2. Phases of prenatal life

Prenatal life can be divided into three main stages, i.e., pre-embryonic, embryonic and fetal. The pre-embryonic phase is the period during which the small collection of cells gets differentiated to form three germ layers, i.e., ectoderm, mesoderm and endoderm by the process of gastrulation. The body axes, i.e., anteroposterior, dorsoventral, left and right also established during gastrulation. The embryonic phase lasts from 5 to 8 weeks.

The pre-embryonic and embryonic phases are the times during which a single cell progresses to form the organ primordia which is the first 8 weeks of human development.

The final fetal stage after 8 weeks leads to rapid overall growth and maturation of the embryo into a viable human fetus. The integration of complex phenomena, which leads to the formation of a viable infant, has generated interest in understanding the molecular and structural aspect of this process [1].

Various comparative and evolutionary studies have been done to understand this fascinating developmental phenomenon. Many experiments were done to trace cells during their development. Transparent embryos were observed and even living cells stained with vital dyes were used to observe their fate. Later radiographic labels and autoradiography techniques were used.

Grafting experiments using quail cells into chick embryos at early stages of development were some of the pioneering efforts in monitoring cell fates that lead to origin of different organs and tissues. Other grafting experiments were grafting the primitive node from its normal position on the body axis to another site could induce formation of a second body axis.

It was also concluded that a piece of tissue from posterior axial border of one limb if grafted to the anterior border of another limb then digits on the host limb are duplicated. This posterior signaling was called zone of polarizing activity and now the signaling molecule is identified as Sonic Hedgehog. Thus the advent and progression in the field of molecular biology has led to better understanding of embryology of normal and abnormal developmental processes.
