Anatomy and Physiology – Bone Structure, Growth and Skeletal Organization

Anatomy and Physiology – Bone Structure, Growth and Skeletal Organization

INTRODUCTION TO THE SKELETAL SYSTEM

• Humans are vertebrates, animals having a vertabral column or backbone.
• They rely on a sturdy internal frame that is centered on a prominent spine.
• The human skeletal system consists of bones, cartilage, ligaments and
• tendons and accounts for about 20 percent of the body weight.
• The living bones in our bodies use oxygen and give off waste products in
• metabolism. They contain active tissues that consume nutrients, require a
• blood supply and change shape or remodel in response to variations in
• mechanical stress.
• Bones provide a rigid framework, known as the skeleton, that support and
• protect the soft organs of the body.
• The skeleton supports the body against the pull of gravity. The large bones
• of the lower limbs support the trunk when standing.
• The skeleton also protects the soft body parts. The fused bones of the
• cranium surround the brain to make it less vulnerable to injury. Vertebrae
• surround and protect the spinal cord and bones of the rib cage help protect
• the heart and lungs of the thorax.
• Bones work together with muscles as simple mechanical lever systems to
• produce body movement.
• Bones contain more calcium than any other organ. The intercellular matrix
• of bone contains large amounts of calcium salts, the most important being
• calcium phosphate.
• When blood calcium levels decrease below normal, calcium is released from
• the bones so that there will be an adequate supply for metabolic needs.
• When blood calcium levels are increased, the excess calcium is stored in the
• bone matrix. The dynamic process of releasing and storing calcium goes on
• almost continuously.
• Hematopoiesis, the formation of blood cells, mostly takes place in the red
• marrow of the bones.
• In infants, red marrow is found in the bone cavities. With age, it is largely
• replaced by yellow marrow for fat storage. In adults, red marrow is limited
• to the spongy bone in the skull, ribs, sternum, clavicles, vertebrae and pelvis.
• Red marrow functions in the formation of red blood cells, white blood cells

and blood platelets.

STRUCTURE OF BONE TISSUE

• There are two types of bone tissue: compact and spongy. The names imply
• that the two types differ in density, or how tightly the tissue is packed
• together. There are three types of cells that contribute to bone homeostasis.
• Osteoblasts are bone-forming cell, osteoclasts resorb or break down bone,
• and osteocytes are mature bone cells. An equilibrium between osteoblasts

and osteoclasts maintains bone tissue.

Compact Bone

• Compact bone consists of closely packed osteons or haversian systems. The
• osteon consists of a central canal called the osteonic (haversian) canal,
• which is surrounded by concentric rings (lamellae) of matrix. Between the
• rings of matrix, the bone cells (osteocytes) are located in spaces called
• lacunae. Small channels (canaliculi) radiate from the lacunae to the osteonic
• (haversian) canal to provide passageways through the hard matrix. In
• compact bone, the haversian systems are packed tightly together to form
• what appears to be a solid mass. The osteonic canals contain blood vessels
• that are parallel to the long axis of the bone. These blood vessels
• interconnect, by way of perforating canals, with vessels on the surface of the

bone.

Spongy (Cancellous) Bone

• Spongy (cancellous) bone is lighter and less dense than compact bone.
• Spongy bone consists of plates (trabeculae) and bars of bone adjacent to
• small, irregular cavities that contain red bone marrow. The canaliculi
• connect to the adjacent cavities, instead of a central haversian canal, to
• receive their blood supply. It may appear that the trabeculae are arranged in
• a haphazard manner, but they are organized to provide maximum strength
• similar to braces that are used to support a building. The trabeculae of
• spongy bone follow the lines of stress and can realign if the direction of
• stress changes.
• Microscopic Structure of Bones
• The skeleton makes up about 30-40% of an adult‘s body mass. The
• skeleton‘s mass is made up of nonliving bone matrix and many tiny bone
• cells. Roughly half of the bone matrix‘s mass is water, while the other half
• is collagen protein and solid crystals of calcium carbonate and calcium
• phosphate.
• Living bone cells are found on the edges of bones and in small cavities
• inside of the bone matrix. Although these cells make up very little of the
• total bone mass, they have several very important roles in the functions of
• the skeletal system. The bone cells allow bones to:
• Grow and develop
• Be repaired following an injury or daily wear
• Be broken down to release their stored minerals

