The Skeletal System and The Osteon

This chapter will expand upon what you learned in the Introduction to Bone Tissue chapter. In this chapter, we will review osseous tissue (bone tissue) and its role in the skeletal organ system. Osseous tissue is a hard , strong tissue where the cells (osteocytes) are surrounded by a solid extracellular matrix enriched with minerals (calcium and phosphate) and collagen fibers that run in a determined direction to resist fracture.

Types of Bone Tissue

Osseous tissue is divided into two types: compact bone and spongy (also known as cancellous or trabecular) bone.

Compact bone is incredibly strong at resisting tension and compression, which makes it perfect for our skeleton. Compact bone consists of closely packed structures called osteons. Osteons consist of a central (Haversian) canal surrounded by concentric rings/layers of bone matrix called lamellae, where collagen fibers run in alternating directions. The central canal provides an opening for blood vessels and nerves. Figures 1 and 2 show illustrations of lateral and superior views of compact bone under low power magnification. For simplicity the vessels and nerves found within the central and perforating canals are shown as red lines in Figure 1.

Sitting between the lamellae are pockets called lacunae in which the osteocytes are located (shown in blue in Figures 1, 2, and 4). Radiating out from the lacunae are tiny channels called canaliculi that allow communication between osteocytes and the central canal (allowing access to oxygen and nutrients) (Figure 3). The central canals are connecting to one another by perpendicular canals called perforating (Volkmann’s) canals.

Figure 1: Longitudinal view of compact bone at low power showing the central and perforating canals of several osteon

Figure 2: Cross-sectional view of compact bone at low power showing the central and perforating canals of several osteon

Figure 3: Osseous tissue with and without illustration overlay

Figure 4: Superior (cross-sectional) view of compact bone at high power showing the structural composition of a single osteon

Spongy bone is less dense/heavy than compact bone because it consists of very small plates/spikes of bone matrix called trabeculae. Between the trabeculae are spaces filled with red bone marrow for blood cell production.

Check out our YouTube video to help you understand Osseous Tissue:

YouTube Video – Osseous Tissue

Skeletal Organ System Organization

Our bones are part of our skeletal organ system, but they contain more than just osseous tissue. In addition to osseous tissue, our skeleton contains blood vessels, nerves, and several types of connective tissue. We will use long bones as an example of the general internal structure of bone. The ends of the long bone that articulate with other bones are covered by articular cartilage (hyaline).  Long bones have a long central shaft (diaphysis) containing the  marrow-containing medullary cavity surrounded by compact bone. The osteons in this compact bone run parallel with the long-axis of the bone. For example, the osteons in the humerus or femur would run vertically up your arm or leg, respectively. The ends of long bone [epiphysis (singular) or epiphyses (pleural)] contain a core of spongy bone surrounded by an outer covering of compact bone.

Figure 5 shows an illustration of the external and internal features of a long bone (femur). The ends of the femur are the epiphyses (singular, epiphysis) the shaft is the diaphysis. The external view (Figure 5, left panel) shows the articular (joint) surfaces of proximal and distal epiphysis covered with hyaline cartilage (blue).  Spanning between the epiphyses is the diaphysis, which contains yellow bone marrow in the medullary cavity (shown on right panel). The right panel of Figure 5 highlights the internal spongy bone composition of the epiphyses, as well as the connective tissue coverings (periosteum and endosteum).

Figure 5: Illustration of the external and internal structures of a long bone

Membranes of Bone

The internal surfaces are lined by connective tissue called the endosteum. The endosteum can be seen in Figure 4 and Figure 5. The external, non-articular surfaces are lined by connective tissue called the periosteum. The periosteum can be seen in Figure 5. Both of these layers contain osteogenic cells and osteoblasts, which deposit bone matrix.

Chapter Illustrations by:

Jaylan Richardson

Soma Mukhopadhyay, Ph.D.