Skeletal System
Sunday, November 3, 2024 2:28 PM
The adult human skeleton is composed of around 206 bones from 270 at birth
after they fuse together.
The skeletal system is divided into two;
1. Axial
2. Appendicular
The axial skeleton includes the
bones found on the central axis of
the body
The appendicular skeleton
consists of the bones that are
attached to the central axis
Bones
Bones are very diverse. The smallest bones in the body are the "Ossicles" found in the ear.
The largest bone is the "Femur" which weighs about
250g.
Bones and Cartilage are categorized as supportive connective tissues.
Classification of bones by shape
Sutural Small flat oddly shaped bones found
Bones between the flat bones of the skull.
Suture means to 'stitch'
Irregular They have complex shapes with short flat
Bones notched or ridged surfaces.
Short Bones These are boxlike in appearance. Ex.
Carpal bones (wrists)
Flat Bones They have thin parallel surfaces. They
form the roof of the skull, the ribs etc.
They provide protection for the underlying
soft tissue.
Long Bones These are relatively long and slender. They
are found in the arm, thigh and legs,
fingers and toes.
Sesamoid Usually small, round and flat. They are
Bones found near joints of the knees, hands and
feet.
Long Bones
Long bones form most of the appendicular skeleton.
• Mostly spongy bone
is found in the
epiphysis
• It is covered with
compact bone
• Articulates with
other bones
• In the diaphysis, a
heavy wall of compact
bone or dense bone is
found.
• A central space called
the marrow cavity or
medullary cavity
The Skull
The skull is made up of sutural bones and flat bones.
In infants, the bones of the skull are in
fragments to allow the squeeze through
the birth canal.
Ossification takes place between 3-24
months post birth.
Composition of Bone
Components Percentage
Calcium 39%
Potassium 0%
Sodium 1%
Magnesium 0%
Carbonate 10%
Phosphate 17%
Organic Compounds ( mostly collagen) 33%
There is an essential balance between the amount of Collagen and Minerals that are found
inside bones.
There should be a ratio of 1: 2 between collagen and minerals for it to be healthy.
Collagen in the bones is what provides its structure and elasticity to bone tissue. Collagen
also plays a role in maintaining the bone's strength.
• Osteogenesis Imperfecta which is a
brittle bone disease, is caused by lack
With collagen of collagen which leads to brittle,
easily broken bones.
Without collagen
Minerals found in the bones are calcium, phosphate, magnesium, sodium, potassium and
citrate ions. These minerals ensure the healthy maintenance and growth of the bones in
our body.
Cartilage
Cartilage is a tough, flexible tissue that lines joints and gives structure to the nose, ears,
larynx and other parts of the body.
It is firm but compressible for shock absorbance and slippery for friction reduction.
A loss of cartilage leads to exposed bone surfaces rubbing against each other and can be
very painful.
It is a translucent type It is a type of It is the most
of cartilage present in connective tissue that flexible cartilage. It
the joints, the provides structural supports parts that
respiratory tract and support for the need to bend and
the immature skeleton. musculoskeletal system. move to function
Ossification
The bones of the skeleton starts to form in the fetus as cartilage. As it obtains
calcium, it deposits in the bones and the bones become ossified and rigid. Growth only
stops after puberty.
Ossification is the formation of bone by osteoblasts.
There are three cell types involved in the development , growth and remodeling of
bones. They are;
1. Osteoblasts - Bone forming cells
2. Osteocytes - Mature bone cells
3. Osteoclasts - Break down and reabsorb bone
There are two types of ossification;
1. Intramembranous ossification
2. Endochondral Ossification
Intramembranous Ossification
It involves the replacement of sheet-like connective tissue membranes with bony tissue.
This can bee seen in certain flat bones of the skull and some of the irregular bones.
The bones are first formed as connective tissue membranes after which osteoblasts
would migrate to the membranes and deposit bony matrix around themselves. When the
osteoblasts are surrounded by the matrix, they are known as osteocytes.
