Organogenesis of the Musculoskeletal System in Horse Embryos and Early Fetuses.

Musculoskeletal system development involves heterotypical inductive interactions between tendons, muscles, and cartilage and knowledge on organogenesis is required for clarification of its function. The aim of this study was to describe the organogenesis of horse musculoskeletal system between 21 and 105 days of gestation, using detailed macroscopic and histological analyses focusing on essential developmental steps. At day 21 of gestation the skin was translucid, but epithelial condensation and fibrocartilaginous tissues were observed on day 25 of pregnancy. Smooth muscle was seen in lymphatic and blood vessel walls and the beginning of cartilaginous chondrocranium was detected at day 30 of gestation. At day 45, typical chondroblasts and chondrocytes were observed and at day 55, mandibular processes expanded toward the ventral midline of the pharynx. At day 75, muscles became thicker and muscle fibers were seen developing in carpal and metacarpal joints with the beginning of the ossification process. At day 105, major muscle groups, similar to those seen in an adult equine, were observed. The caudal area of the nasal capsule and trabecular cartilages increased in size and became ossified, developing into the ethmoid bone. The presence of nasal, frontal, parietal, and occipital bones was observed. In conclusion, novel features of equine musculoskeletal system development have been described here and each process was linked with an early musculoskeletal event. Data presented herein will facilitate a better understanding of the equine muscular system organogenesis and aid in the detection of congenital deformities. Anat Rec, 299:722-729, 2016.