[12/30, 8:57 AM] [email protected]: A "complex miniorgan," the hair follicle generates hair shafts from terminally differentiated keratinocytes12. The hair follicle forms the pilosebaceous unit along with the sebaceous gland, apocrine gland, and arrector pili muscle (APM). It has also recently been demonstrated that eccrine glands are incorporated into the pilosebaceous unit13, 14. Scalp hair follicles aggregate to create compound pilosebaceous units, which are made up of one or more secondary follicles connected to a single APM and sebaceous gland in addition to one parent follicle11. A common infundibulum allows each follicular unit's hairs to emerge collectively11.
A person's hair follicle count ranges from 2 to 5 million on average, with 100,000 of those being on the scalp. Generally speaking, Caucasians with blonde hair have a higher hair density than those with dark hair, who have a
[12/30, 8:58 AM] [email protected]: A "complex miniorgan," the hair follicle generates hair shafts from terminally differentiated keratinocytes12. in connection with the arrector, apocrine gland, and sebaceous gland
A person's hair follicle count ranges from 2 to 5 million on average, with 100,000 of those being on the scalp. Caucasians with blonde hair usually have more hair than those with dark hair, who in turn have more hair than those with red hair, African Americans, and Asians15–18. Ectoderm, neuroectoderm, and mesoderm tissues are integrated into the complex structure of the hair follicle. Anagen, catagen, and telogen are the stages of the hair cycle that epidermal and mesenchymal cells cycle through as they integrate, multiply, differentiate, and proliferate. During anagen, the hair fiber develops and lengthens; it is then held during catagen and early telogen.
[12/30, 8:59 AM] [email protected]: A "complex miniorgan," the hair follicle generates hair shafts from terminally differentiated keratinocytes12. in connection with the arrector, apocrine gland, and sebaceous gland
Humans typically have two to five million hair strands, which are exogenously shed mid-telogen. Kenogen refers to the late-telogen phase, where the follicle is empty, following exogen and preceding the start of the subsequent anagen12,19–22. The primary factor influencing hair length is the duration of the anagen phase, since the rate of linear hair growth is rather stable throughout life. Hair density is influenced by kenogen time, but not by length.
During catagen, the lower, or "bulb," part of the hair follicle is remodeled, a process known as hair cycling. The higher, non-cycling section of the hair follicle is home to the
[12/30, 9:00 AM] [email protected]: A "complex miniorgan," the hair follicle generates hair shafts from terminally differentiated keratinocytes12. in connection with the sebaceous gland, apocrine gland, and the sebaceous gland duct that divides the isthmus and infundibulum. At the lowest part of the isthmus, where the APM enters, the hair bulge is situated on the outer root sheath (ORS). The inner root sheath creates a protective milieu for the bulge and acts as a mechanical barrier between the isthmus and the outer world. For hair follicle keratinocytes, melanocytes, and arrector pili myocytes, the bulge serves as a stem cell niche23, 24. Bulge keratinocyte stem cells encourage bulb regeneration by activating during the telogen-anagen transition. Transient amplifying cells move downward and multiply quickly.
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