Muscle fiber nuclei

Satellite cells

The solution to this paradox (mitosis without easily visible chromosomes) is the fact that several types of nuclei occur among the several hundred located along each centimetre of fiber length.

True muscle fiber nuclei (a) are located within the sarcoplasm. But, although the nuclei of satellite cells appear by light microscopy to be located within the sarcoplasm, electron microscopy shows that satellite cells are located in depressions in the fiber surface.

Thus, the nucleus of a satellite cell (b) is separated from the sarcoplasm of its muscle fiber by a satellite cell membrane and a muscle fiber membrane, which cannot be seen easily by light microscopy, as in image B.).

The existence of satellite cells was established properly by electron microscopy (although some light microscopists had already found them).

1 = satellite cell, 2 = depressed muscle fiber, and 3 = endomysium wrapping around both the muscle fiber and its satellite cells.

The other types of nuceli that may appear are fibroblasts (c) involved in making the connective tissue fibers of the endomysium.

Endothelial cells (d), here shown forming a small capillary on the muscle fiber surface.

Pericytes (e) are mesodermal cells found around very small blood vessels. They contain actomyosin and are probably capable of contraction and phagocytosis.

Perhaps an important point to stress is that finding the membrane around the satellite cell is tricky, even with electron microscopy.

Here the separation runs from top left corner to bottom right corner, with the satellite cell nucleus on the right, and this is a good separation!

Increase in number