Final answer:
Vellus hair converts to terminal hair primarily due to hormonal changes, which are often linked to puberty, pregnancy, or specific hormonal conditions. This transition is influenced by androgens and varies among individuals.
Step-by-step explanation:
The only way vellus hair will convert to terminal hair is through hormonal changes, such as those associated with puberty, pregnancy, or conditions that alter hormone levels. This process is a part of the normal growth cycle controlled by androgens, which are a group of hormones including testosterone. Vellus hair is soft, fine, and short, often referred to as 'peach fuzz,' while terminal hair is thicker, longer, and darker.
Hair growth begins inside a follicle where stem cells divide and produce keratinocytes, which are the cells that make up the bulk of the hair shaft. The transition of hair types is influenced by genetic factors as well as the hormonal environment, which can differ substantially among individuals. For instance, during puberty, increased androgen levels typically cause vellus hair in certain body areas to transition into terminal hair.