I’ve watched TV documentaries that show people’s pets receiving chemotherapy. It doesn’t seem to cause them to lose their fur or hair, unlike humans. Why?
• Follicles go through cycles of growth, degeneration and rest, and then regeneration and another growth phase. During growth, the follicle cells divide to create the hair shaft. The natural length of this shaft is controlled by the extent of this phase. To conserve energy, in most animals, growth stops when the shaft reaches an appropriate length, the follicle degenerates, the hair shaft is held in the skin and the follicle goes into its rest phase.
Chemotherapy stops cells from dividing and therefore doesn’t damage resting hair follicles, which is probably why animals don’t usually lose hair. However, they might if treatment took place during their moulting cycle when new hairs are growing.
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The human head is unusual in that hair growth cycles are asynchronous. At any time, only about 10 per cent of follicles are resting, with the remainder in a growing phase that continues for up to seven years. This is why human head hair, unlike that of most other animals, can reach more than a metre in length if left uncut, and why chemotherapy in humans affects these follicles, resulting in hair loss.
Paul Kemp, Manchester, UK
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