Skeletal System
Overview
• The skeletal system composed of bones, cartilages, joints, and ligaments, accounts for about 20% of the
body mass (i.e., about 30 pounds in a 160-pound person).
o Bones make up most of the skeleton
o Cartilages occur only in isolated areas, such as the nose, parts of ribs, and the joints
o Ligaments connect bones and reinforce joints, allowing required movements while restricting
motions in other directions.
o Joints are the junctions between bones which provide for the mobility of the skeleton
Skeletal Cartilages
• Human skeleton initially made up of cartilages and fibrous membranes; most are soon replaced with bone
• In adults, the few areas where cartilage remains are mainly where flexible skeletal tissue is needed.
• Cartilage tissue consists mainly of water—approximately 80%; high water content allows cartilage to be
resilient (i.e., spring back to its original shape after being compressed).
• Cartilage contains no nerves or blood vessels.
• Perichondrium (“around the cartilage”) is dense irregular connective tissue; surrounds the cartilage and
acts like a girdle to resist outward expansion when cartilage is compressed.
o Perichondrium contains the blood vessels from which nutrients diffuse through the matrix to
reach the cartilage cells. This mode of nutrient delivery limits cartilage thickness.
• Three types of Cartilage Tissue in body
o All three have cells called chondrocytes encased in small cavities (called lacunae) within an
extracellular matrix containing a jellylike ground substance and fibers.
o Skeletal cartilages contain representatives from all three types.
Hyaline cartilages
• Looks like frosted glass
• Most abundant skeletal cartilages
• Their chondrocytes appear spherical
• Only fiber type in their matrix is fine collagen (undetectable microscopically)
• Skeletal hyaline cartilages include:
o Articular Cartilages—cover ends of most bones at movable joints
o Costal cartilages—connect ribs to sternum
o Respiratory cartilages—form skeleton of the larynx (voicebox) and
reinforce other respiratory passages.
o Nasal cartilage—support the external nose
Elastin cartilages
• Similar to hyaline cartilage AND contain more stretchy elastic fibers
• Stand up better to repeated bending
• Located in only two places:
o External ear
o Epiglottis—flap that bends to cover the opening of the larynx each time
we swallow
Fibrocartilages
• Highly compressible and have great tensile strength
• Intermediate between hyaline and elastic cartilages
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Consist of roughly parallel rows of chondrocytes alternating with thick collagen
fibers
Located in places with heavy pressure and stretch:
o Meniscus—padlike cartilages of the knee joint
o Intervertebral disc
Growth of Cartilage
o Unlike bone, which has a hard matrix, cartilage has a flexible matrix which can accommodate
mitosis; ideal environment to lay down embryonic skeleton and provide new skeletal growth.
o Typically, cartilage growth ends during adolescence when skeleton stops growing.
o Cartilage grows in two ways
Appositional growth—“growth from outside”
• Cartilage forming cells in the surrounding perichondrium secrete new matrix
against the external face of the existing cartilage tissue.
Interstitial growth—“growth from inside”
• Lacunae bound chondrocytes divide and secrete new matrix, expanding the
cartilage from within.
Classification of Bones
• An adult human has 206 named bones divided into two groups
o Axial skeleton
Forms the long axis of the body
Bones of skull, vertebral column, and rib cage
o Appendicular skeleton
Form upper and lower limbs and girdles that attach limbs to axial skeleton
• Classification based on shape
o Long bones
Bones that are longer than they are wide
All limb bones except patella, carpals and tarsals
o Short bones
Cube shaped
Carpals and tarsals
Sesamoid bones—special short bones that form in a tendon; “shaped like a sesame seed”
• Patella
o Flat bones
Thin, flattened and usually somewhat curved
Sternum, scapulae, ribs, most skull bones
o Irregular bones
Complicated shapes that don’t fit any other class
Vertebrae, hip bones
Functions of Bones
• Support
o Bones provide framework for body and cradle its soft organs
• Protection
o Bones of skull protect brain; vertebrae protect spinal cord; rib cage protects vital organs of thorax
• Movement
Skeletal muscles, which attach to bones via tendons, use bones as levers to move the body and its
parts.
Mineral & growth factor storage
o Bone is reservoir for minerals; most importantly, calcium & phosphate
o Minerals are released into blood stream as needed
o Mineralized bone matrix stores various growth factors as well
Blood cell formation
o Hematopoiesis—blood cell formation; occurs in the marrow cavities of certain bones
Triglyceride (fat) storage
o Fat is stored in bone cavities and is a source of stored energy for the body.
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Bone Structure
• Compact Bone
o Dense outer layer of bone that looks smooth and solid to the naked eye
o Haversian system or Osteon—structural unit of compact bone
Each osteon is an elongated cylinder oriented parallel to the long axis of the bone
Functionally, osteons are tiny weight-bearing pillars
• An osteon is a group of hollow tubes of bone matrix, one placed outside the next
like the growth rings of a tree trunk.
