Bone structure and function

• The lecture will describe the functions of the bones and the structure of a long bone.
• It will also explain the function of different bone cells and homeostasis in bone cell function.
• Furthermore it will explain the action of factors affecting bone.
• The components of the skeletal system are bones, joints, cartilages, and ligaments.
• The skeletal system is divided into the axial and appendicular divisions.
• The axial skeleton includes bones around the body axis such as skull bones, the hyoid, ribs, sternum, and vertebrae.
• The appendicular skeleton includes bones of the upper and lower limbs plus the shoulder and hip bones: clavicle, humerus, radius, ulna, and femur.
• Bones support the body, protect soft organs, assist in movement, maintain mineral homeostasis, produce blood cells, and store triglycerides.
• The adult skeleton has 206 bones.
• The two basic types of bone tissue are compact and spongy.
• Compact bone tissue contains few spaces and is arranged in repeating structural units called osteons or haversian systems.
• Spongy bone tissue is formed from trabecullus bone tissue and does not contain osteons.

Osteon (Haversian System)

• An osteon is a unit of bone.

Central (Haversian) Canal

• The central canal carries blood vessels, nerves, and lymphatic vessels.

Perforating (Volkman's) Canal

• The perforating canal is perpendicular to the central canal and carries blood vessels and nerves.
• Long bones are typically longer than they are wide: femur and humerus.
• Short bones are generally cube-shaped: carpals and tarsals.
• Flat bones are thin and flattened, usually curved, and have thin layers of compact bone around a layer of spongy bone: sternum, scapulae, ribs, and most skull bones.
• Irregular bones are irregular in shape and do not fit into other bone classification categories: vertebrae and hip bones.

Diaphysis

• The diaphysis is the shaft of a long bone.
• It is composed of compact bone.

Epiphysis

• The epiphysis is the ends of the bone.
• It is composed of mostly spongy bone (cancellous).

Metaphysis

• The metaphysis is the region in a mature bone where the diaphysis joins the epiphyses.

Articular Cartilage

• Articular cartilage covers the external surface of the epiphyses.
• It is made of hyaline cartilage and decreases friction at joint surfaces.

Periosteum

• The periosteum is the outside covering of the diaphysis and is a fibrous connective tissue membrane.

Sharpey's Fibres

• Sharpey's fibres secure periosteum to underlying bone.

Arteries

• Arteries supply bone cells with nutrients.

Endosteum

• The endosteum is a membrane lining the medullary cavity.

Medullary Cavity

• The medullary cavity is the cavity of the shaft.
• It contains yellow marrow (mostly fat) in adults.
• It contains red marrow (for blood cell formation) in infants.
• Bone markings are surface features of bones such as projections, processes, depressions or cavities and indentations.
• They are sites of attachments for muscles, tendons, and ligaments as well as passage ways for nerves and blood vessels.
• There are bone forming cells called osteoblasts.
• The are mature bone cells called osteocytes.
• There are bone-destroying cells called osteoclasts that break down bone matrix for remodeling and release of calcium.
• Bone remodeling is a process by both osteoblasts and osteoclasts.
• Active bone -forming cells produce collagenase bone matrix.
• Secrete enzyme alkaline phosphatase promotes deposition of calcium phosphate salts in the matrix to calcify bone.
• Osteocytes are mature bone cells.
• Multinucleated cells concerned with bone resorption.
• Remove bone matrix by phagocytosis and dissolve bone salts.
• These release calcium and phosphate ions in circulation.
• Bone formation is known as ossification.
• The is an initial bone development in embryo and foetus, growth of bone into adulthood, remodeling (replacement of old bone) and repair if fractures occur.
• Mesenchyme (early connective tissue) model.
• Initially this skeleton model is replaced by bone tissue beginning at 6 weeks of embryonic life.
• There are two different methods of ossification that each result in similar bone tissue.
• Intramembranous: bone forms within sheets of mesenchyme that resemble membranes.
• A few bones form by this process: flat bones of the skull, lower jawbone (mandible), and part of clavicle (collarbone).
• Endochondral: mesenchyme forms hyaline cartilage which then develops into bone.
• All other bones form by this process.

