The Skeletal System

THE SKELETAL SYSTEM

The skeletal system is composed of three types of organs – bones, cartilages and ligaments tightly joined together to form a strong flexible organ system. 

Cartilage is the forerunner of host bones in embryonic and childhood development and also covers many joints surfaces in a mature skeleton.

Ligament: holds bones together at the joints.

Tendons: are structurally similar to Iigament but attach muscle to bones.

ANATOMICAL TERMS

The following terms are projections that are sites of muscle and ligament attachment:

• Tuberosity; large rounded projection and may be roughened.
• Crest: narrow ridge of bones usually prominent,
• Trochanter: very large blunt, irregularly shaped process (the only examples are on the femur).
• Tubercle: small rounded projection or process.
• Spine: sharp, slender, often pointed projection.
• Process: any bony prominence.
• Styloid process: This is a downward projection of bone which gives attachment to muscles and ligament.

Projections that help to form joints

• Facet: smooth, nearly flat articular surface.
• Condyle: rounded articular projection.

Depression and openings allowing blood vessels and nerves to pass.

• Meatus: canal-like pathway.
• Sinus: cavity within a bone filled with air and lined with mucous membrane.
• Fossae: shallow, basin-like depression in a bone often serving as an articular surrace.
• Fissure or Cleft: narrow, slit-like opening.
• Foramina/Foramen: round or oval opening through a bone

Other terms

• Articulation: This is a joint between two or more bones
• Suture: This is the name given to an immovable joint e.g, between the bones of the skull.
• Articulating surface: This is a part of a bone which enters into the formation of a joint.

The Skeleton

The skeleton is the bony framework of the body. It is often stated that there are 206 bones in the skeleton, but this is only a typical adult count, as at birth, there are about 270 bones.

Functions of the Skeleton

The skeleton performs numerous functions which include:

Support

Bones of the legs, pelvis and vertebral column hold up the body, the mandible supports the teeth; nearly all other soft organs are directly or indirectly supported by nearby bones.

Protection 

The cranium encloses the brain; the spinal column encloses the spinal cord, the ribcage encloses the heart and lungs; the pelvis encloses organs of the reproductive system and lower urinary and digestive tracts; most bones enclose bone marrow.

Movement

Skeletal muscle would serve no useful purpose if not for the rigid attachment and leverage provided by bones. Leg and arm movements are the most obvious examples. A less obvious one is that ventilation of the lungs depends on movement of the ribs by skeletal muscles.

Blood formation

Red bone marrow is the major producer of blood cells, including most cells of the immune system

Electrolyte balance

The skeleton is the body’s main mineral reservoir. It stores calcium and phosphate and release them according to the body’s physiological needs 

Acid-base balance

Bone buffers the blood against excessive pH changes by absorbing or releasing alkaline mineral salts.

Detoxification

Bone tissue removes heavy metals and other foreign elements from the blood and helps reduce their effects on nervous and other tissues. It can later release these more slowly for excretion.

Mineral Storage

The skeleton stores essential minerals like calcium and phosphorus, releasing them into the bloodstream when needed for bodily functions.

Energy Storage

Fat is stored in yellow marrow found in certain bones, serving as an energy reserve for the body.

Endocrine Regulation

Bones secrete hormones that help regulate calcium levels in the body, impacting overall hormonal balance.

Balance and Stability

The skeleton provides balance and stability during activities like walking, running, and standing.

Joint Formation

Bones form joints, enabling smooth movements between different parts of the body.

Reduction of Friction

Cartilage at the joints reduces friction and prevents bone damage during movement.

Thermal Regulation

Bones play a role in maintaining body temperature through their interactions with the muscular and nervous systems.

Storage of Growth Factors

Growth factors and cytokines are stored in bone matrix, aiding in tissue repair and remodeling.

Sound Transduction

Bones in the ear participate in sound transmission, allowing us to hear.

Dental Anchorage

The jawbone anchors teeth, providing a stable foundation for chewing and speaking.

Posture Maintenance

The skeleton’s structure contributes to maintaining an upright posture.

Reservoir for Toxins

Bones can accumulate and store certain toxins and heavy metals, reducing their harmful effects on other organs.

Mechanical Protection

In addition to protecting organs, bones shield nerves and blood vessels from external pressure and injury.

Joint Stability

The skeletal structure ensures joint stability, preventing dislocation and injury during movements.

