Author details

Bilgen Erdoğan

hair and finally shed. The follicle remains in this stage until the hair germ which is responsive to anagen initiating signals from the dermal papilla, starts to show enhanced proliferative and

Telogen is one of the main targets of hair cycle which is influenced by several modulatory agents like androgens, prolactin, ACTH, retinoids and thyroid hormones [40]. No unique molecular markers associated with the telogen follicle are determined yet; however, estrogen receptor expression is reported to be limited to the telogen papilla fibroblasts. Germ cells of telogen follicles also express basonuclin and FGF-5 [33]. The bone morphogenic protein-4 (BMP-4) as a growth factor plays an essential role in suppressing follicular growth and differ-

The macro-environment surrounding the hair follicle also takes part in regulating cycle transitions. BMPs in the subcutaneous fat are capable of maintaining follicles in a "refractory" telogen, and cessation of this inhibitory activity by BMPs enables the follicle to progress to a "competent" telogen with a hair germ that is responsive to anagen-initiation signals and

There is less interest for the mechanism of the hair shedding but from the patient's perspective it is probably the most important part of the hair growth. It is not unusual for human telogen hairs to be retained from more than one follicular cycle and this suggests that anagen and exogen phases are independent. The shedding period is believed to be an active process and independent of telogen and anagen thus this distinct shedding phase is named exogen

Based on the observations: the hair follicle has no need for intact innervation, vascularization or other extrafollicular components to maintain cycling, and the basic oscillator system which controls hair cycling is located presumably in the follicle [42]. The principal challenge is to define the underlying "oscillator" system. Probably, the hair cycle clock is controlled by regulating the balance of the interactions between the follicle epithelium and the surrounding mesenchyme. This might be provided by the rhythmic secretions of growth/modulatory signals from follicle epithelium or mesenchyme as well as the rhythmic alterations in the expres-

In this chapter, the basic anatomy and the amazing and complicated biology of the hair follicle is reviewed. Enhanced knowledge on the normal dynamics of the hair provides understanding the basis of how the follicle behaves during a disease. However recent progress in our understanding of the biology and pathology of hair follicles should lead more effective therapies for

transcriptional activity in late telogen, leading to the initiation of anagen [2, 39].

entiation at telogen stage [16].

4.1.4. Exogen

24 Hair and Scalp Disorders

[16, 33].

4.2. Hair cycle clock

5. Conclusion

hair disorders.

sions of corresponding receptors [40].

capable of entering a new anagen phase [2, 41].

Address all correspondence to: bcakil@hotmail.com

Bakırköy Dr. Sadi Konuk Research and Training Hospital, İstanbul, Turkey
