almost as large as the blood pressure in arteries! How are the thin walls of arteries able to withstand such strong pressure? And if capillaries (made only of endothelial cells) are able to withstand so much pressure, then why do the walls of arteries need thick layers of smooth muscle cells and collagen fibers? The answer is that Pressure=Curvature*Tension. The high curvature of the capillary walls enables them to hold back very strong pressures with small amounts of tension This is one of many examples where you need to know about P=C*T in order to make sense of important embryological and anatomical phenomena. Few textbooks even explain the definition of curvature, in a mechanical and mathematical sense. This same physical principle is why hydraulic equipment like back-hoes can be powered by high pressure fluids driven through narrow rubber pipes. Heart development is the beginning of the stage we call organogenesis. The vertebrate body plan has been achieved. Despite varying ways of developing around varying amounts of yolk, all vertebrates come to a stage where they have an endodermal tube surrounded by mesoderm with its several divisions, surrounded by ectoderm. A neural tube is found dorsally, and a coelom is found ventro-laterally. Not all vertebrates look the same at this stage in development, as they exhibit heterochrony: premature or delayed development of certain parts of their bodies, relative to other vertebrates. Organogenesis begins with the heart and circulatory system. Most of which, like the heart, is derived from splanchnic mesoderm. Blood vessels begin to transport blood cells that were produced in the yolk sac of amniotes.
EndodermThe endoderm is that tissue that surrounds the primitive gut. After gastrulation, this is the innermost layer. It is surrounded by splanchnic mesoderm, the coelom, somatic mesoderm, and ectoderm. In amniotes, the ventral midgut is open to the yolk sac and the ventral hindgut is open to the allantois. The anterior and posterior ends are blind-ended tubes. That is, they are not open to the exterior as they are in the adult.The morphogenesis of the endoderm is characterized by evagination. This reorganization involves an out pocketing or herniation of the wall of the endodermal tube to produce what is called a diverticulum. The Architecture of the GI tract (alimentary canal): In cross section the gut can be seen as a multilayered tube. Endoderm forms the epithelial part of organs arising from the primitive gut. Epithelia are invested with other tissues such as connective tissues, blood vessels, and muscles that are derived from the splanchnic mesoderm. The outermost layer lines the abdominal cavity (coelom). From anterior to posterior, the endodermal tube produces several organs: esophagus, stomach, small intestine, and large intestine. The liver, pancreas, and gall bladder are not part of the tube but are associated with it through ducts. The ducts hint at the embryological origin of these organs. They form as evaginations from the gut. Regionalization of the gut along the A-P axis. The endoderm initially signals to the LPM, recruiting cells to surround the endoderm: the splanchnic mesoderm. Once the mesoderm surrounds the endoderm, it instructs the endoderm how to differentiate. Recombination experiments suggest the mesoderm patterns the gut. For example, midgut endoderm will form foregut structures if placed in contact with foregut mesoderm. Hox gene expression is found in both layers of the LPM and is largely independent of that in the somites. Hox expression perhaps regulates mesenchymal-to-endodermal specification. How the endoderm is pre-patterned into fore- mid-, and hindgut is not entirely clear. In summary, the endoderm's three regions are further "segmented" by hox-patterned mesodermal signals. The Liver and Pancreas: The liver forms as an evagination of the primitive gut. The endoderm at the end of the diverticulum/bud undergoes proliferation to form the tissue of the liver. The gall bladder forms as a bud from the liver bud. The pancreas forms around this same region of the endodermal tube. It has two diverticula, one ventral and one dorsal. They likewise undergo proliferation to produce the endocrine and exocrine portions of the pancreas. As development progresses, the two pancreases fuse together.
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