# Interstitial Growth --- **Interstitial** or **longitudinal growth** occurs in [[long bone|long bones]] and is what ultimately determines our *height*. This form of growth takes place at the [[long bone|epiphyseal plate]], commonly referred to as the **growth plate**. The epiphyseal plate is a section of [[cartilage]] left between the primary and secondary ossification centers at the end of [[ossification|endochondral ossification]]. The epiphyseal plate is divided into five sections, or *zones*. The first zone is located at the "top" or the edge nearest the tip of the epiphysis. ![[epiphyseal plate.png]] - Zone 1 is the **resting zone**. Chondrocytes just chill here, and nothing much happens. - Zone 2 is the **proliferative zone** where chondrocytes undergo [[mitosis|mitotic division]] rapidly, pushing downward and forming column-like structures. This is identifiable on slides because they move downward in rows. - Zone 3 is the **hypertrophy zone** where the chondrocytes grow in size. The proliferative and hypertrophy zone are the zones that are responsible for the real length gain of the bone. - Zone 4 is the **calcification zone** where the matrix has calcified. *Before the matrix can ossify, the cartilage matrix must calcify*. Calcification cuts off chondrocytes from their supply of nutrients, and, much like in [[ossification|endochondral ossification]], [[capillaries]] can now invade the space and and [[bone cells|osteoblasts]] now have room to move in and get to work. - The **ossification zone** is where the cartilage matrix ultimately ossify and become bone. Over the course of growth, the rate of ossification is slightly faster than the rate of the mitotic division, which means when this is finished, the epiphyseal plate disappears and the only remnant of it is the **epiphyseal line**. A failure of this process to work results the condition **achondroplastic dwarfism**. Where a person has long bones that are not proportional to the rest of their body. ## Effects of Hormones on Interstitial Growth - [[HGH]] triggers growth in zones 2-3. - [[thyroid hormone]] influences basal metabolic rate of bone cells, and regulates normal activity at epiphyseal plates - The sex hormones: [[estrogen]] and [[testosterone]] both have an influence because they are released in large amounts during puberty and both dramatically accelerate growth *and* closure of the growth plate. - [[glucocorticoids]] like **cortisol**, inhibits bone cell activity, high amounts increase bone loss and it impairs growth at epiphyseal plate in children. - [[serotonin]] most bones have serotonin receptors, and it plays a role on the rate and regulation of [[bone remodeling]]. Incidentally, this means that [[SSRIs]] can impact bone health. if levels are too high, [[bone cells|osteogenic cells]] are prevented from differentiating into osteoblasts, and it could be linked to low bone density disorder Related: See [[calcium#Hormones that Effect Blood Calcium Levels]] ___