Biology 441 Spring 2012

Embryology   Biology 441   Spring 2012   Albert Harris


Lecture notes for Wednesday, February 15, 2012



Somatic Ectoderm: What tissues, organs and structures develop from somatic ectoderm

The process of neurulation subdivides the ectoderm into
    * neural tube ectoderm
    * neural crest ectoderm
    * somatic ectoderm

Most of the somatic ectoderm cells differentiate into epidermal cells (the outer layer of skin), which are epithelial.

[NOTE that most of the skin, by weight, volume and strength is dermis, from mesoderm, the cells of which are mesenchymal, and which secrete a dense network of collagen fibers. (Collagen fibers are outside of cells, and provide most of the strength of the skin. Leather is the skin of vertebrates (mostly the dermis) whose collagen fibers have been artificially cross-linked to increase strength.]

In the head region, several areas of somatic ectoderm fold inward to form special organs.

The stomodeum folds inward to form the mouth and throat.

The innermost part fuses with the tip of the archenteron, which has in-folded from the rear. These epithelia should fuse and open a connection from throat to esophagus and lungs.

A birth defect sometimes occurs that is caused by failure of this fusion, so that immediate surgery has to be done to connect the throat to the lungs and esophagus.

Normally, there is no remnant of the location of the fusion.

The outer layer of teeth is called enamel, and secreted by special type called ameloblasts. These form as a row of infoldings of somatic ectoderm along the edges of the stomodeum.

The inner layer of teeth is secreted by a special kind of mesenchymal cell called odontoblasts. This inner layer is called dentine, and is very much like bone (a tight mixture of 1/3 collagen fibers and 2/3 calcium phosphate (with some fluoride ions, normally).

Odontoblasts are neural crest ectoderm; ameloblasts are somatic ectoderm.

The earliest birds had teeth (Archeopterex, for example), but evolution eliminated teeth in all modern birds, probably to reduce weight, thereby helping flight.
Which genes would you guess would be lost or inactivated, to eliminate tooth formation?
Many answers are possible: It need not be genes for either enamel or dentine! It might be genes needed for ameloblasts to induce formation of odontoblasts, Or it might be genes needed for odontoblasts to induce formation of ameloblasts. In other words, cells might be waiting to be told where to make the teeth, but never get the signal.

Experiments indicate this must be what happened: failure of the inductive signal. Kollar and Fisher dissected out mouth tissues from chicken embryos and mouse embryos, And co-cultured chicken neural crest cells with mouse stomodeal cells, and the reverse. With one of these combinations, teeth formed! These teeth were partly made by chicken cells!

Think about it: for millions of years, every time a bird embryo develops, cells in its mouth are waiting to be induced to differentiate into tooth-producing cells, but never get the message. Please think about which signal doesn’t get sent? What if some mouse neural crest cells were unintentionally included in the graft? How could you be sure some such mistake wasn’t made, if you were a researcher? Whether it might be possible to induce snake embryos to form legs? Humans to form gills? Or fins? Or scales? Or alligators to form feathers?

An epithelial outfolding from the roof of the mouth buds off and forms part of the pituitary gland. This outfolding is called Rathke’s pocket, and an embryologist with whom I shared an office in graduate school and I used to joke that it was the quintessence of embryological knowledge: "Rathke’s pocket" and all that! If he reads this, 'Hi, hope you are doing well.'
The rest of the pituitary develops from a downward epithelial fold from the floor of the brain (which, of course, in neural tube ectoderm).

Notice how often embryos make structures by epithelial folding! They do it A LOT. (Theoreticians seldom pay attention to this aspect of reality.)

In mammals, two masses of cells fold inward from the sides of the stomodeum, fuse in the middle to form the roof of your mouth, called the palate. Birds and reptiles don’t have palates, if you don’t count alligators, which have a similar structure that forms in a different way.

An all-too common birth defect called "cleft palate" results from failure of this fusion. It is analogous to two other birth defects, "cleft lip" and "spina bifida". When the neural folds don’t fuse completely, it looks as if the spine had been split open; but really it’s failure of fusion. The human face is formed by fusion of several blocks of somatic ectoderm, and the little groove under everybody’s nose is sort of a natural scar produced by one of these epithelial fusions. Sometimes these fusions fail to occur, which is the cause of "cleft lip", which used to be called "hare lip" because it looked somewhat like a rabbit’s upper lip & nose.

The nostrils are formed by two infoldings of the somatic ectoderm. ("Olfactory Placodes") Some of their cells differentiate into nerve cells that sense smell, and axons from these nerves "grow" axons back to the front of the brain. [Please realize that axon "growth" is cell locomotion]

These infoldings eventually fuse with the roof of the stomodeum, in the space above the palate. But the original purpose of the nostrils is smelling, not breathing; in young tadpoles, they do not connect to the mouth or throat. They are blind-ended invaginations.

The lenses of the eyes develop as infoldings of somatic ectoderm. "Lens placodes" These bud off completely from the surface of the head, forming closed epithelial sacks.

The otic placodes are also infoldings of somatic ectoderm, that separate completely from the surface. In cross sections of embryo heads, they look almost the same as the lenses.

