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A germ layer is a primary layer of cells that forms during embryonic development. [1] The three germ layers in vertebrates are particularly pronounced; however, all eumetazoans ( animals that are sister taxa to the sponges ) produce two or three primary germ layers.
These three germ layers are the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer). [3] [4] In diploblastic organisms, such as Cnidaria and Ctenophora, the gastrula has only ectoderm and endoderm. The two layers are also sometimes referred to as the hypoblast and epiblast. [5] Sponges do not go through the gastrula stage.
The mesoderm is the middle layer of the three germ layers, between the outer ectoderm and inner endoderm. During the third week of embryonic development the lateral plate mesoderm splits into two layers forming the intraembryonic coelom .
The blastoderm now consists of three layers, an outer ectoderm, a middle mesoderm, and an inner endoderm; each has distinctive characteristics and gives rise to certain tissues of the body. For many mammals, it is sometime during formation of the germ layers that implantation of the embryo in the uterus of the mother occurs. [18] [20]
The mesoderm is the middle layer of the three germ layers that develops during gastrulation in the very early development of the embryo of most animals. The outer layer is the ectoderm, and the inner layer is the endoderm. [1] [2] The mesoderm forms mesenchyme, mesothelium and coelomocytes. Mesothelium lines coeloms.
A germ layer is a collection of cells, formed during animal and mammalian embryogenesis. Germ layers are typically pronounced within vertebrate organisms; however, animals or mammals more complex than sponges ( eumetazoans and agnotozoans ) produce two or three primary tissue layers.
The endoderm is the inner most germ layer of the embryo which gives rise to gastrointestinal and respiratory organs by forming epithelial linings and organs such as the liver, lungs, and pancreas. [5] The mesoderm or middle germ layer of the embryo will form the blood, heart, kidney, muscles, and connective tissues. [5]
Intermediate mesoderm or intermediate mesenchyme is a narrow section of the mesoderm (one of the three primary germ layers) located between the paraxial mesoderm and the lateral plate of the developing embryo. [1] The intermediate mesoderm develops into vital parts of the urogenital system (kidneys, gonads and respective tracts).