LAB FOUR: BONE

A. Compact Bone (ground bone, slide 21) The basic functional unit of compact bone is the osteon or Haversian system (shown below). Osteons consist of bone that has been deposited in concentric rings (lamellae) around a Haversian canal (H) that is occupied by blood vessels, nerves, and lymphatics. Between concentric lamellae the cell bodies of bone cells (osteocytes) occupy lacunae (L) and send fine cytoplasmic projections toward either adjacent osteocytes or toward the Haversian canal in small canals termed canaliculi (C). Still other channels, called Volkmann's canals, enter the bone perpendicular to the haversian canals from the periosteum and endosteum.

B. Cancellous (spongy) Bone (slide 20) Cancellous (spongy) bone is in many ways similar to compact bone; however, the ossified trabeculae are thin and surrounded by bone marrow and blood vessels. Osteons are mostly absent, and the spaces between spicules are filled with hemopoietic tissue. In a typical long bone, the shaft, or diaphysis, is composed predominantly of compact bone. The terminal ends, the epiphyses, are essentially cancellous bone covered with a thin shell of compact bone.

C. Endochondral Bone Formation (slide 20) Endochondral bone formation is a process in which a cartilage model of bone is formed prior to actual bone deposition. Bone growth depends on continued proliferation of cartilage cells (chondrocytes). Endochondral bone growth occurs at the ends, or epiphyses, of long bone in an area known as the epiphyseal growth plate. The epiphyseal growth plate is divided into five zones, starting from the epiphyseal side of cartilage: 1) The resting zone (not shown) consists of hyaline cartilage without morphologic changes in the cells. 2) In the proliferative zone, chondrocytes divide rapidly and form columns of stacked cells parallel to the axis of the bone. 3) The hypertrophic cartilage zone contains chondrocytes (and their lacunae) that have enlarged. 4) In the zone of calcification, chondrocytes die leaving large empty holes. Osteoblasts and blood vessels invade the area and begin secreting bone matrix. 5) In the zone of ossification, osteoid, or bone matrix, is formed..

D. Intramembranous Bone Formation (slide 23) Intramembranous bone formation is the mechanism used to form most of the flat bones in the body, particularly those of the face and skull. In intramembranous bone formation, osteoblasts secrete bone matrix (osteoid); the matrix then mineralizes to form bone proper. Some of the osteoblasts become trapped in the forming bone and become osteocytes. Osteoclasts are large, multinucleated cells that function in bone breakdown and absorption.