title: Two waves of anisotropic growth generate enlarged follicles in the spiny mouse.
abstract: BACKGROUND
  Mammals exhibit a remarkable variety of phenotypes and comparative studies using novel model species are needed to uncover the evolutionary developmental mechanisms generating this diversity. Here, we undertake a developmental biology and numerical modeling approach to investigate the development of skin appendages in the spiny mouse, Acomys dimidiatus.
  
  
  RESULTS
  We demonstrate that Acomys spines, possibly involved in display and protection, are enlarged awl hairs with a concave morphology. The Acomys spines originate from enlarged placodes that are characterized by a rapid downwards growth which results in voluminous follicles. The dermal condensation (dermal papilla) at the core of the follicle is very large and exhibits a curved geometry. Given its off-centered position, the dermal papilla generates two waves of anisotropic proliferation, first of the posterior matrix, then of the anterior inner root sheath (IRS). Higher in the follicle, the posterior and anterior cortex cross-section areas substantially decrease due to cortex cell elongation and accumulation of keratin intermediate filaments. Milder keratinization in the medulla gives rise to a foamy material that eventually collapses under the combined compression of the anterior IRS and elongation of the cortex cells. Simulations, using linear elasticity theory and the finite-element method, indicate that these processes are sufficient to replicate the time evolution of the Acomys spine layers and the final shape of the emerging spine shaft.
  
  
  CONCLUSIONS
  Our analyses reveal how hair follicle morphogenesis has been altered during the evolution of the Acomys lineage, resulting in a shift from ancestral awl follicles to enlarged asymmetrical spines. This study contributes to a better understanding of the evolutionary developmental mechanisms that generated the great diversity of skin appendage phenotypes observed in mammals.
authors.affiliation: Department of Genetics & Evolution, Laboratory of Artificial & Natural Evolution (LANE), University of Geneva, Sciences III, 30, Quai Ernest-Ansermet, Geneva 4 1211, Switzerland.
authors.forename: Sophie A
authors.initials: SA
authors.lastname: Montandon
authors.affiliation: Department of Genetics & Evolution, Laboratory of Artificial & Natural Evolution (LANE), University of Geneva, Sciences III, 30, Quai Ernest-Ansermet, Geneva 4 1211, Switzerland.
authors.forename: Athanasia C
authors.initials: AC
authors.lastname: Tzika
authors.affiliation: Department of Genetics & Evolution, Laboratory of Artificial & Natural Evolution (LANE), University of Geneva, Sciences III, 30, Quai Ernest-Ansermet, Geneva 4 1211, Switzerland.
authors.forename: António F
authors.initials: AF
authors.lastname: Martins
authors.affiliation: Department of Computer Science, Scientific and Parallel Computing Group, University of Geneva, Geneva, Switzerland.
authors.forename: Bastien
authors.initials: B
authors.lastname: Chopard
authors.affiliation: Department of Genetics & Evolution, Laboratory of Artificial & Natural Evolution (LANE), University of Geneva, Sciences III, 30, Quai Ernest-Ansermet, Geneva 4 1211, Switzerland.
authors.forename: Michel C
authors.initials: MC
authors.lastname: Milinkovitch
keywords: Acomys; Evolutionary developmental biology; Hair; Spines; Spiny mouse