Mdka overexpression enlarges the MFP at the expense of the notochord. Lateral views (a,c) and transverse sections (b, posterior trunk) of noninjected 14s control embryos. (d-f) In embryos injected with 50 pg mdka RNA, the MFP is enlarged (expanded shh in MFP, n = 165/191, 86%; f-spondin, in n = 36/46, 76%; col2a1, n = 134/145, 92% [data not shown]) and the notochord is reduced (reduced shh in the notochord in n = 20/38, 53%; col2a1 in n = 27/88, 31%; and ntl in n = 11/45, 24%). (g-i) Severely affected embryo with complete block of notochord formation in the posterior trunk (lacking shh expression in n = 16/38, 42%; col2a1 in n = 41/88, 46%, and ntl in n = 30/45, 67%). White lines in a, d, and g indicate border between the notochord and MFP. Sections in b, e, and h are at similar positions in the posterior trunk. (j) Endogenous mdka transcription in the paraxial mesoderm extends up to the tailbud. (k) mdka overexpression resulted in a significant decrease of notochord width: n = 10; (**) p < 0.01 versus control for anterior trunk; (***) p < 0.001 versus control for posterior trunk; Mann-Whitney U-test (see Supplementary Fig. S3). (l-o) Transverse sections of 18s embryos showing enlarged MFP (expanded col2a1 in n = 37/44, 84% [o]), but normal expression of nkx2.2b in the lateral floor plate (n = 27/29, 93% [n]) and olig2 in motoneuron domain (n = 43/44, 98% [o]; n = 10, p = 0.796; see Supplementary Fig. S1) in mdka-injected embryos. Anterior is left in lateral views. Bars: j, 20 μm; o, 10 μm. (lfp) Lateral floor plate, (mfp) medial floor plate, (no) notochord, (pmn) motoneuron progenitor domain.
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