The zone of Ranvier has been identified as a stem cell niche located directly next to the epiphyseal growth plate in the knee structure, which has drawn attention as a potential area of study in stem cell therapy treatments for degenerative cartilage diseases, such as osteoarthritis. Cathepsin K, a cysteine protease, is a stem cell marker used to identify stem cells and the niches they reside in. This research aims to analyse Cathepsin K expression in the zone of Ranvier in 10 day postnatal mice, as well as zebrafish of three ages (3 and 5 days post fertilisation, and adult). Staining through immunohistochemistry, and imaging with fluorescent microscopy were the methods chosen to visualise the expression of each marker as they allow a preservation of the structural integrity of the sample. The images showed a nuclear expression of Cathepsin K in the zone of Ranvier. Meanwhile in zebrafish, it was concentrated in both nuclei and the extracellular matrix of jaw bones. There were no compelling differences in the expression between the developmental stages. The counterstaining performed with known stem cell niche markers helped identify both the location of the stem cell niches where Cathepsin K would be expected and the differences of expression between murine and fish tissues. The findings of this research showed Cathepsin K as one of the markers expressed in the zone of Ranvier, which gives further insight into this stem cell niche and its possible use in the treatment of osteoarthritis and other cartilage degenerative diseases.