Developmental Biology

DEVELOPMENTAL BIOLOGY IS the science that studies the mechanisms of development of animals. Some would also include the development of plants within its scope, but as mechanisms differ very substantially between animals and plants, in practice the term developmental biology is usually used to refer only to animal developmental biology.

The subject represents a fusion between three traditions of 20th—century biological science. Experimental embryology emerged in the early 20th century, being initially based mostly on microsur—gical experiments using amphibian and sea urchin embryos. Developmental genetics grew up mid—century, based mosdy on the genetics of the fruit fly Drosopbila and the laboratory mouse. Molecular biology arose from the discovery of the structure of DNA in 1953, and by the 1980s a group of new techniques enabled the isolation of individual genes by molecular cloning, the determination of their nucleotide sequences, and the study of gene expression in cells and embryos. Modern developmental biology took shape in the 1980s by combining the concepts and techniques of these three areas.

Development of animals takes place mostly during embryonic life, so developmental biology is mostly concerned with embryology. However, there are other developmental processes; for example, those associated with postnatal development, tissue renewal, regeneration, wound healing, and in some types of animal, metamorphosis.

Much use is made of “model organisms” in developmental biology research. These are particular species that have some technical advantages for one or more types of experimental work, and with which a large community of scientists agrees to work. Concentration of effort on a few model organisms has brought rapid progress because it enables the elucidation of whole genome sequences and the sharing of clones for specific genes or antibodies for specific gene products and generally speeds the development of techniques for working with the organism. The main model organisms used in developmental biology research are the nematode Caenorbabditis elegans, the fruit fly [Page 160]Drosopbila, the zebrafish, the frog Xenopus, the chick embryo, and the mouse. Each has its own strengths and weaknesses for experimental work. In general, C. elegans, Drosopbila, and the mouse are good for genetic experiments, and Xenopus and the chick are good for microsurgical methods; the zebrafish occupies an intermediate position.

The specific problem areas addressed by developmental biology have been regional specification, morphogenesis, and cell differentiation and growth. Regional specification refers to the mechanism by which a uniform ball or sheet of cells becomes programmed such that different cell types arise at different positions. The earliest such event usually depends on the localization of a regulatory substance, or determinant, in a particular position in the cytoplasm of the fertilized egg. Following cleavage of the egg to form a cell mass, certain cells will inherit the determinant, and it then causes activation of specific genes in those cells.

Subsequent events of regional specification normally involve extracellular inducing factors, which are secreted from one group of cells (an organizing or signaling center) and diffuse away, forming a concentration gradient. The surrounding cells are competent to respond by activating or repressing specific regulatory genes in response to particular concentrations of the inducing factor. Several cycles of this process bring about subdivision of a simple cell mass into a highly complex body pattern. The inducing factors belong to a few classes: chiefly, the fibroblast growth factor, Wnt, hedgehog, and bone morphogenetic protein families.

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