The classical model of tissue renewal posits that small numbers of quiescent stem cells (SCs) give rise to proliferating transit-amplifying cells before terminal differentiation. However, many organs house pools of SCs with proliferative and differentiation potentials that diverge from this template. Resolving SC identity and organization is therefore central to understanding tissue renewal. Here, using a combination of single-cell RNA sequencing (scRNA-seq), mouse genetics and tissue injury approaches, we uncover cellular hierarchies and mechanisms that underlie the maintenance and repair of the continuously growing mouse incisor. Our results reveal that, during homeostasis, a group of actively cycling epithelial progenitors generates enamel-producing ameloblasts and adjacent layers of non-ameloblast cells. After injury, tissue repair was achieved through transient increases in progenitor-cell proliferation and through direct conversion of Notch1-expressing cells to ameloblasts. We elucidate epithelial SC identity, position and function, providing a mechanistic basis for the homeostasis and repair of a fast-turnover ectodermal appendage.
An eigenvalue problem encountered in the dynamical theory of chain molecules is ∫ −11α′′(s)(|r−s|)−12ds=−λα(r),α′(±1)=0.This is solved by three methods: use of a Fourier series for α, expansion of α in associated Legendre polynomials Pm2, and by a variation method. The eight smallest eigenvalues are calculated explicitly and an approximate formula is found for the remaining ones. Formulas are found also for the eigenfunctions.
Integrals of the form where 0 :-::; k< 1, a nd j is a positive integer, occ ur in a r adiation fi cld problem. Expressions for [loCk ) and [ll (k ) have been derived in terms of co mpl ete ellip t ic integrals of t he first a nd second kinds. Using these values, and t he r ecursion formula[l iCk ) can be found for all values of j and k. A number of useful se ri es expansions and other r elations ar e given for [l ;(k ), a nd tables a re included for 0 :-::;j( l ) :-::; 9 a nd 0 ~ k2(0.01):-::; 0.99.
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