Long bones, such as our femur, tibia, or ulna, primarily grow by elongation of the diaphysis, which happens at the growth plate. The growth plate itself is a transient structure that eventually disappears in humans after undergoing several stages of maturation until they die and are replaced completely by bones nearing the end of puberty. Elongation is achieved through a process of proliferation and hypertrophy of chondrocytes that migrate medially from the ends of the bone resulting in expansion along the longitudinal axis. This initial cartilage matrix will eventually be converted to bone in a process known as endochondral ossification.

The growth plate consists of distinct zones that are maintained by a complex interplay of genetic and chemical factors. One key chemical component is the release of PTHrP by chondrocytes of the resting zone (most distal zone) that sustains the division of MSC’s in this zone and begins their migration through the subsequent zones of cartilage. The other key component is Indian Hedgehog factor (IHhh) which is released by chondrocytes just beginning to hypertrophy that interacts with the cells of the resting zone in a negative feedback loop to maintain the growth plate. The result is a polarized structure characterized by stem cells on the epiphysis and hypertrophic chondrocytes at the distal end - which, consequently, makes this structure hard to mimic in cell culture.

From the above summary, we know that 2D cultures are insufficient to accurately mimic the native microenvironment of the growth plate. This is where the allure for the use of a bioreactor for 3-D culture in tissue engineering arises. This idea is not novel and has been used in many applications where 3D scaffolds are utilized for culturing stem cells because it better recapitulates the spatiotemporal environment of the native tissue.