*2.4.2 Tissue-specific factors*

The mechanism, by which control cellular multiplication, differentiation, and organ development during embryonic period of fetal life, remain a mystery. The concept that classic hormones directly regulate growth and maturation has been replaced by the hypothesis, that growth is controlled by variety of growth factors. Those factors production is stimulated by the classic hormones, these considered as mediator hormones because they mediate some of the biological actions of classic hormone [37].
