Final answer:
The cortex is the middle layer of the hair responsible for hair's length and texture, and hair follicles are the source of hair growth. Hair is composed mainly of dead keratinocytes, and its structure influences various homeostatic functions. Cutting or removing hair does not cause pain because the hair shaft is composed entirely of dead cells.
Step-by-step explanation:
The cortex in hair is the second or middle layer of keratinocytes originating from the hair matrix, which is largely responsible for the hair's length and texture. Surrounding the cortex is the cuticle, the outermost layer of keratinocytes that serves as a protective barrier. The basal cells of the hair bulb in the follicle divide and push cells outward, forming the hair shaft as the hair grows. The hair texture, whether straight or curly, is influenced by the shape and structure of the cortex and the medulla, when present.
Hair growth begins with the production of keratinocytes by the basal cells of the hair bulb. These cells are then keratinized as they are pushed toward the surface, forming the hair shaft. As hair is composed entirely of dead keratinocytes filled with keratin, it lacks sensation, which is why cutting hair does not cause pain. Different hair removal techniques affect the hair structure differently, with some targeting the hair bulb to prevent regrowth.
Hair follicles are the structures where hairs originate, with each hair consisting mainly of tightly packed, dead keratinocytes. The hair provides various homeostatic functions including insulation, protection from UV radiation, trapping particles and microorganisms, and sensory input. Additionally, characteristics like hair color and texture are genetically controlled.