The myeloid, or bone marrow, period of hematopoiesis begins during the fourth month. The bone marrow is the principal site of all blood cell formation during the last 3 months before birth, at which time only residual hematopoiesis occurs in the liver and spleen.

SYMPHYSEAL JOINTS

In the first vertebrates, the skeleton evolved as an axial skeleton, the vertebral column. The segmentation that evolved in the increasingly substantial column allowed the necessary swimming movements that the flexible notochord afforded the prevertebrates. Intervening regions between the firmer segments of the column became pliable cartilage that allowed very limited and yet every possible type of motion between the firmer segments. Thus, in humans, intervertebral discs between the vertebral bodies allow a limited degree of twisting and bending in all directions. However, the sum total of a given motion occurring between the vertebral bodies throughout the column is considerable.

The multiaxial joint between the vertebral bodies is known as a symphysis because of its structure. A central portion of fibrocartilage, including the nucleus pulposus, blends with a layer of hyaline cartilage lining the surface of each of the two vertebral bodies bordering the joint. The only symphysis of the appendicular skeleton is the pubic symphysis (see Plate 1-7).

Because a symphyseal joint has limited motion, it is an amphiarthrosis. A central cleft containing fluid occurs in some symphyses, such as the pubic and manubriosternal (sternal angle) joints, but true gliding surfaces do not develop (see Plate 1-5). This is an intermediate phase in the evolution of synovial joints.

Although the majority of articulations of the appendicular skeleton are synovial joints, many of the articulations of the axial skeleton are also typical synovial joints. For example, the numerous joints between the articular processes of the vertebral arches are synovial joints of the plane variety in that their apposed articular surfaces are fairly flat (see Plate 1-4).

SYNOVIAL JOINTS

Synovial, or diarthrodial, joints have a wide range of motion; they link cartilaginous bones with one another and with certain membrane bones, such as the mandible and clavicle.

The earliest mesenchymal rudiments of long bones are essentially continuous. As the rudiments pass into the precartilage stage, the sites of the future joints can be discerned as intervals of less concentrated mesenchyme (see Plate 1-15). When the mesenchymal rudiments transform into cartilage, the mesenchymal cells in the future joint region become flattened in the center. At the periphery of the future joint, these flattened cells are continuous with the investing perichondrium; this perichondral investment becomes the joint capsule.

During the third month, the joint cavity arises from a cleft that appears in the circumferential part of the mesenchyme. The mesenchymal cells in the center of the developing joint disappear, allowing the cartilage rudiments to come into direct contact with each other, and, for a time, a transitory fusion may result in a small area of direct cartilaginous union. Soon, all the remaining mesenchymal cells undergo dissolution and a distinct joint cavity is formed. The surrounding joint capsule maintains its continuity with the perichondrium when it becomes transformed into periosteum as the cartilage rudiments become bones. The original cartilage of the rudiment forming the joint surface is retained as the articular hyaline cartilage.