The Skeletal System (part 1 of 2)

The human skeleton is made up of 2 types of connective tissue: bone and cartilage.

The primary purpose of the skeleton is to carry the weight and maintain the shape of the body, protect and support the internal organs, and provide an area of support where muscles can attach for movement to occur.

The skeleton is strong enough to absorb reasonable amounts of shock without fracturing, and at the same time it is flexible and light enough to allow movement.

Skeletal bones move in response to muscles that work like levers. Bones act as levers to which muscles are attached. When the muscles contract the bones produce movement.

At birth, the human body is made up of 270 bones, due to the fusion of separate bones; the mature skeleton has only 206 bones.

There is considerable difference between the male and female skeletons. The bones of the male are generally larger and heavier than those of the female. The ends of the bones are thicker in relation to the shafts in males. The points of attachment of muscles are larger in males due to the fact that males have larger muscles. The pelvic girdle of females is designed to allow the birth of children.

Bone replaces itself throughout adult life. Worn or injured bone is removed and replaced by new tissue.

Bone replaces itself at different intervals. The distal end (furthest from the center) of the femur is replaced about every 4 months; however the bone in parts of the shaft of the femur may not be completely replaced during a lifetime. This process allows the bone to serve as the body’s storage are for calcium.

Bones are the major site of mineral storage, particularly calcium and phosphorous, these minerals can be distributed to other parts of the body on demand.

– Long bones are longer than they are wide and they are slightly curved for strength.
– A bone that is curved is structurally able to absorb the stress of the body at several points so the stress will be evenly distributed.
– The straight bone would not be able to evenly distribute the weight of the body and the bone would break more easily.
– Long bones are shaped like tubes with rounded ends that are designed to fit into other bones to form JOINTS.
– The ends of long bones are filled with spongy bone; this makes them light but strong.
– Long bones can be found in the thigh, lower leg, toes, arms, forearms and fingers.
– Short bones are cubed-shaped and nearly equal in length and width.
– They are a spongy bone except at the surface, where there is a thin layer of compact bone.
– Short bones can be found in the wrist and ankle.
– Flat bones are usually thin and are composed of several plates of compact bone over a layer of spongy bone.
– They provide considerable protection and provide ample area for muscle attachment.
– cranial bones, the sternum, ribs and the scapula are flat bones.
– Irregular bones have complex shapes and cannot be grouped into any of the other 3 categories.
– The vertebrae and some of the facial bones are irregular.

Most of the human skeleton is living tissue that is growing constantly.

The bones are made up of cells embedded in a matrix.

About 20% of living bone is water. The remaining 80% is a MATRIX that consists of minerals (tri-calcium phosphate, magnesium and other elements) and protein (collagen fibers).

When the minerals are deposited, the bone becomes OSSIFIED (hardened).

The protein fibers and the minerals make up the non-living matrix of the bone.

Bone is not a completely solid substance; bone has some spaces between the hard components.

The spaces provide channels for blood vessels that supply bone cells with nutrients and make bones lighter.

The bone matrix has a network of canals called HAVERSAIN CANALS in it. These canals allow blood vessels and nerve fibers to extend into interior bone.

Bone also contains many living cells and blood vessels that provide the movement of nourishment into the cell and the removal of waste from the cells.

The surface of the bone is covered by a tough membrane called the PERIOSTEUM.

The periosteum has many microscopic blood vessels that provide nourishment to the bone.

Living bone cells are found in small spaces in the mineral matrix of the bone. There are 3 types of living bone cells and each has a special function.

– Osteoblasts repair broken bones and produce new bone material.
– They also secrete the mineral and protein compounds that form the matrix.
– Osteoclasts are the “bone breakers”. They are able to dissolve pieces of bone that are in the way of “efficient” skeletal design.
– The destructive work is often followed by the constructive work of the osteoblast in the rebuilding of the bone.
– Osteocytes function as the “caretaker” of the bone tissue. They provide coverall maintenance of the bone.

