Critical Stages of Hair Development and Pigmentation in the Mouse

Abstract

The anagen, or proliferation phase, of the hair cycle is divided into 6 substages. Briefly these are as follows anagen I, first cell divisions appear in previously dormant germ plate; II, down growth of germ plate forms beginning of papilla cavity, and internal sheath first appears as a dome; III, definite bulb of matrix cells now formed from germ, and first melanogenic pigmentary dendritic cells appear in this bulb; IV, hair shaft forms from bulb, and pigment granules are found in cells of bulb around apex of papilla cavity (typical of 6 days after plucking); V, tip of hair is near skin surface, and the papilla cavity is compressed; VI, hair emerges and is produced at rate of nearly 1 mm./day for about 9 days. The subsequent stage, the catagen, is transitional between the anagen and resting telogen. In the catagen, cell divisions in the bulb cease and the bulb essentially inverts. The result is a club, a dormant plate of cells which is the germ, and a sphere of papilla cells below the germ. Emphasis is placed on the similarity of the Malpighian layer of the skin epidermis, the external sheath, and the derived Malpighian layer of the bulb and its germ. The dynamics of development as well as the distr. of pigmentary dendritic cells are basically alike in these Malpighian layers.