Morphological and molecular basis of human ectodermal appendage formation | The morphological and molecular basis of human ectodermal appendage formation

The presence of hair is a key trait of mammals. Hair follicles develop in the skin, with early follicles composed of an epidermal placode and a dermal condensate. Most experimental studies of hair follicle development have used the mouse as a model. This thesis aimed to define the development of human hair follicles at a molecular and morphological level, and to assess the suitability of mouse primary hair follicles as a model for human development. Variations in the developmental mechanisms exist between distinct types of mouse pelage hair follicles, and as clear physiological differences can be seen between adult human terminal and vellus hair follicles, I aimed to elucidate any differences in their initial development. Using morphological measurements, localisation of protein and gene expression, and RNA sequencing to make these comparisons, no significant differences were found between presumptive terminal and vellus hair follicles. The differences in hair follicle development that I detected were instead related to anatomical region of the body. I report that human follicle development shares many similarities with mouse, with their morphology, spatial scale, and cell signalling being largely conserved. Some key differences were identified, including in the expression of the DKK4 and EDA genes in the placode, and the timing of the initiation of SOX2 expression in the dermal condensate. Analysis of sweat gland and primary fingerprint ridge development, showed that many signalling factors and the early morphology are conserved, with different appendage fates being specified after the initiation. Embryonic chicken skin has been used extensively as a model for epithelial appendage development, and requires sufficient dermal cell density to permit appendage formation. I aimed to determine if, in mammals, dermal cell density and proliferation contribute to the formation of hair follicles, analogous to their requirement in avian skin. I found little distinction between dense and loose dermis in mouse and human at different anatomical sites, in contrast to the structure of embryonic avian skin, and that the dermal density does not play as important a role in hair placode induction as in chicken feather formation. Density of the epidermal layer however, strongly correlated with the onset of primary fingerprint ridge formation, and my findings support cell proliferation as a driver of downgrowth in hair follicles and fingerprints. Novel human induced pluripotent stem cell-derived 3D skin organoids present a new model for studying human skin and hair follicle development, permitting experimental interventions not possible in intact tissues. I aimed to replicate a published protocol and evaluate the suitability of this system as a model for human hair follicle and skin development through comparisons to foetal skin. The organoids show similar responses in proliferation and LEF1 expression to cultured ex vivo skin when WNT and BMP pathways were manipulated, while donated organoids showed differences in hair follicle morphological measurements. In conclusion, I find that human hair follicle types, though exhibiting profound differences after birth, cannot be distinguished at their initial development. My findings show that mouse primary hair follicles present an appropriate model for most aspects of human hair follicle development, but with some key characteristics that must be taken into account when extrapolating from experimental results between these species. Mammalian hair follicle development is not correlated in the same way as feather formation with a dense dermis, and fingerprint ridge development is likely to be more reliant on epidermal cell density changes than hair follicles, with proliferation a key driver of downgrowth in both.