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Mane Growth and Induction

Normal Mouse Hair Formation

Signaling inductions between the germinal layer of the epithelium and the dermal mesenchyme interact to create the cutaneous appendages, such as the formation hair follicles (Gilbert, 2003). This interaction appears to be initiated by a signal originating from the dermis that instructs the ectodermal cells to form placodes. These placodes, which are local thickenings, form as adhesion molecules, NCAM, signal them to thicken and aggregate. As the dermal cells continue to condense, they in turn instruct the epidermis to make a hair shaft (Gilbert, 2003).

(Click on Image to view Original Source)

The cells in the basal layer of the epidermis aggregate to form the hair follicle primordium or the hair germ, which is directed by the underlying dermal fibroblast cells. This process appears to be able to occur at varying times and places throughout embryonic development. Dermal fibroblasts respond to the ingression of epidermal cells by forming the dermal papillae beneath the hair germ due to a series of paracrine factors, FGFs, SHH, and BMP2. Sonic hedge hog (SHH) has been shown to play a crucial role in the induction of dermal papillae formation, through knockout mouse experiments in which this formation has been disrupted (Chiang et al., 1999). In response to the signal from the dermal papillae, the hair germ proliferates down into the mesenchyme, forming the hair shaft. As the inner hair root develops above the papillae, subsequent interactions between paracrine factors and the dermal papillae, induces the papillae to push upward against the hair cells stimulating them to divide more rapidly (Gilbert, 2003).

Initial hair patterns are lanugo, which can be characterized as thin hairs closely spaced on the skin, but in the adult these hairs are lost and replaced by new follicles that produce vellus, characterized as short and silky and similar in appearance to adult hairless regions. However, in many areas of the body terminal hairs can develop from these vellus regions, for instance at the onset of puberty, while in some instances terminal hair can revert back to vellus as is the case in male pattern baldness (Gilbert, 2003, Tosney, 2005). Further regulation of hair placement and type can be attributed to subsequent interactions between SHH and BMPs.

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What Happens in Pierson's Puppeteer?

While SHH stimulates dermal condensations, BMP inhibits them, thus allowing for the over expression of SHH in the thick and coarse mane seen in the Pierson’s Puppeteer. However, SHH acts locally in high concentrations in the hair germ to induce the hair follicle formation in the areas between the two necks, along the spine and the top portions of the hip joints. The appearance of excessive hair in places such as the spine and hip joints may be due to the phenomenon of hypertrichosis, in which local or general excessive hairiness is induced by the augmented development of hair follicles or the overgrowth of the lanugo-vellus set of hair follicles (Tosney, 2005). The brown color of the mane can be attributed to initial melanoblast ingression, resulting in differentiated melanocytes in these regions. These melanocytes ultimately transfer their pigment to the growing hair, thus expressing the pigment color in the hair.

Image of hypertrichosis

(Click on Image to view Original Source)

Sources:

Chiang C. Swan R.Z. Grachtchouk M. Bolinger M. Litingtung Y. Robertson E.K. Cooper M.K. Gaffield W. Westphal H. Beachy P.A. Dlugosz A.A. (1999, January 1). Essential Role for Sonic hedgehog during hair follicle morphogenesis. Developmental Biology. 205(1):1-9.

Gilbert S.F. Development Biology. 7^th Editon. Massachusetts: Sinauer Associates, Inc., 2003. p. 418-419.

Muhammed K, Safia B. Cornelia de Lange syndrome . Indian J Dermatol Venereol Leprol. 2003 [cited 2005 Dec 4];69:229-231. Available at: http://www.ijdvl.com/article.asp?issn=0378-6323;year=2003;volume=69;issue=3;spage=229;epage=231;aulast=Muhammed

Tosney K.W. “Epithelial-Mesenchymal Interactions.” Biology 208: Embryology, Ann Arbor. November 3, 2005.