Fig. 5. Contribution of Isl1⁺ progenitors and their progeny to a subset of branchiomeric muscles in the mouse. (A,A') Expression of the lacZ reporter in the mesodermal core (white arrowhead) of BA1 in E10.5 Isl1-Cre/R26R whole-mount (A) or sectioned (A') embryo. Dashed line in A indicates plane of section. (B) Illustration of the various sections of the E16.5 Isl1-Cre/R26R embryos, modified from the Atlas of Mouse Development (Kaufman, 1992). (C-F) X-Gal staining for the lacZ reporter in transverse sections of an E16.5 Isl1-Cre/R26R embryo. The Isl1 lineage gives rise to multiple tissues including branchiomeric muscles (C,D,E), the salivary glands (arrowhead), and the oropharynx (arrow). (G-H') Immunostaining for β-gal (G,H) and MF20 (MHC; G',H') on serial frontal paraffin sections of an E16.5 Isl1-Cre/R26R embryo (shown in B). For clarity, a half-section of β-gal was combined with the adjacent half of the MF20 staining. Note the co-expression of β-gal and MF20 in a subset of branchiomeric muscles, but not in tongue or extraocular muscles (eom, marked by squares in G,G'). The masseter (ma) has both superficial and deep masses and the pterygoid (pt) has medial and lateral masses (H'). nt, neural tube; ov, otic vesicle; nc, neural crest; mes, mesoderm; t, tongue; my, mylohyoid; ad, anterior digastric; g, genioglossal; co, constrictor; st, stylohyoid; pd, posterior digastric; tm, temporalis.