Blood Vessel Protection

Bones protect blood vessels that pass through or alongside them, preventing compression and damage.

Growth and Development of skeleton

The skeleton plays a crucial role in the growth and development of the body, particularly during childhood and adolescence.

Bone

Bone is a rigid form of connective tissue that forms most of the skeleton and is the chief supporting tissue of the body. It is composed of water 20% organic material 30-40% and inorganic material 40-50%

Bone Development

This is also referred to as ossification or osteogenesis. In embryonic stage, this process leads to formation of the bony skeleton. Later growth goes on until early childhood as the body continuous to increase in size. Bones are capable of growing in thickness throughout life. However, Ossification in adults serves mainly for bone remodeling and repair. This process begins before 8 weeks of foetal life and is not complete until about age 25.

The formation of bones is in two forms which are

• Intramembranous ossification
• Endochondral ossification

Intramembranous Ossification

This results in the formation of cranial bones of the skull (frontal, parietal, occipital and temporal bones) and the clavicles. All bones formed by this process are flat bones. Fibrous Connective tissue membranes formed by mesenchymal cells are the supporting structure on which ossification begins at about 8th week of development.

Endochondral Ossification

This mechanism of bone formation is characterized by the presence of cartilaginous model of the developing bones. The term endochondral refers to the close association of the developing bone with the pre-existing hyaline cartilage model of the bone.

Except for the clavicles, essentially all bones of the skeleton below the base of the skull are formed by endochondral ossification

 In the long bones the focal points from which ossification begin are small areas of osteogenesis cells or centers of ossification in the cartilage models. This is accompanied by development of bone collar at about 8 weeks of gestation. Later the blood supply develops and bone tissue replaces cartilage as osteoblasts secrete osteoid components in the shaft. The bone lengthens as ossification continues and spreads to the epiphyses. Around birth, secondary centers of ossification develop in the epiphyses and medullary canal forms when osteoclasts break down the central bone tissue in the middle of the shaft. After birth, the bone grows in length by ossification of the diaphyseal surface of the epiphyseal cartilages and growth is complete when the cartilages become completely ossified.

Bone Growth

Bones increase in size only by appositional growth, the formation of new bone on the surface of older bone or cartilage. For example trabeculae grow in size by the deposition of new bone matrix by osteoblasts onto the surface of the trabeculae.

Growth in bone Length

Long bones and bony projections increase in length because of growth at the epiphyseal plate.

In a long bone, the epiphyseal plate separates the epiphysis from the diaphysis. Long projections of bones, such as the processes of vertebrae also have epiphyseal plates.

Growth followed by appositional bone growth on the surface of the cartilage.

Growth at Articular cartilage

Growth at the epiphyseal plate involves the formation of new cartilage by interstitial cartilage

Epiphyses increase in size because of growth increase at the articular cartilage. In addition growth at the articular cartilage increases the size of bones that do not have an epiphysis. such as short bones. The process of growth in articular cartilage is similar to that occurring in the Epiphyseal plate, except that the chondrocyte columns are not as obvious. The chondrocytes near surface of the articular cartilage are similar to those in the zone of resting cartilage of epiphyseal plate. In the deepest part of the articular cartilage, nearer bone tissue, the cartilage is calcified, dies and is ossified to form new bone. When the epiphyses reach their full size, the growth of cartilage and its replacement by bone ceases. The articular cartilage, however, persists throughout life and does not become ossified as does the epiphyseal plate.

Growth in Bone Width

Long bones increases in width and other bones increase in size or thickness because of appositional bone growth beneath the periosteum. When bone growth in width is rapid, osteoblast from the periosteum lay down bone to form a series of ridges with grooves between them. The periosteum covers the bone ridges and extends down into the bottom of the grooves, and one or more blood vessels of the periosteum lies within each groove. As the osteoblasts continue to produce bone, the ridges increase in size, extend toward each other and meet to change the groove into a tunnel. The name of the periosteum in the tunnel changes to endosteum because the membrane now lines an internal bone surface. Osteoblast from the endosteum lay down bone to form a concentric lamella. The production of additional lamellae fill in the tunnel, encloses the blood vessel and produces an osteon. When bone growth in width is slow, the surface of the bone becomes smooth as osteoblasts the periosteum lay down even layers of bone to form circumferential lamellae. The circumferential lamellae are broken down during remodeling to form osteons.

