By Levi Clancy for Student Reader on
Clarifying Note: Splanchnic mesoderm & somatic mesoderm are layers of lateral plate mesoderm adjacent to the endoderm & ectoderm, respectively.
Note that all three germ layers are derived from the epiblast. Furthermore, epithelium is derived from all three germ layers: endoderm (epithelial lining inside viscera); mesoderm (mesothelial lining outside of viscera); and ectoderm (skin epithelium).
The ectoderm gives rise to the skin and its differentiated structures: hair, nails, feathers, scales, mammary glands and teeth. Ectodermal placodes give rise to the eye, ear and nose. Much of this organogenesis requires interaction between the ectoderm and the underlying mesoderm, referred to as epithelio-mesenchymal interactions.
The surface of the ectoderm and its neural plate give rise to different tissues.
|Surface||Epidermis and its appendages.|
|Neuroectoderm||Neurons, neuroglia, neurohypophysis, pineal gland (NNNN).|
|Neural Crest||Glial cells (of peripheral ganglia), arachnoid and pia mater, melanocytes, enteric ganglia, Schwann cells (GAMES).|
There are dorsal/ventral differences in what the mesoderm gives rise to. This is induced by differences in dorsal and ventral endoderm. Organizer cells self-diffrentiate into dorsal mesoderm (notochord), organizer cells dorsalize adjacent mesoderm to for paraxial mesoderm (somites) and organizer cells secrete dorsalizing signals (noggin, chordin, nodal-related, etc) to induce the neural plate.
The cardiovascular system, reproductive/excretory organs, connective tissues, vessels and skeleton all derive from the mesoderm. In mnemonic form: mesothelium (peritoneal, pleural, pericardial), muscle (striated, smooth, cardiac), spleen, soft tissues, serous linings, sarcoma, somite, osseous tissue, outer layer of suprarenal gland (cortex), ovaries, dura mater, dducts of genitalia, endothelium, renal (kidney), microglia, mesenchyme, male gonad (MMSSSSSOOODDERMMM).
In amphibians, signals from the vegetal portion of the egg establish mesoderm; various growth factors play a role in signaling to the marginal zone cells to cause them to become mesoderm precursors. For example, we discussed the role of Xnr (Xenopus nodal-related) in mesoderm formation.
Expression of nodal-related is also required for proper function of the node in birds and mammals, specifically for induction of axial mesoderm. We have seen that, in vertebrates, the most dorsal mesoderm forms the notochord, and that BMP inhibitors such as chordin and noggin are required for notochord formation.
After gastrulation, the endoderm is the innermost germlayer. Gives rise to epithelial lining of respiratory tract and GI tract.
|Lower vertebrates||The endoderm is a hollow tube called the archenteron (the primitive gut). The archenteron is lined with a cell monolayer that will later give rise to the gut, lungs, liver, pancreas and other structures.|
|Birds and mammals||The endoderm initially forms the open endoderm (aka primitive endoderm), a disk that floats atop the yolk sac. The disk invaginates along the anterior-posterior axis to form a closed epithelial cylinder (forming the foregut, midgut and hindgut) surrounded by splanchnic mesoderm (forming smooth muscle). Read more here|