Depending on the size and distribution of the spaces in the bone, the regions of the bone can be categorized as COMPACT or SPONGY.
– Compact bone is dense tissue (thicker) with few spaces.
– It is deposited in a layer over the spongy bone tissue.
– Compact bone tissue provides protection and support and helps long bones resist the stress of weight placed on them. It can resist considerable weight and stress.
– Spongy bone is less dense and contains many large spaces. It is filled with soft tissue called RED MARROW, which makes red blood cells and helps the body store fat.
– Red bone marrow consists of immature blood cells, called STEM CELLS, fat cells, and macrophages.
– Red marrow produces red blood cells, some white blood cells and platelets.
– Spongy bone make up most of the short, flat and irregularly shaped bones. It also makes up most of the ends of long bones.

– Cartilage is both firm and flexible, unlike bone that is harder and more brittle.
– Cartilage is usually located where firmness and flexibility are needed, like joints, the nose, the ears, trachea, and larynx and in between the vertebrate.
– Cartilage is made up of circular cells embedded in a rubbery matrix that has supporting fibers.
– During the development of the skeletal system of humans, embryos begin with a cartilaginous skeleton. Gradually most of the cartilage is replaced by bone.

The axial skeleton includes the bones that lie along the longitudinal axis. It provides the protective functions of the skeleton.

– All of the bones of the head make up the skull. There are 2 regions of the skull; the CRANIUM and the FACE.
– The skull is made up of 22 flat and irregular shaped bones.
– There are 8 bones in the cranium; their function is to protect the brain.
– The facial region is made up of 14 bones and it protects the eyes, nose, mouth and ears.
– The SINUSES are air space in the facial bones. They aid in reducing the weight of the skull.
– The 3 bones in the middle ear (the HAMMER, the ANVIL, and the STIRRUP) function in
transmitting sound to the inner ear and are the smallest bones in the body.

– The vertebral column has 26 bones that are called VERTEBREA.
– At birth the vertebral column has 33 bones.
– 5 SACRAL VERTEBREA. The 5 sacral bones fuse into one large triangular bone, called the SACRUM, located at the back of the pelvis.
– 4 CAUDAL/COCCYGEAL (tail) VERTEBREA. The 4 coccygeal bones fuse into a single COCCYX.
– The vertebrae column is called the BACKBONE; it is made flexible by the cartilage and ligaments that join the individual vertebrae.
– This flexibility allows movement of the head and the trunk of the body.
– DISC of CARTILAGE separate the individual vertebrae. The disc prevent friction due to the rubbing of the bones and act as shock absorbers.

– Just below the neck is the THORACIC BASKET (12 pair of ribs attached to the vertebrae column).
– The 1st 10 pair of ribs are attached to the STERNUM (BREASTBONE) by cartilage strips.
– The 11th and 12th pair of ribs are called the FLOATING RIBS, because they are attached to the vertebrae column, but not to the sternum.
– The loose connections of the ribs to the vertebrae and the flexible cartilage connections at the sternum allow the ribs to move when the lungs are inflated.

– Appendicular is the adjective of the word appendage.
– An appendage is an attachment to a main body or structure. Arms and legs are attached to the axial skeleton.
– The moveable limbs attached to the axial skeleton make up the appendicular skeleton.
– The appendicular skeleton forms a system of levers, providing movement and dexterity.
– The arms and hands, the legs and feet, and the bones of the shoulders and the pelvis make up the appendicular skeleton.
– The sites where the arms and legs are attached to the axial skeleton are bones referred to a GIRDLES.
– The PECTORAL(shoulder)GIRDLE, 2 SCAPULA(shoulder blades) and 2 CLAVICAL(collar bones) hold the arm to the axial skeleton.
– The legs are attached to the PELVIC GIRDLE, which is formed by the fusion of 3 bones. The ILIUM, the ISCHIUM, and the PUBIS on each side of the midline of the body.
– The PELVIC GIRDLE receives the weight of the upper body from the vertebral column and transfers it to the leg bones or the surface when seated.
– The arched shape of the foot allow it to withstand tremendous force and weight.

