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Web Topic 6.2: Principles of Embryonic Development

[Referenced on textbook p. 157]

It is beyond the scope of this book to give a detailed account of embryonic development, but we should briefly mention some important themes here.

Large numbers of developmental genes orchestrate body layout and organ and tissue development. The prime movers are the Hox genes, four sets of genes that are expressed very early in development and which control the basic body plan. Individual Hox genes are expressed in restricted, though overlapping, territories within the early embryo. The products of Hox genes are transcription factors. Within each territory, these transcription factors switch on further sets of genes that control the development of a particular body part, such as a head or a leg.

Organs often form by means of morphogenetic movements, in which sheets or masses of cells invaginate, evaginate, or otherwise deform to produce a new structure. An example is the formation of the central nervous system from the ectoderm (see Figure 1): the ectoderm invaginates along the midline of the embryo, forming a groove; the groove pinches off, forming a tube; and the tube then gives rise to the spinal cord and brain. Similarly, the lungs develop by evagination from the developing gut, and the retinas by evagination from the developing brain. Morphogenetic movements are instigated by chemical cues; for example, the invagination of the ectoderm to form the neural tube is triggered by a chemical released from a portion of the underlying mesoderm known as the notochord.

Figure 1  Invagination of the midline ectoderm forms the neural tube—the precursor of the spinal cord and brain. When this process fails to take place normally—as may occur when the mother is severely deficient in folic acid during early pregnancy—severe neurological conditions such as spina bifida or anencephaly (absence of most of the brain) may result.

Cell migration—the passage of individual cells through preexisting tissue—also contributes to organ development. For example, some ectodermal cells (“neural crest cells”) migrate a long distance through the body, giving rise to a variety of tissues, including some glandular, neural, and connective tissue structures. Similarly, the entire cerebral cortex is formed from cells that migrate from the lining of the ventricles, deep within the brain.

Cell death plays an important role in development. We have already seen an example at the very beginning of development in the formation of the first and second polar bodies: cells labeled for destruction from the moment they were formed. During embryonic life, cell death occurs on a massive scale, helping to sculpt structures such as the fingers. (If cells don’t die in sufficient numbers, the fingers remain “webbed.”) Cell death also occurs in the brain, eliminating many neurons that have made wrong connections.

During development, there is a progressive restriction of cell fate. Before the formation of the blastocyst, all cells are totipotent: that is, they can become any tissue in the embryo or in the extraembryonic structures such as the amnion. Some cells in the inner cell mass of the blastocyst are pluripotent: they can give rise to any embryonic tissue, but not to extraembryonic structures. These cells—referred to as embryonic stem cells—have been isolated from human blastocysts (Talbot & Lin, 2011). (Embryonic stem cells may have important medical applications, but ethical concerns have delayed research in this area.) Later in embryonic development, groups of cells become restricted to forming certain tissues; these cells are called neural stem cells, muscle stem cells, and so on. The progressive restriction of cell fate involves the switching on of genes needed for given tissues and the inactivation of genes needed for other tissues. This process is regulated by a variety of chemical factors, such as cytokines. (It has proved possible to reverse and redirect cell fate in the laboratory under some circumstances, so it may eventually be possible to obtain, say, new nerve cells from skin cells.)

Cell differentiation is the process by which cells that have adopted a certain fate actually turn into the functional cell types they are supposed to be. This process can involve dramatic changes in cell morphology (generating the intricately shaped cells of the nervous system, for example; see Figure 2) and metabolism (such as developing the ability to synthesize a particular hormone). Cell differentiation involves an interaction between the genes expressed in a particular cell and the local environment to which that cell is exposed.

Figure 2  Cell differentiation is most obvious in the brain, where developing neurons adopt a bewildering variety of shapes. The giant, fan-shaped cell in the top image is a cerebellar Purkinje cell; the small spidery cells in the bottom image are cerebellar granule cells. The large object at the bottom is a blood vessel. (Micrographs by Simon LeVay.)

References

Talbot, P. & Lin, S. (2011). Mouse and human embryonic stem cells: can they improve human health by preventing disease? Curr Top Med Chem 11: 1638–1652.

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