Bone Remodelling

Just as we renovate or remodel our homes when they become outdated, when bone becomes old , it’s replaced with new bone in a process called bone remodelling. In this process, osteoclast removes old bone and osteoblast deposits new bone. Bone remodelling converts Woven bone into lamellar bone, and it is involved in

• bone growth
• changes in bone shape
• the adjustment of the bone to stress bone repair and 
• calcium ion regulation in the body

For example, as a long bone increases in length and diameter, the size of the medullary cavity also increase. Otherwise, the bone would be very heavy.

Another type of remodelling is a change in size and contour to accommodate the growing body. The femurs become longer, the curvature of the cranium changes to accommodate a large brain. Remodelling also adopts bones to the amount of gravitational force and muscular tension exerted on them. The prominence of bone processes and the density of bone depend on the amount of stress to which the bone has been subjected. On average, the bones have more density and mass in athletics people than sedentary ones.

Homeostasis of Bone

Bones are dynarmic, living organs and they are continually restructured throughout life by the removal of calcium salts by osteoclast and deposition of new bone matrix by osteoblasts. Physical activity causes the density and volume to maintained or increased, while inactivity results in a reduction in bone density and volume. Calcium salts may be removed from bones to meet body needs when dietary calcium is inadequate. When dietary calcium salts return to a sufficient level, they are used to form bone matrix.

Children have a relatively large amount of protein fibres in their bone matrix, which makes their bones somewhat flexible. But as people age, the amount of protein gradually decreases. This trend causes older people to have brittle bones that are prone to fractures. Older persons may also experience a gradual loss of calcium salts (osteoporosis). which reduces the strength of the bones.

Mineral Deposition/Mineralization

Deposition or mineralization is the process by which calcium and phosphate ions from the blood plasma are deposited in the bone tissue.

Mineral Resorption

Resorption is the process of dissolving bone so that its minerals can be released into the blood and used for other purpose in the body. It is done by osteocytes and osteoclasts under the influence of hormone.

FACTORS AFFECTING BONE PHYSIOLOGY

1. Hormones

Parathyroid hormone: Raises blood calcium concentration by stimulating osteoclasts and osteocytes to resorb bone, inhibiting urinary excretion and promoting urinary excretion, intestinal absorption and calcitriol synthesis.

Calcitonin: Almost no effects in adults, Promotes mineralization and lower blood calcium concentration in children; may prevent bone loss in pregnant and lactating women.

Growth Hormone: stimulates bone elongation and cartilage proliferation at epiphyseal plate, increases urinary excretion of calcium ion which compensates for the loss.

Thyroid Hormones: Essential to bone growth; enhance effects of growth hormone but excess can cause hypercalcaemia.

Oestrogen: stimulate osteoblasts and prevent osteoporosis.

Testosterone: stimulates osteoblasts and promotes protein synthesis, thus promoting epiphyseal growth and closure.

Insulin:stimulates bone formation; significant bones loss occurs in untreated diabetes mellitus.

1. Nutrition

Vitamin A: Promotes synthesis of chondrocytes.

Vitamin C: Is necessary for collagen synthesis by osteoblasts.

Vitamin D: ls necessary for normal absorption of calcium from the intestine. The body can synthesize or ingest vitamin D. Its rate of synthesis increases when the skin is exposed to sunlight.

CLASSIFICATION OF BONES

The bones of the skeleton are classified into five principal types on the basis of shape rather than their size. The five classes are long bones, short bones, flat bones, irregular and sesamoid bones.

Long Bones: These are longer than they are wide and function as levers. Most of the bones of upper and lower extremities are of this type (e.g. the humerus, radius etc.)

Short bone: short bones of the wrist and ankle bones are somewhat cube-shaped and are found in confined spaces, where they transfer forces.

Flat bones: short bones such as the cranium, ribs and bones of the shoulder girdle have broad, dense surface for muscle attachment or protection of underlying organs.

Sesamoid bone: They are small nodules of bones that are found in certain tendons where they rub over bony surfaces. The greater part of a sesamoid bone is buried in tendon, and the Free surtace is covered with cartilage. The largest sesamoid bone is the patella which is located in the tendon of quadriceps femoris.

Irregular Bones: Irregular bones like the vertebrae and certain bones of the skull have varied shapes and many surface markings for muscle attachment or articulation.

