Introduction

Abstract

Physiology is the study of the specific functions carried out by the various organs and tissues which make up a multicellular organism. The organs and tissues themselves are the result of a number of cells adhering to one another and aggregating in specific patterns to form organized structures. The form these structures (organs and tissues) will take is determined partly by the genetic makeup of each cell and partly by the environmental factors acting upon the cells during embryonic development and throughout the life of the organism. In order, therefore, to gain some insight into the functions of cells and tissues, it is necessary to study their growth, the changes which occur in them during development and differentiation and the ways in which they convert available nutrients into energy and other products of metabolism. An obvious way to study these problems is to place tissues or organs in an environment which resembles, as closely as possible, the environment within the organism, but which can be varied or controlled as desired. When a piece of tissue or an organ is placed in a medium under in vitro conditions it will generally behave in one of two ways: It may (1) continue to grow, develop, differentiate and maintain fairly closely, normal physiological functions, much as it would have done in the whole animal, or (2) its cells may multiply rapidly, migrate away from the explant and exist as more or less independent units. In such cultures, the explant sooner or later loses its organization and the cells, after a time, adapt themselves to their new surroundings, lose many of their specific functions and become “dedifferentiated”.