Skeletal system Flashcards
What does the skeletal system provide us with?
shape and movement
blood cells
a storage area for minerals
protection for some internal organs
What are the four tissues of the skeleton?
bone, cartilage, ligaments, tendons
What is bone?
highly vascular connective tissue
contains osteogenic (bone-producing) cells separated by bone matrix
collagen fibres form 90-95% of the organic part of bone
inorganic crystals (e.g. calcium phosphate, calcium carbonate, citrate) form 50-70% of bone
Collagen is more elastic than bone and consists of which two cell types?
immature chondroblasts - secrete the components of cartilage
chondrocytes - mature cartilage cells derived from chondroblasts trapped within lacunae
What are the three types of cartilage found in the body?
hyaline (transparent), fibrous, elastic
What is hyaline cartilage?
covers the ends of synovial joints
connects the ribs to the sternum
forms the larynx and part of the nose
reinforces the trachea and bronchi
What is fibrous cartilage?
contains more collagen fibres which makes it compressible and able to resist high pressures
found in areas of high stress (e.g. intervertebral discs and knee joints)
What is elastic cartilage?
contains large amounts of elastic fibres which provide flexibility
found in the external ear and the epiglottis
What are ligaments?
tough fibrous bands of dense collagen fibres and fibrocytes (spindle-shaped cells)
support internal organs and attach bones to bones, holding them together at joints
What are tendons?
composed of dense fibrous connective tissue
attach muscle to bone
How many bones does the adult human skeleton contain?
206
The human skeleton is divided into which two parts?
axial skeleton and appendicular skeleton
What is the axial skeleton?
consists of 80 bones
includes bones of the skull, vertebral column, ribs and sternum
these bones form the central axis of the body and support the head, neck and torso
What is the appendicular skeleton?
consists of 126 bones
includes bones of the arms and legs and bones that attach them to the axial skeleton (pectoral and pelvic girdles)
What are the five main functions of bone?
support, movement, storage, protection, blood cell formation
How do bones provide structural support?
they provide a rigid framework to which soft tissues and organs are attached
How do bones enable movement?
bones and muscles with joints enable movement
tendons and ligaments determine the direction and the degree of bone movement
this depends on whether skeletal muscle generates fine or gross motor movements
How do bones provide storage?
bones store essential minerals (e.g. calcium, magnesium, phosphorus)
bones release or absorb minerals in response to the body’s demands under the influence of hormones
hypercalcaemia causes excess calcium to be deposited in bone
hypocalcaemia causes calcium to be released from the bone into the bloodstream
lipids are stored within yellow bone marrow which can be released and used as an energy source as required
How do bones provide protection?
they provide a rigid structure that gives protection to most of the internal organs and soft tissues within the body cavities
cranium (skull) - protects the brain
spinal column of vertebrae - protects the spinal cord
thoracic cavity (sternum and ribs) - protects the heart and lungs
pelvic cavity - protects the bladder, rectum and anus, and reproductive organs
How are bones involved in blood cell formation?
blood cell formation occurs in red bone marrow through a process called haemopoiesis
in newborns, all bone marrow is red
in adults, red bone marrow is only found in certain bones (e.g. skull, vertebrae, ribs, sternum, pelvis, ends of the femur and humerus)
red bone marrow outside these bones changes from red to yellow and stops producing these cells through ageing
What is a long bone?
a bone that is longer in length than width
A long bone consists of which parts?
diaphysis, epiphysis, metaphysis, articular cartilage, periosteum, medullary cavity, endosteum
What is the diaphysis?
the shaft and main portion of bone
long and cylindrical shape
What are the epiphyses?
the proximal and distal ends of bone
What are the metaphyses?
between the diaphysis and epiphyses
each metaphysis contains the epiphyseal (growth) plate made of hyaline cartilage where the diaphysis can grow in length
when the bone stops growing (aged 18-21) the hyaline cartilage is replaced by bone to form the epiphyseal line
What is the articular cartilage?
