Hair cycle and structure of the pilosebaceous unit in carnivores
SEBACEOUS AND SWEAT GLANDS
HAIR CYCLE
The sebaceous glands of domestic animals are much more developed than those of humans, and sebaceous secretions are largely responsible for the lustrous and supple appearance of the skin of dogs and cats. The sebaceous glands are located next to the hair follicles, where their excretory ducts end. These glands are widely distributed throughout the body surface, except for the footpads and the nasal plane, and secrete a mixture of lipids consisting primarily of triglycerides and free fatty acids. Secretion is fundamentally regulated by hormones, and is stimulated by androgens and attenuated by corticosteroids.
The hair follicle undergoes a growth cycle consisting of sequential phases of growth (anagen), regression (catagen), rest (telogen), and shedding (exogen). Another phase (kenogen, during which the follicle is empty) may soon be added to this sequence (Fig. 7). In response to this cycle of activity, the portion of the hair located beneath the arrector pili muscle undergoes significant changes, including keratinocyte apoptosis and subsequent regeneration. Hair growth is influenced by multiple factors, including age, circadian rhythm, and photoperiod cycles, which in turn are mediated by the hypothalamus, hypophysis, and pineal gland, altering levels of various hormones (e.g. melatonin, prolactin, and gonadal, thyroid, and adrenal hormones) (Table 1).
Epitrichial or apocrine sweat glands are distributed across the body surface, except for the footpads and the nasal plane. They are located below the sebaceous glands and their excretory duct ends in the isthmus of the follicular canal. These glands secrete a lipid suspension that mixes with sebum.
Catagen phase
1
Epidermis
2
Dermis Subcutaneous tissue
3
Late anagen phase
Hair
Telogen phase
New hair 1
Shedding of old hair
2
2
3
2 3
Figure 7. Schematic showing
hair cycle in dogs and cats. After the telogen phase, the hair follicle may shed the hair shaft and enter kenogen phase for an indefinite period of time, or alternatively progress directly to the anagen phase. Shedding of the dead shaft (exogen phase) can occur at any point during the next cycle, but tends to occur during anagen phase.
1
1
3
Exogen phase Kenogen phase
Old hair Early anagen phase
Empty follicle
1 2 3
Initiation of new hair growth
1 2 3
7