General Biology Lecture BIO 211
Skeletal System Regis University
Spg 1992
B. Rife
1/4
Chapter 5 Skeletal System
The internal endoskeleton of vertebrates is largely composed of cartilage and bone that grows with the animal.
These tissues are storage areas for calcium and phosphorus; they are also the site of blood cell production in adult
animals.
Functions of Skeletal System
Support : provides structural support for entire body. Individual bones provide a framework for the attachment of soft
tissues and organs.
Storage of minerals and (energy reserves) lipids: The Ca salts of bone represent a valuable mineral reserve that
maintains normal concentrations of Ca and phosphate ions in body fluids. Ca is the most abundant mineral in
the human body.
Ca needed for normal nerve and muscle function
Blood Cell Production: red bone marrow (blood cell forming connective tissue) Hemopoiesis - process of blood cell
formation
Greek hemo means blood
poiesis means to make
Protection: examples; skull protects the brain
breastbone and ribs protect vital organs (heart / lungs)
vertebrae shield the spinal cord
pelvis cradles delicate digestive & reproductive organs
Leverage: Many bones function as levers that can change the magnitude and direction of the forces generated by
skeletal muscles.
Example: delicate motion of the fingertip
Human Skeleton
The human skeleton may be divided into two parts:
the axial skeleton and the appendicular skeleton.
1. Axial Skeleton
The axial skeleton includes the skull, the vertebrae, the ribs, and the sternum.
The skull, or cranium, is composed of many bones fitted tightly together in adults.
In newborns, certain bones are not completely formed and instead are joined by membranous regions called fontanels,
all of which usually close by the age of 16 months.
The vertebral column extends from the skull to the pelvis and forms a dorsal backbone that protectsthe spinal cord.
Normally, the vertebral column has four curvatures that provide more resiliency and strength than a straight
column could. It is composed of many parts, called vertebrae, that are held together by bony facets, muscles,
and strong ligaments.
The vertebral column, directly or indirectly, serves as an anchor for all the other bones (ribs, sternum) of the skeleton.
Axial Skeleton: consists of the bones of the skull, thorax, and vertebral column (form the longitudinal axis of the body)
80 bones (40% of the bones in the body)
Skull - 22 bones total - 8 cranial bones
Vertebral Column - 24 vertebrae, the sacrum, and the coccyx
Cervical - (7) neck
Thoracic - (12) chest
Lumbar - (5) lower back
Sacrum - (5 fused vertebrae)
Coccyx (3-5 fused vertebrae) tailbone
Thoracic Cage - 24 ribs and the sacrum
True Ribs - (1-7) attach directly with sternum
False Ribs - (8-12) 11 and 12 are floating ribs - don’t connect w/ribs attach indirectly with sternum
Function of the axial skeleton is to create a framework that supports and protects organs in the dorsal and
ventral body cavities.
2. Appendicular Skeleton
The appendicular skeleton consists of bones within the pectoral and pelvic girdles and the attached appendages.
The components (Coxal/Ilium, Ischium, Pubis) of the pectoral girdle are loosely linked together by ligaments rather
than firm joints.
Each clavicle (collar bone) connects with the sternum in front and the scapula (shoulder blade) behind, but the scapula
is freely movable and held in place by muscles.
General Biology Lecture BIO 211
Skeletal System Regis University
Spg 1992
B. Rife
2/4
The single long bone in the upper arm, the humerus, has a smoothly rounded head that fits into a socket of the scapula.
The socket is very shallow and much smaller than the head in almost any direction, there is little stability.
Therefore this is the joint that is most apt to dislocate.
The opposite end of the humerus meets the two bones of the lower arm, the ulna and the radius, at the elbow.
The wrist has eight carpal bones that look like small pebbles. From these, five metacarpal bones fan out to form a
framework for the palm.
Beyond the metacarpals are the phalanges, the bones of the fingers and the thumb.
The pelvic girdle consists of two heavy, large innominate bones, or hipbones. The innominate bones are anchored to
the sacrum and together these bones form a hollow cavity, the pelvis.
The largest bone in the body is the femur, or thigh bone.
In the lower leg the larger of the two bones, the tibia, has a ridge we call the shin. Both of the bones of the lower leg
have a prominence that contributes to the ankle - the tibia on the inside of the ankle and the fibula on the
outside of the ankle.
Although there are seven tarsal bones in the ankle, only one receives the weight and passes it on to the heel and the ball
of the foot.
The metatarsal bones form the arches of the foot.
If the tissues that bind the metatarsals together become weakened, flatfeet are apt to result.
The bones of the toes are called phalanges, just like those of the fingers, but in the foot the phalanges
those of the fingers, but in the foot the phalanges are stout and extremely sturdy.
just like
Differences Between a Man’s and a Woman’s Skeleton: pg. 101
* Males Skeleton Larger Structure
Shape of pelvic girdle shaped like a funnel
* Female Skeleton - Lighter bone weight
teeth / mandible are smaller
Oval / Round shaped pelvic inlet
functional importance - to carry and deliver newborn
Types of Bones: There are four types of bones
1. Long - humerus
Structure: Diaphysis (Shaft)
Medullary cavity - hollow area; contains soft yellow bone marrow
Epiphyses - ends of the bone (red marrow cavities in spongy bone)
Periosteum - strong fibrous membrane covering a long bone except at joint surfaces.
