Molecular Biology of the Cell - Volume 3 - Alberts B., Bray D., Lewis J., Raff M., Roberts K., Watson J. 1994

From Cells to Multicellular Organisms
Maintaining Normal Tissue Organization

Within a few days or weeks, a single fertilized egg develops into a complex multicellular Organism composed of differentiated Cells whose spatial arrangement is strictly determined. As a rule, this Organization is initially established on a small scale, followed by subsequent growth. During embryonic development, various Cell types are determined, each in its appropriate Location. Then, during the growth period, the cells proliferate, but with few exceptions, their specialization remains more or less constant. An organism may continue to grow throughout its entire life, as in most crustaceans and fish, or it may cease growing upon reaching a specific size, as in birds and mammals. In some animals with fixed body sizes, such as flies and nematodes, the proliferation ¹ of somatic cells ceases as soon as the adult state is reached. In many other cases, including higher vertebrates, cells continue to divide in the adult organism to replace dying cells.

When cells in Tissues such as Skin, Blood, or Lungs wear out and die in vertebrates, they are replaced by new Cells of the corresponding type. Thus, the adult organism can be likened to a stable ecosystem in which one generation of individuals succeeds another, while the overall ORGANIZATION OF THE system remains unchanged. This chapter is devoted to the problems of tissue maintenance and renewal in higher vertebrates—here we will become acquainted, to some extent, with the striking diversity of structures, Functions, and life cycles of specialized cells in these animals.