Otic placode cells differentiate to form the semi-circular canals (for balance), the cochlea (for hearing, and distinguishing between sounds of different frequencies, and some other cells that secrete rock-like otoliths, the weight of which are a key part of our gravity detection system, by which we distinguish down vs. other directions

Fish and amphibians (but not reptiles birds or mammals) develop another set of placodes and hollow infoldings behind the ears. These are called lateral lines or lateral line organs. These detect water flow, pressure differences down the sides of animals that have them, and evidence has indicated they also detect electric fields. Many (all?) fish can detect electric fields; some use electric pulses as mating calls, some use them analogous to radar to detect other animals for food. Electric eels are one of seven groups of fish that have evolved the ability generate electric shocks. (The word "torpedo" originally meant a species of electric pulse-producing ray fish. Better they should produce electric rays!) For many years, lots of biologists considered that Darwinian evolution could not possibly have produced the ability to produce strong electric fields. The key point was that small voltages wouldn’t have produced any advantage, at all. So what selective pressure could lead toward the ability to produce 600 volts?
An answer (the answer?) is that fish use small electric pulses to detect food organisms, and as the equivalent of mating calls.

Eventually, some of the modern anti-Darwinists are going to realize that this paradox once existed, and make a fuss. It is one of many examples of serious scientific criticisms of Darwin’s theory, that biologists carefully considered before they eventually became convinced by the evidence that Darwin was mostly correct (except about the "pan-genes"). Creationists should read the history of research on evolution. All the issues they invented recently were thought of more than a century ago. Give us some credit, please.

"Neuromast cells", sometimes called "hair cells" (nothing to do with hair, like on your head). These are a special group of differentiated cells that use filopodia to detect microscopic sideways pressures or water flow. Neoromast cells are scattered along the inside of lateral line canals, and there are bunches of neuromast cells inside semi-circular canals, under the otoliths, and inside the cochlea. They are how we detect gravity, intertia and sound vibrations.

Have you ever wondered why the inner ear detects such different things? Why have our sound detecting cells in an adjacent part of the same organ that detects gravity and inertia?
Because we use neuromast cells to detect all of these. Originally, they were water flow detectors.

If short lengths of a lateral line got separated from the surface, and their ends connected to each other, what you would have would be equivalent to a semi-circular canal. Please visualize it.

My guess is that the inner ear evolved from part of the lateral line.

Scales, hair, feathers

Fish scales look like reptile scales, but are embryologically different (dermal, NOT ectodermal)

Hair is made of special somatic ectodermal cells, tightly packed together, and each so full of keratin protein that they die. Locations and structural organization of hairs are induced by mesenchymal (and mesodermal) dermal cells that become tightly packed together in little balls of a hundred or more cells. These are hair papillae. Equivalent balls of mesodermal cells contol locations and sizes of bird feathers and reptile scales (and also scales in those birds and mammals which have scales). I would expect porcupine skin to contain some gigantic papillae.

The base of each hair is wrapped around its papilla, somewhat as if each oak tree in a forest had its roots wrapped around a boulder. The actual hairs are made of cells that invaginated from the epidermis at the skin surface. Without the dermal papilla, the epidermis won’t invaginate to form hair papillae (or equivalent dermal condensations that control epidermal formation of feathers and scales.

Much research on hair papilla genetics (in sheep) has been done in Australia. If skin were to form twice as many papillae, fur would be twice as dense.

Geometric locations of bird feathers and reptile scales have the degree of spatial regularity one associates with crystals and marching Prussians. They are EXACTLY in the same pattern each time an embryo of a given species develops. For some reason, textbooks call their pattern hexagonal, but really it is rhomboidal, positioned like the corners of lots of squares packed tightly together. Four small muscles cross-connect the bases of each feather papilla (equals "feather germ")

The next time you are eating barbecued chicken, scrape the side of a fork lightly across the inside surface of some skin where there had been feathers. An array of exact squares will be revealed, About a millimeter or less across.

Feather papillae develop by pulling-together of dermal fibroblasts. For many years, scientists believed they were created by having more mitoses at certain locations; but it is entirely caused by bunching together. Mitosis per cell is actually slightly lower at these sites of papilla formation.

Feather papillae do not form simultaneously, but in a regular spatial and time sequence. It looks as if somites were forming in two dimensions. Originally, the clock and wave-front hypothesis was invented as an explanation for the special and time control of formation of feather papillae, and then later applied to somite formation. Possibly, it might be the true explanation for both, or for neither.

It is not entirely logical for embryologists always to hypothesize that spatial patterns first form as gradients or other concentration variations of some diffusible chemical "morphogen". 99% of embryologists always hypothesize this. I think its because they think of genes as being chemical, so causal chains from genes to anatomy are assumed to start with chemicals.

Several research-plus-mathematics papers have been published showing (arguing?) that regular spatial patterns can be generated directly by physical forces becoming unbalanced. We might have convinced more critics if we had concentrated more on bird skin and somites, with less speculation about humerus, radius+ulna, digits. Much more research should be done on what happens when you change the amount of tension in skin during the time when papillae are condensing. The patterns become A LOT different. Slack tension results in random patterns.


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