Cell multiplication and differentiation.

Embryonic and fetal growth involves a complex interaction of mitogenesis, tissue induction, cell migration and differentiation which occurs within a temporal programme. These tissue interactions are controlled by the extracellular matrix, cell-cell recognition systems, and the release of hormonal messengers such as peptide growth factors. The growth factors include peptides which potentiate growth, such as the insulin-like growth factors (IGFs), and also growth inhibitors, such as transforming growth factor-p. Additionally, some growth factors potentiate and some block tissue differentiation. Studies of whole embryos have shown that growth factors are present and are withdrawn in particular structures at predetermined times. However, discrepancies may exist between sites of synthesis and action due to growth factor distribution being influenced by cellular binding mechanisms. A picture emerges of a paracrine system of growth control which may be controlled by external influences, including insulin and nutritional metabolites. The most obvious parameter of embryonic and fetal growth is the size of the conceptus. However, increasing mass is only one aspect of a complex series of events that constitute early growth. During blastulation inductive effects cause the appearance of the mesoderm from the primitive ectoderm. From this point on the growth of the embryo is asymmetrical, some tissues remaining in a rapidly growing and relatively undifferentiated state while others are differentiated and functionally active. For instance, the anterior somite blocks form and differentiate while the posterior somites have yet to condense. The events of mitogenesis, induction and differentiation therefore take place within the additional developmental dimension of time. At least three primary control mechanisms have been identified; the deposition and subsequent modification of extracellular matrix molecules, the expression and withdrawal of cell-cell recognition molecules, such as the cadherins, and the selective expression of humoral intercellular messengers, such as peptide growth factors.