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Fig. 7. A model combining ventral elongation of the somites and somitic
Fgf10 expression with maturation of the surface ectoderm and the
acquisition of a mammary cell fate at the position of mammary placode 3.
(A) A model for the molecular interactions leading to a mammary cell
fate in the surface ectoderm. GLI3 (purple dots) and an unidentified factor or
factors X act upstream of Fgf10 (pink dots) in the somites. Somitic
FGF10 activates ectodermal FGFR2B (green), via diffusion and/or delamination
of somitic cells. Sufficient levels of activation are required for
Wnt10b/TOPGAL expression (blue) and multilayering of the surface
ectodermal cells as hallmarks of a mammary cell identity. (B) Schematic
drawings of body halves of progressively older embryos, illustrating the
correlation between somitic elongation, somitic Fgf10 expression and
ectodermal maturation. For convenience, the body size is kept the same for all
ages. Broken vertical lines indicate the mammary line (ml) and Wolffian ridge
(WR). Increasingly darker shades of pink indicate increasingly higher levels
of Fgf10 expression. The surface ectodermal cells are cuboidal
(thicker black line) at the WR, whereas dorsal and ventral to the WR they are
squamous (thinner black line). Ventrally on the flank, ectodermal cells become
cylindrical (green) and enlarge (orange) coinciding with low levels of
Wnt10b and TOPGAL expression. These cells give rise to a stratum
intermedium (blue), with elevated Wnt10b and TOPGAL expression,
indicating a mammary cell fate. (C) Model of cumulative FGF10/FGFR2B
signaling in the flank leading to progressive maturation of the ectoderm, and
eventually conversion into mammary epithelium. See main text for explanation.
Color coding corresponds to color coding of B. Abbreviations: G, dorsal root
ganglion; som, somite.
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