Abstract:

This is a preprint synthesis of articles "Ectoderm," "Endoderm," "Mesoderm," and "Notochord" originally published in K. Lee Lerner and Brenda Wilmoth Lerner, eds. World of Anatomy and Physiology. Thomson | Gale. 2002.

Ectoderm

Ectoderm is one of three principal germinal layers of cells that are formed early in embryonic development. Ectoderm comprises the outermost germinal layer from which the nervous system, eyes, ears, epidermis, and integumentary elements (glands, hair, and nails) develop. Membranes derived from ectoderm are in contact with endoderm derived structures at membranes of the mouth and anus.

In the embryonic disk, ectoderm and endoderm sandwich mesoderm, the third primitive germinal layer. When the embryonic disk ultimately folds into a tube, the basic "tube within a tube" plan of development becomes evident. A core endodermal tube establishes a primitive digestive pathway bounded by an oral orifice and an anal orifice. Around that innermost tube is an outer tube comprised of ectoderm. The ectoderm serves as a protective layer and the layer from which the nervous system and sense organs develop. Mesodermal cells fill the space between the inner (endodermal) and outer (ectodermal) tube. Mesodermal cells ultimately contribute to the muscles, organs, and other internal body structures.

About a week following fertilization, the human embryonic blastocyst is embedded in (download to read more)