Neural factor involvement in hair growth

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Neural controls of hair growth

That the neural factors play key role in the hair growth process is proved by the fact that the hair follicle is richly innervated. The follicular growth cycle coincides with a constant remodeling of these innervations. Transplantation studies of skin in rats and of hair follicle relocations in human scalps have established that cycling process is never disrupted by the temporary denervation. In the organ culture also follicles have been seen to complete part of the growth cycle. This indicates hair cycling does not depend on intact neural system and it is not also critically dependent on neural influences. However, scientists didn’t either brush off the possibility of a direct or indirect regulating influence of the nerves on hair growth.

As shown by the observations, the neurotransmitters and neuropeptides play a role in hair cycle control. It has also been seen that stimulation of the neuropeptides and neurotransmitters can alter the proliferation and differentiation keratinocytes. When substance P was administered to mice, it induced development of anagen and catagen.

The early anagen stage involved the epithelial stem cell region of the hair follicles express beta – adrenoreceptors. It has been observed that the beta – adrenoreceptors aganist isoprotenerol causes cycling progression from anagen stage 3 to 4 in skin organ culture of mice and norepinophrine depletion induces premature anagen onset. This proves the relevance of neuropeptides and neurotransmitters in hair cycle control.

Loss of hair follicles can also be caused by Trauma or toxic degeneration of the peripheral nerves. Administration of capsacin to rat skin was followed by a sensory denervation, which led to a retardation of hair shaft thickness. On the other hand, major thoracic surgery is usually followed by an increase in hair growth. This is because of peripheral nerve damage - posttraumatic sympathetic hyperinnervation is the probable reason behind it. Significant hair loss can also be caused by severe psycho emotional stress.

The nervous system also influences the immunology of the skin which again indirectly affects hair growth. The neuropeptide substance P in perifollicular nerve endings has been observed to produce mast cell degranulation in mice skin organ culture. Murine hair cycling has been found to fluctuate when the mast cell to nerve fiber contacts were increased or decreased. Besides, both anagen and catagen development in mice seem to be dependent on mast cell degranulation. These observations lead to the premise that hair growth may well be affected by neuropeptides released by the sensory nerve fibers that act on mast cells and in turn the mast cel degranulation response controls hair growth cycling.

Neuroendocrine influence is also important for hair growth. Hair growth cycle is often influenced by Neurohormones such as prolactin, melatonin and ACTH. The pilosebaceous unit itself can produce neurohormones in different degrees over the different stages of hair cycling. Assays with skin organ cultures have indicated that neurotrophins and their receptors play a definite role in hair follicle morphogenesis and cycling. Hair follicles produce neurotrophins NGF, neurotensin—3, NT – 4 and brain derived neurotrophic factor (BDNF). The follicle cycling is also affected by these neurotrophines.

The influence of neurotrophins over hair cycle control can be summed up in the following way:

  • Neurotrophins which are expressed by the cells in the bulge region of the hair follicle, stimulate the cognate receptors in the hair follicle epithelium and mesenchyme.
  • Production of neurotrophins depends on the hair cycle. This in turn influences hair cycling.
  • The mast cells or macrophages are affected by Follicle or glia - derived neurotrophins—they are essential immunological elements influencing hair cycling.

All these above mentioned factors collectively establish a strong connection between the nervous system and hair follicle cycling.

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