Intramembranous Ossification

• Mesenchyme cells → osteogenic → osteoblasts.
• Osteoblasts secrete organic matrix.
• Cells become osteocytes.
• In lacunae they extend cytoplasmic processes to each other.
• Calcium & other mineral salts are deposited.

Intramembranous covering

• Blood vessels grow in and red marrow is formed
• Periosteum covering the bone forms from mesenchyme

Endochondral Ossification

• Development of cartilage model: mesenchyme cells develop into chondroblasts.
• Cartilage "bone" grows as chondroblasts secrete cartilage matrix.
• Chondrocytes increase in size, matrix around them calcifies.
• Chondrocytes die as they are cut off from nutrients, leaving small spaces (lacunae).
• Perichondrium sends nutrient artery inwards into disintegrating cartilage.
• Osteogenic cells in perichondrium become osteoblasts that deposit bony matrix over remnants of calcified cartilage and the spongy bone forms in center of the model.
• As perichondrium starts to form bone, the membrane becomes periosteum.
• Spongy bone in center of the model grows towards ends of model.
• Octeoclasts break down some of new spongy bone forming a medullary cavity through most of diaphysis.
• Most of the wall of the diaphysis is replaced by a collar of compact bone.
• Nutrient arteries enter ends (epiphyses) of bones and osteoblasts deposit bony matrix → spongy bone forms in epiphyses from center outwards (about time of birth).
• At the cartilage ends of epiphyses become articular cartilage.
• The epiphyseal (growth) plate of cartilage remains between epiphysis and diaphysis until bone growth ceases.
• The skeleton is primarily hyaline cartilage during embryos.
• During development, much of this cartilage is replaced by bone.
• Cartilage remains in isolated areas such as the bridge of the nose, parts of ribs and joints.
• Epiphyseal plates allow for growth of long bone during childhood.
• New cartilage is continuously formed, older cartilage becomes ossified, broken down, and bone replaces cartilage.

Bone Growth

• Bones are remodeled and lengthened until growth stops.
• Bones change shape somewhat and grow in width.

Homeostasis in Bone

• Bone Resorption is action of osteoclasts and parathyroid hormone (PTH)
• Bone Deposition is action of osteoblasts and calcitonin.
• This occurs by the direction of the thyroid and parathyroid glands.
• Ca2+ blood levels are controlled by feedback loops.
• Parathyroid hormone (PTH) increases osteoclast activity and also decreases loss of Ca2+ in urine.
• Calcitonin decreases osteoclast activity.
• Bone tissue can alter its strength in response to changes in mechanical stress.
• Under stress, bone tissue becomes stronger through increased deposition of mineral salts and production of collagen fibres by osteoblasts.
• Mechanical stress is the pull of skeletal muscles and gravity.
• Necessary factors for bone development, growth and repair are minerals (Ca, P, Mg), vitamins (A, C, D), and hormones.
• Hormones such as hGH + insulin-like growth factors are required before puberty.
• Thyroid hormone and insulin are also required, and sex hormones contribute to adolescent growth spurt.
• Weight-bearing activity is also a necessary factor.
• Some factors affecting bone development, growth and repair are:
• Deficiency of Vitamin A which retards bone development.
• Deficiency of Vitamin C results in fragile bones.
• Deficiency of Vitamin D causes rickets and osteomalacia.
• Insufficient Growth Hormone causes dwarfism.
• Excessive Growth Hormone causes gigantism, acromegaly
• Insufficient Thyroid Hormone delays bone growth.
• Sex Hormones promote bone formation and stimulate ossification of epiphyseal plates.
• Physical Stress stimulates bone growth.
• Glucocorticoids activate osteoclasts and resorption of bone.