Bone and Cartilage Histology
Copyright 2010
Introduction
Bones are a modified connective tissue and consist of cells and extracellular matrix.
Bones are also considered individual organs and are composed of multiple tissue types including;
bone (osseous )
fibrous tissue
cartilage
adipose tissue
vascular structures
lymphatics
nervous tissue
The dense outer layer is called compact boneand the inner layer is called spongy bone
There are two main types of cells;
osteoblasts
osteoclasts
osteocytes
Bone is made up of both inorganic and organic components. The organic components are type I collagen. The inorganic components are hydroxyapatite or calcium-phosphate crystals.
The epiphysis is the end of the bone where there is articular cartilage, a secondary center of ossification, and an epiphyseal disc.
The metaphysis is the bone trabeculae at the “bottom” of the epiphyseal disc and extending to the shaft or diaphysis of the bone.
The diaphysis is the shaft of the bone. This extends from one metaphysis to the other metaphysic of the same bone.
There are three types of bone tissue; compact, spongy/cancellous, and the marrow cavity. The compact bone is dense and makes up the walls of the shaft of the bone. There is a thin cortical layer surrounding the bone. Compact bone forms an intricate system around vessels known as Haversian systems or osteons. Volkman’s canals help to connect the Haversian systems. The spongy or cancellous bone is very porous. It fills the interior of the epiphysis and extends to the diaphysis. Spongy bone has vessels around the bone. The marrow cavity is filled with bone marrow components and fat. Hematopoesis occurs in the marrow.
Periosteum is a tough dense connective tissue that adheres to the underlying bone. It is located circumferentially around the bone.
Bone and cartilage both contain collagen. There are three types of collagen; hyaline, elastic, and fibrocartilage. Only hyaline and fibrocartilage are found in the upper and lower extremity.
Hyaline cartilage is made up primarily of type II collagen but also contains type XI, IX, and X. It contains cartilage chondroitin-sulfate proteoglycans seen by electron microscopy as small granules. Hyaluronic acid helps to bind the chondroitin-sulfate proteoglycan molecules. Chondrocytes are surrounded by perichondrium unless covering an articular surface. When hyaline cartilage covers an articular surface, it is known as articular cartilage. Hyaline cartilage functions to resist compression and provide a cushioning, smooth and low friction surface for joints. It is the foundation for endochondral bone formation and growth. It undergoes calcification during endochondral bone formation. Major cells types present are chondroblasts and chondrocytes.
Fibrocartilage is made up of hyaline cartilage and dense regular connective tissue. There is both type I and type II collagen. The chondrocytes are dispersed among the collagen fibers. There is less cartilage matrix material in fibrocartilage than hyaline cartilage. There is no periochondrium surrounding the chondrocytes. It is located in interverterbal discs, the pubic symphysis, articular discs of the sternoclavicular joint and temporomandibular joint, knee menisci, triangular fibrocartilage complex of the wrist, and some places where tendons attach to bones. Its function is to resist deformation when stressed. It can become calcified. Major cell types present are chondrocytes and fibroblasts.
The function of bone is to provide protection for vital organs and neurovascular structures and to rigidly support the extremities. Another function is to mobilize stored minerals specifically calcium.
There are three major cell types in bone; osteoblasts, osteocytes, and osteoclasts. Osteoblasts help to make the osteoid matrix of the bone. Osteocytes are mature bone cells that maintain the bone matrix. Osteoclasts are multinucleate giant cells that help to resorb bone.
Bone growth can occur by two different methods; endochondral or intramembranous ossification. Endochondral ossification occurs when cartilage is replaced by bone. Intramembranous ossification is when bone is formed directly in loose connective tissue.
References
Lieberman J (ed), AAOS Comprehensive Orthopaedic Review, American Academy of Orthopaedic Surgeons, 2008.
Moore K, Dalley A (eds), Clinically Oriented Anatomy (5th edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Ross MH, Pawlina W, Histology: A Text and Atlas with Correlated Cell and Molecular Biology, Lippincott Williams & Wilkins, Philadelphia, PA, 2006.