Mesoderm is one of three principal germinal layers of cells that are formed in early in embryonic development. Mesoderm comprises the middle germinal layer that lies between the ectodermal and endodermal layers. Mesodermal cells ultimately form tissues, organs, and systems including blood vessels, blood, connective tissues, muscles, cartilage, bone, lymph glands, pericardium, peritoneum, pleura, gonads, and kidneys.
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.
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. Ectodermal cells lie on the dorsal side of the embryonic disk, the endodermal layer lies on the ventral side of the disk. Initially there are only two germ layers but by 18 days following 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.
The mesodermal germ layer divides into an outer layer of somatic mesoderm and inner layer of splanchnic mesoderm. Intraembryonic mesoderm also becomes divided by a longitudinal groove into paraxial (along the axis) and lateral plate mesoderm. The paraxial mesoderm further divides by segmentation into a series of blocks termed somites
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