Mammalian Merkel cells are descended from the epidermal lineage

Abstract

Merkel cells are specialized cells in the skin that are important for proper neural encoding of light touch stimuli. Conflicting evidence suggests that these cells are lineally descended from either the skin or the neural crest. To address this question, we used epidermal (Krt14(Cre)) and neural crest (Wnt1(Cre)) Cre-driver lines to conditionally delete Atoh1 specifically from the skin or neural crest lineages, respectively, of mice. Deletion of Atoh1 from the skin lineage resulted in loss of Merkel cells from all regions of the skin, while deletion from the neural crest lineage had no effect on this cell population. Thus, mammalian Merkel cells are derived from the skin lineage.

Fig. 1

β-galactosidase expression driven from the Atoh1^LacZ locus is altered in the skin of Atoh1^LacZ/LacZ embryos. (A, B, G, H) Whisker pads from E14.5 Atoh1^LacZ/+ (A, B) and Atoh1^LacZ/LacZ (G, H) embryos. (C, D, I, J) Whisker pads from E15.5 Atoh1^LacZ/+ (C, D) and Atoh1^LacZ/LacZ (I, J) embryos. Note the increased amount of X-gal staining in both genotypes compared to E14.5. (E, F, K, L) Forelimb, body skin and touch domes of E15.5 Atoh1^LacZ/+ (E, F) and Atoh1^LacZ/LacZ (K, L) embryos. Head is at the top of (E, K), and dorsal is to the right. Arrowheads in (B, D, F) denote single cells that are completely labeled by X-gal reaction product. Only punctate X-gal staining is present in whisker follicles and touch domes of Atoh1^LacZ/LacZ embryos. Scale bars: 1mm (A, C, G, I), 2.5 μm (E, K), 25 m (B, H), 100 μm (D, J), 30 μm (F, L).

Fig. 2

Merkel cells are absent from the skin of Atoh1^LacZ/LacZ embryos. Tissue sections from whisker pads of E15.5 Atoh1^LacZ/+ and Atoh^LacZ/LacZ embryos immunostained for β-galactosidase (A′, B′) and keratin 8 (A″, B″). Arrowheads denote double-labeled cells; arrows denote β-galactosidase-positive, keratin 8-negative cells; brackets in (B′-B‴) denote the region of punctate β-galactosidase immunoreactivity; dotted lines identify the epidermal/dermal boundary; boxes denote regions shown in subpanels. Epi – epidermis, WF – whisker follicle. Scale bar: 30 μm (panels), 22.5 μm (subpanels).

Fig. 3

Merkel cells are present in the skin of Wnt1; Atoh1^CKO animals. (A-B″) Wholemount X-gal staining of E16.5 Wnt1^+/+; Atoh1^LacZ/flox (Atoh1^LacZ/+; A-A″) and Wnt1; Atoh1^CKO (B-B″) embryos. Boxed areas in (A) and (B) are shown in (A′, A″) and (B′, B″), respectively. (C-F‴) Whisker pad (C-D‴) and touch dome (E-F‴) sections from P0 Wnt1; Atoh1^CKO animals stained with X-gal (C, E) or immunostained with β-galactosidase and keratin 8 (D-D‴, F-F‴). Dotted lines demarcate the regions shown in subpanels. Merkel cells are present in both tissues. HS – hair shaft. Scale bar: 50 μm C-F‴ main panels, 25 μm subpanels.

Fig. 4

Merkel cells are absent from the skin of adult Krt14; Atoh1^CKO animals. All tissue was obtained from 22–25 day old mice. Tissue was prepared for wholemount X-gal staining (A, B, D, E, G, H, J, K, M, N, P, Q) or immunostaining (C-C‴, F-F‴, I-I‴, L-L‴, O-O‴, R-R‴). (A-F‴) Back skin from Krt14^+/+; Atoh1^LacZ/flox (Atoh1^LacZ/+; A-C‴) and Krt14; Atoh1^CKO (D-F‴) animals. (H-L‴) Whisker pad from Krt14^+/+; Atoh1^LacZ/flox (G-I‴) and Krt14; Atoh1^CKO (J-L‴) animals. (M-R‴) Foot pad from Krt14^+/+; Atoh1^LacZ/flox (M-O‴) and Krt14; Atoh1^CKO (P-R‴) animals. Boxes in (A, D, G, J, M, P) denote areas shown in (B, E, H, K, N, Q), respectively; boxes in immunostaining panels denote regions shown in subpanels. Arrowheads in subpanels denote double-labeled cells. No keratin 8-positive cells were found in Krt14; Atoh1^CKO animals in any skin region. Punctate X-gal and β-galactosidase staining (brackets) is present in epidermal cells of touch domes, whisker follicles and foot pads in Krt14; Atoh1^CKO animals. Scale bars: 300 μm (A, D, G, J, M, P), 50 μm all other panels, 25 μm subpanels.

Fig. 5

Rab3c and VGLUT2 are not expressed in touch domes of Krt14; Atoh1^CKO animals. Hairy skin from P25 Krt14^+/+; Atoh1^LacZ/+ (Atoh1^LacZ/+) (A-A‴, C-C‴) and Krt14; Atoh1^CKO (B-B‴, D-D‴) mice was immunostained for β-galactosidase, Rab3c, and/or VGLUT2. Punctate β-galactosidase staining is present in epidermal cells overlying touch domes in both genotypes, but Rab3c and VGLUT2 labeled cells are absent in Krt14; Atoh1^CKO animals. Brackets denote touch domes. Scale bar: 50 μm.