Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson
OUTLINE OF FIFTEENTH LECTURE: Feb 20, 2006, by Corey JohnsonNeurulation and axial structures II, continued, and then Mesoderm
- Adrenal medulla, part or all of: parathyroid, thyroid, thymus - Melanocytes of the skin - Many connective tissues of the head including bone - Outflow tract of heart (pulmonary trunk/aorta separation, aortic endothelium) Vertebrae formation Each pair of sclerotomes divides into anterior and posterior divisions. Each half will form a vertebra in between somites. Somite pair 8 will split into anterior and posterior halves. The anterior halves will join with the posterior halves of somite pair 7; the posterior halves will join with the anterior halves of somite pair 9. If you missed lecture, draw it... it'll make sense. So, each vertebra will form from contributions of 4 somites (2 pairs).
Dermatome: forms dermis in horizontal "bands" across the body. For each segment (anterior-posterior level) of the body, the somite forms bone, dermis, and muscle. There are spinal nerves that form from the NT and NC that are associated with a somite and its derivatives so that in the adult, there is a spatial coordination between the nervous system and its sensory and motor innervations. Myotomes form two groups of muscles. Epaxial muscles that extend the spine (dorsally) and hypaxial muscles that flex the spine (ventrally) as well as form the intercostals muscles, between ribs. The limb musculature also migrates out from the myotomes.
The Sclerotomes: Vertebrae formation
Animation of vertebrae formation Somites differentiate forming these distinct regions in response to inductions from their neighbors: notochord/floor plate, neural tube, ectoderm, lateral plate mesoderm
Scroll down on this page for some pictures of somites The structures near the neural tube produced by the neural crest include the dorsal root ganglia and the ganglia of the sympathetic chain. Migratory paths run dorsal-lateral to the somites and also through the somites. The presence of these cells participates in the development of the vertebrae. If the neural crest is removed, part of the vertebrae fuse together. Interestingly, somites do not form in the head, but instead somites coalesce and disperse before they really finish forming what looks like a somite in the rest of the body. These immature somites are called somitomeres. One idea (well, the only idea proposed) is that somitomeres are immature somites that never fully form. Some have taken to calling immature somites in the rest of the body, somitomeres. Somitomeres has come to mean immature somite, but also refers to the head mesoderm. So the common usage of the word implies that they are the same, but we don't know for sure. Head mesoderm gives rise to some of the muscles of the head. The patterning of the somites (which we'll discuss on Friday) involves the activation of genes, but no one really knows how they come together to form blocks. Sure, genes are turned on in a segmental fashion. People seem to think genes cause things to happen. Genes don't cause morphogenesis... they produce proteins that regulate the expression of other proteins that interact to change cell behaviors that cause tissues to interact in different ways or produce forces that are ultimately responsible for morphogenesis. So genes turn tell a group of cells to become somites, but how they execute the plan remains a mystery. Nobody knows what the plan is, exactly. Intermediate mesoderm: Briefly, it forms ventrolaterally to the somites. It forms 4 basic structures from anterior to posterior:
2) Mesonephros - functional kidney of most fishes and amphibians and embryonic amniotes 3) Metanephros - functional kidney of amniotes starting late in embryonic life 4) Gonads and some associated ducts Lateral plate mesoderm The LPM forms laterally to the somites. It divides distally to form two layers:
- The combined somatic mes. and ectoderm forms a layer called the somatopleure 2) Splanchnic mesoderm: heart, lymphatic system, visceral serosa (this is the membrane that lines organs like the lungs and GI tract. -The combined splanchnic mes. and endoderm forms a layer called the splanchnopleure 3) The space in between is called the coelom: abdominal, pleural, and pericardial cavities.
(...just to tie up some loose ends): The currently accepted theory about the formation of the notochord is that each cell extends pseudopods sideways, and then actively contracts these extensions, thereby squeezing themselves sideways and pushing forward and backward. The notochords cell get flattened, like a stack of coins.
Collagen fibers get wrapped around the cells in many pairs of helical spirals (each with fibers oriented 90 degrees to the others) When I took gross anatomy, I was taught that the notochord remains as part of the intervertebral discs. I don't buy it; the notochord probably degenerates, and the intervertebral discs develop where it was. Nobody has actually studied this to my knowledge.
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