Embryology Biology 441 Spring 2009 Albert Harris
|
Lecture notes for Friday, March 6, and Monday, March 16, 2009The parts of the body that are made of ectodermal cells:Cells that don’t move to the interior during gastrulation are (therefore?!) considered to be ectoderm. It is a good question why decisions about future differentiation of cells are so often made by mechanical rearrangements ("Morphogenetic Movements"). It’s a rather odd way to make such decisions, and it's also odd that it gets ignored by most people inventing theories about the signals that control which embryonic cells will differentiate into which cell types.
The ectodermal cells themselves are then sub-divided into three categories
(#1) Somatic ectoderm
and fingernails, hair, feathers, scales (of reptiles & birds) (teleost scales are mesodermal, however) Lens of eye, Inner ear [cochlea etc.] (formed from Otic placode) Olfactory placode -> Sensory nerves of nose Other Placodes which form nerves in head area. Other Placodes that form the Lateral Line of fish & amphibia
Neuromast cells have special filopodia that detect even very small bending forces, which alter the frequency of nerve impulses sent to the brain. These filopodia are supported by fibers of the muscle protein actin (and are much smaller than hairs on your head, to which they are only similar in shape. &&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&&& While we are on the subject of neuromast cells and the inner ear:
Inertia is detected by water flow in the 3 semi-circular canals bending filopodia of sets of neuromast cells located in parts of these canals.
In mammals, the sound-detecting part of the inner ear is lengthened and coiled to form a structure named the cochlea. Only mammals have a cochlea, but all vertebrates have part of their inner ears specialized to detect sound vibrations. Frog inner ears tend to be tuned to resonate only to certain frequencies, I have read.
******************************************************************* Back to subdivisions of the ectoderm:
(#2) Neural tube ectoderm
Spinal cord segmental motor nerves (nerves that stimulate muscles to contract) (but NOT post-ganglionic autonomic nerves, which are neural crest) This origin of this name is that the spine seems to be split in two. But it isn't split; what happens is that they two sides fail to fuse. Spina bifida often results in partial paralysis & can result in non-formation of the brain. 50% of cases of spina bifida are caused by deficiency in the vitamin named folic acid. (& would not occur if women made sure to get enough of this vitamin early in pregnancy) An early diagnosis of spina bifida is based on finding certain molecules in the amniotic fluid that are normally found only inside the neural tube (in the neurocoel). ___________________________________________________________________
Also formed by out-foldings from the sides of the brain are the optic cups, the outermost edges of which bend back inward to form the neural retina. The part of the optic cup just inward from the neural retina differentiates into a thin epithelium full of melanin pigment granules, called the pigmented retina.
Small areas of somatic ectoderm are induced by the optic cups to fold inward and differentiate into the lens of the eye. Lens cells elongate very much.
Eye cup with lens __________________________________________________________________ In the mechanical sense, expansion of vertebrate brains is equivalent to inflation of several closely spaces balloons. Cells pump water into the neurocoel, creating enough water pressure to expand the anterior part of the neural tube. The fore-brain, mid-brain andhind-brain are areas which stretch more than the part of the brain between and behind them. Eventually, there is much more cell growth in these areas; But at first, they are thin-walled, water-inflated & balloon-like Experiments have shown that the brain won't expand if the fluid pressure is released. The eye-ball also depends on hydraulic expansion for its enlargement. (and won't enlarge if punctured)
_________________________________________________________________
(#3) Neural crest ectoderm (sometimes called "ectomesenchyme")
postganglionic autonomic nerves (including the medulla part of the adrenal gland) Pigment cells of skin ("melanocytes" in Mammals) (plus amphibia, fish, birds & reptiles also have other pigments than melanin and the cells that made these other pigments also develop from neural crest)
Odontoblasts which are special mesenchymal cells that secrete the inner, dentin layer of the teeth.
Thin section through tooth development on a mouse embryo.
Lining of mouth (and also nasal cavity) (posterior to oro-pharyngeal membrane) The palate is the roof of your mouth (but not in birds, reptiles, amphibia) The palate is formed by fusion of right and left palatal shelves. Enamel (outside layer of teeth: secreted by ameloblasts) Rathke's pocket -> Anterior Pituitary Gland (That secretes FSH, LH, etc.) At least one of the three pairs of salivary glands
Can you find the lens, neural retina, pigmented retina, otic vesicle, and Rathke's pocket, and other embryonic structures in this photograph.. _____________________________________________________________________
A side comment:
Two secret weapons of the vertebrate subphylum are myelination and
the random DNA splicing clonal selection immune system.
(In case you ever wonder why squids aren't running the planet!) [or are they?] ______________________________________________________________________ Review questions, that you should be able to answer on future exams: a) Are all sensory nerves derived from the neural crest? What about nerves from the nose, eye and inner ear? b) Are all motor nerves derived from the neural tube? What is a big exception? c) Are all autonomic nerves derived from neural crest? (what about pre-ganglionic nerves) d) Do all vertebrate have lateral line systems? What are some that do? & don't? e) How are placodes related to the somatic ectoderm? f) Is skin ectodermal or mesodermal? (what about the thickest, heaviest, strongest parts) g) Why do the early stages of development of the inner ear look so much like the lens? h) What is an example of a placode that retains an open connection to the outside surface. i) What is the similarity (at the causal level) between these three birth defects "Spina Bifida", "Cleft Palate", "Cleft Lip" (= so called "Hare Lip") j) There are human genetic syndromes that combine some or all of the following symptoms: defective tooth dentine; defective autonomic nerves; irregular skin pigmentation, facial skeletal abnormalities, defective adrenal gland. Deduce a possible single underlying cause, based on embryological cell origins. k) What are the three main subdivisions of the ectoderm? (and name at least two organs that develop from each) l) If you were in a debate club, and assigned to argue in favor of the idea that there are "really" 4 or 5 subdivisions of the ectoderm, then what would these 5 be? And what arguments could you make in favor of them being as different as the 3? m) Suggest arguments (pro or con) that the semi-circular canals must have evolved originally as parts of the lateral line system that became separated from the rest of this system, and entirely internalized. n) Suggest further arguments that the sound frequency-detecting and gravity-detecting parts of the inner ear probably evolved from the semi-circular canals. o) Think of as many examples as you can in which sharp folds in epithelial cell sheets become boundaries between cells that differentiate into different cell types. p) Imagine that you are an embryonic ectodermal cell who aspires to differentiate into some kind of a nerve cell: If you were located in the wrong place to have become part of the neural tube, then do you have any further chances of differentiating into a nerve? (List as many further opportunities as you can) q) * Do all neural tube cells become nerve cells, anyway? r) * Are myelin sheaths formed by Schwann cells crawling around and around nerve axons, or is there another possibility? What about in the formation of myelin sheaths formed by oligodendrocytes? s)** One trillion dollars is about how many dollars per United States citizen? t)** About how many lives could be saved by the money it costs to save a bank? Compare total funds ever spent for cancer research with costs of subsidizing one particular insurance company. u) More questions to be added later: Suggested questions will be appreciated and used.
|