5.1Early Embryonic Development in Humans
Human development begins with fertilization—the fusion of an egg and a sperm. The sperm and the egg each have 23 (i.e., half) of the 46 chromosomes present in the somatic cells that make up the bodies of the parents. Therefore, fusion of the sperm and egg forms a new cell that has 46 chromosomes, just like the parental somatic cells. The fertilized egg soon begins vigorous cell division and the number of cells increases. The cell division of the fertilized egg is called "cleavage."
After cleavage has progressed and the number of cells has increased, a large hollow forms inside the embryo, which is then referred to as a blastula (Fig. 5-1). The blastulae of mammals, including humans, are called "blastocysts." The blastocyst is shaped like a hollow ball surrounded by a structure called a "trophoderm." The blastocyst also contains a cluster of cells called a "blastocyst inner cell mass," from which the body is created.
After the blastula stage, the body begins to form. First, two cell layers develop from the inner cell mass. These two layers are referred to as the "primitive ectoderm" and the "primitive endoderm," and together they form the "blastodisc." Next, cells separate from the primitive ectoderm that forms the blastodisc and move into the space between the two cell layers. Consequently, three cell layers are formed in the blastodisc area. These three cell layers are called the "three germ layers" (the ectoderm, mesoderm, and endoderm), and the embryo during the period of their formation is called the "gastrula." Our body then forms various structures based on the structure of these three germ layers. This formation of germ layers is common to the development of all organisms, from invertebrates to vertebrates, including humans, and is thought to be absolutely necessary for the formation of the bodies of animals.
Fig. 5-1. Early Human Development
(A) When a human egg cell is fertilized, it divides repeatedly, becomes a morula, and then forms a blastula. The primitive ectoderm and endoderm are formed later from an inner cell mass inside the blastula, and a blastodisc forms from the region where the ectoderm and endoderm adhere to each other. Cells that have detached from the primitive ectoderm then move between the two cell layers and form three germ layers (the ectoderm, mesoderm, and endoderm). All structures of the body are formed from these three germ layers.