Beginning at the moment of conception, the human organism depends upon adequate nutrition for growth, development, and survival. In the first week from the fertilization, the zygote produces a series of blastomeres and, because of further cellular divisions, the morula, that contains about 10-30 cells. The morula stage proceeds to the formation of a fluid filled cavity, the early blastocyst. Inside the blastocyst is an inner cell mass or embryoblast (future embryo), and the outer cell mass or trophoectoderm (future placenta).

Embryogenesis (three to eight weeks) involves three major processes: morphogenesis (generation of shape), pattern formation (biologic-spatial cell organization), and differentiation (specialization in specific phenotypes). Morphogenesis and pattern formation are regulated by bone morphogenetic proteins and homeobox genes. During embryogenesis tissues and organs develop. The most important events of the embryonic period can be resumed in: (1) gastrulation, a process able to convert the bilaminar...