The skeletal SystemCLS 224:

Essentials of Human Anatomy and PhysiologyDeemah M. Dabbagh

Skeleton is Subdivided into two divisions

• Axial Skeleton: Bones that form the longitudinal axis of the body (skull, vertebral column and ribcage)

• Appendicular Skeleton: Bones of the upper and lower limbs, shoulder and hip

The Skull

• The skull is formed by cranial and facial bones

• 8 cranial bones protect the brain: frontal, occipital, ethmoid, sphenoid and the pairs of parietal and temporal bones

• The 14 facial bones are all paired except the mandible and the vomer

• The skull of newborns contains fontanels (membranous areas), which allow for brain growth and ease birth passage.

Sphenoid

The Bony Thorax

• It is formed from the sternum and 12 pairs of ribs• It encloses the lungs and the heart

The Vertebral Column• The vertebral column is formed from 24 vertebrae, the sacrum and the

coccyx.• There are 7 cervical vertebrae, 12 thoracic vertebrae and 5 lumbar

vertebrae• Vertebrae are separated by fibrocartilage discs that allow the column to

be flexible.• The vertebral column is S-shaped to allow for up-right posture. At birth,

the vertebral column is C-shaped.• Spinal curvatures present at birth are the thoracic and sacral curvatures

(primary curvatures). Secondary curvatures (cervical and lumbar)develop after birth.

Abnormal Spinal Curvatures

The Structure (gross anatomy) of a Long Bone

1. Compact bone: dense bone tissue

2. Spongy Bone: Small pieces of bone with lots of space

Classification of bone on the basis of shape• Long Bones: usually have a shaft with heads at both ends. Composed

mostly of compact bone.

• Short bones: cube-shaped and contain mostly spongy bone.

• Flat bones: thin and curved. Composed of 2 thin layers of compact bone sandwiching a layer of spongy bone between them

• Irregular bones: bones having an irregular shape

Classification of bone on the basis of shape

Microscopic Anatomy of Compact Bone

• Compact bone has a complex structure filled with nerves, blood vessels which provide bone cells with nutrients and a route for waste disposal

• Calcium salts deposited in the matrix give bone its hardness. The organic parts especially collagen fibers, provide the bone with flexibility

• Mature bone cells “osteocytes” are found in cavities within the matrix called lacunae.

• Lacunae are arranged in concentric circles called lamellae around central (haversian) canals – the main nutrient supply. Each complex consisting of central canal and lamellae is called an osteon.

• Central canals run lengthwise through the bony matrix carrying blood vessels and nerves to all areas of the bone

• Tiny canals “canaliculi” act as a transportation system connecting neighboring bone cells to the main nutrient supply.

• This elaborate network of canals causes the bones to be well nourished despite the hardness of the matrix.

Microscopic Anatomy of Compact Bone

Formation of the Human Skeleton• The Skeleton is formed from the most supportive tissues in the body: cartilage and bone.

• For most bones, hyaline cartilage is used as the “model” upon which osteoblasts build bone tissue in a process called ossification. An exception is flat bones which use fibrous connective tissue as a model for forming bone.

• In embryos, the skeleton is primarily made of hyaline cartilage.

• During development, much of this cartilage is replaced by bone.

• Cartilage remains in isolated areas:• bridge of the nose• Parts of ribs• joints.

Bone growth (ossification)

• There are 2 types of bone growth:

1. Growing lengthwise, which is called interstitial growth• Occurs in the epiphyseal plates during childhood and throughout adolescence• New cartilage is continuously formed• Old cartilage becomes ossified through the action of osteoblasts

2. Growing in width, appositional growth• Occurs within the periosteum• Osteoblasts add bone tissue to the outside of the bone shaft• At the same time, osteoclasts remove bone from medullary cavity along the

endosteum (to maintain the thickness of the bone

Interstitial Bone Growth

Appositional Bone Growth

Bone Remodeling

• Cell types found in bone tissue and the roles that they play:

• Osteoblasts (bone-forming cells): connective tissue cells found at the surface of bone. They can be stimulated to proliferate and differentiate as osteocytes.

• Osteocytes (mature bone cells): Osteocytes manufacture type I collagen and other substances that make up the bone extracellular matrix. Osteocytes are found enclosed in bone matrix.

• Osteoclasts (bone-resorbing cells): "clast" means to break; osteoclasts break down bone. They are large, multinucleated cells that form through the fusion of precursor cells.

Bone Remodeling cont.

• Bones are continually remodeled in response to changes in two factors:1. Calcium levels in the blood2. Pull of gravity and muscles on the skeleton (moving, exercising..etc)

• When blood calcium levels drop, the parathyroid glands are stimulated to secrete parathyroid hormone (PTH)• PTH activates bone resorption by osteoclasts and release calcium ions into

the blood• When blood calcium levels are high (hypercalcemia), calcium is deposited

into bone tissue as hard calcium salts and new bone tissue is made.

Bone Fractures• There are 2 main types of fractures• Closed fractures: a fracture in which bone breaks, but does not

penetrate the skin• Open (compound) fracture: a fracture in which the broken bone ends

penetrate through the skin