Adhesion Molecules-28

Advances and Experimental-2

Aging-9

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Antisense-4

Apoptosis-49

Atherosclerosis-12

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Channels and Transporters-79

Chaperones-11

Circadian Rhythms-13

Coagulation-14

Cytokines/Growth Factors-52

Development-223

DNA-56

DNA Surveillance and Repair-42

Drug Design-17

Endocrine-66

Evolution-33

Extracellular Matrix-30

Gastroenterology-52

Gene Expression-178

Gene Therapy-53

Heart-61

Heat Shock Proteins-19

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Immunology-93

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Protein-12

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RNA-152

Signal Transduction-186

Stem Cells-80

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T-Cell Activation-12

Tissue Engineering-116

Transplant-51

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Viruses-85

Chapter category: Development

Epithelial Morphogenesis: A Physico-Evolutionary Interpretation

Chapter authors:
Stuart A. Newman

The molding of living tissues that occurs during development, regeneration, wound heal-ing, and various pathological processes is referred to as morphogenesis. During morphogenetic events tissue masses may disperse, form internal foci of cell condensation, lengthen or shorten, or acquire lumens. They can also form sheets which may invaginate or evaginate, or develop one or more internal boundaries across which cell mixing is selective or prohibited. Such compartmentalized tissues can physically separate, or remain attached, where they may engulf, or become engulfed by one another. The outcomes of these processes are the various body plans and organ forms characteristic of metazoan organisms, as well as tumors, abnormal polyps and fibrotic lesions.

While mechanisms of morphogenesis, like other biological processes, are typically studied by attempting to isolate single determining factors while holding everything else constant, the interactive and regulative nature of developing systems frequently limits the causal information that can be obtained by such strategies. For example, the apical ectoderm of the developing vertebrate limb bud forms a raised ridge over the bud's distal margin.¹ How the ridge forms involves mesodermal effects on the ectoderm, which in turn elicit ectodermal factors which act on the mesoderm.² Together the two tissues produce an atypical basal lamina at the limb bud margin^3–5 which appears to influence the ectodermal cell shape. Exactly how many reciprocal cycles of interaction are required to achieve the end point, and in which tissue the causal chain is initiated, are not clear. It must also be recognized that this morphogenetic event is a product of evolution and, during the course of its attaining its contemporary means of generation, the timing of expression of key genes may been reversed from the order in which their influence was first exerted phylogenetically. This phenomenon, referred to as "heterochrony,"⁶ can clearly confound experimental assignment of cause and effect. Similarly, whereas the "knocking-out" of a specific gene from an animal's germ line would seem to be an elegant and straightforward way of assessing its developmental and physiological roles, the readjustment of gene expression and gene product function in the resulting organism can often thwart the interpretation of such experiments.

Multicellular organisms first arose as early as 600 million years ago.⁷ By approximately 540 million years ago, at the end of the "Cambrian explosion," virtually all the "bauplans" or body types seen in modern organisms already existed.^8-10 The original multicellular forms were established with cells that were metabolically and structurally sophisticated—the first single celled organisms appeared two or perhaps three billion years earlier. There is no reason, however, to assume that the earliest metazoa were morphogenetically sophisticated—that they generated complex forms using the baroque, hierarchical molecular machinery that guides such morphogenesis in modern organisms.¹¹ In particular, the defining characteristic of a multicellular organism is simply the existence of a mechanism of adhesion, whereby cells may remain attached to one another after they divide. The precise chemical or physical nature of the adhesive interaction is immaterial, as long as it serves to keep the organism's cells from entirely dispersing. Therefore the appearance of simple multicellular forms in the fossil record may have been a relatively straightforward matter, and was certainly not dependent on the evolution of any complex developmental schemes.

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