Endochondral Ossification
This involves replacing hyaline cartilage with bony tissue. In this process, the bones are
first formed as Hyaline cartilage models.
There is a membrane around the surface of cartilage called Perichondrium. This would
infiltrate the blood vessels and osteoblasts which causes a change into periosteum. This
usually happens after 3 months of conception.
Osteoblasts will then create a collar of compact bone around the diaphysis. The cartilage
at the center of it will then begin to break down. Osteoblasts will then move inside,
replacing it with spongy bone. Ossification will spread towards the Epiphyses (bone end).
The cartilage here will continue growing which extends the bone's lengths.
Compact bones and Spongy Bones
Compact or Cortical Bones Spongy or Cancellous Trabecular Bones
Strong and Heavy Less Dense
Brittle More flexible
Higher mineral content Large surface area
Forms the outer region of the bone Forms the inner region
Provides strength and support Contains red bone marrow
Layered Forms a lattice-like meshwork of bundles.
Joints
Joints, also known as articulations, are the
points where two bones meet. They are
three types of joints.
1. Synarthroses: Joints that don't move at all.
They mostly provide structural support.
2. Amphiarthroses: Joints with limited
movement that give you a mix of stability
and some motion.
3. Diarthroses: Joints that you can freely move
in most directions.
Calcium (Ca2+) Homeostasis
Calcium Homeostasis in the body is monitored by the thyroid and parathyroid glands.
Hypercalcemia
Hypocalcemia
The skeletal system plays a key role in maintaining the body's calcium levels. Calcium is the
most abundant mineral not only in the bone but in the whole human body. Calcium is
important due to multiple reasons.
i) Bone mineralization
ii) Tooth health
iii) Regulation of the heart rate
iv) Strength of contraction
v) Blood coagulation
vi) Nerve impulse reduction
The normal level of calcium in the blood is about 10mg/ dL. When the body cannot
maintain this level, we will experience either hypocalcemia or hypercalcemia.
Hypocalcemia is when there are abnormally low levels of calcium in the body. This can be
caused by hormonal imbalances, improper diets, vitamin D deficiency etc. Hypocalcemia can
lead to heart failures, muscle cramps, seizures, fatigue etc.
When Ca2 levels reduce, the parathyroid glands will release parathyroid hormones (PTH)
which causes osteoclasts to release Ca2 from the bones. This calcium is reabsorbed from
urine by the kidneys. Ca2 absorption also increases in the small intestines through Vitamin
D synthesis. This would cause Ca2 levels in the blood to increase bringing back to
homeostasis of Ca2 levels.
Hypercalcemia is a condition where calcium levels in the body are abnormally higher. This
can be caused when Parathyroid hormones and vitamin D levels are abnormally high.
Hypocalcemia will cause the kidney to work harder to filter all the calcium, leading to
serious thirst and frequent urination. It can also cause digestive issues like constipation
and vomiting. The extra calcium in the blood may weaken the bones causing bone pain and
muscle weakness.
Our body would try to control this situation by releasing calcitonin through the thyroid
glands. This would cause the osteoclast activity to inhibit and the Ca2 reabsorption in the
kidney to decrease.
The Ca2 level in the blood will then decrease, bringing back to the homeostasis of Ca2
levels.
Bone Cells
Bone cells make up 2% of the total bone mass.
The bone cells in our body are divided into cells which;
i) Create bones
ii) Maintain bones
iii) Destroy bones
Osteogenic Cells
These are the stem cells that divide, producing
osteoblasts. This leads to osteogenesis.
Osteoblast cells
Osteoblast cells are responsible for bone
growth and forming new bone tissue which
adds to the growth of existing bones.
Osteoclast cells
Cells that secretes acids and enzymes to dissolve
bone matrix. This bone resorption helps maintain
bone integrity and calcium metabolism
Osteocytes
These are mature bone cells that
maintains the bone matrix.
