In frog development, a zygote undergoes a series of cell divisions that result in the formation of a blastula, a fluid-filled ball of cells. Based on the pattern of cleavage, the cells of the blastula, called blastomeres, contain cytoplasm with slightly different contents. Some of these blastomeres are essential in triggering the next phase of development, called gastrulation.
In gastrulation, the blastula rearranges, with sheets of blastomeres from the outside of the embryo entering the embryo's interior. Cells move into contact with new cells, allowing unique intercellular communications that lead to cell determination and differentiation. By the end of gastrulation, three embryonic germ layers—endoderm, mesoderm, and ectoderm—take their positions in the embryo. These layers ultimately give rise to specific tissues and organs that make up the adult body plan.
The three colors represent the three tissue layers that become defined early in embryogenesis. Yellow indicates endoderm, red indicates mesoderm, and blue indicates ectoderm.
At the beginning of gastrulation, a few surface cells, called bottle cells, move into the interior of the embryo, followed by other surface cells. We can track the movement of cells into the embryo if we add dye to a few surface cells.
The movement of cells into the embryo creates a lip, called the dorsal lip, over which sheets of cells continue to move inside. At the same time, the ectoderm extends around the embryo's surface in a process called epiboly. As gastrulation proceeds, a cavity, called the archenteron, forms while the blastocoel shrinks.
The archenteron is the primitive gut and is completely surrounded by endodermal tissue. The endoderm at the roof of the cavity originated from the outside of the embryo. The cavity is continuous with the outside, via the blastopore, which eventually becomes the anus of the animal.
As the ectoderm extends around the embryo, another set of bottle cells forms. These cells migrate into the embryo, and other surface cells follow them, creating the ventral lip of the blastopore.
By the end of gastrulation, the ectoderm has surrounded the embryo, endoderm lines the inside, and mesoderm lies between the two. Additionally, the fates of specific regions have become determined.
The endoderm gives rise to the digestive and respiratory tracts and associated structures. The mesoderm gives rise to the skeleton, circulatory system, muscles, excretory system, and most of the reproductive system. The ectoderm gives rise to the skin, sense organs, and nervous system.
Developmental biologist Lewis Wolpert said about gastrulation, "It is not birth, marriage, or death, but gastrulation, which is truly the most important time of your life."
Although humans and frogs differ greatly in the structure of the blastula (called a blastocyst in humans) and in the process of gastrulation, in each case gastrulation sets the stage for the formation of the adult body. In gastrulation, the three embryonic tissue layers move into the positions where they begin to develop into the adult's organs and tissues.
In the accompanying animation, we examined gastrulation in the frog. In this process, endodermal tissue migrates from the surface of the embryo to the interior. Ectodermal tissue extends around the embryo by the process of epiboly. Mesodermal tissue extends from a band of tissue to take its place between the endoderm and ectoderm. The primitive gut—the archenteron—forms while the blastocoel recedes. In frogs and other animals of the deuterostomate lineage, the blastopore eventually develops into the anus of the animal.
The following is a list of the adult tissues and organs that develop from the three embryonic tissue layers:
Ectoderm
Brain and nervous system; lens of the eye; inner ear; lining of mouth and nasal canal; epidermis of skin; hair and nails; sweat glands; oil glands; milk secretory glands
Mesoderm
Skeletal system (bones, cartilage, notochord); gonads; muscle; outer coverings of internal organs; dermis of skin; circulatory system (heart, blood vessels, blood cells); kidneys
Endoderm
Inner linings of gut; respiratory tract (including lungs); liver; pancreas; thyroid; urinary bladder