BONE DEVELOPMENT & GROWTH

• The terms osteogenesis and ossification are often used synonymously to
• indicate the process of bone formation. Parts of the skeleton form during the
• first few weeks after conception. By the end of the eighth week after
• conception, the skeletal pattern is formed in cartilage and connective tissue
• membranes and ossification begins.
• Bone development continues throughout adulthood. Even after adult stature
• is attained, bone development continues for repair of fractures and for
• remodeling to meet changing lifestyles. Osteoblasts, osteocytes and
• osteoclasts are the three cell types involved in the development, growth and
• remodeling of bones. Osteoblasts are bone-forming cells, osteocytes are
• mature bone cells and osteoclasts break down and reabsorb bone.
• There are two types of ossification: intramembranous and endochondral.
• Intramembranous
• Intramembranous ossification involves the replacement of sheet-like
• connective tissue membranes with bony tissue. Bones formed in this manner
• are called intramembranous bones. They include certain flat bones of the
• skull and some of the irregular bones. The future bones are first formed as
• connective tissue membranes. Osteoblasts migrate to the membranes and
• deposit bony matrix around themselves. When the osteoblasts are

surrounded by matrix they are called osteocytes.

Endochondral Ossification

• Endochondral ossification involves the replacement of hyaline cartilage with
• bony tissue. Most of the bones of the skeleton are formed in this manner.
• These bones are called endochondral bones. In this process, the future bones
• are first formed as hyaline cartilage models. During the third month after
• conception, the perichondriumthat surrounds the hyaline cartilage "models"
• becomes infiltrated with blood vessels and osteoblasts and changes into a
• periosteum. The osteoblasts form a collar of compact bone around the
• diaphysis. At the same time, the cartilage in the center of the diaphysis
• begins to disintegrate. Osteoblasts penetrate the disintegrating cartilage and
• replace it with spongy bone. This forms a primary ossification center.
• Ossification continues from this center toward the ends of the bones. After
• spongy bone is formed in the diaphysis, osteoclasts break down the newly
• formed bone to open up the medullary cavity.
• The cartilage in the epiphyses continues to grow so the developing bone
• increases in length. Later, usually after birth, secondary ossification centers
• form in the epiphyses. Ossification in the epiphyses is similar to that in the
• diaphysis except that the spongy bone is retained instead of being broken
• down to form a medullary cavity. When secondary ossification is complete,
• the hyaline cartilage is totally replaced by bone except in two areas. A
• region of hyaline cartilage remains over the surface of the epiphysis as the
• articular cartilage and another area of cartilage remains between the

epiphysis and diaphysis. This is the epiphyseal plate or growth region.

Bone Growth

• Bones grow in length at the epiphyseal plate by a process that is similar to
• endochondral ossification. The cartilage in the region of the epiphyseal plate
• next to the epiphysis continues to grow by mitosis. The chondrocytes, in the
• region next to the diaphysis, age and degenerate. Osteoblasts move in and
• ossify the matrix to form bone. This process continues throughout childhood
• and the adolescent years until the cartilage growth slows and finally stops.
• When cartilage growth ceases, usually in the early twenties, the epiphyseal
• plate completely ossifies so that only a thin epiphyseal line remains and the
• bones can no longer grow in length. Bone growth is under the influence of
• growth hormone from the anterior pituitary gland and sex hormones from
• the ovaries and testes.
• Even though bones stop growing in length in early adulthood, they can
• continue to increase in thickness or diameter throughout life in response to
• stress from increased muscle activity or to weight. The increase in diameter
• is called appositional growth. Osteoblasts in the periosteum form compact
• bone around the external bone surface. At the same time, osteoclasts in the
• endosteum break down bone on the internal bone surface, around the
• medullary cavity. These two processes together increase the diameter of the
• bone and, at the same time, keep the bone from becoming excessively heavy
• and bulky.
• Types of Bones
• All of the bones of the body can be broken down into five types: long, short,
• flat, irregular, and sesamoid.
• Long. Long bones are longer than they are wide and are the major bones of
• the limbs. Long bones grow more than the other classes of bone throughout
• childhood and so are responsible for the bulk of our height as adults. A
• hollow medullary cavity is found in the center of long bones and serves as a
• storage area for bone marrow. Examples of long bones include the femur,
• tibia, fibula, metatarsals, and phalanges.
• Short. Short bones are about as long as they are wide and are often cubed or
• round in shape. The carpal bones of the wrist and the tarsal bones of the foot
• are examples of short bones.
• Flat. Flat bones vary greatly in size and shape, but have the common feature
• of being very thin in one direction. Because they are thin, flat bones do not
• have a medullary cavity like the long bones. The frontal, parietal,
• and occipital bones of the cranium—along with the ribs and hip bones—are
• all examples of flat bones.
• Irregular. Irregular bones have a shape that does not fit the pattern of the
• long, short, or flat bones. The vertebrae, sacrum, and coccyx of the spine—
• as well as the sphenoid, ethmoid, and zygomatic bones of the skull—are all
• irregular bones.
• Sesamoid. The sesamoid bones are formed after birth inside of tendons that
• run across joints. Sesamoid bones grow to protect the tendon from stresses
• and strains at the joint and can help to give a mechanical advantage to
• muscles pulling on the tendon. The patella and the pisiform bone of the
• carpals are the only sesamoid bones that are counted as part of the 206 bones
• of the body. Other sesamoid bones can form in the joints of the hands and

