Embryo and Embryonic Development

Embryo and Embryonic Development

Embryonic development in the Rana pipiens

In vitro fertilization and stages of frog development

Tissue specific differentiation during embryogenesis

Resources

An embryo is a stage directly after fertilization that signifies the early stages of growth and development of an organism. In humans, this stage ends during the third month of pregnancy, and the embryo is then called a fetus. Plants and invertebrates as well as vertebrate animals have an embryonic stage of development. For example, the embryo of the common North American leopard frog, Rana pipiens has been studied extensively because it is common, relatively easy to induce ovulation in females, and developmental progress can be monitored in a glass dish due to the fact that the embryo is neither within a shell (as with reptiles and birds) nor within the body of the mother (as with mammals).

Much of what is known today about the stages of embryonic development and characterization of the embryo has been accomplished through research contributions related to the development of the frog. In fact, the technique of removing male and female sex cells to be used for fertilization and then replace the fertilized egg into the appropriate place in the female, also known as in vitro fertilization was first demonstrated in this species. This technique has revolutionized reproductive technologies in humans and has allowed some infertile couples to have children. Although there are many differences, human development is much like the frog in terms of the basic embryological terminology as well as distinct stages of development. In Rana pipiens, development is divided into an embryonic period that occurs prior to hatching from the jelly membranes that enclose the embryo, and larval development which is the period of the free-living feeding tadpole.

Embryonic development in the Rana pipiens

Embryonic development begins with the formation of the fertilized egg called the zygote. In North American leopard frogs, zygote formation occurs in breeding ponds or wetlands in the early spring. During winter, the frogs reside in the cold water of northern lakes.

When the ice melts and the days grow longer, the frogs leave the cold lakes and seek shallow ephemeral bodies of water. Water has a high specific heat, which means that lakes resist a change in temperature. The shallow water of breeding ponds warms readily, which is essential for both ovulation (the release of eggs) and embryo development. Male frogs clasp mature female frogs and this may encourage egg release. The clasping male releases sperm as the eggs are extruded. A female may release as many as 3, 000 eggs which, when fertilized, results in 3, 000 potential embryos.

In vitro fertilization and stages of frog 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, which causes 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) within 3.5 hours into two cells 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 hollow ball of cells. The mature blastula has about 3, 000 cells, which can form within the first 24 hours in the laboratory.

Blastula cell sizes depend on the amount of yolky granules they contain. Very yolky cells are large; cells with minimal yolk are tiny but clearly visible under the microscope. Regardless of size, the cells are considered to be developmentally equivalent until they begin 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 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; this 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.

Tissue specific differentiation during embryogenesis

Specific tissue types form during embryo development. A portion of the ectoderm on the dorsal side of the embryo rolls up into a tube, which forms into the central nervous system. The anterior portion of the tube becomes the brain and the posterior portion becomes the spinal cord. Some mesoderm cells become specialized to form muscle and the muscle functions well before feeding occurs. This can be observed by muscular activity (bending and twisting of the embryo body) in the as yet unhatched embryo in its jelly membranes. Careful examination of the embryo at this time reveals a pulsation. The pulsation is the beating of the heart muscle, which begins to circulate embryonic blood cells. Embryonic gills are exposed on either side of the head. The structure of the gills is so delicate that blood cells, with a dissecting microscope, can be seen surging in synchrony with the beating heart.

During embryonic development, the excretory system and the digestive system begin their formation. Within six days in the laboratory, all embryonic systems have begun developing. Hatching occurs at about this time, which marks the end of the embryonic period and the beginning of larval development. Larvae feed in about a week after fertilization. Embryonic development has been characterized by differentiation of organ systems but no increase in mass. Feeding begins and the tadpole grows enormously in size compared with its origin. Feeding and growing will continue until the larval tadpole begins its metamorphosis into a juvenile frog.

See also Embryo transfer; Embryology; Sexual reproduction.

Resources

BOOKS

Larsen, William J. Human Embryology, 3rd. ed. Philadelphia: Elsevier Science, 2001.

OTHER

The Visible Embryo. “The Visible Embryo” <http://www.visembryo.com/baby/index.html> (accessed November 21, 2006).

Nova Online. “Morphing Embryos” <http://www.pbs.org/wgbh/nova/odyssey/clips> (accessed November 21, 2006).

University of New South Wales, Sydney, Australia. “UNSW Embryology” <http://embryology.med.unsw.edu.au/> (accessed November 21, 2006).

Bryan H. Cobb, PhD