Embryo and Embryonic Development

The processes leading to fertilization can be accomplished in the laboratory. Laboratory frogs are ordinarily kept cold until embryo formation is required. Female frogs brought to laboratory temperature (18°C or about 65° F) are injected with a pituitary extract combined with progesterone causing ovulation within 48 hours. The release of sperm is induced with an injection of human chorionic gonadotropin hormone. A zygote is formed when freshly ovulated ova are combined with freshly released sperm. The zygote cleaves (divides) into two cells within 3.5 hours called blastomeres. It divides again into four cells, then eight, and continues until a ball of cells forms known as a morula. With continued division, the morula gives rise to the blastula. The frog blastula is also a ball of cells but it is hollow. The mature blastula has about 3,000 cells, which can form within the first 24 hours in the laboratory.

Blastula cells vary in size depending on the amount of yolky granules they contain. Very yolky cells are large; cells with minimal yolk are tiny but clearly visible in the microscope. Regardless of size, the cells are considered to be developmentally equivalent. That means that they have not begun the process of specialization known as differentiation. Classic grafting experiments in the early 1900s demonstrated the lack of specialization by grafting small populations of blastula cells to a new site within an embryo with no effect on development. While 3,000 cells are present at the definitive blastula stage, there has been no net growth and, therefore, no increase in mass. A frog blastula has the same diameter as a zygote. Cleavage gives rise to an increasing number of cells which partitions the former zygote into ever smaller compartments (cells).

Gastrulation, which precedes the blastula stage, is a time of developmental change. Many living cells can migrate within a developing organism and the first migrations are observed during gastrulation. Some cells on the A human embryo at five to six weeks of development. Photograph by Petit Fromat/Nestle. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. surface migrate to the interior. In the process of migration, the former population of equivalent and unspecialized cells becomes a structured sphere with a complicated interior with the potential to differentiate. The three primary germ layers may be detected at this time; the external ectoderm, the internal endoderm, and the intermediate mesoderm. The rearrangement of cells that result from migration forms an area that invaginates and cells move toward the interior. The site of movement inwards is known as the blastopore. The blastopore will eventually become the posterior opening of the digestive system and the positioning of the blastopore within the gastrula permits identification of an anterior and posterior axis as well as left and right sides.

Differentiation occurs during gastrulation. Cells begin their specialization, limiting their competence to differentiate into all of the different possible cell types. Thus, when a graft is made, which exchanges populations of cells from one area to another, the grafts develop in their new locations as if they had not been moved. For example, when cells destined to form nerve structures are grafted to a skin-forming area, they do not form skin but continue on their pathway to differentiate neural cells.