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The mesenchyme is not one of the primary germ layers (ectoderm, endoderm, mesoderm) of the developing embryo). It is a special type of undifferentiated connective tissue found in the early embryo. Mesenchymal cells are fairly uniform small spindle-shaped cells. They are connected to other cells by processes that form a three dimensional cellular network.
Most mesenchyme is derived from the mesoderm. Some mesenchyme is derived from neural crest cells and thus originates from the ectoderm (many embryologists use the term mesenchyme only for those cells that develop from the mesoderm).
All connective tissue of the body is derived from the mesenchyme. Muscle, vascular and urogenital systems, and the serous membranes of body cavities are also derived from mesenchyme. Mesenchymal cells have the character of stem cells and can differentiate into osteoblasts, adipocytes, myoblasts, or chondroblasts (Prockop, 1997; Pittenger et al, 1999).
The term mesenchyme essentially refers to the morphology of embryonic cells. Mesenchymal cells can migrate easily, unlike epithelial cells, which do not show great mobility and which are polygonal in shape, polarized in an apical-basal orientation, and organized into closely adherent sheets.
Mesenchymal cells perform a function that is essential for the progression of tissue development as they can migrate to a particular region of the developing embryo that will give rise to a particular tissue. The epiblast part of the inner cell mass of a developing embryo is at first an epithelial cell sheet and thus consists of relatively immobile cells.
Epithelial cells have been shown to possess the capacity of changing into mobile mesenchymal cells. Likewise, mesenchymal cells can change into epithelial cells. This is accompanied by the controlled downregulation (EMT) or re-expression (MET) of cell adhesion molecules (Duband et al, 1995; Hau et al, 1995; Viebahn et al, 1995). The distinctive characteristics of epithelial cells are lost not only when they change into mesenchymal cells during embryogenesis but also after neoplasia into invasive carcinoma cells.
The change of epithelial cells into mesenchymal cell type is known as epithelial-mesenchymal transition. Frequently, the terms epithelial-mesenchymal interactions, epithelial-mesenchymal transdifferentiation or epithelial-mesenchymal transformation (abbr. EMT) are used interchangeably to refer to the same process. The change of mesenchymal cells into epithelial cells is being referred to as mesenchymal-epithelial transformation (abbr. MET) or mesenchymal-epithelial transition. Note that the term "transition" is the more neutral one as transformation carries the connotation of oncogenic conversion, whereas transdifferentiation classically refers to differentiated cells changing into other differentiated cells.
Krawetz and Kelly (2008) have shown that the development of primitive endoderm from cells in the inner cell mass is regulated by one member of the Wnt family of structurally related genes that encode secreted extracellular signaling factors, Wnt-6. Sites of epithelial remodelling and epithelial-mesenchymal transition are marked by Wnt-6 expression (Schubert et al, 2002).
For related information of interest see also: Cell types, Cell lines in Cytokine Research, Cell culture.
Copyright © 2012 by H IBELGAUFTS. All rights reserved.
ENTRY LAST MODIFIED: April 2008
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