Embryology and Early Developmental Physiology

Abstract

One of the wonders of life is the manner in which the single-cell fertilized ovum develops into a sentient human being, with several trillion cells of over 200 individual types each with 10,000 or more proteins and other metabolites interacting in a well-regulated manner. Following ovulation and fertilization, the latter of which normally occurs in the fallopian tube within minutes of ovulation, the zygote (the cell that results from fertilization) divides sequentially to form the morula (a solid ball with 16 cells), then the blastocyst in which a fluid filled cavity forms. At the end of the first week post conception upon reaching the uterus, the blastocyst implants into the endometrium/decidua that lines the uterine cavity. The impetus for implantation is derived from the blastocyst and its metabolic products. The blastocyst invades the uterine decidua and surrounding small blood vessels to obtain essential nutriments to grow. By week three post conception, cell division continues with the inner cell mass giving rise to the embryo. The blastocyst outer cell mass develops into trophoblast cells, the early placenta. Soon the gastrula develops with three layers of cells (germ layers from which body organs and tissues arise). These include the innermost layer, the endoderm, which gives rise to the lungs, digestive organs, and other intra-abdominal viscera; the middle layer, the mesoderm, from which arises the skeleton, muscles, and blood system; and the outer cell layers, or ectoderm, which gives rise to the nervous system, and skin. Recognition of pregnancy by the maternal organism includes a number of processes, including prolongation of the life-span of the ovarian corpus luteum to ensure secretion of progesterone and tolerance by the maternal decidua of the semi-allogenic graft of the placenta and fetus. The embryonic period extends until the end of the seventh week of gestation, at which time the major organs have commenced their development. From the eighth week onward, the developing conceptus is referred to as a fetus.