Catagen in the hairless house mouse

A morphological and histochemical study was made of some aspects of the catagen phase of the first hair cycle in hairless (hr) mice. The histochemical tests used included those for sulphydryl and disulfide groups, collagen, and periodic acid‐Schiff‐reactive materials. Measurements were made of skin thickness, follicle length and for follicles in catagen VI and VII, epithelial column length, length of hair shaft in the skin, the width of glassy membrane and connective tissue sheath, and the width of collagen in the glassy membrane. The results showed that: The mutant epithelial column shortens at a slower than normal rate so that at catagen VII, the total follicle length is greater than normal. The position of the mutant hair shaft in the skin is normal at this stage. The keratinization sequence of the mutant club appears normal. The failure to form an anchoring brush is associated with a morphological variation in the cell shapes and fibrils of the club. The inner root sheath of the mutant follicle, although apparently keratinizing in the normal sequence, extends further up the follicle than normal and also further down, enclosing the club. The glassy membrane and connective tissue sheath of the mutant are abnormally narrow due mainly to a marked reduction in collagen, and to a narrower band of PAS‐positive material.