feet, but are not present in all people.

CLASSIFICATION OF BONES

Long Bones

• The bones of the body come in a variety of sizes and shapes. The four
• principal types of bones are long, short, flat and irregular. Bones that are
• longer than they are wide are called long bones. They consist of a long shaft
• with two bulky ends or extremities. They are primarily compact bone but
• may have a large amount of spongy bone at the ends or extremities. Long

bones include bones of the thigh, leg, arm, and forearm.

Short Bones

• Short bones are roughly cube shaped with vertical and horizontal dimensions
• approximately equal. They consist primarily of spongy bone, which is
• covered by a thin layer of compact bone. Short bones include the bones of
• the wrist and ankle.

Flat Bones

• Flat bones are thin, flattened, and usually curved. Most of the bones of the

cranium are flat bones.

Irregular Bones

• Bones that are not in any of the above three categories are classified as
• irregular bones. They are primarily spongy bone that is covered with a thin
• layer of compact bone. The vertebrae and some of the bones in the skull are
• irregular bones.
• All bones have surface markings and characteristics that make a specific
• bone unique. There are holes, depressions, smooth facets, lines, projections
• and other markings. These usually represent passageways for vessels and
• nerves, points of articulation with other bones or points of attachment for

tendons and ligaments.

DIVISIONS OF THE SKELETON

• The adult human skeleton usually consists of 206 named bones. These bones
• can be grouped in two divisions: axial skeleton and appendicular skeleton.
• The 80 bones of the axial skeleton form the vertical axis of the body. They
• include the bones of the head, vertebral column, ribs and breastbone or
• sternum. The appendicular skeleton consists of 126 bones and includes the
• free appendages and their attachments to the axial skeleton. The free
• appendages are the upper and lower extremities, or limbs, and their
• attachments which are called girdles. The named bones of the body are listed

below by category.

AXIAL SKELETON (80 BONES)

Skull (28)

Cranial Bones

• Parietal (2)
• Temporal (2)
• Frontal (1)

 Occipital (1)

• Ethmoid (1)
• Sphenoid (1)

Facial Bones

• Maxilla (2)
• Zygomatic (2)

 Mandible (1)

• Nasal (2)
• Platine (2)
• Inferior nasal concha (2)
• Lacrimal (2)
• Vomer (1)

Auditory Ossicles

• Malleus (2)
• Incus (2)
• Stapes (2)

Hyoid (1)

Vertebral Column

• Cervical vertebrae (7)
• Thoracic vertebrae (12)
• Lumbar vertebrae (5)

 Sacrum (1)

• Coccyx (1)

Thoracic Cage

• Sternum (1)
• Ribs (24)

APPENDICULAR SKELETON (126 BONES)

Pectoral girdles

 Clavicle (2)

• Scapula (2)

Upper Extremity

• Humerus (2)
• Radius (2)
• Ulna (2)

 Carpals (16)

• Metacarpals (10)
• Phalanges (28)

Pelvic Girdle

• Coxal, innominate, or hip bones (2)

Lower Extremity

 Femur (2)

• Tibia (2)
• Fibula (2)
• Patella (2)
• Tarsals (14)
• Metatarsals (10)

 Phalanges (28)

