Explaining The "Horseshoe" Shape of Male Pattern Baldness

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“The role of mast cells in male-pattern baldness is unknown, but the large numbers often present is a striking feature.”[1] 

"We postulate that because of the underlying anatomy, there is a relative microvascular insufficiency to regions of the scalp that lose hair in male pattern baldness and that is associated with local tissue hypoxia in those regions. The vascular supply of the scalp is derived from branches of the internal carotid artery and branches of the external carotid artery. The frontal region of the scalp, which loses hair in male pattern baldness, is supplied by the supraorbital and the supratrochlear arteries. These are relatively small branches of the internal carotid artery system. The temporal [sides] and occipital [back of head] regions of the scalp, which do not lose hair in male pattern baldness, are supplied by larger branches of the external carotid artery. Specifically, these are the superficial temporal, posterior auricular, and occipital arteries. Further, the frontal and vertex regions of the scalp overlie the galea aponeurotica, which is relatively avascular. The temporal and occipital regions of the scalp overlie the temporalis and occipitalis muscles, which provide a rich network of musculocutaneous perforator blood vessels. These anatomic differences contribute to the tenuous nature of the dermal blood supply to the frontal and crown regions of the scalp."[2]

"We hypothesized that this difference in pattern of prostaglandin D-synthase expression may constitute a developmental pattern inherent to normal as well as alopecic scalp skin, thus defining a ‘field’ vulnerable to acquired hair loss." “These data indicate that scalp is spatially programmed via mast cell prostaglandin D-synthase distribution in a manner reminiscent of the pattern seen in androgenetic alopecia.” "In a prior study of male pattern alopecia, increased numbers of mast cells have been seen in balding vertices compared to non-balding occipital scalp and, in fact, this pattern was also observed in five control subjects studied, though there were greater numbers of mast cells in the patients with alopecia.” "In the 1990’s mast cells were found to be actively degranulating in the inflammatory infiltrates of scalp with male pattern alopecia and this was proposed to contribute to perifollicular fibrosis.”[3] 

"Mast cells express the high-affinity estrogen receptor and studies have shown that estrogens augment their activities: in the presence of high levels of estrogens, mast cell responses to compound 48/80 are increased, leading to more substantial degranulation and release of histamine and serotonin.” "Progesterone is necessary for the maintenance of pregnancy and plays a key role in maintaining cervical integrity prior to labour induction. Progesterone can prevent the migration of mast cells in response to chemokines and down-regulate surface chemokine receptor expression. In addition, mast cell function can be altered by the presence of high concentrations of progesterone. For example, progesterone inhibits the secretion of histamine from mast cells. Notably, these observations would suggest that mast cells present within the uterus during pregnancy are quiescent and inhibited by high levels of progesterone, and also that recruitment of mast cell progenitors from the circulation may be limited.” "At present the prevalence of allergies, including allergic rhinitis, hayfever, eczema, food allergies and urticaria, is rising." [4]

"Results from this study provide the first evidence of a unique regulatory mechanism by which CO2 inhibits mast cell degranulation and histamine release by repressing stimulated increases in intracellular calcium. Thus, our data provide a plausible explanation for the reported therapeutic benefit of noninhaled intranasal delivery of 100% CO2 to treat allergic rhinitis." [5]

1. Male pattern alopecia a histopathologic and histochemical study (1975) 
2. Transcutaneous PO2 of the scalp in male pattern baldness: a new piece to the puzzle (1996)
3. A prostaglandin D-synthase-positive mast cell gradient characterizes scalp patterning (2014) 
4. The role of mast cells and their mediators in reproduction, pregnancy and labour (2010) 
5. Treatment of mast cells with carbon dioxide suppresses degranulation via a novel mechanism involving repression of increased intracellular calcium levels (2011)