Cartilage and bone histology

Fibrocartilage

Fibrocartilage is a type of cartilage that should be viewed as a blend between hyaline cartilage and dense fibrous connective tissue. It is a white, densely arranged, opaque, tufted tissue with a mixture of both chondrocytes and fibroblasts. The former typically lie between bundles of type I and type II collagen fibers. Due to its unique composition and anatomical location, it is resistant to mechanical stress and has a great tensile strength. Fibrocartilage is found in the intervertebral discs, in the socket of the temporomandibular joint or in the menisci in the knee joint.

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Hyaline cartilage

Hyaline cartilage is the most common type of cartilage in the human body. It is typically characterized by a firm consistency and a smooth surface. In contrast to fibrocartilage, there is no dense and fibrillar ECM in hyaline cartilage. Light microscopy usually reveals many densely grouped chondrocytes with strongly stained cartilage capsules in hyaline cartilage. Between them, there is an extracellular ground substance made up of proteoglycans and glycoproteins, which appears much brighter and more homogeneous. The proteins in the ground substance have a high water-binding capacity, which allows the hyaline cartilage to carry out its function.

Hyaline cartilage is usually surrounded by a perichondrium. It is found in joints as articular cartilage, in costal cartilage, in the cartilaginous framework of the nose, the larynx (most parts), the trachea, the bronchi and in growing bones.

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Elastic cartilage

Elastic cartilage has a similar composition as hyaline cartilage. However, the chondrocytes present are rather isolated and there is an abundance of elastic fibers embedded in the ground substance. These fiber networks typically appear yellowish in a fresh specimen, while in histological slides, they can be easily visualized using special stains.

Elastic cartilage is always surrounded by a perichondrium. It is found in the auricle, the external auditory canal, the ear trumpet, the epiglottis, the larynx and the small bronchi, where its flexibility and resistance to bending forces and deformation is functionally important.

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Bone tissue

Bone tissue consists of various cell types such as osteoblasts, osteocytes and osteoclasts, as well as a mineralized extracellular matrix. Osteoblasts deposit bone material (protein fibers) into the bone matrix, the osteocytes are the bone cells that lie in the lacunae of the bone matrix and osteoclasts can dissolve and break down the mineralized bone matrix again.

Flat bones, like those of the cranium, consist of a layer of spongy bone, covered on either side by a layer of compact bone. Long bones are made mostly of compact bone, with lesser amounts of spongy bone and bone marrow.

Every time bone is formed, woven bones are formed first, in which collagen fibrils and bone cells are still irregularly arranged. Mature bones are called lamellar bones, where osteons form the functional units and collagen fibrils are arranged in an organized manner in the lamellar system.

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Bone tissue formation

Osteogenesis refers to the formation of bones, while ossification describes the development of bone tissue. There are two pathways of ossification: intramembranous and endochondral.

During intramembranous, direct ossification, mesenchymal tissue is directly converted into woven bone. This type of bone formation takes place in the calvarium (skull), in parts of the mandible and in the clavicle.

However, most bones are formed through endochondral, indirect ossification. Cartilage tissue (hyaline cartilage) is initially formed from aggregated mesenchymal cells, which is later broken down and ossified.

Both types of ossification play an essential role in the formation of long bones, while only endochondral ossification takes place in short bones. 

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Related articles

Articles within this topic:

• Articular cartilage
• Bone formation: Ossification
• Bone histology
• Bone matrix
• Cartilage
• Elastic cartilage histology
• Endochondral ossification
• Fibrocartilage
• Hyaline cartilage histology
• Medullary cavity
• Osteoblast
• Osteoclast
• Osteoid
• Perichondrium
• Sharpey's fibers
• Spongy bone
• Types and parts of microscopes
• Yellow bone marrow