ARTICULATIONS

• An articulation, or joint, is where two bones come together. In terms of the
• amount of movement they allow, there are three types of joints: immovable,
• slightly movable and freely movable.
• Synarthroses
• Synarthroses are immovable joints. The singular form is synarthrosis. In
• these joints, the bones come in very close contact and are separated only by
• a thin layer of fibrous connective tissue. The sutures in the skull are
• examples of immovable joints.
• Amphiarthroses
• Slightly movable joints are called amphiarthroses. The singular form is
• amphiarthrosis. In this type of joint, the bones are connected by hyaline
• cartilage or fibrocartilage. The ribs connected to the sternum by costal
• cartilages are slightly movable joints connected by hyaline cartilage. The
• symphysis pubis is a slightly movable joint in which there is a fibrocartilage
• pad between the two bones. The joints between the vertebrae and the
• intervertebral disks are also of this type.
• Diarthroses
• Most joints in the adult body are diarthroses, or freely movable joints. The
• singular form is diarthrosis. In this type of joint, the ends of the opposing
• bones are covered with hyaline cartilage, the articular cartilage, and they are
• separated by a space called the joint cavity. The components of the joints are
• enclosed in a dense fibrous joint capsule. The outer layer of the capsule
• consists of the ligaments that hold the bones together. The inner layer is the
• synovial membrane that secretes synovial fluid into the joint cavity for
• lubrication. Because all of these joints have a synovial membrane, they are
• sometimes called synovial joints.
• Parts of Bones
• The long bones of the body contain many distinct regions due to the way in
• which they develop. At birth, each long bone is made of three individual
• bones separated by hyaline cartilage. Each end bone is called
• an epiphysis (epi = on; physis = to grow) while the middle bone is called a
• diaphysis (dia = passing through). The epiphyses and diaphysis grow
• towards one another and eventually fuse into one bone. The region of growth
• and eventual fusion in between the epiphysis and diaphysis is called the
• metaphysis (meta = after). Once the long bone parts have fused together, the
• only hyaline cartilage left in the bone is found as articular cartilage on the
• ends of the bone that form joints with other bones. The articular
• cartilage acts as a shock absorber and gliding surface between the bones to
• facilitate movement at the joint.
• Looking at a bone in cross section, there are several distinct layered regions
• that make up a bone. The outside of a bone is covered in a thin layer of
• dense irregular connective tissue called the periosteum. The periosteum
• contains many strong collagen fibers that are used to firmly anchor tendons
• and muscles to the bone for movement. Stem cells and osteoblast cells in the
• periosteum are involved in the growth and repair of the outside of the bone
• due to stress and injury. Blood vessels present in the periosteum provide
• energy to the cells on the surface of the bone and penetrate into the bone
• itself to nourish the cells inside of the bone. The periosteum also contains
• nervous tissue and many nerve endings to give bone its sensitivity to pain
• when injured.
• Deep to the periosteum is the compact bone that makes up the hard,
• mineralized portion of the bone. Compact bone is made of a matrix of hard
• mineral salts reinforced with tough collagen fibers. Many tiny cells called
• osteocytes live in small spaces in the matrix and help to maintain the
• strength and integrity of the compact bone.
• Deep to the compact bone layer is a region of spongy bone where the bone
• tissue grows in thin columns called trabeculae with spaces for red bone
• marrow in between. The trabeculae grow in a specific pattern to resist
• outside stresses with the least amount of mass possible, keeping bones light
• but strong. Long bones have a spongy bone on their ends but have a hollow
• medullary cavity in the middle of the diaphysis. The medullary cavity
• contains red bone marrow during childhood, eventually turning into yellow
• bone marrow after puberty.
• Articulations
• An articulation, or joint, is a point of contact between bones, between a bone
• and cartilage, or between a bone and a tooth. Synovial joints are the most
• common type of articulation and feature a small gap between the bones. This
• gap allows a free range of motion and space for synovial fluid to lubricate
• the joint. Fibrous joints exist where bones are very tightly joined and offer
• little to no movement between the bones. Fibrous joints also hold teeth in
• their bony sockets. Finally, cartilaginous joints are formed where bone meets
• cartilage or where there is a layer of cartilage between two bones. These
• joints provide a small amount of flexibility in the joint due to the gel-like
• consistency of cartilage.
• Support and Protection
• The skeletal system‘s primary function is to form a solid framework that
• supports and protects the body's organs and anchors the skeletal muscles.
• The bones of the axial skeleton act as a hard shell to protect the internal
• organs—such as the brain and the heart—from damage caused by external
• forces. The bones of the appendicle skeleton provide support and flexibility
• at the joints and anchor the muscles that move the limbs.
• Movement
• The bones of the skeletal system act as attachment points for the skeletal
