Answer: Germ layer, any of three primary cell layers, formed in the earliest stages of embryonic development, consisting of the endoderm (inner layer), the ectoderm (outer layer), and the mesoderm (middle layer). The germ layers form during the process of gastrulation, when the hollow ball of cells that constitutes the blastula begins to differentiate into more-specialized cells that become layered across the developing embryo. The germ layers represent some of the first lineage-specific (multipotent) stem cells (e.g., cells destined to contribute to specific types of tissue, such as muscle or blood) in embryonic development. Hence, each germ layer eventually gives rise to certain tissue types in the body.
The endoderm is so called because it is the innermost of the three germ layers. Cells derived from the endoderm eventually form many of the internal linings of the body, including the lining of most of the gastrointestinal tract, the lungs, the liver, the pancreas and other glands that open into the gastrointestinal tract, and certain other organs, such as the upper urogenital tract and female vagina. Endoderm cells give rise to certain organs, among them the colon, the stomach, the intestines, the lungs, the liver, and the pancreas. The ectoderm, on the other hand, eventually forms certain “outer linings” of the body, including the epidermis (outermost skin layer) and hair. The ectoderm also is the precursor to mammary glands and the central and peripheral nervous systems.
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