Embryology - Biology 104, Spring 2006 - Albert Harris and Corey Johnson
OUTLINE OF FIRST LECTURE: Jan 11, 2006, by Corey JohnsonBasics of EmbryologyWhat happens in development: fertilization, cleavage, differentiation, rearrangements
I. What is Embryology?How does a single cell become a multicelllular organism with complex tissues and organs... anatomy. It is the study of transition from a "symmetrical" entity containing all of the blueprints for life, to an anatomically complex entity capable of producing more of itself. What's more, this anatomy made of cells in different configurations and producing different proteins, is made by cells.The scope of the subject of embryology extends from fertilization/zygote formation, to birth (or hatching, or the end of metamorphosis) How does DNA translate into anatomy? DNA-->proteins-->specialized (differentiated) cells. What controls this process?
Protein synthesis Self-assembly of proteins 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)
II. What mediates morphogenesis (the change in form of an embryo)?Cellular Processes1. differentiation - cells express genes that make them different from their neighbors(selective transcription of certain subsets of genes) a. Special proteins that bind to DNA and control gene transcription = "Transcription factors" Example: red blood cells are the only differentiated cell type that transcribe the genes for hemoglobin
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
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
(like the sonic hedgehog protein, that you will learn about later. 2. changes in shape/size of tissues In addition to factors that control cell differentiation according to location in the embryo, what else determines the anatomical locations of differentiated cell types?
(for example, to cause active bending of epithelial cell sheets, for example in gastrulation). Example given in class: contraction along in the early embryo forces cause a change in tissue shape (neurulation), where a flat sheet of cells becomes a tube
b) Locomotion of cells from one part of the body to another (one part of an embryo to another) c) Exertion of traction forces , that pull other materials past cell surfaces. d) Osmotic pressure (and sometimes pressure of water pumped into a cavity)
e) Electro-Osmotic pressure (that makes cartilage swell, and elongates bones, etc.)
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.
3. cell division - provides greater numbers of cells to contribute to structures 4. cell death - 1) loss of unused structures, 2) change shape of structures, 3) regulation of cell number and gene expression
Structural changesThe processes and stages that occur during development include the following:Fertilization --> Zygote --> cleavage --> blastula --> gastrulation --> gastrula --> neurulation --> neurula / embryo --> organogenesis --> birth, hatching, etc 1. Cleavage - division of cells Cleavage refers to the first divisions of the zygote. These usually occur without growth of the zygote, so the volume stays essentially the same but is divided among 2, 4, 8 then 16 cells. The initial mass of cells, roughly equal in size, is called the morula. The next stage is the blastula, when the cells form a hollow ball. 2. Gastrulation - cells segregate into 3 germ layers - ectoderm, mesoderm, endoderm These processes will be discussed more extensively in future lectures, but you might want to look now at chapters 6 and 7 of your book for diagrams and micrographs. 3. Neurulation - formation of neural tube (spinal cord and brain) 4. Organogenesis - Rearrangements and specializations - forming different combinations of cell types and spatial arrangements to produce organs and adult structures Other things are happening too. The above are readily observable stages. Embryologists have invented words to describe what's happening within - what regulates the above changes.
1. Restriction/determination - the zygote is totipotent, then pluripotent, etc. as cells decrease the range of differentiated cell types that they are able to become.
Examples: the eyes are formed by outpockets from the developing brain. Ectoderm overlying these outpockets is induced to form an inpocket that eventually breaks off to become the lens of the eye 3. regulation and regeneration - repair to damaged or loss of structures/tissues Regulation has a very specific meanining in an embryological context, referring to the ability to form a complete embryo from only some of the cells arising from cleavage. If one cell is removed from a 16-cell embryo, the others form a normal embryo. If the cells of a 2-cell embryo are separated, each forms a complete embryo (although smaller). But if one cell of a 2-cell embryo is killed but left attached to the other, only half an embryo is formed.
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