Growth in length:
A long bone grows in length my multiplication of cells in the epiphyseal plate of cartilage. The cartilage cells divide and increase in number. The zone of active division in the epiphyseal plate of cartilage lies towards the epiphysis (end of the bone). This means that newly formed cartilage cells will push the older, larger cells towards the diaphysis (shaft of the bone). Eventually these cartilage cells are replaced by osteocytes (bone cells), thus increasing the length of the bone. It should be kept in mind that after puberty, when the epiphyseal plate of cartilage no more exists, the growth in length of a bone stops completely.
Growth in thickness:
A long bone grows in thickness by multiplication of cells in the deeper layer of periosteum. The cells lying in the deeper layer of periosteum are known as osteoblasts (bone forming cells). These cells divide continuously and form the osteocytes, thus increase the thickness of bone.
Remodeling of bone:
The growth take place by deposition of new bony tissue on the surface and at the ends. The is process of bone deposition by osteoblasts is called appositional growth or surface accretion. The appositional growth can result in shape alterations, however, in order to maintain the shape, the unwanted bone must be removed. This job is done by another class of cells known as octeoclasts. The process of reshaping the bone is called remodeling and it is one of the major factors responsible for increasing size of marrow cavity.