The construction of the tissues of the body involves developmental phenomena at two levels of organization. One is the level of individual cells, in which the cells that make up a tissue undergo increasing specialization through a process called cytodifferentiation…
Congenital malformations have attracted attention since the dawn of human history. When seen in humans or animals, malformations were often interpreted as omens of good or evil. Because of the great significance attached to congenital malformations, they were frequently represented…
One of the most characteristic features of human embryonic development is the intimate relationship between the embryo and the mother. The fertilized egg brings little with it except genetic material. To survive and grow during intrauterine life, the embryo must…
After gastrulation is complete, the embryo proper consists of a flat, three-layered disk containing the ectodermal, mesodermal, and endodermal germ layers. Its cephalocaudal axis is defined by the location of the primitive streak. Because of the pattern of cellular migration…
While it is implanting into the uterine wall, the embryo undergoes profound changes in its organization. Until the time of implantation, the blastocyst consists of the inner cell mass, from which the body of the embryo proper arises, and the…
The act of fertilization releases the ovulated egg from a depressed metabolism and prevents its ultimate disintegration within the female reproductive tract. Immediately after fertilization, the zygote undergoes a pronounced shift in metabolism and begins several days of cleavage. During…
The application of new techniques in molecular biology and genetics continues to revolutionize the understanding of the mechanisms underlying both normal and abnormal embryonic development. It is impossible to have a contemporary understanding of embryonic development without integrating fundamental molecular…
Sexual reproduction is based upon the union of an egg and a sperm. This chapter explains how the egg and sperm cells come together in the female reproductive tract so that fertilization can occur. It then outlines the complex set…
Human pregnancy begins with the fusion of an egg and a sperm within the female reproductive tract, but extensive preparation precedes this event. First, both male and female sex cells must pass through a long series of changes ( gametogenesis…