Bone Remodeling: Skeletal Homeostasis
Skeletal homeostasis involves maintaining a balance between the formation (osteoblasts)
and resorption (osteoclasts) of the bone tissue in the body.
Bone building and recycling must maintain a balance.
More breakdown than building may cause the bones to weaken, causing Osteoporosis.
Osteopetrosis also known as Stone Bone, can be caused by malfunctioning of missing
osteoclasts, which leads to thick, brittle bones as they have not being broken down.
Paget's disease is cause by an imbalance of bone cell activity. Excessive breakdown and
excessive re-forming leads to misshapen bones. In this case, the new bones are weak and
brittle.
Functions of the Skeletal System Factors affecting the bone strength
1. Support: The skeleton provides structure 1. Age: As we grow older, our bones become
for the body and helps keep it upright. thinner and weaker.
2. Protection: Skeleton protects the vital 2. Physical Stress: Bones become thicker
organs such as the brain, heart, lungs and and stronger with stress. If we are
spinal cord. inactive, 1/3 of the bone mass can be
lost.
3. Mineral Storage: Bones store minerals like
calcium and vitamin D, which are released 3. Hormone Levels: Sex hormones help
when the body needs them. maintain bone mass. Women are
vulnerable for bone related issues after
4. Blood cell production: Red bone marrow in menopause due to this.
some bones produces red blood cells, white
blood cells and platelets. 4. Calcium and Phosphorus levels
5. Movement: The skeleton allows the body to 5. Genetic and environmental factors.
move as a whole and in its individual parts.
Bones act as levers, and muscles pull on
them to create movement.
Marfan Syndrome
This is a genetic disorder affecting all connective tissue in the body. It causes excessive
cartilage formation at the epiphyseal cartilages.
Hormones like Growth Hormones from the pituitary gland greatly influences bone growth.
Any problems may lead to dwarfism or gigantism.
Calcitriol
Calcitriol is a hormone made in the kidneys and is the active form of vitamin D.
When we are exposed to the UV lights from the sung light, our body converts 7
dehydrocholesterol in the skin to Vitamin D3.
In the liver and kidney, the vitamin D3 is converted to calcifediol.
In the kidney, calcifediol is formed into calcitriol. This causes the parathyroid hormones
to be stimulated when calcium levels in the blood are low.
Calcitriol increases the absorption of calcium and phosphate from the intestines, promotes
the reabsorption of calcium in the kidneys and stimulates the release of calcium from
bones.
Red Marrow
Red marrow is a soft tissue located at the end of long bones and contains the stem cells
that form blood.
These can be found in the ribs, breastbone, shoulder blades, collarbones, hip bones, skull
and spine.
As people age, red marrow is replaces with yellow bone marrow which stores fat and
connective tissue.
Bone Fractures
A bone fracture is the medical definition for a broken bone. It can be caused by falls,
accidents or sport injuries.
Comminuted fractures : This is when the bone breaks into more than two pieces or is
crushed.
Compression fractures: This is when vertebrae is over compressed.
Spiral fractures: This occurs when the leg is twisted while fixed in place.
Epiphyseal fractures: When the growing part of a child's bone is broken.
Greenstick fractures: This is when one side is broken and the other is bent.
Fractures also become more likely with age.
Fractures leading to hospital admissions in the
elderly is a common end of life series of events.
Males have higher bone mass in general due
to testosterone.
On the other hand, females have a
protective effect of oestrogen but this
wears off at menopause.
Reflective Journal
In the lesson I learned about bone remodeling which I found quite interesting. I also
learned the different types of bone cells and their functions including Osteocytes,
Osteoclasts, Osteoblasts and Osteogenic cells. Furthermore, I learned about how there
are two layers in the bone; compact and spongy bones. Even though I had seen them when
eating chicken for example, I never realized it was a part of the bone, but now I know.
