The follicular cycle in a human portrayed through photomicrographs


View Figure  		Fig. 1  Embryogenesis of epithelial structures of adnexa at about 10 weeks" gestation. On either side of an eccrine germ, and continuous with the evolving epidermis, are germs and papillae of future folliculo-sebaceous-apocrine units.

View Figure  		Fig. 2  Early anagen. At the base of the isthmus is seated a follicular germ, and beneath it resides a follicular papilla. Those two structures, requisite for development of an inferior segment of a follicle, are analogous to the germ and papilla in an embryo that together are responsible for development of an entire folliculo-sebaceous-apocrine unit. Note that bulges of the isthmus are situated well above the follicular germ and papilla.

View Figure  		Fig. 3  Fully developed anagen. The bulb of a follicle at the prime of anagen encompasses a follicular papilla and consists of matrix, a not yet cornified hair, an inner sheath that displays trichohyalin granules, and an outer sheath that consists of pale and clear cells. Hair and inner sheath represent maturation of matrical cells, but the outer sheath does not derive from the matrix; it results from maturation of germinative cells seated at the base of the isthmus and at the periphery of it and it descends to ensheath the inner sheath and the matrix.

View Figure  		Fig. 4  Early catagen. The follicular papilla no longer is enveloped partially by a bulb in ball and claw fashion; the interface between papilla and shortened column of lower segment of a follicle has become flat. A thickened basement membrane surrounds the involuting follicle, especially the lower part of it.

View Figure  		Fig. 5  Moderately advanced catagen. A thickened corrugated basement membrane is present along a shortened, thin column of retreating lower segment of a follicle and separates it from scattered fibrocytes of a poorly defined follicular papilla.

View Figure  		Fig. 6  Far advanced catagen. A pincer of epithelial cells at the base of a markedly shrunken lower segment of a follicle encloses a follicular papilla. Beneath the unit of epithelial and nonepithelial cells, which is surrounded by a strikingly thickened, corrugated basement membrane, stretches a tract of fibrous tissue.

View Figure  		Fig. 7  Late catagen. What is left of the lower segment of a follicle resembles a normal isthmus. No discrete papilla can be identified, only scattered fibrocytes at the base of the paltry remains of the lower segment.

View Figure  		Fig. 8  Telogen. A follicle at rest consists only of an upper segment, namely, infundibulum and isthmus. Columnar cells at the periphery of the isthmus are aligned in a palisade. At the base of the isthmus are positioned a few scattered fibrocytes that constitute an ill-defined follicular papilla. The short bent columns of undifferentiated epithelial cells that emanate from the junction of infundibulum and isthmus are mantles, i.e., anlagen and residua of sebaceous glands and ducts.

View Figure  		Fig. 9  Very early anagen. Beneath a follicular germ-like structure situated at the base of an isthmus resides a follicular papilla. The germ-like structure is characterized at its periphery by columnar cells aligned in a palisade, and in its center by cells with crowded monomorphous oval nuclei, at least one of which is in mitosis. The structure resembles a germ in an embryo that eventuates in a fully formed folliculo-sebaceous-apocrine unit.

View Figure  		Fig. 10  Early anagen. An elongated follicular germ-like structure is continuous with the isthmus and its arc-like base is contiguous with a discrete follicular papilla.