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What parts of the body develop from ECTODERM: 3 subdivisions: (see figure #12.1 on page 392 of the textbook) 1) Somatic ectoderm mostly becomes epidermis;
But some forms placodes-> olfactory nerve, inner ear
Another part of the somatic ectoderm is the stomodeum
2)Neural crest ("ectomesenchyme")
Autonomic nerves (postganglionic autonomic nerves) Adrenal medulla (which is made of postganglionic autonomic nerves) Schwann cells (that form myelin sheaths around nerve fibers) Melanocytes (and other pigment cells of the skin) Odontoblasts (that make dentine of the teeth) Facial cartilage cells (and maybe other mesoderm-like cells of face?)
3) Neural tube (often very close to 50% of total ectoderm)
Spinal cord Motor nerves (the ones that stimulate contraction of voluntary muscles)
Neural retina, and also the pigmented retina, of the eye.
This is a long-standing riddle of embryology; What do these different kinds of cells have in common? Why or how did the neural crest evolve. One theory was based on chemical similarities between the hormone adrenaline and the pigment melanin. Adrenaline is synthesized from the amino acid tyrosine and melanin is a polymer of many tyrosines strung together. But nobody can think of a connection between these and facial bone cells and teeth. Actually, even more tissues develop from the neural crest than are listed here. Researchers used the Japanese Quail tissue grafting method to discover that several additional kinds of cells also develop from neural crest cells - such as cells that form the septum between the right and left ventricles and right and left atria of the heart. (to be discussed on page 441 of the textbook) Notice also that people even use an antibody that binds specifically to quail cells, which makes it easier to find them. (but otherwise can't really be quite as specific a marker as nuclei) Birth defects that result from failure of cell sheets to fuse together
Spina bifida (results from the neural folds not fusing completely)
Cleft palate (results from the palatal shelves not fusing) Cleft lip (=hare lip) (results from plates of facial tissue not fusing) Septal defects (of many different kinds) in the heart, result from failures of fusion of tissues that split apart the right and left ventricles and also the right and left atrial chambers of the heart. Mammal hearts eventually have 4 chambers, but they begin as one large chamber, that gets split into right and left halves which get constricted to separate atria from ventricles. If a woman has too little folic acid (vitamin B12), and she gets pregnant, that increases the chance of her baby having spina bifida; As many as 10% of people have genes that make them deficient in folic acid. The number of babies born with spina bifida could be decreased by as much as 50%, if women regularly took folic acid pills. Does it mean anything important that some vertebrates neurulate by hollowing-out a solid rod of cells, instead of folding + fusing? For example, teleosts neurulate by "cavitation", and the rear-most 20% of bird neural tubes form by cavitation! Our textbook's approach to this paradox by means of vocabulary:
"primary neurulation" is the kind caused by epithelial folding. But the author isn't very clear whether he thinks these are two different mechanisms, or two results of the same mechanism! Notice that is yet another example of embryonic tissues being able to arrive at the same end result by means of different paths. What I called "path independence" in a previous lecture, and which the textbook author and many others sometimes interpret as evidence for "thermodynamic" "minimum free energy" mechanisms (that maybe maximize cell-cell adhesion, or something) I have done some research dissociating amphibian neural plates into random piles of single cells, and watching them rearrange into hollow balls, with the cells' apical surfaces toward the middle. The results support the idea that the key even in "cavitation" is the reorientation of all the cells so that their apical ends are all together, inside the cell mass, and all stick to each other so as to seal off an inner space, and then cells pump fluid into this space. Some comments on connections between cell differentiation and mechanical forces
(a quotation) "Pay no attention to the man behind the curtain "
What is the physical mechanism of neurulation? What evidence there is suggests that the mechanism of epithelial folding is sideways contraction of the neural plate cells, with this contraction becoming concentrated toward the concave surface (which is the apical surface of the epithelium) Just like a piece of wood would warp if one side contracted. One of the strange things about embryology is that mechanical bending, and other active movements of cells, are part of how cells decide which cells will differentiate to form what organs.
Take a moment to notice how odd this is, as a decision method! All the cell fate decisions of early embryos use some kind of active physical rearrangements of cells to separate them in groups. Gastrulation subdivides the whole embryo into 3 germ layers
Neurulation subdivides the ectoderm into 3 parts
Rearrangements of mesoderm cells subdivide them into 4 groups
In each somite, active rearrangements separate cells into 3 parts The brain subdivides into 5 parts by bending its walls The rhombomeres in the hind-brain separate by folds. The sensory parts of the cerebrum separate by a large fold (sulcus) from the motor parts. If you look in older editions of textbooks, formation of the rhombomeres was explained as being caused by wrinkling due to mechanical pressure. But then when it was discovered that the cells of each rhombomere differentiated into special kinds of nerve cells everyone decided that the folds must be secondary effects of some kind of chemical signals. (as if nothing important could be caused by mechanical pressure) Almost everyone assumes that mechanical forces are unimportantsecondary side-effects of the real controls of cell fates. And that you don't need to be able to map pressures and tensions in order to understand the mechanisms that control cell fates.
Human builders don't use cracks to decide where to put windows. Imagine if embryos used mechanical forces as a method of quick long-range coordination of cell fates! What would that be like? Whenever cells needed to be subdivided into groups, this would be done by means of some kind of geometric rearrangements.
But almost everyone would assume that these rearrangements were just weird side effects of the real control mechanisms, which have to be some kind of chemical gradients. Somatic ectoderm develops into the following
Placodes (in the head, forming lens, inner ear, olfactory nerve)
notice the importance of neuromast cells in detecting
"Rathke's Pocket" --> Anterior pituitary gland Probably at least one pair of salivary glands
The oropharyngeal membrane is the boundary between endoderm and stomodeum (ectoderm)
Epidermis SKIN Somatic ectoderm The outer, epithelial layer of the skin is derived from somatic ectoderm; so are reptile scales, bird feathers, claws and hair in mammals
Scales in teleost fish are mesodermal, and are essentially bones The dermis is the leathery, mesodermal inner layer of the skin, made of mesenchymal cells, and tightly-woven collagen protein Tight aggregations of dermal mesenchymal cells (dermal papillae) control the locations where epidermal cells will differentiate to form hair, feathers, scales, or claws.
The rapidly growing bottom part of each hair, feather etc. are wrapped around their dermal papilla.
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