In the formation of membrane bone, individual shafts of bone, known as trabeculae, are laid down (see Plate 1-10). Trabeculae increase in length and thickness and join each other at various points to produce a lattice framework of primary trabecular bone. At the outer surface of the bone rudiment, the dense sheath of connective tissue acquires an inner layer of osteoblasts to become the periosteum. The osteoblastic layer lays down bone in the form of subperiosteal layers, or lamellae. The coalescing trabeculae in the deeper parts of the rudiment surround capillaries and nerves. Bone is laid down in layers on these trabeculae to constitute the lamellae of primary trabecular bone. Up to the time of birth, the bones of the fetal skeleton are made up chiefly of this type of bone, but near the time of birth, this primary trabecular bone begins to transform into compact bone (see Plate 1-11).

The transformation from trabecular to compact bone is essentially the reduction in the size of the marrow spaces containing mesenchymal cells, capillaries, and nerve fibers. The relatively large marrow spaces with their surrounding bony trabeculae are known as primary osteons. The osteoblasts lining the trabeculae surrounding a marrow space (which contains one or two capillaries, some perivascular cells, and a nonmyelinated and occasionally a myelinated nerve fiber) lay down bone in concentric layers, or lamellae. This process continues until the marrow space is nearly obliterated, leaving a small central osteonal, or haversian, canal. The canal is about 50 µm in diameter and usually contains a single capillary and nerve fiber and some perivascular cells in the center of what is known as a secondary osteon (haversian system). There are from 4 to 20 (usually 6 or less) concentric lamellae that are each 3 to 7 µm thick. The formation of many such adjacent secondary osteons converts what was originally trabecular bone into compact bone. In the central core of a membrane bone, the marrow cavities persist and their mesenchymal tissue develops into hematopoietic red bone marrow. Thus, in a fully formed, flat bone of the calvaria, there is an inner and outer table of compact bone, between which is trabecular bone surrounding a marrow cavity, the diploë.

The secondary osteons of compact bone usually run the length of a bone. In cross section, the outer limit of each osteon is clearly demarcated by a narrow refractile ring known as a cement line, which lacks collagen fibrils and is highly mineralized. The central haversian canals are connected to one another and communicate with the periosteal surface as well as with the marrow cavity via transverse and oblique channels known as Volkmann’s canals. The blood flows through the compact bone from the inner marrow cavity via vessels in Volkmann’s and haversian canals until it emerges at the periosteal surface.

Cartilage Bone. The cartilage rudiments of bones of endochondral origin are temporary miniatures of the future adult bone. With the exception of the clavicle, the long bones are of endochondral origin. The first of two or more ossification centers of a long bone appears in the shaft, or diaphysis (see Plate 1-12). Diaphyseal ossification is actually a form of intramembranous ossification, because bone is laid down by the connective tissue outer sheath of the cartilage rudiment known as the perichondrium. The perichondrium becomes known as the periosteum once it starts to lay down bone in the form of a delicate collar surrounding the center of the diaphysis of the cartilage rudiment. Deep to this collar of bone, the cartilage matrix becomes calcified and the chondrocytes hypertrophy.