The lower inset, an enlargement of the right circle in lower figure, reveals several osteoclasts (Ocl). They are in apposition to the spicule, which is mostly cartilage. A small amount of bone is evident, based on the red-staining material in this inset. Note the light area (arrow) representing the ruffled border of the osteoclast. Examination of lower figure reveals a number of other osteoclasts (Ocl).
• KEY CM, zone of calcified matrix C, calcified cartilage EB, endochondral bone HC, zone of hypertrophic cartilage M, marrow Ob, osteoblast Oc, osteocyte Ocl, osteoclast PC, zone of proliferating cartilage RC, zone of reserve cartilage arrow, ruffled border of osteoclast R, zone of resorption
Intramembranous bone formation is limited to those bones that are not required to perform an early supporting function, e.g., the flat bones of the skull. This process requires the proliferation and differentiation of cells of the mesenchyme to become osteoblasts, the bone-forming cells. They produce ground substance and collagen. This initial matrix, called osteoid, calcifies to form bone. As the osteoblasts continue to secrete their product, some are entrapped within their matrix and are then known as osteocytes. They are responsible for maintenance of the newly formed bone tissue. The remaining osteoblasts continue the bone deposition process at the bone surface. They are capable of reproducing to maintain an adequate population for continued growth. This newly formed bone appears first as spicules that enlarge and interconnect as growth proceeds, creating a three-dimensional trabecu-lar structure similar in shape to the future mature bone. The interstices contain blood vessels and connective tissue (mesenchyme). As the bone continues to grow, remodeling occurs. This involves resorption of localized areas of bone tissue by osteoclasts in order to maintain ap-propriate shape in relation to size and to permit vascular nourishment during the growth process.
Intramembranous bone formation, fetal head, human, Mallory trichrome ×45.
A cross section of the developing mandible, 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
Intramembranous bone formation, fetal head, human, Mallory trichrome ×350.
This higher-magnification micrograph of a portion of the field in lower left figure 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 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.
Intramembranous bone formation, fetal head, human, Mallory trichrome ×175.