Abstract
Hair cycle disturbances are common in dogs and comparable to some alopecic disorders in humans. A normal hair cycle is maintained by follicular stem cells which are predominately found in an area known as the bulge. Due to similar morphological characteristics of the bulge area in humans and dogs, the shared particularity of compound hair follicles as well as similarities in follicular biomarker expression, the dog is a promising model to study human hair cycle and stem cell disorders. To gain insight into the spatial distribution of follicular keratinocytes with stem cell potential in canine compound follicles, we microdissected hair follicles in anagen and telogen from skin samples of freshly euthanized dogs. The keratinocytes isolated from different locations were investigated for their colony forming efficiency, growth and differentiation potential as well as clonal growth. Our results indicate that i) compound and single hair follicles exhibit a comparable spatial distribution pattern with respect to cells with high growth potential and stem cell-like characteristics, ii) the lower isthmus (comprising the bulge) harbors most cells with high growth potential in both, the anagen and the telogen hair cycle stage, iii) unlike in other species, colonies with highest growth potential are rather small with an irregular perimeter and iv) the keratinocytes derived from the bulbar region exhibit characteristics of actively dividing transit amplifying cells. Our results now provide the basis to conduct comparative studies of normal dogs and those with hair cycle disorders with the possibility to extend relevant findings to human patients.
Highlights
In the dog, like in humans and mice, hair follicles (HFs) self-renew and undergo recurrent phases of controlled growth, regression of the lower portion of the follicle through apoptosis of keratinocytes, and relative quiescence [1]
To reach our goal we investigated the growth potential of keratinocytes derived from compound and single canine HFs in healthy dogs by 1) assessing the colony forming efficiency of cells derived from defined locations of HFs; upper isthmus, lower isthmus and bulbar region, 2) investigating the growth potential compared to premature differentiation of resulting colonies using proliferation and differentiation markers; 3) comparing colony growth during different hair cycle (HC) stages and 4) investigating clonal expansion of canine follicular keratinocytes
All experiments were performed with single HFs and compound HFs from anagen (19 and 11 dogs, respectively) and telogen HC stages with comparable results
Summary
Like in humans and mice, hair follicles (HFs) self-renew and undergo recurrent phases of controlled growth (anagen), regression of the lower portion of the follicle through apoptosis of keratinocytes (catagen), and relative quiescence (telogen) [1]. The best characterized SC compartment in the mouse is located in the lower half of the isthmus in a morphologically visible trochanter-like extrusion called the bulge [5, 9,10], defined by the attachment site of the arrector pili muscle [11]. In humans and dogs the bulge area has no distinct morphological structure It is identified in the anagen HC by the attachment of the arrector pili muscle and in the telogen stage due to its location at the lower end of the HF [7, 9, 12]. Besides the afore-mentioned morphological distinctions, differences exist with respect to biomarker expression in the bulge area of humans and dogs compared to the mouse [10, 14,15]
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