PRENATAL GROWTH AND DEVELOPMENT

Human development is considered to begin with fertiliza­tion, the union of sperm and ovum resulting in a zygote (Fig. 2-3). The process begins with the intermingling of a haploid number of paternal (23,X or Y) and maternal (23,X) chromosomes in the ampulla of the oviduct that fuse to form a zygote. Within 24 hours, the unicellular or­ganism becomes a two-cell organism and, within 72 hours, it is a 16-cell organism called a morula. This series of mi-totic divisions is called cleavage. During cleavage, the rapidly developing cell mass travels down the oviduct to the uterus by a series of peristaltic movements. The morula enters the uterus approximately 3 days after fertilization. On the fourth day, the morula is separated into two parts by fluid from the uterus. The outer layer gives rise to the placenta (trophoblast), and the inner layer gives rise to the

 

Fertilization               

Cleavage

Implantation 

embryo (embryoblast). The structure is now called a blasto-cyst. By the sixth day, the blastocyst attaches to the endo-metrium. This is the beginning of implantation, and it is completed during the second week of development.⁸

Prenatal development is divided into two main peri­ods. The first, or embryonic, period begins during the sec­ond week and continues through the eighth week after fertilization.⁶,⁸ During the embryonic period, the main or­gan systems are developed, and many function at a mini­mal level (see Chapter 7). The second, or fetal, period begins during the ninth week. During the fetal period, the growth and differentiation of the body and organ systems occur.