Skeletal System: Bones and Tissues

• The skeletal system includes bones (skeleton), joints, cartilages, and ligaments.
• The skeletal system's two divisions are the axial (bones around the body axis) and appendicular (bones of the upper and lower limbs & shoulder & hip bones).

Functions of Bones

• Support of the body
• Protection of soft organs
• Assistance in movement
• Mineral homeostasis
• Blood cell production
• Triglyceride storage
• The adult skeleton has 206 bones.
• There are two basic types of bone tissue: compact and spongy.
• Compact bone tissue contains few spaces and is arranged in repeating structural units called osteons/Haversian systems.
• Spongy bone tissue is formed of trabecular bone tissue and does not contain osteons.

The Osteon (Haversian System)

• It is a unit of bone, a cylindrical structure that helps give bones strength.
• It consists of a Central (Haversian) canal, which carries blood vessels, nerves, & lymphatic vessels.
• A Perforating (Volkman's) canal is perpendicular to the central canal & carries blood vessels and nerves

Classification of Bones

• Long bones are typically longer than they are wide, have a shaft with heads at both ends, and contain mostly compact bone. Examples are the femur and humerus.
• Short bones are generally cube-shaped and contain mostly spongy bone. Examples are carpals (wrist bones) and tarsals (ankle bones).
• Flat bones are thin, flattened, and usually curved, with thin layers of compact bone around a layer of spongy bone. Examples are the sternum (breastbone), scapulae (shoulder blades), ribs & most of skull bones.
• Irregular bones are irregular in shape, do not fit into other bone classification categories. Examples are vertebrae & hip bones.

Gross Anatomy of a Long Bone

• Diaphysis (shaft) is the long, middle part of the bone made of compact bone, which gives strength.
• Epiphysis is the ends of the bone and mostly made of spongy bone (cancellous bone).
• Metaphysis is the regions in a mature bone where the diaphysis joins the epiphyses (the region between the diaphysis & epiphysis).

Structure of Long Bone

• Articular cartilage covers the external surface of the epiphyses, is made of hyaline cartilage, and decreases friction at joint surfaces.
• Periosteum is the outside covering of the diaphysis and a fibrous connective tissue membrane.
• Sharpey's fibers secure the periosteum to the underlying bone.
• Arteries supply bone cells with nutrients.
• Endosteum is a membrane lining the medullary cavity.
• Medullary cavity is the cavity of the shaft that contains yellow marrow (mostly fat) in adults and red marrow (for blood cell formation) in infants.
• Bone markings are special features on the surface of bones and include projections & processes (parts of the bone that stick out) and depressions or cavities (indentations in the bone). These markings provide sites of attachments for muscles, tendons & ligaments and passages for nerves & blood vessels.

Types of Bone Cells

• Osteoblasts are bone-forming cells that create new bone, produce collagen for the bone matrix, and secrete alkaline phosphatase, which promotes the deposition/removal of calcium phosphate salts to calcify/harden the bone.
• Osteocytes are mature bone cells that maintain bone structure.
• Osteoclasts are bone-destroying cells that break down bone matrix for remodeling and release of calcium and are multinucleated and responsible for bone resorption.

Bone Matrix

• It is the framework of bone made up of minerals like calcium and collagen fibres.
• Bone remodeling is a process by both osteoblasts & osteoclasts that builds new bone & break down of old bone.

Bone Formation (Ossification)

• Embryo & foetus: initial bone development, where bone first starts to develop
• Childhood & adulthood: bone continues to grow up until adulthood
• Remodelling: is replacement of old bone throughout life
• Fracture repair: if a bone breaks, the body repairs it
• Mesenchyme (early connective tissue) model is when, in early embryo, the skeleton is made of soft connective tissue called mesenchyme. At around 6 weeks of embryonic life, this soft model starts getting replaced by hard bone tissue.
• Bones can form (ossification) in two ways/methods, but both result in the similar hard bone tissue:

