Mucoid connective tissue

Mesenchyme
Transmission electron micrograph of mesenchyme displaying the ultrastructure of a typical cell and matrix.
Mesenchyme (pointer) stained with H&E
Details
Carnegie stage	6b
Precursor	Primarily mesoderm
Identifiers
Code	TE E5.16.4.0.3.0.18
Anatomical terminology []

Mesenchyme, also known as mucous connective tissue or mucoid connective tissue, is a type of connective tissue found mostly during development of the embryo. It is composed mainly of ground substance with few cells or fibers. It can also be referred to as a group of mucoproteins found in certain types of cysts (etc.), resembling mucus. It is most easily found as a component of Wharton's jelly.

Mucous connective tissue forms the umbilical cord.

The vitreous of the eyeball is a similar tissue.

The mesenchyme originates from the mesoderm. From the mesoderm, the mesenchyme appears as an embryologically primitive "soup". This "soup" exists as a combination of the mesenchymal cells plus serous fluid plus the many different tissue proteins. Serous fluid is typically stocked with the many serous elements, such as sodium and chloride. The mesenchyme develops into the tissues of the lymphatic and circulatory systems, as well as the musculoskeletal system. This latter system is characterized as connective tissues throughout the body, such as bone, muscle and cartilage. A malignant cancer of mesenchymal cells is a type of sarcoma.

Mesenchyme is characterized morphologically by a prominent ground substance matrix containing a loose aggregate of reticular fibrils and unspecialized cells. Mesenchymal cells can migrate easily, in contrast to epithelial cells, which lack mobility and are organized into closely adherent sheets, and are polarized in an apical-basal orientation.

The first emergence of mesenchyme occurs during gastrulation from the epithelial–mesenchymal transition process. This transition occurs through the loss of epithelial cadherin, tight junctions, and adherens junctions on the cell membranes of epithelial cells. The surface molecules undergo endocytosis and the microtubule cytoskeleton loses shape, enabling mesenchyme to migrate along the extracellular matrix (ECM). Epithelial–mesenchymal transition occurs in embryonic cells that require migration through or over tissue, and can be followed with a mesenchymal–epithelial transition to produce secondary epithelial tissues. Embryological mesenchymal cells expresses fibroblast-specific protein (Fsp1), which is indicative of their shared properties with the migratory adult fibroblasts, and c-Fos, an oncogene associated with epithelial cadherin, down-regulation. Both formation of the primitive streak and mesenchymal tissue is dependent on the Wnt/β-catenin pathway. Specific markers of mesenchymal tissue include the additional expression of ECM factors such as fibronectin and vitronectin.