Plate 15 Intramembranous Bone Formation

Figure 1, fetal head, human, Mallory trichrome x45.

A cross section of the developing lower jaw bone, as seen at this relatively early stage of development, consists of bone spicules (BS) of various sizes and shapes. The bone spicules interconnect and, in three dimensions, have the general shape of the mandible. Other structures present that will assist in orientation include developing teeth (DT), the tip of Meckel's cartilage (MC), also referred to as the mandibular process, seen on the left side, and the oral cavity (OC). The bottom surface of the specimen shows the epidermis (Ep) of the underside of the chin. A large portion of the developing tongue is seen in the upper half of the figure. The tongue consists largely of developing striated visceral muscle fibers arranged in a three-dimensional orthogonal array that is characteristic of this organ.

Figure 2, fetal head, human, Mallory trichrome x 175.

This higher-magnification view of the boxed area in Figure 1 shows the interconnections of the bone spicules (BS) of the developing mandible. Within and around the spaces enclosed by the developing spicules is mesenchymal tissue. These mesenchymal cells will give rise to new osteoblasts

Figure 3, fetal head, human, Mallory trichrome x350.

This higher-magnification micrograph of a portion of the field in Figure 2 shows to advantage the distinction between newly deposited osteoid, which stains blue, and mineralized bone, which stains red. Osteoblasts are seen in two different levels of activity. Those that are relatively inactive and are in apposition to well-formed osteoid (Ob1) exhibit elongate nuclear profiles and appear to be flattened on the surface of the osteoid. Those osteoblasts (Ob-) that are actively secreting new osteoid appear as tall, columnar-like as well to the cells that will form the vascular components of the bone. The more dense connective tissue (CT) will differentiate into the periosteum on one side of the developing mandible. Other structures shown in the field include numerous blood vessels (BV) and the enamel organ (EO) of a developing tooth.

cells adjacent to osteoid. One of the spicules shows a cell completely surrounded by bone matrix; this is an osteoblast that has become trapped in its own secretions and is now an osteocyte (OC). At this magnification, the very loose connective tissue characteristics of the mesenchyme and the sparseness of the mesenchymal cells (MC) are well demonstrated. The highly cellular connective tissue (CT) on the right margin of the figure is the developing perichondrium. Some of its cells will also develop into osteoblasts to allow growth of the bone at its surface.

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