STRUCTURE OF A TYPICAL LONG BONE

With few exceptions, all long bones have the same general structure.

Diaphysis: A tubular diaphysis or shaft forms the long axis of bone. It is constructed of a relatively thick collar of compact bone that surrounds a central medullary cavity or marrow cavity. In adults, the medullary cavity contains fat (yellow marrow) and is called the yellow bone marrow cavity.

Epiphyses: The epiphyses (singular-epiphsis) are bone ends (epi-upon). In many cases, they are more expanded than the diaphysis. Compact bone forms the exterior of epiphysis and their interior contains spongy bone.

The joint surface of each epiphysis is covered with a thin layer of articular (hyaline) cartilage, which cushions the opposing bone ends during joint movement and absorbs stress. Between the diaphysis and each epiphysis of an adult long bone is an epiphyseal line, a remnant of the epiphyseal plate, a disc of hyaline cartilage that grows during childhood to lengthen the bone. The region where the diaphysis and epiphysis meet, whether it is the epiphyseal plate or line is sometimes called the metaphysis.

Membrane : The external surface of the entire bone except the joint surfaces is covered by a glistening white double-layered membrane called the periosteum. The outer fibrous layer is dense irregular connective tissue. The inner osteogenic layer, covering the bone surface, consists primarily of bone-forming cells, osteoblast (bone germinators) and bone-destroying cells, Osteoclast (bone breakers).

The periosteum is richly supplied with nerve fibres, Iymphatic vessels and blood vessels, which enter the diaphysis via a nutrient foramen – i.e. small openings into the bone that allow for nourishment of the living tissue. The periosteum is secured to the underlying bone by perforating fibres of collagen fibres that extend from its fibrous layer into the bone matrix.

The periosteum also provides anchoring points for tendons and ligaments.

Internal bone surfaces are covered with a delicate connective tissue membrane called the endosteum. The endosteum covers the trabeculae of spongy bone and lines the canal that pass through the compact bone. Like the periosteum, the endosteum contains both osteoblasts and osteoclasts.

STRUCTURE OF IRREGULAR, SHORT AND-FLAT BONE

Short, irregular and flat bones share a simple design; they all consist of thin plates periosteum covered compact bone on the outside and endosteum covered spongy bone within. However, these bones are not cylindrical and so they have no shaft or epiphysis. They contain  bone marrow (between their trabeculae), but no marrow cavity is present. In flat bones, spongy bones are called the diploe and the whole arrangement resembles a sandwich.

Bone marrow (haemopoietic tissue)

Bone marrow occupies the marrow cavity in long and short bones and the inter cancellous bone in flat and irregular bones. At birth the marrow of all the bones of the body is red and haemopoietic. This blood-forming activity gradually lessens with age short bones and the red marrow is replaced by yellow marrow. At 7 years of age the red marrow gradually moves proximally, so that by the time the person becomes adult, red marrow is restricted to the bones of the skull, the vertebral column, the thoracic cage, the girdle bones and the head of the humerus and femur.

Bone Repair

Despite their remarkable strength, bones are susceptible to fracture or breaks. During youthful life, most of fractures result from exceptional trauma that twists or smashes the bones (sports injuries, automobile, accidents and falls for example). In old age, most fractures occur as bones thin and weaken.

TYPES OF FRACTURE

• Greenstick fracture
• Pott’s fracture
• Colle’s fracture
• Open displaced fracture
• Comminuted fracture
• Linear fracture
• Transverse, nondisplaced fracture.
• Spiral fracture

Healing of fractures

A fracture heals in about 8 to 12 weeks in uncomplicated cases. Complex fractures take longer time and all fractures heal more rapidly in young people than in older people. The healing process occurs in the following stages.

Hematoma formation

A bone fracture also breaks blood vessels of the bone and periosteum, causing bleeding and the formation of a clot (fracture hematoma)

Formation of granulation tissue

This soft tissue forms as blood vessels grow into the hematoma. Macrophages arrive by way of these vessels and clean up tissue debris. Osteoclasts, osteogenic cells and fibroblasts also migrate into the tissue from the periosteal and medullary sides of the fracture.