• muscles of the body. Almost every skeletal muscle works by pulling two or
• more bones either closer together or further apart. Joints act as pivot points
• for the movement of the bones. The regions of each bone where muscles
• attach to the bone grow larger and stronger to support the additional force of
• the muscle. In addition, the overall mass and thickness of a bone increase
• when it is under a lot of stress from lifting weights or supporting body
• weight.
• Hematopoiesis
• Red bone marrow produces red and white blood cells in a process known as
• hematopoietic. Red bone marrow is found in the hollow space inside of
• bones known as the medullary cavity. Children tend to have more red bone
• marrow compared to their body size than adults do, due to their body‘s
• constant growth and development. The amount of red bone marrow drops
• off at the end of puberty, replaced by yellow bone marrow.
• Storage
• The skeletal system stores many different types of essential substances to
• facilitate growth and repair of the body. The skeletal system‘s cell matrix
• acts as our calcium bank by storing and releasing calcium ions into the blood
• as needed. Proper levels of calcium ions in the blood are essential to the
• proper function of the nervous and muscular systems. Bone cells also release
• osteocalcin, a hormone that helps regulate blood sugar and fat deposition.
• The yellow bone marrow inside of our hollow long bones is used to store
• energy in the form of lipids. Finally, red bone marrow stores some iron in
• the form of the molecule ferritin and uses this iron to form hemoglobin in
• red blood cells.
• Growth and Development
• The skeleton begins to form early in fetal development as a flexible skeleton
• made of hyaline cartilage and dense irregular fibrous connective tissue.
• These tissues act as a soft, growing framework and placeholder for the bony
• skeleton that will replace them. As development progresses, blood vessels
• begin to grow into the soft fetal skeleton, bringing stem cells and nutrients
• for bone growth. Osseous tissue slowly replaces the cartilage and fibrous
• tissue in a process called calcification. The calcified areas spread out from
• their blood vessels replacing the old tissues until they reach the border of
• another bony area. At birth, the skeleton of a newborn has more than 300
• bones; as a person ages, these bones grow together and fuse into larger
• bones, leaving adults with only 206 bones.
• Flat bones follow the process of intramembranous ossification where the
• young bones grow from a primary ossification center in fibrous membranes
• and leave a small region of fibrous tissue in between each other. In the skull
• these soft spots are known as fontanels, and give the skull flexibility and
• room for the bones to grow. Bone slowly replaces the fontanels until the
• individual bones of the skull fuse together to form a rigid adult skull.
• Long bones follow the process of endochondral ossification where the
• diaphysis grows inside of cartilage from a primary ossification center until it
• forms most of the bone. The epiphyses then grow from secondary
• ossification centers on the ends of the bone. A small band of hyaline
• cartilage remains in between the bones as a growth plate. As we grow
• through childhood, the growth plates grow under the influence of growth
• and sex hormones, slowly separating the bones. At the same time the bones
• grow larger by growing back into the growth plates. This process continues
• until the end of puberty, when the growth plate stops growing and the bones
• fuse permanently into a single bone. The vast difference in height and limb
• length between birth and adulthood are mainly the result of endochondral
• ossification in the long bones.
• Here is what we have learned from Introduction to the Skeletal System:
• The human skeleton is well-adapted for the functions it must perform.
• Functions of bones include support, protection, movement, mineral
• storage, and formation of blood cells.
• There are two types of bone tissue: compact and spongy. Compact
• bone consists of closely packed osteons, or haversian system. Spongy
• bone consists of plates of bone, called trabeculae, around irregular
• spaces that contain red bone marrow.
• Osteogenesis is the process of bone formation. Three types of cells,
• osteoblasts, osteocytes, and osteoclasts, are involved in bone
• formation and remodeling.
• In intramembranous ossification, connective tissue membranes are
• replaced by bone. This process occurs in the flat bones of the skull. In
• endochondral ossification, bone tissue replaces hyaline cartilage
• models. Most bones are formed in this manner.
• Bones grow in length at the epiphyseal plate between the diaphysis
• and the epiphysis. When the epiphyseal plate completely ossifies,
• bones no longer increase in length.
• Bones may be classified as long, short, flat, or irregular. The diaphysis
• of a long bone is the central shaft. There is an epiphysis at each end of
• the diaphysis.
• The adult human skeleton usually consists of 206 named bones and
• these bones can be grouped in two divisions: axial skeleton and
• appendicular skeleton.
• The bones of the skeleton are grouped in two divisions: axial skeleton
• and appendicular skeleton.
• There are three types of joints in terms of the amount of movement
• they allow: synarthroses (immovable), amphiarthroses (slightly
• movable), and diarthroses (freely movable).

Anatomical Structure of human body