THE SKELETAL SYSTEM

The Skeletal System represents the body's framework, made up of bones and other connective tissues

(associated cartilages, joints) that protect and sustain the tissues of the body and the internal organs, produces blood cells, and stores minerals. The human skeleton comprises 206 bones, six of which are the tiny middle ear bones (three in each ear) that serve in hearing.

Every bone is a complex living organ composed of various cells, protein fibers, and minerals. 

The skeleton acts as a scaffold by supporting and protecting the soft tissues which make up the rest of the body. 

The skeletal system also provides muscles with attachment points to facilitate movements at the joints. 

The red bone marrow inside our bones is creating new blood cells.

The skeleton is the body's structure, which protects the softer tissues and provides attachment points for most skeletal muscles.

The skeleton supplies mechanical shielding for many internal organs of the body, which reduces the risk of injury to them. Cranial bones, for example, protect the spine, vertebrae protect 

the spinal cord and ribcage preserve the heart and lungs.

Skeletal muscles are bound to the bones; thus, they cause bones to move as the corresponding muscles contract.

Several minerals are contained in the bone tissues, including calcium (Ca) and phosphorus (P). 

Bone releases minerals into the blood if needed-promoting mineral balance within the body.

The red bone marrow inside some larger bones (bone marrow cells - lymphocytes - are activated by other lymphoid tissue) blood cells are produced.

Most bone marrow transitions from ' redbone marrow ' to ' yellow bone marrow 'happen with increasing age. 

The yellow bone marrow primarily consists of adipose cells and some blood cells. This is a significant reserve of chemical resources.

• There are axial and appendicular bones.
• (The appendages are the arms and legs, which contain approx. 30 bones each.)
• There are typically 22 bones in the head.
• There are 33 bones in the spine.
• These include:
  • 7 cervix (neck)
  • 12 thorax
  • 5 lumbar
  • 5 sacral
  • 4 coccyx.
• The pelvic girdle is fused to the sacrum at the sacroiliac joint.
• The pelvis is the part that is added to the spine.
• The thorax (chest) consists of 12 pairs of ribs:
  • 7 pairs 'true' ribs - joined directly to the sternum ("breast-bone"),
  • 3 pairs 'false' ribs - joined to the sternum ("breast-bone") by cartilage,
  • 2 pairs 'floating' ribs (not connected to the sternum ("breast-bone") at all, connected to the diaphragm.
• The shoulder girdle consists of the scapula (shoulder blade) and the clavicle ("collar bone").

The following table summarises the five main categories of bones, together with another type (sutural bones).

Long bones have a greater length than width and consist of a shaft and a variable number of endings (extremities).T hey are usually somewhat curved for strength.
Examples include 

• femur, 
• tibia,
• fibula,
• humerus, 
• ulna, 
• radius.

Short bones are roughly cube-shaped and have approximately equal length and width.

 Examples include:

• ankle 
• wrist bones.

Flat bones have a thin shape/structure and provide considerable mechanical protection and extensive surfaces for muscle attachments.

Examples include 

1. cranial bones (protecting the brain), 
2. the sternum and ribs (protecting the organs in the thorax), 
3. the scapulae (shoulder blades).

Irregular bones have complicated shapes and so cannot be classified into any of the above (shape-based) categories. Their shapes are due to the functions they fulfill within the body, e.g., providing significant mechanical support for the body yet also protecting the spinal cord (in the case of the vertebrae).

  Examples include 

• the vertebrae and some facial bones.

For certain tendons, sesamoid bones grow for places where friction, strain, and physical stress are significant. 

Therefore, they can develop in the palms of the hands and the soles of the feet; however, their existence and quantity differ significantly from person to person.

•  Examples popular to everybody represent kneecaps (the patellae).

Sutural bones are categorized by position and not shape. 

They are small bones in between the cranial bones inside the sutural joints. 

The number of sutural bones varies greatly from person to person; thus, these are labeled as unidentified bones.

Bones grow from their extremities.

Under normal circumstances, bones stop growing when the owner reaches his/her late teens or early twenties.

