MC HS is important for stability and diameter control of vessels in the skin. Endomucin (red) and collagen IV (green) staining of embryonic skin revealed regressing blood vessel in EXT1^MCko embryos compared with control littermates (A-I). White arrowheads represent regressing vessels lacking EC marker endomucin but positively stain for BM component CIV. Higher magnification images (G-I) of the boxed area in panel D show disconnected ECs that lack proper formation of lumen (arrowheads). Quantification for CIV-positive and endomucin-negative vessels confirmed significant increase in regressing blood vessels of EXT1^MCko (J), whereas the density of the vascular network in the skin is unaffected (J, red bars). (J) n > 15 images/embryo; 2 embryos/group. Measurement of vessel diameter reveals a significant increase in diameter variability, whereas the mean diameter is identical between control and mutant samples. This conspicuous variation in vessel diameter was consistently observed in all mutant samples; n > 5. (K) Measurement of 650 vessel profiles (n > 15 images/embryo), reflecting the variability in vessel diameter in EXT1^MCko embryos; n = 2. ***F < 0.0001 (F test, variance of SD). ns indicates not significant.