Unit Six. Animal Life
Cleavage: Setting the Stage for Development
Fertilization begins a carefully orchestrated series of developmental events. Table 31.1 traces the major stages of mammalian development, beginning with fertilization. Follow down the table as the stages of development are discussed here.
TABLE 31.1. STAGES OF MAMMALIAN DEVELOPMENT
The first major event in human embryonic development is the rapid division of the zygote into a larger and larger number of smaller and smaller cells, becoming first 2 cells, then 4, then 8, and so on. The first of these divisions occurs about 30 hours after union of the egg and the sperm, and the second, 30 hours later. During this period of division, called cleavage, the overall size does not increase from that of the zygote. The resulting tightly packed mass of about 32 cells is called a morula, and each individual cell in the morula is referred to as a blastomere. The cells of the morula continue to divide, each cell secreting a fluid into the center of the cell mass. Eventually, a hollow ball of 500 to 2,000 cells is formed. This is the blastocyst, which contains a fluid-filled cavity called the blastocoel. Within the ball is an inner cell mass concentrated at one pole that goes on to form the developing embryo. The outer sphere of cells, called the trophoblast, releases the hCG hormone, discussed earlier.
During cleavage, the morula journeys down the mother’s fallopian tube. On about the sixth day, the blastocyst has formed and reaches the uterus; it attaches to the uterine lining and penetrates into the tissue of the lining. The blastocyst now begins to grow rapidly, initiating the formation of the membranes that will later surround, protect, and nourish it. One of these membranes, the amnion, will enclose the developing embryo, whereas another, the chorion, which forms from the trophoblast, will interact with uterine tissue to form the placenta, which will nourish the growing embryo (see figures 20.23 and 31.21). The placenta connects the developing embryo to the blood supply of the mother. Fully 61 of the cells at the 64-celled stage develop into the trophoblast and only 3 into the embryo proper.
Gastrulation: The Onset of Developmental Change
Ten to eleven days after fertilization, certain groups of cells move inward from the surface of the cell mass in a carefully orchestrated migration called gastrulation. First, the lower cell layer of the blastocyst cell mass differentiates into endoderm, one of the three primary embryonic tissues, and the upper layer into ectoderm. Just after this differentiation, much of the mesoderm arises by the invagination of cells that move from the upper layer of the cell mass inward, along the edges of a furrow that appears at the embryo midline, the primitive streak.
During gastrulation, about half of the cells of the blastocyst cell mass move into the interior of the human embryo. This movement largely determines the future development of the embryo. By the end of gastrulation, distribution of cells into the three primary germ layers has been completed. The ectoderm is destined to form the epidermis and neural tissue. The mesoderm is destined to form the connective tissue, muscle, and vascular elements. The endoderm forms the lining of the gut and its derivative organs. The fates of all three primary germ layers are:
Neurulation: Determination of Body Architecture
In the third week of embryonic development, the three primary cell types begin their development into the tissues and organs of the body. This stage in development is called neurulation.
The first characteristic vertebrate feature to form is the notochord, a flexible rod. Soon after gastrulation is complete, it forms along the midline of the embryo, below its dorsal surface. Then the second characteristic vertebrate feature, the neural tube, forms above the notochord and later differentiates into the spinal cord and brain. Just before the neural tube closes, two strips of cells break away and form the neural crest. These neural crest cells give rise to neural structures found in the vertebrate body.
While the neural tube is forming from ectoderm, the rest of the basic architecture of the human body is being rapidly determined by changes in the mesoderm. On either side of the developing notochord, segmented blocks of tissue form. Ultimately, these blocks, or somites, give rise to the muscles, vertebrae, and connective tissues. As development continues, more and more somites are formed. Within another strip of mesoderm that runs alongside the somites, many of the significant glands of the body, including the kidneys, adrenal glands, and gonads, develop. The remainder of the mesoderm layer moves out and around the inner endoderm layer of cells and eventually surrounds it entirely. As a result, the mesoderm forms two layers. The outer layer is associated with the body wall and the inner layer is associated with the gut. Between these two layers of mesoderm is the coelom, which becomes the body cavity of the adult.
By the end of the third week, over a dozen somites are evident, and the blood vessels and gut have begun to develop. At this point the embryo is about 2 millimeters (less than a tenth of an inch) long.
Key Learning Outcome 31.6. The vertebrate embryo develops in three stages: cleavage, a hollow ball of cells forms; gastrulation, cells move into the interior, forming the primary tissues; neurulation, organs begin to form.