Bone marrow produces stem cells, such as erythrocytes (red blood cells) and leucocytes (white blood cells).

• Compact (also known as "compact") tissue forms the outer shell of bones. It consists of a sturdy (virtually solid) mass of bony tissue arranged in concentric layers (Haversian systems).

• Cancellous (also known as "spongy") tissue is located beneath the compact bone and consists of a meshwork of bony bars (trabeculae) with many interconnecting spaces containing bone marrow.

  The diagram labels the basic components of a typical long bone

• articulatory (also known as 'articular')
• cartilage;
• spongy bone;
• bone marrow;
• endosteum;
• compact bone;
• periosteum;
• medullary cavity, 
• a blood vessel (indicating blood supply and circulation within bones).

The Functions of Components of Long Bone:

• Articulatory (or articular) cartilage

reduces friction and absorbs shock at freely moveable joints.

• Endosteum is the membrane that lines the cavity of bones.
• Periosteum is a tough fibrous membrane that surrounds the outside of bones wherever they are not covered by articulatory cartilage.
• In adults, the medullary cavity contains fatty yellow bone marrow. 

Components:

1) bone

2)skeletal cartilage: surrounded by dense irregular connective tissue which acts to enclose the

cartilage to prevent it from deforming too much under stress

• ƒ hyaline cartilage – the precursor of endochondral bones; flexible and resilient
• ƒ elastic cartilage – cartilage subjected to repeated bending
• ƒ fibrocartilage – highly compressible with great tensile strength

Major Functions of the skeletal system

 1. support – provides a hard framework to support the body and cradle its soft organs

 2. protection – provides a protective framework encasing for body structures/organs

 3. movement – used as a lever system to move the body and its parts; the arrangement of bones and design of joints determine types of movement possible

 4. mineral storage – retrievable storage for calcium and phosphate for release into the blood

 5. blood cell formation – most occurs within marrow cavities of certain bones (ribs, sternum, long bones)

Ways to Classify Bones:

    a. Based on Shape

(1) long bones – longer than wide; has shaft plus two ends (ex. limb bones)

(2) short bones – cube-shaped bones (ex. carpal and tarsal elements and sesamoid bones that form

within tendons)

(3) flat bones – are thinly flattened and usually a bit curved (ex. skull roofing bones, sternum and

scapula)

(4) irregular bones – bones that have complicated shapes (ex. includes vertebrae and braincase)

     b. Based on the Formation

(1) membrane or dermal bones – bones that form within a collagen membrane (ex. skull roofing

bones frontals and parietals)

(2) endochondral bones – bones that are preformed in hyaline cartilage and then transformed into

bone (ex. long bones, vertebrae)

Structure and Histology of a Long bone

• structure:

• diaphysis – the shaft of the bone cross-section shows from outside toward inside the following layers:

(1) periosteum

(2) compact bone

(3) the medullary cavity for marrow

• epiphyses – ends of bones that form articular surfaces; have thin layer of compact bone that is

underlain by cancellous or spongy bone

• periosteum – outer wrapping of bone made up of collagen (dense irregular connective tissue);

inner layers are osteogenic and contain osteoblasts (bone-forming cells), osteoclasts (bone

destroying cells; this layer is richly supplied by blood vessels, nerves, and lymphatic vessels

• articular cartilage – covers joint surfaces of epiphyses; made up of hyaline cartilage; acts to

cushion stresses during joint movement

• epiphyseal line – remnants of the epiphyseal growth plate, a band of actively dividing hyaline

the cartilage that acts to lengthen the bone

• medullary cavity – marrow cavity that contains blood-forming tissue (red marrow) or yellow or

fat marrow; lined with an endosteal membrane

• histology:

• outer layer = periosteum – layers of collagen that surround bone; during growth will have bone

forming cells and fibrocytes

• inside epiphyses = membrane that lines medullary cavity and trabecular system inside bone =

endosteum; contains osteoblasts and osteocytes

Structure of Short, Irregular, and Flat Bones

• ƒ consist of thin plates of periosteum covered compact bone on the outside
• ƒ endosteum covered spongy bone is internal
• ƒ no shaft or epiphyses
• ƒ contain some marrow but no marrow cavity

