Endoderm
Endoderm is one of three principal germinal layers of cells that are formed in early in embryonic development. Endoderm is also referred to as entoderm.
The other principal germinal layers (e.g., layers of cells from which body tissues develop) are ectoderm and mesoderm. Endoderm comprises the innermost germinal layer from which the epithelium of the digestive system and most of the respiratory system are derived. Endodermal cells also develop into parts of the urethra and bladder.
Membranes derived from endoderm are in contact with ectoderm derived membranes at the mouth and anus.
In the embryonic disk ectoderm and endoderm border a third intermediate mesodermal 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 the endometrium of the uterus. The blastocyst is a proliferating ball of cells with a cavity termed a blastocoele. At one pole of the blastocyst there is a thickened mass of cells termed the inner cell mass. The inner cell mass contains communicating slit-like openings that form the amniotic cavity and the embryonic disk.
Endoderm lies on the ventral side of the embryonic disk. At the cellular or histological level, endoderm is comprised of columnar cells. The ectodermal layer lies on the dorsal side of the disk. Initially there are only two germ layers, but by 18 days post fertilization a thickening occurs in the ectoderm to form a primitive streak. The walls of the primitive groove continue to thicken and, at the anterior (cephalic) end of the groove, expand into a primitive knot also known as Henson's node. Anterior to the primitive knot, primitive ectoderm and endoderm are in direct contact with each other to form a prochordal plate. The primitive streak also establishes the general head-to tail (cephalo-caudal) axis for subsequent development. Starting at the primitive knot, cells from the ectodermal layer migrate into the primitive groove and invaginate into the space between the ectoderm and endoderm to form mesodermal cells. Other cells, derived from other fetal membranes also contribute cells to the mesodermal layers. As ectodermal cells stream invaginate to form a trilaminar embryonic disk, a head process (ultimately to become the notochord) forms in the middle mesodermal layer.
Endodermal cells ultimately provide the cells that form the tissues comprising the majority of the epithelial lining of the alimentary canal. Endoderm also provides the linings of glands--except for ectodermal derived salivary glands--that open into the alimentary canal. The liver and pancreas also develop from endoderm. Other organs and systems that contain critical components derived from endoderm include elements of the thyroid gland, parathyroid glands, thymus, pharynx, trachea alveoli, bladder, prostate and urethra.
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