Callus formation

Fibroblasts deposit collagen in granulation tissue, while some osteogenic cells become chondroblasts and produce patches of fibrocartilage. The result is a soft callus, which subsequently becomes calcified into a hard (bony) callus. The hard callus forms a collar around the periosteal and endosteal surfaces of the fracture acting like a splint to join the broken ends of the bone to each other. It takes about 4 to 6 weeks for a bony callus to form. During this period, it is important that a broken bone be immobilized by fraction or a cast to prevent refractures.

Remodeling

The bony callus persists for about 3 to 4 months as osteoclasts dissolve small fragments of broken bone and osteoblasts bridge the gap between the broken ends with spongy bone. This spongy bone is subsequently remodeled into compact bone. Usually the fracture caves a slight thickening of the bone visible by x-ray, but in some cases healing is complete that no trace of the fracture can be found.

Diseases of Bones

Imbalances between bone deposit and bone resorption underlie nearly every disease that affects the adult skeleton, Some of these diseases are

Osteoporosis

A condition where bones become weak and brittle, increasing the risk of fractures.

Osteoarthritis

A degenerative joint disease that causes the breakdown of cartilage in the joints, leading to pain and stiffness.

Rheumatoid Arthritis

An autoimmune disease where the body’s immune system attacks the joints, causing inflammation and joint damage.

Paget’s Disease of Bone

A disorder in which bones become enlarged, misshapen, and weak due to abnormal bone remodeling.

Osteogenesis Imperfecta

Also known as brittle bone disease, it is a genetic disorder characterized by fragile bones that break easily.

Bone Cancer

Various types of cancer can originate in the bones, such as osteosarcoma, chondrosarcoma, and Ewing sarcoma.

Gout

A form of arthritis caused by the buildup of uric acid crystals in the joints, leading to inflammation and pain.

Ankylosing Spondylitis

An inflammatory arthritis that primarily affects the spine and can lead to fusion of the vertebrae.

Fibrous Dysplasia

A rare bone disorder where fibrous tissue replaces normal bone, weakening the affected bones.

Osteomyelitis

An infection of the bone, often caused by bacteria, leading to bone pain, fever, and inflammation.

BONES OF THE SKELETON

The bones of the skeleton are 206 divided into two groups; the axial skeleton and the appendicular skeleton.

Axial Skeleton

Consists of the skull, vertebral column, ribs and sternum. That is the bony and cartilaginous parts that support and protect the organs of the head, neck and trunk. Together the bones forming the structures constitute the central bony core of the body. The squamous suture (one on each side) where a parietal and temporal bone meet on the lateral aspect of the skull.

Temporal Bones

The two temporal bones are located on the lateral skull surface. They lie inferior to the parietal bones ones and meet them at the squamous sutures. The bones form the inferio-lateral aspects of the skull and parts of the cranial floor.

Each temporal bone has a complicated shape and is described in terms of its four major regions the squamous, tympanic, mastoid and petrous regions.

The squamous region is thin fan-shaped part that articulates with the parietal bone. It has a barlike zygomatic process that meets the zygomatic bone of the face anteriorly. Together, these two bony structures form the zygomatic arch, which you can feel as the projection of your cheek. The small, oval mandibular fossa on the inferior surface of the zygomatic process receives the condyle of the mandible (lower jawbone), forming the freely movable temporomandibular joint. The tempanic region surrounds the external auditory (acoustic) meatus or external ear. The external auditory meatus and the ear drum at its deep end are part of the external ear. Below the external auditory meatus is the needle-like styloid process, an attachment point for several tongue and neck muscles and for a ligament that secures the hyoid bone of the neck to the skull.

The mastoid region of the temporal bone exhibits the conspicuous mastoid process, an anchoring site for some neck muscles. The mastoid process can be felt as a lump just posterior to the ear. The stylomastoid foramen between the styloid and mastoid processes allows cranial nerve VII to leave the skull.

The petrous region: The petrous region can be seen in the floor of the cranium. The structures of the middle ear and inner ear are housed in dense part of the temporal bone. The carotid canal and the jugular foramen border on medial side of the petrous part at the junction of temporal and occipital bones. The carotid canal allows blood into the brain via the internal carotid artery, and the jugular vein. Three cranial nerves also pass through the jugular foramen.

The mastoid process of the temporal bone can be easily palpated as a bony knob immediately behind the earlobe. The process contains a number of small air-filled spaces called mastoid cells that can become infected in mastoiditis, as a result, for example, of a prolonged middle-ear infection.