Embryology Biology 441 Spring 2007 Albert Harris


Basics of Embryology

                               Embryology (~= "Developmental Biology" )
Genetics -> -> -> -> -> -> -> -> -> -> -> -> -> -> -> -> -> -> -> -> Anatomy

DNA base pair sequences
ACTGGCCTTAA, etc. Somehow cause formation of bones, muscles, brains, livers etc.
TGACCGGAATT How do genes cause anatomical structures to form?

What are some of the most important processes that cause embryonic development?

Protein synthesis             (of course!)
Self-assembly of proteins         (of course!)
Selective cell-cell adhesion molecules (by which liver cells stick to other liver cells, better than to heart cells, and heart cells stick better to other heart cells)

Cell Differentiation (selective transcription of certain subsets of genes)
(Red blood cells are the only differentiated cell type that transcribes the genes for hemoglobin)

    Anatomy is a certain geometric arrangement of differentiated cells.
    The human body is made of about 250 differentiated cell types
    (the following are random specific examples of what is meant by a differentiated cell type

      red blood cells are one example of a differentiated cell type
      cardiac muscle cells are another example of a differentiated cell type
      B-lymphocytes are another example....etc.
      the blue-sensitive cone cells in the retina of your eye count as one cell type
      the green-sensitive cone cells in your retina counts as another cell type
      so do the red-sensitive cone cells in your retina
      so do the "ganglion cells ", whose axons connect the retina to the brain
      "pigmented retina epithelial cells " are another cell type,
      not to be confused with the "mesenchymal pigment cells" in your skin
      because they are an entirely different cell type

    Sponges have about a dozen differentiated cell types; Hydra may have 15 or 20

What causes and controls cell differentiation?

1) Special proteins that bind to DNA and control gene transcription = "Transcription factors"

2) Extracellular proteins that signal cells what differentiated cell type to differentiate into.
(like the sonic hedgehog protein, that you will learn about later.

In addition to factors that control cell differentiation according to location in the embryo, what else determines the anatomical locations of differentiated cell types?

a) Acto-myosin contractions along one surface of cells or sheets of cells,
(for example, to cause active bending of epithelial cell sheets, for example in gastrulation).

b) Locomotion of cells from one part of the body to another (one part of an embryo to another)
(analogous to amoeboid locomotion, but more like the crawling of Dictyostelium amoebae)

c) Exertion of traction forces , that pull other materials past cell surfaces.

What about growth? Does the enlargement of cells exert forces that create anatomical structures?
People have always expected such things to occur!
But actually growth turns out not to be a significant driving force in embryonic development!
In every case that I know of, specific examples of "growth pressure" have turned out to be caused by one of the other 5 kinds of forces (a-e) listed above. Not even the eruption of teeth is really caused by growth pressure.

This is perhaps somewhat strange, because so much growth occurs in many embryos. You would expect that growth would produce some pressure, and that this pressure would be put to use building structures, or changing arrangements of cells. But every case that has been carefully studied turned out to be driven by some different force.


back to syllabus

back to index page