[Current concepts on the "dark" cells of human and animal brains]

The latest current literature is reviewed on the dark cells of the brain and internal organs of animals and man. The following questions are considered: shrinked and picnomorphic neurons; dark neurons and glyocytes--structure and function; and perspectives of research into the problem of dark cells. A conclusion has been made that in most cases dark cells of the animal and human nervous tissue constitute functionally conditioned elements emerging during the life span.