Bone Tissue – Tutorial
Please read Unit 2 – Introduction to Connective Tissues prior to completing the activities in this chapter.
Introduction to Bone Tissue
Bone tissue (sometimes referred to as osseous tissue) is a supporting connective tissue that forms the rigid structural framework of the body. Bone tissue by volume consists of very few cells (less than 2%) scattered throughout vast expanses of extracellular matrix (approximately 98%). The extracellular matrix of bone tissue consists of approximately two-thirds hardened calcium salts (mainly calcium phosphate), one-third collagen fibers, and less than 1% liquid ground substance. Together, the matrix of hardened calcium salts permeated with strong yet flexible collagen fibers allows bone to be rigid enough to resist compression yet flexible enough to resist shattering.
The primary types of cells found in bone tissue are osteoblasts, osteocytes, and osteoclasts. Osteoblasts actively secrete inorganic substances that harden into calcium salts and proteins that assemble into collagen fibers within the extracellular matrix. After an osteoblast becomes completely surrounded by solid matrix it is referred to as an osteocyte (mature bone cell) which exists in a tiny space known as a lacuna. Osteocytes continue to secrete substances that support and maintain healthy bone matrix. It is important to note that an osteocyte can revert back to an osteoblast and actively work to build new bone matrix if it is released from its lacuna by a fracture or dissolution of its surrounding matrix. Osteoclasts are large multinucleated cells that are more closely related to macrophages than they are to other bone cells. Osteoclasts work to dissolve or break down bone matrix in a process known as osteolysis which releases dissolved calcium salts into interstitial fluid.
Two distinct forms of bone tissue are found in human bones. Dense (compact) bone forms the superficial outer shell of all bones. As its name implies, dense bone is densely packed with ossified matrix with very little open space. Spongy (cancellous) bone is found deep to dense bone in most bones of the body. Spongy bone exhibits many open spaces between arches of bone matrix. The open spaces of spongy bone are filled with red bone marrow which is a mixture of blood stem cells, blood vessels, and adipocytes. Dense bone and spongy bone will be examined separately in the tutorial activities that follow.
