Fertilization and Embryology

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. This is not the place to consider the details of embryonic development; however, a brief overview may be appropriate. Following ovulation and fertilization of the ovum, 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, which result in invasion of the uterine decidua and small blood vessels. By week 3 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 blastocyst develops three layers of cells, the germ layers from which body organs and tissues arise. These include the innermost layer, endoderm (or entoderm), that gives rise to the epithelial cells of the lungs, digestive organs and other intra-abdominal viscera; the middle layer, mesoderm, from which arise the skeleton, muscles, cardiovascular system, reproductive system, and connective tissues; and the outer cell layer, ectoderm, which gives rise to the brain, nervous system, and integument including the 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 semiallogenic graft of the placenta and fetus. The Embryonic period extends until the end of the seventh week of gestation, at which time the major organ systems have commenced their development. From the eighth week onward, the developing conceptus is referred to as a fetus.