Haarbulbus

Haarbulbus, literally “hair bulb,” is the enlarged base of a hair follicle located within the dermis or at its upper boundary. It surrounds the hair papilla and the hair matrix, the region where cells actively divide to form the hair shaft and the inner root sheath. The bulb also contains melanocytes that contribute to hair color by distributing pigment to the growing shaft.

The dermal papilla within the Haarbulbus is a small, highly vascular cluster of fibroblasts and other cells that supplies nutrients and signaling molecules to regulate follicle growth and cycling. The hair matrix, composed of rapidly proliferating keratinocytes, differentiates as it moves upward to produce the outer and inner root sheaths and ultimately the keratinized hair shaft.

Throughout the hair growth cycle, the Haarbulbus undergoes dynamic changes. In the anagen phase, matrix cells proliferate to build the hair shaft; in catagen and telogen phases, activity declines and the follicle rests or sheds the club hair. The interaction between the matrix and the dermal papilla helps determine cycle timing, follicle size, and pigment incorporation.

Clinical relevance exists because the integrity and activity of the Haarbulbus are essential for sustained hair production. Damage to the bulb region can lead to hair shedding or alopecia, and external agents that affect rapidly dividing matrix cells, such as certain chemotherapies, commonly cause transient hair loss. Aging can also reduce matrix activity, contributing to thinning hair.