The different phases of the hair cycle


Hair biology
Home Contact Us


Hair cycling

Research findings have recognized three main phases of the hair growth cycle – a growth phase (anagen phase I-VI), a regression phase (catagen), and resting phase (telogen). Finely tuned changes in the local signalling milieu in the skin control these cyclic transformations, while the cycles are based on changes in expression of cytokines, hormones, neurotransmitters and their receptors as well as transcription factors and enzymes, which act via endocrine, paracrine or autocrine routes.

The hair cycling procedure parallels morphogenesis in that it involves multiple signalling events incorporating developmental pathways - the same reoccur during the different hair cycle stages. The hair cycle includes a complex remodelling and regenerating of the complete non-permanent portion of the hair follicle. In addition to the hair follicle epithelium, the mesenchyme, the extracellular matrix, vasculature and nerve innervation, and the hair follicle-associated cell populations also undergo dramatic transformations. In the species that have synchronized hair follicle cycling (like mice and rats), the architecture and physiology of the entire skin, including its immune and gene expression status undergo substantial changes. Here the changes are the result of prior switches of multiple hair follicles from one stage of their cycling phase to another.

Anagen

The anagen phase, experiences the active growth of hair and deposit of materials in the hair shaft by cells found in the hair follicle. There are certain metabolically active and dividing cells above and around the dermal papilla of the follicle - they grow upward during this phase to form the hair shaft. The anagen phase is chracterized by the growth of the hair, but more importantly, it is characterized by a highly increased proliferative rate of all hair follicle cells in all epithelial compartments with the highest activity observed in matrix cells. Anagen duration, which is genetically predetermined, varies with the size of the hair follicle and part of the body where it is located. An anagen growth phase may last several years and the terminal hair follicle on the scalp is a proof of this. It may also persist for just a few weeks in terminal hair follicles on the extremities – particularly on the legs.

Catagen

The anagen phase is followed by catagen phase which is short stage in which the hair follicle regresses and dismantles the hair growing part of the hair follicle. This phase can be divided into eight subphases starting with late anagen and ending in early telogen. The hair follicles undergo regression during this phase and this phenomenon is characterized by a cessation of protein and pigment production, involution of the hair follicle and a fundamental restructuring of the extracellular matrix. Also, massive apotosis in the bulbar, transient, portion of the hair follicle contributes to regression of the hair follicle and the formation of a fibrous streamer in the skin. The onset of these apoptotic events seems to be predetermined and finely orchestrated, and as such the events in this phase can be more appropriately described by the term, “programmed cell death”. Club hair is formed when the part of the hair follicle in contact with the lower portion of the hair becomes attached to the hair shaft. Catagen is the first component of the initial hair cycle occurring after first morphogenesis of the hair follicle.

Telogen

The regression continues with the telogen phase - now the hair follicle has regressed to about half of its previous size and does not extend beyond the upper dermis. Morphologically, a “finger” of epithelial cells overlies a cluster of dermal fibroblasts; the remains of the dermal papilla. In this phase the dermal papilla ceases to be enveloped by surrounding epithelial cells. It now sits as a small ball of cells in close association with the epithelial cell finger. The epithelial cells of the lower telogen follicle are characterised by the absence of DNA or RNA synthesis. There is also no trace of any synthesis of proteins characteristic of the anagen follicle, such as trichohyalin and the hair cortical keratins. It is however noteworthy that, Keratin 14 (K14) synthesis still continues in the epithelial sac to which the telogen hair fiber anchors. In telogen follicles the volume of the dermal papilla extracellular matrix is noticeably reduced. Then the dermal papilla cells become relatively quiescent having scant cytoplasm. The telogen hair shaft, the club hair, can be retained for months in this epithelial sac.

Exogen

Current research views the shedding of the hair fiber as a highly controlled, active process. According to modern research views, the process differs from the quiescence normally found during the telogen phase. As such, the term “exogen” is used to describe this event as a specific process of its own during hair follicle cycling. The former studies in this regard assumed that the newly formed hair fiber pushes the resting shaft outward to effect hair shedding - the recent more detailed studies proves it as unlikely. Major morphological differences become apparent when a shed hair shaft root is compared to a plucked telogen hair shaft root by light or electron microscopy. The telogen root is composed of packed nucleated cells which show intracytoplasmic fractures surrounding a cornified core making up the shaft. On the other hand, the exogen root is made of very few cells which are separated at their outer edge by intercellular cleavage. The morphology of the hair root suggests that the exogen process involves a proteolytic event which occurs between the moving cells of the telogen shaft base. What makes this shedding process possible is still unknown. A possible role of desmoglein and proteolytic events was suggested by Koch et al. (1998).

Kenogen

Scientists observed a novel phenomenon in hair cycling when they used phototrichograms to study hair growth - the empty hair follicles were noticed even after shedding of the hair fiber. Thus the term “kenogen” was coined to describe the interval of the hair cycle in which the hair follicle remains empty after the telogen hair has been extruded and before a new anagen hair reappears. It is normal to observe Kenogen in healthy skin; but in men and women with androgenetic alopecia the frequency and duration of kenogen have been reported to be greater.

  Copyright Hair Biology Com
Disclaimer | Copyright | Privacy Policy
Hair Biology