Neonatal vitamin A but not retinoic acid administration increases intramuscular adipocyte number in sheep by promoting vascularization

This study investigated whether vitamin A (VA) administration during the neonatal stage could increase the number of intramuscular adipocytes in Hu sheep by promoting vascularity. A total of 56 newborn male Hu sheep were divided into four groups and received intramuscular injections of either 0, 7500 IU retinoic acid (RA), 7500 IU VA, or a combination of 7500 IU VA and 5 mg SU5416 (an angiogenic inhibitor), at 1, 7, 14, and 21 days of age. At 15 days of age, 6 sheep from each group were randomly selected and sacrificed for intramuscular adipogenic capacity analysis. The remaining 8 sheep in each group were raised until they were 8 months old. VA-treated sheep exhibited an increase in preadipocytes, elevated expression of adipogenic genes (CCAAT enhancer binding protein alpha [<i>CEBPA</i>] and CCAAT enhancer binding protein beta [<i>CEBPB</i>]) and angiogenic genes (vascular endothelial growth factor A [<i>VEGFA</i>]), and stromal vascular fraction cells in the longissimus dorsi (LD) muscle with enhanced adipogenic capacity (<i>P</i> ＜ 0.05). These effects were entirely negated by SU5416. Upon slaughter, VA increased final weight, carcass weight, and average daily gain (<i>P</i> ＜ 0.05) but did not affect feed intake at 21 to 32 weeks (<i>P</i> = 0.824). VA increased the number of intramuscular adipocytes in the LD and semitendinosus (ST) muscle (<i>P</i> ＜ 0.05) without changing the adipocyte number of the omentum, perirenal and subcutaneous fats (<i>P</i> ＞ 0.05). VA injections also increased intramuscular triglyceride (TG) content (<i>P</i> = 0.016) without changing the omentum fat weight or subcutaneous fat thickness (<i>P</i> ＞ 0.05), but it did increase the perirenal fat weight (<i>P</i> = 0.011). Consistently, SU5416 mitigated the effects of VA on intramuscular TG content and adipocyte count, correlating with a decrease in vascularity. In contrast, RA injections didn't affect the intramuscular fat (<i>P</i> = 0.744) but reduced the TG content of the omentum and perirenal fat (<i>P</i> ＜ 0.05). In conclusion, intramuscular injections of VA but not RA at the neonatal stage improved the growth performance of Hu sheep, increasing the number of intramuscular adipocytes and marbling by promoting angiogenesis.