Intramembranous Ossification

• Bone forms within sheets of mesenchyme that resembles membranes. Only a few bones form by this process, which are the bones of the skull, lower jawbone (mandible), & a part of the clavicle (collarbone).
• Development of ossification center: Mesenchyme cells become osteogenic osteoblasts, which secrete organic matrix.
• Calcification: Cells become osteocytes. In lacunae (small spaces), they extend cytoplasmic processes to each other and deposit (accumulate or store) calcium & other mineral salts.
• Formation of trabeculae (spongy bone): Blood vessels grow in & red marrow is formed.
• Periosteum covering the bone forms from mesenchyme: The outer covering of the bone (periosteum) develops from mesenchyme.

Endochondral Ossification

• Mesenchyme forms a hyaline cartilage which then develops into bone and all other bones form by this process. (starts as cartilage before becoming bone)
• Development of cartilage model of the "bone": Mesenchyme cells develop into chondroblasts.
• Growth of cartilage model: Cartilage "bone" grows as chondroblasts secrete cartilage matrix, chondrocytes increase in size, and matrix around them calcifies. Chondrocytes die when cut off from nutrients, therefore leaving small spaces (lacunae).
• Development of Primary ossification center: Perichondrium sends a 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, and as perichondrium starts to form bone, the membrane is called periosteum.
• Development of medullary (marrow) cavity: Spongy bone in center of the model grows towards ends of model, osteoclasts break down some of the new spongy bone forming a cavity (marrow) through most of diaphysis, and most of the wall of the diaphysis is replaced by a collar of compact bone.
• Development of secondary ossification center: Like step 3 except that nutrient arteries enter ends (epiphyses) of bones and osteoblasts deposit bony matrix spongy bone forms in epiphyses from center outwards and occurs about time of birth.
• Formation of articular cartilage is when hyaline cartilage at ends of epiphyses becomes articular cartilage and the epiphyseal (growth) plate of cartilage remains between epiphysis and diaphysis until bone growth ceases.

Changes in the Human Skeleton

• In embryos, the skeleton is mostly hyaline cartilage (soft cartilage).
• During development, most of this cartilage is replaced by bone.
• Some cartilage remains in isolated areas such as the bridge of the nose, parts of ribs & in joints.
• Epiphyseal plates allow for growth of long bone during childhood.
• New cartilage is continuously formed and older cartilage becomes bone (ossification).
• Cartilage is broken down & bone replaces cartilage (making the bone longer).
• Bones are remodelled and lengthened until growth stops (bones change shape & grow in width).

Homeostasis in Bone

• Bone resorption (breaking down bone) involves Osteoclasts breaking down bone & release calcium into the blood & parathyroid hormone (PTH) signals this when the body needs more calcium.
• Bone deposition (building new bone) involves Osteoclasts creating new bone by depositing calcium and Calcitonin from the thyroid helps by lowering blood calcium and promoting bone formation.
• Parathyroid hormone PTH causes an increase in osteoclast activity & also decreases calcium loss in urine.
• Calcitonin causes a decrease in osteoclast activity.
• Bone tissue can become stronger when it undergoes or experiences mechanical stress such as exercise.

Action of Factors Affecting Bone

• Adequate minerals (Ca, P, Mg) and Vitamins A, C & D are needed for bone development.
• Before puberty, hGH + insulin-like growth factors, thyroid hormone and insulin, and sex hormones contribute to teenage growth spurt.
• Weight-bearing activity/exercise
• Deficiency of Vitamin A retards/slows bone development.
• Deficiency of Vitamin C results in fragile bones.
• Deficiency of Vitamin D weakens bones and can result in rickets & osteomalacia.
• Insufficient Growth Hormone can cause dwarfism.
• Excessive Growth Hormone can cause gigantism (tall children) & acromegaly (large hands, feet & face).
• Insufficient Thyroid Hormone low levels delay bone growth.
• Sex Hormones promote bone formation & stimulate ossification of epiphyseal plates.
• Physical Stress stimulates bone growth.
• Glucocorticoids these hormones activate osteoclasts and resorption of bone (bone breakdown).