ORGANOGENESIS BEGINS in the third week of gestation (postfertilization) with the process of gastrulation and ends in the eighth week. During this six-week period, known as the time of embryogenesis, primordia for most of the organ systems are established and the conceptus becomes recognizable as a human fetus. These six weeks are also marked as the period of greatest sensitivity for induction of major congenital malformations and each organ system experiences its own peak(s) of susceptibility to insult during this time frame. By the ninth week, the conceptus enters the fetal period, which is characterized by the growth and continued differentiation of the tissues and organs that were established previously. Thus, most of the tissues and organs are established early in gestation as a result of cell-cell interactions, including inductive events. The remainder of pregnancy is then characterized by continued cell differentiation and growth.

Gastrulation

Gastrulation is the process that establishes the three germ layers, ectoderm, mesoderm, and endoderm, that are responsible for formation of all embryonic structures. Prior to this process, the embryo consists of two cell layers comprising the epiblast dorsally and the hypoblast ventrally (Figure 2.1). On the fourteenth to fifteenth day of gestation, the caudal half of the epiblast becomes marked by a shallow depression, the primitive streak. Its cephalic end is marked by a slight elevation, the primitive node (of Henson), surrounding the primitive pit. Cells of the epiblast migrate toward the streak, detach from this cell layer, and turn inward to lie between the hypoblast and remaining epiblast (Figure 2.2). The first cells to migrate displace the hypoblast and form the embryonic endoderm; later arriving cells form the mesoderm; and cells remaining in the epiblast form the ectoderm. Thus, all three germ layers and, ultimately, all tissues of the embryo are derived from the epiblast layer.

Ectoderm gives rise to the central and peripheral nervous systems; sensory epithelium of the ear, nose, and eye; subcutaneous glands, mammary glands, pituitary, and enamel of the teeth. Mesoderm provides: blood and lymph cells; endothelium of blood vessels and the heart; kidneys and gonads; cortical portion of the suprarenal glands; and the spleen.