In this text the authors use a thick-tissue preparation method for observing hair at various stages of growth. They put forward a hypothesis - the four-stage essential hair cycle - that incorporates their discovery that stem cells are located in the isthmal portion of the sebaceous gland.

In this text the authors use a thick-tissue preparation method for observing hair at various stages of growth, and their findings challenge the three-stage common hair cycle. They put forward another hypothesis, the four-stage essential hair cycle, that incorporates their discovery that stem cells are located in the isthmal portion of the sebaceous gland. The essential hair cycle is then used to review the factors contributing to the process of androgenetic alopecia, or premature balding, as it is commonly called. This is followed by an explanation of some general measures for the prevention of androgenetic alopecia and a description of the treatments available.