The menisci are first seen as triangular aggregations of cells in the 20-day rabbit fetus. At 25-days, a matrix that contains types I, III and V collagens has formed. These collagens are also found in the 1-week neonatal meniscus, but by 3 weeks, type II collagen is present in some regions. By 12 to 14 weeks, typically cartilaginous areas with large cells in lacunae are found and by 2 years, these occupy the central regions of the inner two-thirds of the meniscus. The surface layers of the meniscus contain predominantly type I collagen. From 12 to 14 weeks onwards, there is little overlap between the regions with types I or II collagens, that is, these are discrete regions of type I-containing fibrocartilage and type II-containing cartilage. Types III and V collagens are found throughout the menisci, particularly in the pericellular regions. All the cells in the fetal and early neonatal menisci express the mRNA for type I collagen. At 3 weeks postnatal, cells that express type I collagen mRNA are found throughout the meniscus, but type II collagen mRNA is expressed only in the regions of developing cartilage. At 12- to 14-weeks, only type II collagen mRNA is expressed, except at the periphery next to the ligament where a few cells still express type I collagen mRNA. Rabbit menisci, therefore, undergo profound changes in their content and arrangement of collagens during postnatal development.