Microscopic Anatomy of Bone Tissue

1. compact bone – composed of lamellar and Haversian bone

• osteon – concentric cylinders of bone that usually run in the long axis of bone and support

stress and weight of bone

• osteocyte – ameboid bone cells that maintain the bone matrix

• lacunae – small cavities in which bones cells reside

• lamellae – each layer of the concentric tube of an osteon (Haversian system)

• Haversian canal – central canal of Haverian system that contains blood vessels and nerves

• canaliculi – canals of radiating out from lacunae and housing pseudopods of osteocytes;

mechanism of nutrient transfer from one osteocyte to another

• Volkmann's canal – transversely arranged canals that bring blood vessels into the Haversian

canals

2. spongy bone

•  trabeculae – the system of plates and spicules supporting epiphyses of bones; plates and spicule are arranged along the lines of stress and are only a few layers or lamellae thick

Types of Bone Cells

• osteoblasts – embryonic bone cells that lay down bone matrix

• osteocytes – mature bone cells that are derived from osteoblasts and are trapped in bony matrix

• osteoclasts – bone cells that break down and remodel bone; derived from hemopoietic stem cells and

macrophages; use acids to destroy bone; can phagocytize demineralized and dead osteocytes

Bone Formation and Remodeling

A. endochondral ossification – all bones from the braincase down except the clavicles

• ƒ bone is preformed in hyaline cartilage; ossification begins in the shaft at a primary ossification center (other centers occur in the epiphyses) when chondrocytes near shaft center enlarge and their surrounding matrix calcifies; this kills the chondrocytes which then disintegrate
• ƒ perichondrium becomes infiltrated with blood vessels that break into the eroding cartilage; cells in the lower layers of the periosteal membrane (formerly the perichondrium) are differentiated into osteoblasts – this converts the perichondrium into periosteum, and its inner layer is called the osteogenic layer
• ƒ osteoblasts in osteogenic layer form a bony collar around cartilage; chondrocytes hypertrophy (enlarge) and signal other cartilage cells to secrete osteoid (calcium phosphate matrix); spaces left by disintegrating chondrocytes are invaded by blood vessels
• ƒ most of the cartilage is replaced by bone except in a band on either end of the shaft facing the epiphyses (called the metaphysis)
• ƒ cartilage in the center of forming bone dies and forms the marrow cavity; invading bone cells form spongy areas under the joint surfaces
• ƒ as diaphysis enlarges osteoclasts erode the central portion (filled with spongy bone) and create the marrow cavity
• ƒ length increases then occur at metaphyses; at the shaft end of metaphyses, osteoblasts are continually invading cartilage and replacing it with the bone; at epiphyseal end of metaphyses, new cartilage is produced at the same rate
• ƒ at time of birth, centers of epiphyses begin to calcify, and capillaries and osteoblasts migrate into these areas (called secondary ossification centers); this fills epiphyses with spongy bone, but at the the proximal or distal-most end of the bone, cartilage remains to form articulating cartilage protecting bones from grinding against each other; at metaphyses, cartilage band continues to enable bone length growth (called an epiphyseal plate) with ossification near shaft and cartilage growth near epiphysis

B. intramembranous (dermal) ossification – bone is formed within a fibrous membrane (the periosteum)

• ƒ embryonic cells within the periosteum form osteoblasts within the connective tissue
• ƒ osteoblasts cluster together and secrete organic components of matrix including collagen fibers that form the scaffolding or framework for bone formation and osteoid
• ƒ ossification occurs in the eighth week of development via a process of crystallizing calcium salts and forms an ossification center
• ƒ developing bone grows outward in small struts or spicules and osteoblasts become entrapped and entombed within the bone; they are then called osteocytes
• ƒ new osteoblasts continue to be formed from embryonic cells to continue the process; are supplied by blood vessels that grow between the spicules
• ƒ this forms spongy bone which is then remodeled into compact bone as marrow cavities are formed
• ƒ examples of these bones are clavicles, mandible, patella and the skull roofing bones such as the frontal, parietal and zygomatic

