Metaphysis. The three functions of the metaphysis are vascular invasion of the transverse septa at the bottom of the cartilaginous portion of the growth plate, bone formation, and bone remodeling (see Plate 2-17). The metaphysis begins just distal to the last intact transverse septum at the base of each cell column of the cartilaginous portion of the growth plate and ends at the junction with the diaphysis. In the first part of the metaphysis, PO₂ is low, which, together with the rouleaux formation frequently seen just distal to the last intact transverse septum, indicates that this is a region of vascular stasis.

Electron microscopy shows capillary sprouts, which are lined with a layer of endothelial and perivascular cells, invading the base of the cartilaginous portion of the plate. Cytoplasmic processes from these cells push into the transverse septa and through their proteolytic enzyme activities degrade and remove the nonmineralized transverse septa. This region of the metaphysis is known as the primary spongiosa. The longitudinal septa are partially or completely calcified, with osteoblasts lining up along the calcified bars. Between this layer of osteoblasts and the capillary sprouts are osteoprogenitor cells that contain little cytoplasm but have a prominent ovoid- to spindle-shaped nucleus.

A short distance down the calcified longitudinal septa is the region called the secondary spongiosa. Osteoblasts begin laying down bone by a process called endochondral ossification, or bone formation on a cartilage scaffold. The amount of osteoid and bone tissue formed on the calcified longitudinal septa increases downward and into the metaphysis. At the same time, the calcified septa gradually become thinner until they disappear altogether. Still farther down in the metaphysis, the original woven-fiber bone is replaced with lamellar bone. The gradual replacement of the calcified longitudinal septa with newly formed woven bone, as well as the gradual replacement of woven bone with lamellar bone, is called internal, or histologic, remodeling. Large, irregularly shaped osteoclasts are distributed evenly throughout the metaphysis (except in the primary spongiosa) and subperiosteally around the outside of the metaphysis, where it narrows to meet the diaphysis. This narrowing of the metaphysis is called external, or anatomic, remodeling.

PERIPHERAL FIBROCARTILAGINOUS ELEMENT

Encircling the periphery of the growth plate in a typical long bone are two structures: a wedge-shaped groove of cells, the ossification groove, first described by Ranvier, and a band of fibrous tissue and bone called the perichondral fibrous ring, studied by La Croix (see Plate 2-18). Although both structures are simply different parts of the peripheral fibrocartilaginous element of the growth plate, they can be considered separate entities because of their different functions.

The function of the ossification groove of Ranvier appears to be the contribution of chondrocytes for the increase in width of the growth plate. The groove of Ranvier contains round-to-ovoid cells that, on light microscopy, appear to “flow” from the groove into the cartilage at the level of the reserve zone. The perichondral fibrous ring of La Croix acts as a limiting membrane that provides mechanical support for the bone-cartilage junction of the growth plate. It is a dense fibrous band encircling the growth plate, in which collagen fibers run vertically, obliquely, and circumferentially. The structure is continuous at one end with the ossification groove and at the other end with the periosteum and subperiosteal bone of the metaphysis.

PATHOPHYSIOLOGY

Certain representative disorders whose pathophysiology exemplifies the known functions of the highly synchronized and interrelated zones in the growth plate have been identified and are shown in Plate 2-16.