Bone and Cartilage (Anatomy)

Question 1

Describe the structure of cartilage

Answer 1

• Semi-rigid, strong but slightly flexible.
• The matrix is rich in glycosaminoglycans and contains collagen fibres that range from fine to coarse.
• Avascular so cells in cartilage have to get their oxygen and nutrients by long-range diffusion.

Question 2

Distinguish between hyaline cartilage, fibrocartilage, and elastic cartilage

Answer 2

Hyaline cartilage

• Clear, glassy appearance, which is due to the fineness of its collagen fibres.
• Appears pale blue/purple when stained with H&E.
• Matrix contains collagen type 2 embedded in a hydrated gel of proteoglycans and structural glycoproteins. Chondrocytes sit in the matrix in lacunae.
• Surrounded by a sheath of dense irregular connective tissue called the perichondrium.
• Found on articular surfaces of bone, trachea/bronchus, and the nose.

Fibrocartilage

• Intermediate between dense irregular connective tissue and hyaline cartilage.
• Coarse bundles of collagen.
• Chondrocytes arranged in long rows separated by collagen type 1 fibres.
• No perichondrium.
• Found in tendon insertions, pubic symphysis, and intervertebral disks.

Elastic cartilage

• Contains abundant elastic fibres and collagen type 2 fibres.
• Flexible but able to spring back into position.
• Surrounded by a perichondrium.
• Found in the external ear and epiglottis.

Question 3

Describe the structure of a long bone

Answer 3

• Outer shell of compact bone.
• The compact bone encloses the medullary cavity which contains bone marrow.
• The shaft of a long bone is called the diaphysis, and the expanded head at each end is called the epiphysis.
• The central cavity of each epiphysis is occupied by trabecular bone.
• The epiphyseal line is a line of slightly denser trabecular bone between the epiphysis and diaphysis.
• The joint surface of the bone is covered with a layer of hyaline cartilage called the articular cartilage.
• Blood vessels penetrate into the bone through small holes called nutrient foramina.
• The bone is covered with a sheath called the periosteum consisting of an outer fibrous layer of collagen and an inner layer containing osteopenia cells.
• Some collagen fibres of the periosteum penetrate into the bone matrix as perforating fibres.
• A thin layer of reticular connective tissue and osteogenic cells, called the endosteum, lines the internal cavities of the bone.

Question 4

Describe compact bone

Answer 4

• The basic structural unit of compact bone is the osteon (haversian system).
• An osteon consists of a central canal surrounded by concentric lamellae (layers of matrix).
• The concentric lamellae are connected with each other by canaliculi.
• Central canals of osteons are joined along their length by perforating canals. Both types of canals are lined with endosteum.
• Collagen fibres corkscrew down the matrix of the lamellae.
• Blood vessels and nerves enter bone tissue through nutrient foramina on the surface of bone, which open into the perforating canals that cross the matrix and feed into the central canals.
• The inner and outer boundaries are arranged in circumferential lamellae that run parallel to the bone surface.
• Between osteons there are interstitial lamellae consisting of the remains of old osteons that broke down as the bone grew and remodelled itself.

Question 5

Describe trabecular bone

Answer 5

• Consists of a lattice of spicules and trabeculae.
• Covered with endosteum and permeated by spaces filled with bone marrow.
• The matrix is arranged in lamellae but it has few osteons.
• The trabeculae are arranged along the bone’s lines of stress.

Question 6

Describe the appearance and function of osteoprogenitor cells

Answer 6

• Stem cells that develop from embryonic mesenchyme.
• Develop into most bone cell types.
• They occur in the endosteum and inner layer of the periosteum.

Question 7

Describe the appearance and function of osteoblasts

Answer 7

• Roughly cuboidal.
• Produce the organic elements of the matrix (collagen, proteoglycans, glycoproteins).
• Presence of viable osteoblasts required before inorganic elements can be deposited.
• Secrete the hormone osteocalcin which is a structural protein of bone in addition to stimulating insulin secretion by the pancreas, increasing insulin sensitivity in adipocytes, and limiting the growth of adipose tissue.

Question 8

Describe the appearance and function of osteocytes

Answer 8

• Occur in small spaces in the mineralised matrix called lacunae.
• Lacunae are interconnected by canaliculi which permit flow of oxygen and nutrients through the matrix.
• Cytoplasmic processes of osteocytes extend into the canaliculi.
• Osteocytes maintain the bone matrix and act as strain sensors when a load is applied to the bone.

Question 9

Describe the appearance and function of osteoclasts

Answer 9

• Derive from the monocyte line.
• Very large, motile cells. Multinucleate with 5-50 nuclei.
• The side of the osteoclasts facing the bone surface has a ruffled border with deep infoldings of the plasma membrane. This increases cell surface area and therefore increases the efficiency of bone resorption.
• Bone remodelling results from the combined action of osteoblasts and osteoclasts.