Mesenchymal tissue may be present in the form of loose or dense connective tissue, cartilage, smooth and striated muscle cells, fat cells and occasionally bone. Cartilage is most readily recognized as it occurs in the form of discrete aggregates of chondroid cells surrounded by hyaline matrix (Figures 13.1 and 13.2). Such foci of cartilage-nous differentiation are usually distributed at random and may be adjacent to other mesenchymal derivatives, ectodermal derivatives such as neural tissue or choroid plexus, or endodermal derivatives such as mucin-secreting glands.
Mesenchymal tissues may be composed of nondescript spindle cells, corresponding to fetal or adult connective tissue (Figure 13.3). Within such condensed mesenchymal tissue one may focally see areas of cartilage, bone, or striated muscle formation.
With the use of immunohistochemistry, such foci of abortive differentiation can be better visualized. For example, by using antibodies to desmin and myoglobin one may see foci of striated muscle differentiation (Figure 13.4).
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