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  • The hair follicle is composed by two segments: upper, or permanent, and lower or dynamic, subject to changes that occur during the follicular cycle.

  • The hair shaft consists of three structures: cuticle, cortex, and medulla.

  • The follicular cycle is composed by three phases: anagen or growth phase; catagen or involution phase; and telogen or resting phase.

  • Hair follicles appear around on the scalp in the fourth month of intrauterine life.


  • Hair follicles undergo a growth cycle characterized by periods of activity, during which they produce hair, and periods of rest.

  • The activity of each follicle is independent from that of other neighboring follicles, so the different follicles are in different phases of the cycle. This concept is very important for the diagnosis and management of hair disorders.


  • Treatment should always be leaded by the involved phase of the hair cycle.


  • The hair follicle in the anagen phase is metabolically active and is therefore more susceptible to pathogenic noxae, in a directly proportional way to the severity of the damage itself (anagen effluvium).


  • Knowledge of the anatomy of hair follicle is fundamental to better understand the pathophysiology of hair diseases.


The hair follicle is composed by two segments: upper, or permanent, and lower or dynamic, subject to changes that occur during the follicular cycle.

The upper segment extends from the follicular ostium to the insertion of the erector pili muscle and consists of two parts: the infundibulum and the isthmus, the area where the erector pili muscle attaches and the opening of the sebaceous gland.

The lower segment extends from the insertion of the erector pili muscle to the bulb, located in the hypodermis in the terminal anagen follicles and in the middle dermis in the vellus follicles.



Upper Segment

The infundibulum extends from the skin surface to the point where the sebaceous duct enters the follicle. It is lined with an epithelium that is morphologically similar to that of the epidermis. The portion of the infundibulum that crosses the epidermis is known as acrotrichium and is similar to the intraepidermal part of an eccrine duct.

A saprophytic microbial flora is normally present in the infundibulum, especially in the scalp and upper part of the trunk. The most important commensals of the follicle are Staphylococcus epidermis, Cutibacterium acnes, Malassezia sp. and Demodex sp. The isthmus is delimited above by the insertion of the sebaceous glands and below by the insertion of the erector hair muscle. It represents a very important transition zone during the follicular keratinization process: at the isthmus the internal epithelial sheath desquamates and ...

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