a thin layer of hyaline cartilage that covers the epiphysis of one bone where it forms a joint with another bone
it reduces friction and absorbs shocks at freely movable joints
it does not contain perichondrium or blood vessels which means damage repair is limited
What is the periosteum?
the tough connective tissue membrane that covers the outside of bone (except where there is articular cartilage)
two layers
outer layer consists of dense irregular connective tissue
inner layer consists of osteogenic cells that can develop bone width but not length
What are the key functions of the periosteum?
to protect and nourish the bone tissue
to act as an attachment site for tendons and ligaments
to assist in bone repair
What is the medullary cavity?
a cylindrical cavity within the diaphysis
contains blood vessels and bone marrow
helps to reduce the density and weight of bone
the cylindrical shape allows it to retain strength to withstand forces
What is the endosteum?
a single layer of bone-producing cells which lines the inner surfaces of the cavities within the bone
What is the bone matrix?
an abundant extracellular matrix that surrounds the bone-generating cells
What does the bone matrix consist of?
water, collagen fibres, crystallised mineral fibres
Which is the most abundant mineral in the matrix?
calcium phosphate
Crystals of hydroxyapatite are formed by the combination of which two minerals?
calcium phosphate and calcium hydroxide
Which other minerals are deposited in the matrix during the formation of crystals?
magnesium, fluoride and sulphate
as the minerals crystallise they harden in the process of ossification
What determines the hardness of the bone?
the amount and type of crystallised minerals
What determines the flexibility of the bone?
the number of collagen fibres present within the matrix
calcification only occurs when collagen fibres are present
Where do the mineral salts crystallise?
initially in the spaces between the collagen fibres
around the collagen fibres when the spaces have been filled
Bone tissue consists of which four cell types?
osteogenic cells, osteoblasts, osteocytes, osteoclasts
What are osteogenic cells?
derived from mesenchymal cells (adult stem cells)
they undergo mitosis and develop into osteoblasts
they are found in the periosteum, endosteum, and within the canals that contain blood vessels
What are osteoblasts?
bone-producing cells
produce the bone matrix by synthesising and secreting collagen fibres and other organic components
initiate calcification of the matrix
What are osteocytes?
start as osteoblasts
surrounded by the matrix which makes them trapped
this means they are no longer able to secrete the matrix and become osteocytes
found in mature bone
main cell type in bone
maintain the daily metabolic function of bone by ensuring the exchange of nutrients and waste products with blood
What are osteoclasts?
formed by the fusion of around 50 monocytes and remove old bone
very large, multinucleated and mainly found in the endosteum
the plasma membrane is folded into deep ruffles and faces the bone surface
it secretes lysosomal enzymes and acids that dissolve the protein and mineral matrix
this is called resorption and is part of normal bone development, maintenance and repair
removal of old bone is usually aligned to production of new bone cells by osteoblasts
What are the functions of the many small spaces within bone?
to provide a pathway for blood vessels for exchange of nutrients and waste products
to act as a storage area for red bone marrow
Approximately what percentage of the skeleton is made up of compact bone?
80%
Approximately what percentage of the skeleton is made up of spongy bone?
20%
Approximately what percentage of the axial skeleton is made up of spongy bone?
70%
What is compact bone?
strongest type of bone tissue
What is spongy bone?
it consists of columns of bone (trabeculae)
between the columns are spaces filled with red and yellow bone marrow and blood vessels
spongy bone is always found inside bone and is protected by a layer of compact bone
it is usually found in bones that have low stress levels or where pressures are exerted from a range of directions
the spaces make spongy bone lighter than compact bone
this reduces the weight of bones which allows them to move easily when pulled by skeletal muscle
How does the blood supply pass into the interior of the bone?
via the periosteum
How do periosteal arteries and nerves enter the diaphysis?