C. bone growth

• ƒ post-natal growth:
• ƒ long bones lengthen by interstitial growth of epiphyseal plates
• ƒ all bones grow in thickness by appositional growth
• ƒ all bones except facial bones stop growth in early adulthood 
• ƒ length increases in bone mimic endochondral ossification
• ƒ cartilages stack up at the epiphyseal plate; those cells on top undergo rapid mitosis and push epiphyses away from the diaphysis
• ƒ those on the bottom, hypertrophy (lacunae enlarge and then erode); ossification occurs and leaves long spicules of calcified cartilage at epiphysis/diaphysis junction
• ƒ spicules are invaded by marrow elements from medullary cavity; osteoclasts erode them and they are then covered with bone matrix by osteoblasts to form spongy bone
• ƒ chondroblasts divide less often in plate region at the close of adolescence (18 for females;21 for males)
• ƒ when the bone of epiphysis and bone of diaphysis fuse ("epiphyseal closure"), growth ends

* Effect of hormones on bone growth:

1. growth hormone – released by the anterior pituitary gland and modulated by thyroid hormone;

acts to stimulate epiphyseal plate activity

ƒ too much growth hormone = giantism

ƒ too little growth hormone = dwarfism

2. sex hormones – initially promote growth spurts and masculinization or feminization of

specific parts of the skeleton

D. bone remodeling – occurs continually in response to:

1. Ca+2 levels in the blood

• Ð blood calcium causes the release of parathyroid hormone

• stimulates osteoclasts to reabsorb bone and release calcium into the blood

• turns off calcitonin production

• Ï blood calcium shuts off the release of parathyroid hormone

• stimulates the secretion of calcitonin

• inhibits bone reabsorption and stimulates calcium deposition in the bone matrix

2. stress on bones from gravity and muscles

• the bone grows or remodels in response to forces placed upon it

• weight is put on bones in an asymmetrical way so that bone is stretched on one side and

compressed on other

• in long bones bending stresses are midway down the shaft

• the neck regions also bear the most stress

• cancellous bone best supports compression under joints

Types of Bone Fractures

a. classified by the position of bone ends after fracture:

• non displaced –bone ends are in a natural position

• displaced bone ends are out of alignment

b. by the completeness of break – complete fracture or incomplete fracture.

c. by the orientation of break to the long axis of bone: linear = parallel to the long axis or transverse = perpendicular to the long axis

d. whether bone ends penetrate the skin: open (compound fracture), closed (simple)

Healing of fractures

a. fracture hematoma

• occurs because blood vessels in bone, periosteum, and surrounding tissues are torn and

hemorrhage

• hematoma is the mass of clotted blood that forms at fracture site

• bone cells deprived of nutrition die and site becomes swollen and painful

b. fibrocartilage callus

• granulation tissue forms

• capillaries grow into the hematoma; phagocytotic cells clean up debris

• fibroblasts and osteoblasts migrate into fracture site from periosteum and endosteum and begin

reconstructing bone

• fibroblasts form collagen fibers that connect broken end

• osteoblasts form woven bone

c. bony callus

• osteoblasts lay down new bone, trabeculae in the fibrocartilage callus gets converted into bony

callus

• takes 3-4 weeks

• continues for 2-3 months before stopping

Effects of aging on the skeletal system

a. estrogen – estrogen levels drop during menopause; affects calcium absorption

b. insufficient exercise – no stress leads to bone reabsorption

c. diet poor in calcium and protein leads to osteomalacia – bones inadequately mineralized

d. vitamin D and calcitonin metabolism also leads to osteomalacia

e. smoking – reduces estrogen levels

f. hormone-related conditions (corticosteroid drugs)

Osteoporosis

• bone reabsorption outpaces bone deposition – bone mass is reduced

• estrogen and testosterone help restrain osteoclast activity and promote bone growth

• peak density is reached between 35-40 years

• occurs more frequently in menopausal females because males secrete testosterone throughout life