The palmar plate is a crucial structural part of hand, associated with metacarpophalangeal and interphalangeal joints. Pediatric disorders involving the palmar plate of thumb metacarpophalangeal joint include trigger thumb, hyperextension, instability, and dislocation. While anatomical differences exist between children and adults, detailed microstructure evaluations in infants remain unexplored. In this study, we provide a histological and structural assessment of the previously unresolved microstructure of the palmar plate in the thumb metacarpophalangeal joint of infant Japanese macaques (Cercopithecidae, Macaca fuscata), a relevant model for human development. Histological staining (light microscopy) and scanning electron microscopy were employed to visualize the three-dimensional microstructure. The palmar plate of the infant macaque was found to contain (1) elastic fibers, (2) hyaline cartilage composed of type II collagen, and (3) type I collagen fibers arranged in distinct patterns. The cartilaginous region exhibited a reticulate fiber arrangement on its periphery, while the membranous region displayed dense and complex fibers on the proximal phalanx side and parallel on the metacarpal side, respectively. This is the first comprehensive three-dimensional investigation of the infant's thumb's palmar microanatomy, providing a foundation for understanding its development and implications for pediatric disorders.
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