Where do mitotic cells in the hair begin their growth activity?

The correct answer identifies the matrix as the location where mitotic cells in the hair initiate their growth activity. The matrix is a specific area located at the base of the hair follicle, within the bulb. It is responsible for producing new hair cells through a process known as mitosis, which is essential for hair growth.

Within the matrix, cells rapidly divide and contribute to the formation of the hair shaft. These newly formed cells push older cells upwards, leading to hair elongation as the cells keratinize and die, eventually forming the structure of the hair that we see above the skin's surface.

Although the follicle, bulb, and dermis are all important components related to hair growth, the matrix is specifically where the active cell division that leads to the formation of new hair occurs. The follicle houses the bulb and the matrix, but it is the matrix itself that is crucial for initiating the mitotic activity.