The Skeletal System
SAP2. Students will analyze the interdependence of the integumentary,
skeletal, and muscular systems as these relate to the protection, support and
movement of the human body.
Functions of the Skeletal System
1. Support – structural framework for the body by
supporting soft tissues and providing points of
attachment for skeletal muscles.
2. Protection – protects many internal organs from injury.
What are some examples?
3. Assisting in movement – most skeletal muscles attach
to bones, when muscles contract, they pull on bones.
Together bones and muscles produce movement.
4. Mineral homeostasis – bone tissue stores several
minerals, (calcium and phosphorus). On demand,
bone releases mineral into the blood to maintain
critical mineral balances (homeostasis) and to
distribute the minerals to other parts of the body.
Functions continued
5. Production of blood cells – Red bone marrow produces
red blood cells, white blood cells and platelets in a
process called hemopoiesis (hemo –blood, poiesis –
making).
6. Triglyceride storage – yellow bone marrow consists
mainly of adipose cells (fat) which store triglycerides. The
stored triglycerides are a potential chemical energy
reserve. Yellow bone marrow also contains a few blood
cells.
Types of Bones
1. Long bones – have a greater length than width and
consist of a shaft and a variable number of ends. They
are usually somewhat curved for strength. Ex. Femur,
tibia, fibula, humerus, radius and ulna
2. Short bones – somewhat cube-shaped and nearly equal
in length and width. Ex. Wrist and ankle bones
3. Flat bones – generally thin, provide considerable
protection and extensive surfaces for muscle
attachment. Ex cranial bones, sternum, ribs and scapula
4. Irregular bones – complex shapes and cannot be
grouped into any of the previous categories. Ex
vertebrae and some facial bones.
Macroscopic Structure of Bone
1. Diaphysis – is the bone’s shaft or body – the long,
cylindrical, main portion of the bone
2. Epiphyses – the distal and proximal ends of the bone
3. Metaphyses – where the growth plate is located. As a
bone grows in length, during adolescence new cells are
created to lengthen the bone. Between ages of 18 and
25 cartilage is replaced by a bony structure called the
epiphyseal line. When the line appears bone growth in
length stops. If a bone fracture damages the
epiphyseal plate (prior to adulthood), the fractured
bone may be shorter than normal once adult stature is
reached.
Macroscopic Structure of Bone
4. Articular
cartilage – thin layer of cartilage covering the
epiphysis where the bone forms an articulation (joint) with
another bone. It reduces friction and absorbs shock in
joints. If damaged repair is limited.
5. Periosteum – tough sheath of connective tissue that
surrounds the bone surface where it is not covered with
articular cartilage. It contains bone forming cells the gives
diameter or thickness to the bone. It also protects the
bone and assists in fracture repair, gives nourishment and is
where ligaments and tendons attach to a bone.
6. Medullary cavity – marrow cavity
7. Endosteum – thin membrane that lines the medullary
cavity. It contains a single layer of bone-forming cells.
Microscopic Structure of Bone
Bone – Osseous tissue
Matrix – surrounds cells
 25% water
 25% collagen fibers
 50% crystallized mineral salts
Calcification – when the mineral salts are deposited in the
framework formed by the collagen fibers of the matrix and
the tissue hardens. The crystallization makes the bones
hard, the collagen fibers makes them flexible.
4 types of cells in bone tissue
1. Osteogenic - unspecialized stem cells, they undergo cell
division (which creates osteoblasts), located in the inner
portion of the periosteum
2. Osteoblasts – bone building cells, secrete collagen
fibers. They surround themselves with matrix and
become trapped and become osteocytes
3. Osteocytes – mature bone cells, maintain daily
metabolism (exchange of nutrients and wastes with the
blood)
4. Osteoclasts – huge cells (fusion of as many as 50
monocytes – WBC ), concentrated in the endosteum.
They release lysosomal enzymes and acids that digest
protein and mineral components of bone matrix which
Compact bone
 Contains very few spaces; it is dense
 Haversian canal – where blood vessels, nerves and
lymphatic vessels go through the bone
 Canaliculi (small channels) connect lacunae (small
spaces) creating routes for nutrients and oxygen to
reach osteocytes and for wastes to diffuse away! This
happens very slowly!
 It contains few spaces. It is beneath the periosteum and
makes up most of the diaphysis of long bones. It
provides protection and support and resists the stresses
produced by weight and movement.
Spongy bone
 It consists of units called trabeculae (little beams) an
irregular latticework of thin columns of bone. The
spaces are filled with red bone marrow.
 It makes up most of the bone tissue of short, flat and
irregularly shaped bones. It forms most of the epiphyses
of long bones
 It is light which reduces the overall weight of a bone.
Second, it provides support and protects the red bone
marrow and is the site of blood cell production in adults.
 It is found in the pelvis, ribs, sternum, and vertebrae
Bone formation
Ossification (process that forms bone) begins
during the 6th week of embryonic development
Growth in length – In the epiphyseal plate,
chondrocytes (mature cartilage) are constantly
dividing. As it gets longer new chondrocytes are
formed on the epiphyseal side of the plate while
old chondrocytes on the diaphyseal side are
replaced by bone. When growth stops the
cartilage is replaced by the epiphyseal line
Fractures
Partial – incomplete break across the
bone…a crack
Complete – break across the
bone…broken into 2 or more pieces
Closed (simple) – fractured bone does not
break through the skin
Open (compound) – the broken ends of
the bone protrude through the skin
Compound and Complete fractures
Exercise, bone health and disease
 It is important to exercise to keep healthy!
 Weight bearing activities, such as walking or moderate weightlifting help
build and retain bone mass.
 Astronauts lose bone mass due to weightlessness in space
 Calcium and Phosphorus make bone extracellular matrix hard.
 Osteoporosis – due to a depletion of calcium from the body so bone mass
depletes which causes fractures, shrinkage of vertebrae, hunched backs
and bone pain. More women suffer from this due to decline of estrogens in
women due to menopause. Proper diet and weight-bearing exercise is
important.
Osteoporosis
Exercise, bone health and disease
 Spina Bifida – congenital defect of the vertebral column where it fails to
unite. Membranes or even the spinal cord itself may protrude. It can
cause complete or partial paralysis.
 Scoliosis – A sideways bending of the vertebral column usually in the
thoracic region
The Head
 Foramen magnum (large) – the largest foramen in the skull, where the
medulla oblongata of the brain, connecting to the spinal cord and the
vertebral and spinal arteries. Look on pg 128
 Zygomatic bones – cheek bones
 Vomer – triangle shaped bone on the floor of the nasal cavity. It articulates
with the maxillae and palatine bones. It helps to form the nasal septum
(which divides the nasal cavity into right and left sides.
 Deviated nasal septum – the septum bends causing nasal blockage
(sinusitis, nosebleeds, snoring). It can be corrected with surgery.
Deviated septum and TMJ
Soft spots
 These areas will eventually be
replaced with bone and become
sutures.
 They enable the fetal skull to be
compressed as it passes through
the birth canal and permit rapid
growth of the brain during
infancy.