– The fused bones creating the CRANUIM make up the SKULL.
– The lower TEETH are located in the MANDIBLE(jaw).
– The collar bone is the CLAVICAL.
– The wings in the upper back are called SCAPULAS.
– The bone connecting all of the RIBS in the middle of the chest is the STERNUM.
– The ribs are connected in the back to the VERTEBRAL COLUMN(backbone), which is composed of VERTEBREA.
– The vertebrae in the neck are called CERVICAL VERTEBREA; THORACIC VERTEBREA join with the ribs; LUMBAR VERTEBREA descends from the thoracic vertebrae to the pelvis; and together the fused bones in the PELVIS make up the SACRUM.
– The COCCYX(tail) is made up of CAUDAL VERTEBREA.
– The bone in the upper arm is the HUMERUS and the two bones in the lower arm are the RADIUS and ULNA.
– The wrist bones are the CARPALS.
– At the base of the fingers(the palm of the hand) are the METACARPALS.
– The smaller bones extending out to the fingertips are the PHALANGES.
– The largest bones in the body are the FEMURS, the thigh bone connects the upper legs with the pelvis.
– The upper and lower legs meet at the KNEE covering that is the kneecap, or PATELLA.
– Each lower leg has two long bones, the TIBIA and FIBULA.
– The little bones in the ankles are the TARSALS, that connect to the METATARSALS
– The bones extending into the toe tips are called PHALANGES.

The site where 2 bones come together is called a JOINT.

Most joints allow the bones to move at the connection point.

Some joints are immovable. They are usually bones that are fused together, like the SUTURES located between the skull bones.

Moveable joints are held together with LIGAMENTS, a strong, flexible tissue that connects bone to cartilage.

The more mobile the joint is, the weaker it is.

– No movement is allowed by this type of joint.
– Suture joints in the skull are an example.
– Bones meeting at these joints have some ability to move.
– The spaces between the vertebrae in the back are an example.
– Bones meeting at these joints have the possibility of great movement.
– BALL and SOCKET JOINTS (found in the shoulder), HINGE JOINT (a back and forth movement joint, like the knees and elbows), and PIVOT JOINTS (a rotating joint, like the forearm at the elbow) are all examples of freely moving joints.


Types of Tissue

Tissues are groups of cells with a common structure and function.

They work together to perform a particular task.

– Organs are a collection of 2 or more of the basic body tissue types.
– Multiple tissues adapt as a group to perform specific functions and form structures called ORGANS.

– A collection of 2 or more of the organs that together perform some complex body function.
– The human body is a cooperative of organ systems that are interdependent upon one another, either chemically or physically.

There are 4 main CATEGORIES OF TISSUE:
1.  EPITHELIAL TISSUE (covers and lines the body)
2.  CONNECTIVE TISSUE (binds and supports other tissue)
3.  MUSCLE TISSUE (is involved with movement)
4.  NERVOUS TISSUE (forms a communication network)


Epithelial tissue is covering and lining tissue, it covers body surfaces in general and lines cavities within the body.

It has little or no intercellular material between its cells.

The free surface of this tissue is exposed either to air or fluid.

The base of the cell is attached to a BASEMENT MEMBRANE (a dense layer of extra cellular material).

The cells are closely joined and may act as a barrier against injury, microbial invasion or fluid loss.

These cells may be specialized for absorption or secretion of chemical solutions.

Epithelial tissue is categorized by the number of LAYERS and the SHAPES of the free surface of the cells.

LAYERS can be:
1. SIMPLE: one layer of cells.
2. STRATIFIED: multiple tiers of cells.
3. PSEUDOSTRATIFIED: one layer that appears multiple because the layers vary in length.

SHAPES include:
SIMPLE SQUAMOUS epithelial tissue is thin and leaky.
– These cells aid in the exchange of material by diffusion.
– They line blood vessels and air sacs in the lungs.
STRATIFIED SQUAMOUS tissue regenerates rapidly near the basement membrane.
– New cell are pushed to the free surface as replacements for the cells that are continually sloughed off.
– Stratified Squamous tissue is located on surfaces that are subject to abrasion, like the outer skin.