• Lamella—“little plate”—each matrix tube; compact bone is often called lamellar
bone.
• Think of an osteon’s design as a “twister resister”
o All the collagen fibers in a particular lamella run in a single direction,
collagen fibers in adjacent lamellae always run in different directions.
o The adjacent lamellae reinforce one another to resist twisting.
• Haversian canal, or central canal—running through the core of each osteon
containing small blood vessels and nerve fibers.
• Volkmann’s canals, or perforating canals—lie at right angles to the long axis of
the bone and connect the blood and nerve supply of the periosteum to those in
the central canals and medullary cavity.
• Osteocytes (spider-shaped bone cells) occupy lacunae (little hollows) at the
junctions of the lamellae.
o Osteocytes maintain the bone matrix and act as stress or strain “sensors”
in cases of bone deformation or other damaging stimuli.
• Canaliculi—hairlike canals; connect lacunae to each other and to the central
canal.
• Spongy Bone
o Internal to compact bone
o Contain a honeycomb of small needle-like or flat pieces called trabeculae
Trabeculae align precisely along lines of stress and help bone resist stress as much as
possible.
Trabeculae contain irregularly arranged lamellae and osteocytes interconnected by
canaliculi.
No osteons are present in the spongy bone.
o In living bones the open spaces between trabeculae are filled with red or yellow bone marrow
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Typical Long Bone:
Diaphysis—a tubular shaped shaft forming the long axis of the bone
Constructed of a thick collar of compact bone surrounding a central medullary cavity—
marrow cavity—containing fat (yellow marrow) in adults.
o Epiphyses—bone ends
Compact bone forms the exterior and their interior contains spongy bone.
Joint surface of each epiphysis is covered with hyaline cartilage
In adults, between diaphysis and epiphysis is an epiphyseal line, a remnant of the
epiphyseal plate, a disc of hyaline cartilage that grows during childhood to lengthen the
bone. This region is sometimes called the metaphysis.
o Membranes
Periosteum
• A glistening white, double-layered membrane covering the external surface of the
entire bone except the joint surface
• Outer fibrous layer is dense irregular connective tissue
• Inner osteogenic layer, abutting the bone surface, consists primarily of boneforming cells called osteoblasts, bone-destroying cells called osteoclasts, and
primitive stem cells (osteogenic cells) that give rise to the osteoblasts.
• Periosteum is richly supplied with nerve fibers, lymphatic vessels, and blood
vessels which enter the diaphysis via nutrient foramina.
• Periosteum provides anchoring points for tendons and ligaments.
Endosteum
• A delicate connective tissue membrane covering the internal bone surfaces.
• Covers the trabeculae of spongy bone and lines the canals that pass through the
compact bone.
• Contains osteoblasts, osteoclasts and osteogenic cells.
Short, Irregular & Flat Bones:
o On outside, thin plates of periosteum-covered compact bone
o On inside, endosteum-covered spongy bone
o They contain bone marrow between the spaces of the trabeculae
o These bones are not cylindrical; thus, they do not contain a shaft or epiphyses
o In flat bones, spongy bone is called diploë and resembles a stiffened sandwich
Hematopoietic tissue in bones
o Hematopoietic tissue (i.e., red marrow)
o Found within trabecular cavities of spongy bone of long bones and diploë of flat bones
o In newborn infants, medullary cavity of diaphysis and all areas of spongy bone contain red bone
marrow
o In most adult long bones, yellow bone marrow extends well into the epiphysis and little red
marrow is present in spongy bone cavities.
Blood cell production, in adult long bones, routinely occurs only in the heads of the femur
and humerus.
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Bone Development
• Ossification and osteogenesis = synonyms meaning the process of bone formation
• Formation of Bony Skeleton
o Before week 8, the skeleton of a human embryo is constructed entirely from fibrous membranes
and hyaline cartilage.
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Intramembranous ossification—bone developing from fibrous membranes beginning around
week 8 of development.
Results in the formation of cranial bones (frontal, parietal, occipital and temporal bones)
and clavicles. Most bones formed by this process are flat bones.
Stages of intramembranous ossification
1. Ossification centers appear in the fibrous connective tissue membrane
2. Bone matrix is secreted within the fibrous membrane and calcifies
3. Woven bone and periosteum form
4. Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow
appears.
Endochondral ossification—process by which essentially all bones of the skeleton form below the
base of the skull.
Beginning in the second month of development, this process uses hyaline cartilage
“bones” formed earlier as models, or patterns, for bone construction.
Hyaline cartilage must first be broken down as ossification proceeds
Stages of endochondral ossification
1. A bone collar is laid down around the diaphysis of the hyaline cartilage model
o Perichondrium covering the hyaline cartilage “bone” in infiltrated with
blood vessels converting it to vascularized periosteum
o Osteoblasts are formed from stem cells
o Osteoblasts in the new periosteum secrete bone matrix against the
hyaline cartilage diaphysis (encasing it in bone).
