PROXIMAL ATTACHMENT OF THE ARRECTOR PILI MUSCLE
Follicle bulge cells interact closely with the APM.[8] Tiede et al. introduced an outer root sheath (ORS) protrusion “the trochanter” at the APM insertion site of anagen VI hair follicles.[29] Narisawa et al.[4] have demonstrated the presence of knob-like swellings and villous projections in human terminal and vellus hair follicles. The authors also demonstrated the presence of skirt-like projections in small vellus hair but not in large vellus hairs. The APM-bulge connection persists throughout the hair growth cycle and has been suggested to play an important role in morphogenesis and renewal of hair follicles.[30,31,32,33]
APM differentiation and anchorage to the bulge is thought to be mediated by components of the follicle basement membrane.[34] Tendon genes such as Scx (scleraxis),[35] Mitf, Igfbp5, Fbln1 (fibulin-1), Postn (periostin), Tnc (tenascin-C), Sparc, Igfbp6, and Fgf18[36] are expressed by bulge stem cells. These cells are known to highly express periostin, which is a major component of the tendon extracellular matrix (ECM). Accordingly it has been concluded that bulge stem cells serve as tendon cells for the APM.[7]
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Nephronectin is an ECM protein expressed by stem cells in the bulge.[7] Nephronectin expression in bulge and hair germ cells results in epidermal basement membrane heterogeneity. Nephronectin is seen in telogen and anagen hair, in the basement membranes of the APM proximal attachment site in the bulge, of the APM itself, and of the hair bulb. α8β1 integrin is the nephronectin receptor. α8 is expressed in the APM and the DP, where it is co-localized with nephronectin at the junction of the telogen hair germ. During the development, fibroblasts positive for α8 integrin are thought to be progenitors of the APM. These cells appear to attach to nephronectin in the bulge and then differentiate into α-smooth muscle actin (SMA)-expressing APM cells. In an in vitro assay, α8-positive cells isolated from skin bound to nephronectin and up-regulated expression of α-SMA and smooth muscle protein 22-α (Sm22a). Accordingly, nephronectin provoked expression of APM markers in these cells.[7] α8 integrin also has a role in the differentiation of vascular and intestinal smooth muscles.[37,38]
Major changes in the APM have been observed in the skin of nephronectin knockout mice (Npnt-/-).[7] While in normal mice the APM attaches to the bulge (88.3% of follicles), in Npnt-/- mice it is displaced to an EGFL6-positive zone distal to the bulge (76.3% of follicles). EGFL6 is also a α8β1 ligand. The number of hair follicles with APM, and APM attachments to the follicle also decrease in Npnt-/- skin. Interestingly, piloerection is maintained despite the fact that the APM attachment site is displaced. In α8 integrin knockout mice, the APM attaches to both the nephronectin and EGFL6-positive regions. These mice have normal nephronectin expression in the bulge, but the APM lacks nephronectin.[7] Overall, the phenotypes of both knockout mice indicate that nephronectin and α8β1 integrin play a significant role in determining the proximal attachment site of the APM.
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