Extra cushion for the knee

Stifle joint, anterior (cranial), 1-day-old mouse (C57BL/6). The meniscus is a specialized fibrocartilaginous structure of the stifle joint providing additional cushion and support. The menisci (pl.; medial and lateral) are paired in each stifle joint, forming semilunar pads sandwiched between the femoral and tibial condyles. Menisci are one of two unique fibrocartilaginous structures within synovial joints: the other being the circular articular discs of temporomandibular joints. The normal anterior horn of the meniscus (asterisk) in this 1-day-old mouse is composed entirely of hyaline cartilage and covered by a thin layer of perichondrium. Superficial and outer regions will soon differentiate into fibrocartilage, with production of Type I collagen, rather than Type II. Inner areas of hyaline cartilage often ossify as mice age, with microscopic bone and marrow visible by 4 weeks of age. The posterior (caudal) horn of the meniscus (not pictured) contains more fibrocartilage as mice age (less weight bearing = more vasculature). Gamer LW, et al. Journal of Orthopaedic Research. 2016.