Region specific microstructural complexity of the ovine meniscus root provides an organizational basis for injury susceptibility

Comprehensive information on the meniscus root microstructure is essential to exactly understand its physiological role and susceptibility to injury. We selected the ovine medial meniscus anterior root (MAR) as model to elucidate the intricate spatial arrangement of its enthesis, root ligament and transition into the medial meniscus anterior horn (MMAH), hypothesizing that its microstructure is comparable to humans. We applied different histological, type-I, -II, and -X collagen immunohistochemical, polarization and confocal analyses to investigate its structural complexity. The results reveal unique region-specific patterns. Cell morphology, proteoglycan, and type-II collagen contents differ between regions. The enthesis is avascular while the MAR ligament and red-red zone of the MMAH are well vascularized. The ovine MAR attachment constitutes an enthesis organ together with a bare area below the root ligament covered by adipose tissue. The MAR ligament comprises large longitudinal fascicles that unweave into a complex network when entering the MMAH, changing their orientation towards its white-white zone. The blood vessels that vascularize the MAR ligament enter at its peripheral-femoral side. Only axial MMAH fibers are immunopositive for type-X collagen. This region-specific microstructural complexity of the ovine MAR is largely similar to published findings in humans, providing an organizational basis for injury susceptibility. Thus, the ovine MAR may serve to study the physiopathology of and therapeutic approaches to human root tears.