The early steps of endochondral bone formation are shown in this figure. The structure seen here is the cartilage model of the bone about to be formed. The steps of bone formation are 1. The cartilage (C) cells in the center of the cartilage model become hypertrophic (HC).
2. The matrix of the cartilage becomes calcified (CM). (The calcified matrix stains intensely with hematoxylin and  appears as the darker condensed matrix material between the enlarged cartilage cells.)
Developing bone, fetal fnger, human, H&E ×60.
The bone in this figure shows later events and a continuation of the earlier ones just described. A vascular bud (not shown) and accompanying perivascular cells from the periosteum have invaded the shaft of the cartilage model, resulting in the formation of a cavity (Cav). Examination at higher magnification would reveal that the cavity contains fat cells, hematopoietic tissue (the dark-blue–staining component), and other connective tissue elements. While the new steps of bone formation occur, the earlier steps continue: 1.
Cartilage (C) cells proliferate at the epiphyses. They are responsible for production of new matrix material. It is this process that creates lengthening of the bone.
2.
Periosteal bone (PB) continues to form.
Developing bone, long bone, human, H&E ×60; inset ×200.
This shows an early stage after the invasion of the epiphysis. A secondary ossification center (Os) has formed, and along with this event, the head of the long bone will develop a marrow cavity similar in its content to that of the diaphysis. The 3. A collar of bone forms around the circumference of the center of the cartilage model. This bone is called periosteal bone (PB) because the osteoblasts that have produced the bone material develop from the periosteum. (Note that the periosteal bone is, in fact, intramembranous bone [see Plate 15], because it develops within the connective tissue membrane that immediately surrounds the developing bone and not on a spicule of calcified cartilage.)
PLATE 13 • E N DOCHON DRAL BON E FOR MATION I KEY C, cartilage Cav, marrow cavity CC, calcified cartilage CM, calcified matrix EB, endochondral bone EP, epiphyseal plate HC, hypertrophic cartilage cell JC, joint cavity Os, secondary ossification center PB, periosteal bone 3.
Cartilage cells facing the cavity become hypertrophic.
4.
Cartilage matrix becomes calcified.
5.
Erosion of cartilage occurs, creating spicules of cartilage.
6.
Bone forms on the spicules of the calcified cartilage at the erosion front; this bone is endochondral bone (EB).
As these processes continue in the shaft of the bone, one end of the cartilage model (the epiphysis) is invaded by blood vessels and connective tissue from the periosteum (periosteal bud), and it undergoes the same changes that occurred earlier in the shaft (except that no periosteal bone forms). This same process then occurs at the other end of the bone. Consequently, at each end of the developing long bone, a cartilaginous plate (epiphyseal plate) is created that lies between two sites of bone formation.
cartilage separating the two cavities is the epiphyseal plate (EP). At the early stage shown in this figure, the plate is not well defined. Despite the enlargement of the epiphyseal cavity, the remaining cartilage between the two cavities persists as a disc or plate until growth ceases. The inset shows some calcified cartilage as well as the deposition of endochondral bone (EB) within the secondary ossification center.