Rapid aging, a follow-up

Stifle joint, anterior (cranial), 10-week-old mouse (athymic). A few months have passed in our previously 1-day-old mouse, and the original hyaline cartilage model of the meniscus now looks quite different. The cranial area and superficial layer of hyalin cartilage have differentiated into fibrocartilage (white asterisk), with predominance of type I collagen and decreased proteoglycans. Woven and lamellar bone, with marrow space (black arrow), occupies the central area, formed at some point by endochondral ossification. Fibrocartilage functions to withstand repeated compressive and shear forces, reducing or delaying damage to the articular cartilage of the tibia/femur. Looking at the articular cartilage of both the femur and tibia, the superficial cell layer (only about 1 cel layer thick) has a fibrocartilage, rather than hyaline cartilage, phenotype.