The process by which specialized and diversified structures arise during development of the embryo. The process involves (1) an increase in the number of cell types, and (2) an increase in morphological heterogeneity through the arrangement of cells into increasingly complex structural patterns in the form of tissues and organs.
Differentiation begins in most organisms with fertilization of an egg with a sperm, after which the relatively large egg divides into many smaller cells called blastomeres. The blastomeres receive unequal portions of the cytoplasmic materials of the egg and are therefore initially somewhat different from each other. At the end of cleavage, the blastomeres are organized into a blastula, commonly either a hollow ball of cells or a flattened two-layered disk of cells. The cells of the blastula lie in different relative positions from those that will be occupied by their descendants in the adult organism. By a process known as gastrulation, they move to their approximate final positions and are arranged into three basic layers, called germ layers. However, only two layers form in the simpler multicellular organisms. The outer layer is the ectoderm, from which arise the nervous system and the epidermal layer of the skin. The innermost germ layer, the entoderm, forms the epithelial lining of the digestive tract and contributes the essential tissue of associated organs. In all but the most primitive animals a third germ layer, the mesoderm, is formed by cells which come to lie in the area between the other two layers. In higher animals the mesoderm gives rise to most of the cells of the organism, such as those found in the muscles, skeleton, blood, connective tissue, kidneys, gonads, and certain other organs. The molding of groups of embryonic cells into such diverse tissues and organs proceeds through a variety of morphogenetic processes, such as migration, aggregation, dispersion, delamination, folding, and differential local growth of cells. See Blastulation, Gastrulation, Germ layers
Underlying the visible structural diversification of the embryo is the more fundamental and concomitant process of cellular differentiation (chemodifferentiation), by which embryonic cells are transformed into the highly specialized cells of the adult.
The mechanisms by which the course of cellular differentiation is realized are not precisely known. The factors involved may, however, be divided into two classes: (1) intrinsic, those operating within the cell, and (2) extrinsic, those brought to bear upon the cell from outside. Both classes of factors play a role in the differentiation of every cell. However, the relative importance of these factors varies considerably from one cell strain to another and also within the same cell at different stages in its development.
The fertilized egg begins development with a rich endowment, consisting of a nucleus with a set of paternal and maternal chromosomes together with a complexly organized cytoplasm. The activation of the egg by the sperm sets off a chain of actions and reactions that progressively transform the physical and chemical constitution of each descendant cell. The emergence of new cell characteristics may be attributed to an oscillating interaction between the intrinsic gene makeup of the cell and the surrounding cytoplasm. The dynamic imbalance existing between these interacting components drives the cell along its path of differentiation. In certain kinds of invertebrate embryos, interactions within each separate cell seem sufficient for guiding differentiation to its terminal state. Such embryos exhibit mosaic development. By contrast, in the embryos of vertebrates and certain invertebrates such as echinoderms, influences from adjacent cells are an essential part of the differentiation process. These embryos show regulative development. See Cell lineage, Developmental biology, Embryonic induction