OBJECTIVES: At the end of this laboratory, you should be able to:

1. Identify the types of bone and the components of an osteon.

2. Identify osteoblasts, osteocytes and osteoclasts, and to describe their relationships to each other and their role in bone remodeling.

3. Thoroughly describe the way in which bone develops and grows, including intramembranous versus endochondral ossification.

4. Understand how this mineralized tissue is vascularized.

SLIDES FOR THIS LABORATORY: 11, 69, 70, 74, and Supplemental Slide 109

Bone is specialized connective tissue with a calcified extracellular matrix (bone matrix) and 3 major cell types: the osteoblast, osteocyte, and osteoclast. The first type of bone formed developmentally is primary or woven bone (immature). This immature bone is later replaced by secondary or lamellar bone (mature). Secondary bone is further classified as two types: trabecular bone (also called cancellous or spongy bone) and compact bone (also called dense or cortical bone).

Slide 70 Developing bone.

Primary bone (or woven bone) is characterized by the irregular arrangement of collagen fibers, large cell number, and reduced mineral content. Note the primary bone is deposited on hyaline cartilage. Primary bone is acidophilic while the hyaline cartilage is basophilic.

Slide 69 Bone, femur.

The trabecular bone present in this slide is found mostly within the epiphysis and some in the bone marrow cavity. Osteoblasts are located immediately above the osteoid (newly formed bone matrix). Osteocytes are found within lacunae. Giant multinucleated osteoclasts, which break down bone, are occasionally found in lacunae termed Howship's lacunae. These are readily found in the ossification zone of the growth plate.

The compact bone in this slide surrounds the marrow cavity and spongy bone. Locate the periosteum (external) and endosteum (internal) linings of the bone. Note the separation of these linings is artifact of slide preparation.

Slide 74 Bone, ground preparation.

Observe the Haversian sytems (or osteons) of compact bone in this slide. The lamellae are concentrically located around a central canal (haversian canal) which contained blood vessels, nerves, and loose connective tissue. Volkmann's canals may be seen connecting haversian canals. The other lamellae of compact bone are organized into inner circumferential, outer circumferential, and interstitial lamellae. Only interstitial lamellae are seen in this slide. Also in this section, note the empty lacunae and canaliculi that housed the osteocyte and its cell processes, respectively.

Slide 11 Nasal mucosa.

Intramembranous ossification is visible in the nasal conchae on this slide. Bone arises directly within mesenchymal condensations. This process can be identified by the appearance of bone spicules (islands of bone) among mesenchymal cells. Look for the eosinophilic bone matrix. The surrounding mesenchymal cells are stellate in appearance.

Slide 69 Bone, femur.

Endochondral bone formation is represented in this slide. Bone arises by replacement of a small hyaline cartilage model. Locate the epiphyseal plate; it is the site for bone elongation. First, find the hyaline cartilage and move toward the bone marrow. Identify the 5 overlapping zones:

1. Zone of Reserve or Resting Cartilage - young small cells evenly distributed, appears as typical hyaline cartilage.

2. Zone of Cell Proliferation - chondrocytes divide, forming parallel columns.

3. Zone of Cell Maturation and Hypertrophy - cells produce collagen and ground substance

4. Zone of Cartilage Calcification - septa of cartilage matrix become calcified, cells die.

5. Zone of Ossification - osteoblasts invade cavities, and deposit bone matrix.

Supplemental Slide 109 Developing bone.

Another example of endochondral bone formation.