(1) isolates bone from surrounding tissues
(2) provides a route for circulatory and nervous supply
(3) actively participates in bone growth and repair
Endosteum - lines medullary cavity inside the bone, active during the growth of bone and
whenever repair or remodeling is under way.
2. Short - carpals
3. Flat - frontal bone
4. Irregular - vertebrae
Long Bone Structure
Microscopic Structure of a typical bone:
The skeletal system contains two major types of connective tissue : bone and cartilage.
Both cartilage and bones have cells separated by a matrix. The matrix of cartilage, composed of protein an
proteinaceous fibers, is more flexible than that of bone, which contains a protein framework in which salts of
calcium and phosphorus are embedded.
Bone occurs in two forms - compact bone and spongy bone. Compact bone consists of structural units called
Haversian systems.
General Biology Lecture BIO 211
Skeletal System Regis University
Spg 1992
B. Rife
3/4
Spongy bone contains numerous bony bars and plates separated by irregular spaces. Although lighter than compact
bone, spongy bone is still designed for strength. Most of the spaces in spongy bone are filled with red
marrow, a specialized tissue that produces blood cells. The medullary cavity of a long bone usually contains
yellow marrow, which is a fat-storage tissue.
The matrix of compact bone is very dense and contains deposits of calcium salts.
The matrix contains bone cells, or osteocytes, within pockets, or lacunae. The lacunae are often organized around
blood vessels that branch through the bony matrix.
Calcium Phosphate accounts for almost two thirds of the weight of bone
Collagen fibers contribute roughly one-third of the weight of bone
Canaliculi - permit the diffusion of nutrients and wastes to and from osteocytes (tiny passageways / canals found in
osteon)
Lamellae - each ring / layer within the osteon
Central Canal - found in center of osteon, contains blood vessels
Four Cells found in bones:
1. Osteocytes - mature bone cells that accounts for most of the cell population
Function:
(1) maintain health of the bone
(2) repair of damaged bone
2. Osteoblasts - cells that are actively depositing bone matrix
Function: (1) produce new bone, important in bone growth and repair
3. Osteoclasts - cells that actively remove (digest) bone matrix
Function: (1) necessary in bone remodeling
4. Osteoprogenitor Cells - stem cells whose divisions give rise to osteoblasts
Growth and Development of Bones
Osteogenesis - bone formation
Most of the bones of the skeleton are cartilaginous during prenatal development. Later, bone forming cells
known as osteoblasts replace cartilage with bone.
Bony skeleton begins to form 6 weeks after fertilization (embryo 12mm long)
Prior to this, skeletal elements are cartilaginous
Bone growth continues through adolescence, and portions of the skeleton usually do not stop growing until age
25
Ossification - replacing other tissues with bone
Endochondral ossification - bones are formed from cartilage models
Epiphyseal Plate - separates epiphysis from the diaphysis
Within this plate, the chondrocytes closest to the epiphysis continue to enlarge and divide, adding to the
thickness of the plate.
At puberty, the combination of rising levels of sexual hormones, growth hormone, and thyroid hormones
stimulates bone growth
dramatically
Epiphyseal Fracture - is a fracture at the epiphyseal plate of long bones in early adolescents.
It is caused by overstressing the bone / joint.
In the adult, bone is continually being broken down and then built up again. Bone-absorbing cells, called osteoclasts,
are derived from cells carried in the bloodstream. As they break down bone, they remove worn cells and
deposit calcium in the blood.
The destruction caused by the work of osteoclasts is repaired by osteoblasts. As they form new bone, they take
calcium from the blood. Eventually some of these cells get caught in the matrix they secrete and are converted
to osteocytes, the cells found within Haversian systems.
Many older women, due to a lack of estrogen, suffer from osteoporosis, a condition in which weak and thin bones cause
aches and pains and fracture easily.
Classification of the Joints
Bones are linked at the joints, which are often classified according to the amount of movement they allow.
General Biology Lecture BIO 211
Skeletal System Regis University
Spg 1992
B. Rife
4/4
Joints (Articulations) - connecting bone to bone
Kinds of Joints Synarthroses (no movement) joints in cranial bones
Amphiarthroses (slight movement) joints between body of vertebrae and joint between two pubis
bones
Diathroses (free movement) most of the joints in our body - allow considerable movement
Types of Diarthrotic Joints:
Hinge - knee and elbow
Ball and Socket - shoulder and hip
Saddle - fingers
Gliding - vertebrae
Ligaments, which are composed of fibrous connective tissue, form a capsule that binds the two bones to one another,
holding them in place. In “double-jointed” individual, the ligaments are unusually loose.
Synovial membrane secretes a lubricating fluid that allows easier movement with less friction.
In the knee there are also crescent-shaped pieces of cartilage between the bones, called menisci (sing., meniscus).
Athletes often suffer injury of the menisci, known as torn cartilage. The knee joint also contains 13 fluid-filled
sacs called bursae (sing., bursa) which ease friction between tendons and ligaments and between tendons and
bones. Inflammation of bursae is called burisitis. Tennis elbow is a form of bursitis.