間葉系細胞の発生・分化

Abstract

The mesencyme, known as the embryonic connective tissue, is a non-epithelial component of the mesoderm which is formed by the proliferation of ectodermal cells on the primitive streak and their subsequent migration between the ectodermal and endodermal germ layers. Mesenchymal cells can differentiate into various kinds of tissue cells such as blood cells, fibroblasts, cartilage cells, osteoblasts, muscle cells, and etc. Here some morphological features of cell differentiation from the mesenchymal cell are described and discussed at the level of ultrastructure.The mesenchymal cells are characterized by a large nucleus and a narrow cytoplasm with abundant free ribosomes and few cell organelles. The first sign of differentiation of the mesenchymal cells is an increase of the cytoplasm and a well-development of granular endoplasmic reticulum and the Golgi apparatus. This may well explain that the tissue cells derived from the mesenchyme produce in general abundant intercellular matrix such as collagen fibers, elastic elements, and amorphous proteoglycan, except for striated skeletal muscle cells. In their differentiation into the skeletal muscle cells, the mesencymal cells fuse to form the myotube which is a multinucleated cylindrical cell. In the myotube, myofilaments are increasingly produced among free ribosomes in the cytoplasmic matrix, but granular endoplasmic reticulum is not prominently increased. After formation of myofibrils, however, a few granular endoplasmic reticulum begins to produce the sarcoplasmic reticulum surrounding indivivual myofibrils.