What is described as the lowest part of a hair strand, forming the lower part of the hair root?

The hair bulb is an essential part of the hair structure, located at the lowest part of the hair strand where the hair root begins. It serves as the enlarged, round portion at the base of the hair follicle and is responsible for the generation of new hair cells. In the bulb, dermal papillae are present, which are rich in blood vessels and provide nutrients and signals that promote hair growth. Since the bulb contains actively dividing cells that produce the hair shaft, it is critical to the overall growth cycle of hair. This function establishes the hair bulb as the foundational component from which hair emerges and continues to grow.

In contrast, the hair follicle surrounds the hair bulb and provides a protective sheath, while the hair shaft is the visible part of the hair that extends above the skin's surface. The cortex is the middle layer of the hair shaft that provides strength and color but does not pertain to the hair's root structure. Thus, identifying the hair bulb as the lower part of the hair root accurately captures its biological significance in the hair growth process.