– They are like a cytoplasm filled water balloon.
– They are found where secretion or active absorption of substances is an important function, like the intestines, where they secrete digestive juices or absorb nutrients.
STRATIFIED COLUMNAR epithelial tissue line the inner surface of the urinary bladder.
PSEUDOSTRATIFIED CILIATED COLUMNAR epithelial tissue line the nasal passage.

SIMPLE CUBOIDAL epithelial tissue is specializes for secretion.
– They can be found in the kidney tubules, the thyroid gland and the salivary glands.


Connective tissue binds and supports other tissue.

It has a sparse cell population scattered throughout an extensive extracellular matrix;  it has a lot of intercellular material between its cells.

The matrix contains long, slender rods and connective tissue fiber in a substance similar to soft-set gelatin.

This fiber helps connective tissue to do its job, to directly or indirectly connect body parts together.

3 types of FIBERS make up the various types of connective tissue:
– They are bundles of fibers containing 3 collagen fibers each.
– These fibers are strong and resist stretching.
– The parallel lines on the palm of your hand are collagen bundles.

– They are long threads of the protein, elastin.
– If stretched, this tissue can return to its original shape.

– They are branched and tightly woven.
– They join connective tissue to neighboring tissue.

There are 6 CATEGORIES of connective tissue:
– Contains all 3 fiber types; collagen, elastin and reticular.
– Holds organs in place and attaches the epithelium to underlying tissue.
– Contains 2 types of cells:
1. FIBROBLASTS: secrete proteins of extracellular fibers, like collagen.
2. MACROPHAGES: act as the “attack dogs” of the body’s immune system.

– It is a loose connective tissue that is specialized to store fat.
– The fat is stored in adipose cells distributed throughout its matrix.
– Each adipose cell stores one fat droplet, which can vary in size.
– The stored fat insulates the body and is used for fuel when needed.

– Large numbers of collagenous fibers in parallel bundles makes this tissue very dense.
– This density gives it the great strength needed for tendons (to attach muscle to the bone) and ligaments (to attach bone together at joints).

– It is the strong and flexible connective tissue found in the skeleton of all vertebrate embryos.
– Most vertebrates convert the cartilage to bone, but they retain cartilage in the nose, ears and trachea.
– It is composed of collagenous fibers embedded in chondroitin sulfate (a protein-carbohydrate).

– Bone is hard, but not brittle or completely solid.
– It is mineralized connective tissue.
– OSTEOBLASTS (bone-forming cells) deposit a matrix of collagen and calcium phosphate that hardens into the mineral hydroxyapatite.

– Blood is the only liquid connective tissue in the body.
-It is a liquid extracellular matrix of plasma containing water, salt and proteins.
– Blood contains RED BLOOD CELLS (that transport oxygen), WHITE BLOOD CELLS (for the immune system) and PLATELETS (which are cell fragments that help in the clotting of blood).
– Blood cells are made in the red marrow of long bones.
– Blood vessels and nerve cells occupy slender canals in the bone tissue call HAVERSAIN CANALS.


There is more muscle tissue available in the human body than any other type of tissue.

It consist of long, slender muscle fibers, that contract or shorten to create body movement.

There are 3 types of muscle:
– It is multinucleated and is usually attached to bones by tendons.
– Contractions are voluntary.
– This muscle appears striated under the microscope.

– It is found in the walls of the internal organs and arteries.
– The spindle-shaped, uninucleated cells contract involuntarily.

– It is located only in the wall of the heart.
– The cells are striated, uninucleated, and are joined by intercalated disks.
– Contractions are involuntary.

– It is the major tissue for communication and control within the body’s internal environment.
– It is designed to sense stimuli.
– It communicates by means of NEURONS (nerve cells).
– The neuron conducts impulses or bioelectric signals.
– It transmits signals from one part of the organism to another.