The skeletal System
CLS 224:
Essentials of Human Anatomy and Physiology
Deemah 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 blood
2. 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