Fig. 6. Impaired development of Sox5^–/–/Sox6^–/– intervertebral mesenchyme. (A) In situ hybridization of Pax1 RNA in mid-sagittal sections of control and Sox5^–/–/Sox6^–/– embryos in the lower lumbar vertebral column. At E11.5, Pax1 is expressed in pre-intervertebral cell condensations in both embryos (arrowheads). At E12.5 and E13.5, Pax1 is expressed in control embryos in presumptive annuli (arrowheads) and perichondrium (arrows). These expression domains are wider at E12.5 than at E13.5 in the control, but similar in the mutant. At E15.5, Pax1 expression is restricted to outer annuli in control embryos (asterisk). In Sox5^–/–/Sox6^–/– embryos, Pax1 is still expressed in intervertebral mesenchyme (arrowheads). (B) In situ hybridization of RNAs for extracellular matrix proteins in mid-sagittal sections of the cervical region of E15.5 control and Sox5^–/–/Sox6^–/– embryos. Wild-type inner annulus cells (arrowheads) are differentiated chondroblasts: they are no longer expressing Col3a1, and are expressing Agc1 and Fmod. Sox5^–/–/Sox6^–/– intervertebral cells (arrowheads) are still exhibiting an undifferentiated mesenchymal phenotype: they are expressing Col3a1 at high levels, and Agc1 and Fmod at low or undetectable levels.

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