The Hair

The base of each hair shaft (HS) 

Resides in a multicellular miniorgan called hair follicle (HF) that is invaginated under the surface of the skin. 

The Hair follicle (HF)

The HF has a wide range of functions including:

Thermoregulation, physical and immunological pro- tection against external insults, sensory perception, social inter- actions, and camouflage. 

One of the most characteristic features of HFs is that they self-renew during hair cycle (HC) throughout the entire life of an individual to continuously produce new HSs.

Hair follicle morphogenesis

•HF morphogenesis is induced by a local thickening of the epidermis named the placode. 

•mesenchymal cells (pink color) condense underneath the placode, leading to the formation of the hair germ, also called the follicular hair bud. 

•The hair germ proliferates and invaginates into the dermis to form the hair peg and the bulbous peg. 

•The superficial portions of the hair follicles subsequently develop 2 distinct, asymmetrical accumulations of cells that form the sebaceous gland (purple color) and the bulge representing the location of the presumptive follicular stem cells (green color). 

•The edges of the down- growing follicular epithelium gradually engulf the dermal condensate, which from thereon is called the dermal papilla (DP). 

•Signals from the DP to the adjacent epithelial cells induce their differentiation into the IRS (blue color) in which the future HS will develop. 

•The epithelial cells surrounding the DP start to differentiate into the different components of the HS (black color). 

•IRS, inner root sheath; ORS, outer root sheath; HF, hair follicle; HS, hair shaft.

Follicular Anatomy

The infundibulum, which extends from the opening of the HF to the point at which trichilemmal cornification begins. 

The isthmus, which extends from the distal end of the infundibulum to the interface between the completely cornified IRS and the first noncornified cell of Hux- ley’s layer.

The inferior portion is composed of a suprabulbar and a bulbar region and extends from the last cell in which red trichohyalin granules can be seen to the base of the HF. 

The suprabulbar region is characterized by the noncornified IRS surrounded by the outer root sheath (ORS). The bulbar region is composed of matrix cells with intermingled melanocytes and the DP.

Most omnivores and herbivores (e.g., horses, cattle, pigs, rats, and mice) have simple follicles, in which each infundibulum contains 1 HS.

Some HSs in the compound follicle have a large diameter (primary or guard hairs), whereas the majority of the HSs have a small diameter (secondary or undercoat hairs)

Carnivores (e.g., dogs and cats) and also rabbits have compound follicles, in which each infundibulum contains multiple HSs.

The hair follicle cycles 

Trough periodic stages, which include a growth phase (anagen), a regression phase (catagen), and a quiescent phase (telogen). Thereafter, a new cycle starts. During the next cycle phase, the old club hair is shed in a process called exogen. A fifth hair cycle stage is called kenogen. Kenogen is a lag phase and applies to hair follicles that have passed the telogen stage, lost their hair fiber, and remain empty for a certain time before a new anagen phase is initiated.

A difference can be observed in regard to the ratio of telogen and kenogen follicles between breeds which have a more synchronous HC to prepare for seasonal changes (arctic breeds) and breeds whose HC is asynchronous and therefore hair is shed during the whole year.

Hair loss

Hair loss (hypotrichosis or alopecia) is a common problem.

Hair growth disorders are attributed, at large, to a decreased and abnormal formation, a decreased regeneration, or an increased destruction of HFs, and the consequence is that either the quality or the total density of HSs is altered 

Canine hair cycle arrest disorders 

hyperadrenocorti- cism, hypothyroidism, hyperestrogenism, alopecias of unknown origin and pattern baldness.

Hair loss, hypotrichosis, or alopecia in dogs is a regular cause for consulting a medical doctor or a veterinarian, and the possible underlying causes are numerous.

Alopecia classification

•Focal: affects one or two specific areas of the animal's body.  Typical in infections by dermatophytes (ringworm) and some types of scabies.

•Multifocal: covers multiple small or medium areas.  Classical in pyoderma and some types of scabies, such as that caused by Demodex. 

•Generalized: it extends throughout the body or large areas, such as the trunk.  It is observed in endocrine alopecia, congenital alopecia, follicular dysplasia and advanced cases of atopy.