The development of a complex, multicellular organism such as a human being is accompanied by changes in which a single, unspecialized cell (the fertilized egg or zygote) ultimately gives rise to cells and tissues that are highly specialized in structure and function.
This progressive transition is characterized by four component processes: determination, differentiation, growth, and morphogenesis.
New cells are usually not “committed” to a specific function. For example, following fertilization of an egg cell or ovum by a sperm cell, the resulting zygote divides mitotically many items to produce a ball of cells called a blastula.
Each cell or group of cells in the blastula subsequently gives rise to specific tissues and organs of the developing embryo.
Although a blastula gives rise to a single embryo, should one cell separate from the others at an early blastula stage, that cell may also develop into a complete embryo. In humans, it is this phenomenon that results in identical twins, identical triplets, and so on. At later stages of embryo development, cells that normally become epidermal tissue (i.e., ectoderm) can be surgically transplanted to another part of the embryo and there develop into mesodermal tissue (e.g., muscle) or endodermal tissue (e.g., intestinal epithelium).
At some early stages of an organism’s development, all cells have the potential to develop into any of the variety of different tissue and cell types that characterize that organism. This potential is called totipotency. Generally, at some point in development, cells become committed to a specific course of differentiation, that is, they differentiate into a specific type of cell or tissue. The process that establishes the fate of a cell is called determination. During determination, certain genes become permanently “turned off” and others are sequentially expressed, further and further narrowing the course of differentiation of the cell.
During differentiation cells take on new and specific properties. These can be structural (such as the formation of organized arrays of actin and myosin filaments in muscle cells) or biochemical (as in the appearance of enzymes of a new metabolic pathway). Differentiation can also take the form of loss of existing structures or biochemical processes. For example, in the differentiation of mammalian red blood cells, the nucleus and other cellular organelles are lost, together with the biochemical processes that were provided by these structures.
The determination and differentiation of the cells of a developing organism are accompanied by growth, that is, an increase in the size and number of cells comprising the organism. In humans, for example, embryonic and fetal development proceeds from the hundreds of cells comprising the small blastula to a fetus weighing several kilograms and containing hundreds of millions of cells.
Growth, in turn, is accompanied by morphogenesis—the generation of form and shape in the developing organism. In this process, differentiating and growing cells give rise to the characteristic organizational pattern of the organism. Small masses of cells take on the form and shape of specific and identifiable structures, such as bones, appendages, the brain, and other organs.
The pattern of differentiation of a cell is founded on the nature of the DNA in the cell nucleus. Before a cell can develop into a hair cell of a mammal, a feather cell of a bird, or a scale cell of a reptile, its nucleus must contain a genome whose transcription and translation into enzymes and other proteins allow the cell to differentiate in a specific direction.
Moreover, given the appropriate genetic complement, conditions must allow these genes to be expressed. Gene expression is regulated at three levels. The first level involves molecular or metabolic interactions, such as mass action and allosteric enzyme function. The second level of control is effected through the interaction between the cell nucleus, the cytoplasm, and the cytoplasmic organelles. The third level of control involves the interactions between the cell as a whole and its environment.