2. Cartilage in the center of the diaphysis calcifies and then develops cavities
o Bone collar forms
o Chondrocytes within the shaft enlarge and signal surrounding cartilage
matrix to calcify
Calcified cartilage matrix is impermeable to diffusing nutrients
As a result, chondrocytes die and the matrix begins to deteriorate
This deterioration opens up cavities
3. The periosteal bud invades the internal cavities and spongy bone forms
o In month 3 of development, periosteal bud invades cavities.
Periosteal bud—contains a nutrient artery and vein, lymphatic
vessels, nerve fibers, red marrow elements, osteoblasts, and
osteoclasts.
o Osteoclasts partially erode the calcified cartilage matrix
o Osteoblasts secrete bone matrix around remaining fragments of hyaline
cartilage—forming bone covered trabeculae (i.e., earliest version of
spongy bone is developed).
4. The diaphysis elongates and a medullary cavity forms
o Osteoclasts break down the newly formed spongy bone in the center of
the long bone creating a medullary cavity in the middle of the diaphysis.
o From week 9 to birth, the epiphyses consist only of cartilage.
Hyaline cartilage models continue to elongate at the epiphyses
Ossification “chases” cartilage formation along the length of the
shaft as cartilage calcifies, erodes and then is replaced by bone.
5. The epiphyses ossify
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At birth, most of our long bones have a bony diaphysis surrounding
remnants of spongy bone, a widening medullary cavity, and two
cartilaginous epiphyses.
Secondary ossification centers appear in one or both epiphyses shortly
before or after birth
When secondary ossification is complete
• Hyaline cartilage remains only at two places
o Epiphyseal surfaces as articular cartilage
o Epiphyseal plates, the junction of the diaphysis
and epiphysis
Postnatal Bone Growth
o During infancy and youth
Long bones lengthen by interstitial growth of the epiphyseal plate cartilage and its
replacement by bone
• Longitudinal bone growth ends when the bones of the epiphysis and diaphysis
fuse; this process is called epiphyseal plate closure (occurring around 18 yo in
females, 21 yo in males).
All bones grow in thickness by appositional growth.
Bone Homeostasis: Remodeling & Repair
• Bone Remodeling
o Every week, we recycle 5-7% of our bone mass; yet, in health young adults, total bone mass
remains constant.
o Each day, we have ½ gram of calcium entering or leaving our adult skeleton
Bone deposits occur wherever bone is injured or added bone strength is required
• For optimal bone deposit, a healthy diet rich in proteins, vitamin C, vitamin D,
vitamin A, and several minerals (primarily, calcium, phosphorus, magnesium, and
manganese) is essential.
Bone resorption is accomplished by osteoclasts
• Osteoclasts move along a bone surface, digging gooves as they break down the
bone matrix.
o Spongy bone is replaced every 3-4 years
o Compact bone is replaced every 10 years
When bone remains in place for long periods, more of the calcium salts crystallize and the
bone becomes more brittle.
• Bone Repair
o Fractures—breaks in bone may be classified by
Position of the bone ends after fracture.
• Nondisplaced fractures—the bone ends retain their normal position
• Displaced fractures—bone ends are out of normal alignment
Completeness of break
• Complete fracture—bone broken through
• Incomplete fracture—bone not broken all the way through
Orientation of the break relative to the long axis of the bone
• Linear fracture—break parallels long axis
• Transverse fracture—break is perpendicular to the bone’s long axis
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Whether bone ends penetrate the skin
• Open (compound) fracture—yes, bone penetrates the skin
• Closed (simple) fracture—no, bone does not penetrate the skin
Repair in a simple fracture involves four major stages:
1. A hematoma forms
a. Blood vessels in bone, periosteum and surrounding tissues are torn and
hemorrhage
b. A hematoma—mass of clotted blood—forms at the fracture site
c. Bone cells deprived of nutrition die and tissue at the site becomes
swollen, painful and inflamed.
2. Fibrocartilaginous callus forms
a. Capillaries grow into the hematoma and phagocytic cells invade the area
beginning the clean-up of the debris
b. Fibroblasts and osteoblasts invade the fracture site from the nearby
periosteum and endosteum and begin reconstructing the bone
c. Spongy bone begins to form connecting the broken bone ends
3. Bony callus forms
a. Within a week, new bone trabeculae begin to appear in the
fibrocartilaginous callus and gradually convert it to a bony callus of
spongy bone
b. Bony callus formation continues for about two months until a firm union
is formed
4. Bone remodeling occurs
a. For several months during and after the formation of the bony callus, it is
remodeled.
b. Excess material on the diaphysis exterior and within the medullary cavity
is removed
c. Compact bone is laid down to reconstruct the shaft walls