Elements of Tissue Development

This book began with a reflection on the miracle of development, wherein a single cell transforms into a human. The transformation from fertilized egg to adult results from a complex tapestry of events, which scientists are only beginning to dissect and unravel. Certain processes occur frequently during development; that is, the tapestry is woven from threads of elemental colors and textures. A central assumption of subsequent chapters is that key concepts underlying tissue regeneration first appear during fetal development. The elements of developmental biology are presented in this chapter; more complete descriptions are available in any of several excellent textbooks. The relevance of developmental processes in the study of tissue engineering is detailed in subsequent chapters. One of the most intimidating aspects of developmental biology is the vocabulary; therefore, important words are indicated in small capitals on first occurrence and collected in a glossary at the end of the chapter. Developmental biology is an ancient science. One of the central concepts in developmental biology, EPIGENESIS, came from Aristotle in the fourth century B.C. Epigenesis is a continuous, stepwise process in which a simple initial structure becomes complex. Through much of history between Aristotle and the present, epigenesis was not widely accepted as operating in development; many scientists, particularly during the 17th and 18th centuries, were preformationists who believed that the structure of animals was preformed at conception. To the preformationist, the embryo begins as a small replica of an individual which changes only in size during the course of development. Preformationists differed as to whether the preformed individual resided in the ovum or the sperm, but they agreed that all of the attributes of an adult were present from the outset of development. Epigenesis is now well established and many of the steps underlying epigenesis are understood. Human development is part of a larger cyclic process; fertilized eggs develop into newborns who grow to adults and produce new eggs and sperm. This chapter will introduce some of the mechanisms underlying human development from egg to newborn.