The hair follicle is an integral component of the skin, and each hair is a keratinized product of the follicle. Each and every hair follicle undergoes a cycle of activity - hair grows to a maximum length, then hair growth ceases and the hair is shed and replaced. This characteristic three-phase cyclical growth pattern of hair is the result of follicular activity, and is not obvious by casual inspection. The three phases of the hair growth cycle have been described as:
A - Anagen (active growth - a long period lasting 2 to 8 years and more, depending on ivdividual characteristics);
B - Catagen (regression - a transitional period from growing to resting lasting 2 to 4 weeks);
C - Telogen (rest - a period of inactivity lasting 2-4 months);
Also lately researchers point to three more phases in addition to above "classic" three, differentiating a total of 6 hair growth phases. The fourth one is called Teloptosis or Exogen - an actual shedding of the "old" hair shaft phase, normally a very short period of several days, however sometimes not shedded hair fiber may remain intact as long as 6 months! The control mechanism for Teloptosis or Exogen is quite independent of Catagen and Telogen because it is not unusual for humans to retain the Telogen hairs for more than one follicular cycle. Although Teloptosis or Exogen deals with the shaft base and not directly with the hair follicle, Teloptosis or Exogen and Anagen stages influence one another. The fifth stage is called Kenogen. During Kenogen, the hair follicle simply rests physiologically for certain time without producing new hair growth. In other words, in addition to the classical cycle, the hair follicle may follow an alternative route during which the Telogen phase, not accompanied by a coincident new early Anagen, ends with Teloptosis or Exogen leaving the follicle empty from some days up to even 9 month. And most recently one more phase of Neogen has been established to exist to reflect in full the entire process of degeneration-regeneration of healty follicle. Based on assumption of three cklassic hair follicle cyclses of Anagen, where follicle undergoes succesive steps of fibre production; Catagen, or regression, and Telogen, or rest, which last on average for 3 years, 3 weeks an 3 months each accordingly, while during next side phase of Exogen when "old" fibre is actively released, it has no direct consequence on new Anagen phase of follicle initiation. After old hair is out, latency period is observed in 80% of hair cycles, between elimination of a hair in Exogen and the appearance of the replacement hair in Anagen. The duration of this period, or Kenogen, varies from 2 to 5 months on average. And if regression stage of Catagen designates the shift from Anagen to Telogen, there is also another shift from Telogen to next Anagen, which is represented by regeneration stage of Neogen. So. the entire process of resting, regeneration, fibre production and regression would thus include four main successive phases, namely Telogen, Neogen, Anagen and Catagen. Two of those are very short, Neogen and Catagen, and two are quite long, Telogen and Anagen.

Although we speak of phases in the development of a hair follicle, it should be made clear that these are actually dynamic, flowing processes and the setting up of phases is purely for our understanding of the human hair follicle cycle.

Anagen is an active phase of the hair growth and extends from the termination of the inactive phase of Telogen to the beginning of the regressing phase of Catagen. Anagen involves the complete re-growth or regeneration of the lower, cycling portion of the follicle, i.e., the hair shaft factory. The epidermal cells surrounding the dermal papilla form the germinal matrix or root of the hair. These cells are constantly dividing, and as new cells are formed they push the older ones upwards and eventually out, so eventually hair shaft emerges and dislodges old Telogen hair and comes out over the skin surface. During this phase the hair grows about 1 cm every month. Because there is a limit to the time a follicle stays in Anagen, there is also a limit to the length of its product, the hair shaft.

Catagen is the transitional phase in the hair growth cycle, and there are chemical and structural changes that take place in the hair follicle during this phase. As soon as the growth phase (Anagen) is complete, follicle growth stops and degeneration begins. The hair follicle goes through a highly controlled process of involution, which is a process of progressive decline or degeneration. The involution process largely brings about a burst of programmed cell death (Apoptosis) in the majority of follicular keratinocytes. Follicular melanogenesis (formation of melanin) also ceases during this stage, and some follicular melanocytes undergo Apoptosis as well. Simultaneously there is a progressive shrinkage of the lower follicle, which withdraws as an epithelial strand. Towards the end of the Catagen phase, the dermal papilla condenses and moves upward, coming to rest beneath the hair follicle bulge.

Telogen is considered as the resting phase or the period of quiescence in the hair follicle cycling process. When hair follicles enter Telogen and stop making hair, they dump their last cells on to the end of the fiber. This lump of cells acts as an anchor to hold the hair fiber in the tube of the hair follicle. The hair is now called a "club" hair as the lump of cells on the end gives it a club appearance. This non-living hair is attached to the skin with a "club-like" root, but will eventually be pushed out or pulled out during combing or washing and replaced by a new growing hair. The Telogen phase typically lasts for two to three months before the scalp follicles enter the Anagen phase and the cycle is repeated, normally 5 to 15 percent of scalp follicles can be in the Telogen phase at any one time.

Hair growth stages: 1 - hair dermal pailla; 2 - mature Anagen; 3 - Catagen; 4 - Telogen; 5 - dermal papilla goes to new regrowth Anagen while hair bulb enters shedding; 6 - early new hair regrowth Anagen; 7 - older hair fiber shedding; 8 – new hair mature Anagen.

First diagram is showing regression of a mature Anagen hair follicle. On entering Catagen the dermal papilla condenses as the cells become inactive. With a lack of dermal papilla cell stimulation, the hair fiber and root sheaths stop growing. In Telogen the dermal papilla can become isolated in the dermis and the hair fiber can easily be pulled out (by combing, shampooing, or brushing). Second diagram is showing a resting hair follicle returning from resting Telogen to growing Anagen. If the old fiber has not already fallen out it is pushed out by the new hair fiber growing underneath.