Subdivision of germ layers into different cell typesONE) Our bodies are made of two hundred and fifty (approximately) differentiated cell types, arranged in a certain geometric pattern. In addition, many differentiated cell types (all epithelial cell types, for example) have some kind of polarity. Epithelia have an apical surface and a baso-lateral surface, the proteins of which are different. Apical surfaces face either toward a fluid-filled cavity, or are the outer-most surface of the body, facing outward. Basal surfaces of epithelia face toward mesenchymal cells and networks of (extracellular) collagen fibers.
TWO) Undifferentiated cells become differentiated into these 250 cell types by a consistent branching pathway, which is the same for all vertebrates (and nearly the same for all multicellular animals)
For example, the first branch separates (and creates) ectoderm, mesoderm and endoderm.
Soon after that, the endoderm subdivides into (and creates) * neural tube ectoderm, * neural crest ectoderm, and *epidermal ectoderm. These undergo further branches.
The mesoderm subdivides into *notochord, *somites, *intermediate mesoderm and *lateral plate mesoderm
Each somite subdivides into *dermatome, *myotome and *sclerotome.
All the skeletal muscle cells of the body develop from myotome cells. The geometries of biceps, triceps, gastrocnemius, extensor digitorum etc. are caused by active movement from the two narrow stripes along the back where the myotomes were formed.
Sclerotome cells become bone cells and muscle cells (and so do some lateral plate mesodermal cells.
All kidneys develop from intermediate mesoderm cells.
All heart muscle cells differentiate from lateral plate mesoderm. So do many smooth muscle cells.
Do you see the general pattern? A consistent sequence of branching subdivision.
THREE) The molecular genetic cause for this branching pattern is not known. Maybe all genes expressed specifically in ectodermal cells have some common base sequence in their promoter regions, or something like that. Discovery of this cause will be a major breakthrough.
Notice that our textbook ignores the issue, and prefers diffusion gradients signaling to cells where they are currently located (like a GPS device in each cell, telling one cell to differentiate into a liver cell, telling another cell to differentiate into a heart muscle cell, etc.).
FOUR) Amounts of yolk differ greatly. This distorts the geometry of cleavage and gastrulation.
Mammal eggs have small amounts of yolk evenly distributed.
Mammal, frog and salamander embryos cleave all the way through the embryo: "Holoblastic cleavage"
Bird, reptile and teleost embryos cleave only a tiny fraction of the way through: "Meroblastic cleavage"
(Also platypus and echidna: egg laying mammals. Marsupials are strange in a different way.) As a result, gastrulation occurs in a flat sheet of cells in embryos of birds, reptiles, teleost fish and mammals!
! Mammals? Why don't mammals gastrulate more like frogs and salamanders?
Because we evolved from reptiles, and continued to gastrulate in the reptile pattern (as a flat sheet).
FIVE) Boundaries between subdivisions of embryonic tissues are very often along epithelial folds.
For example, the boundary between somatic ectoderm and neural tube ectoderm.
Six) Many cell types differentiate at one location, and then move actively to completely different and independent locations in the body (skeletal muscle, pigment cells of the skin, Schwann cells, bone marrow cells, endothelial cells). Maybe as many as half the differentiated cells move somewhere else.
Testes and metanephric kidneys move as organs, distances of inches or more. Teeth also migrate.
Sometimes grafted tissues will physically rotate or move short distances, back toward their normal original orientations or locations. For example, if one salamander limb bud is grafted to the side of another salamander limb bud, it will move (or BE moved) either distally or proximally toward whichever part of the other limb bud is equally distal to its own origin. This may be difficult to visualize.
Imagine a salamander arm cut off at the elbow and grafted to the side of the wrist of another individual: that graft would slowly migrate up the arm toward the elbow.
Imagine an arm cut off at the elbow and grafted to the side of the shoulder of another individual: that graft would slowly migrate down the arm toward the elbow.
Imagine an arm cut off at the wrist and grafted to the side of the elbow of another individual: that graft would slowly migrate down the arm toward the wrist.
Like seeks like; and stops migrating when like is touching like. Nardi discovered this at U. Illinois, Urbana. The mechanism is not known. Most embryologists who know about it guess that some kind of adhesive difference is the cause. My guess is that differences in contractile strength are more likely (but I always do guess contraction whenever the majority guess adhesion.). Please invent other possible explanations, and invent experiments by which somebody could prove what idea is correct.
seven) Sponges, Hydra and some other anatomically simple organisms are made of cells that move constantly. Locations of their differentiated cells have NOTHING AT ALL TO DO WITH where they originally differentiated.
Budding in Hydra is geometric rearrangement of cells: from being part of a vertical cylinder to becoming a horizontal cylinder. Budding can occur without growth; and growth can occur without budding,
Budding is as if a blacksmith or a glass-blower started with a cylinder and distorted it into a T-bend.
Richard Campbell discovered this and proved it experimentally. The reason it's not featured in all Developmental Biology textbooks is because people wish it were not true.
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