via Volkmann’s canals
What is the function of periosteal arteries?
to supply blood to periosteum and compact bone
What is the large nutrient artery?
located near the centre of the diaphysis
passes through nutrient foramen and enters medullary cavity
divides into proximal and distal branches
What is the function of the large nutrient artery?
to supply blood to compact and spongy bone and red bone marrow of diaphysis as far as the epiphyseal line
Which two factors affect the number of nutrient foramina in each bone?
bone size
amount of red bone marrow in bone
What is the function of metaphyseal and epiphyseal arteries?
to supply blood to ends of bones and to red bone marrow
What are the three veins that carry blood away from long bones?
nutrient veins - accompany nutrient artery and exit through diaphysis
metaphyseal and epiphyseal veins - accompany their arteries and exit through their respective names
periosteal veins - exit through periosteum
Why do people experience severe pain when they have sustained a fracture or have a bone tumour?
periosteum contains many sensory nerve endings
these produce sensations of pain and are sensitive to tension or tearing
What is ossification?
the process of bone formation
During the first few weeks, the embryonic skeleton is composed of which type of cells with the general shape of bones?
mesenchymal cells (a special type of undifferentiated tissue)
Bone is formed by one of which two processes during embryonic or foetal development?
intramembranous ossification - occurs within connective tissue
endochondral ossification - occurs in cartilage
What is intramembranous ossification?
bone formation occurs with the direct conversion of mesenchymal tissue into bone
this occurs in the flat bones of the skull, the lower jaw and the scapula
What are the four stages of intramembranous ossification?
the ossification centre
calcification
trabeculae formation
development of periosteum
What happens in the first stage of intramembranous ossification?
the ossification centre forms at the site where bone will develop
it occurs when specific chemical messengers cause mesenchymal cells to group together and then differentiate into osteogenic cells and later into osteoblasts
osteoblasts secrete the extracellular matrix until they become surrounded, when it stops
What happens in the second stage of intramembranous ossification?
osteocytes lie in the lacunae and extend into the canaliculi radiating in all directions
over a period of days, calcium and other minerals are deposited and the extracellular matrix begins to harden (calcify)
What happens in the third stage of intramembranous ossification?
trabeculae develop during formation of the extracellular matrix
they fuse with one another to produce spongy bone surrounding blood vessels in connective tissue
What happens in the fourth stage of intramembranous ossification?
mesenchyme at the periphery of the bone condenses and forms the periosteum
a thin layer of compact bone replaces the surface layers of spongy bone (the centre remains spongy)
the newly formed bone undergoes remodelling and eventually becomes the adult shape and size
What is endochondral ossification?
how most bones are formed through replacement of hyaline cartilage with bone
What are the six stages of endochondral ossification?
development of cartilage model
growth of cartilage model
development of primary ossification centre
development of medullary cavity
development of secondary ossification centre
formation of articular cartilage and epiphyseal plate
What happens in the first stage of endochondral ossification?
specific chemical messengers cause mesenchymal cells to group together and then differentiate into chondroblasts
chondroblasts secrete a cartilage extracellular matrix to produce a hyaline cartilage model
perichondrium (membrane) forms around the cartilage model
What happens in the second stage of endochondral ossification?
chondroblasts become surrounded by the matrix and develop into chondrocytes
model grows in strength (interstitial growth) with secretion of the matrix and cell division of chondroblasts
cartilage model grows in thickness (appositional growth) as new chondroblasts secrete the matrix deposited on the cartilage surface
chondrocytes in the middle increase in size (hypertrophy) and deteriorate leaving a cavity in the cartilage
the matrix starts to calcify
nutrients cannot diffuse through the matrix and chondrocytes start to die and lacunae develop in spaces
What happens in the third stage of endochondral ossification?
primary ossification continues from the external surface of the bone inwards
nutrient artery enters perichondrium and calcifying cartilage via a nutrient foramen into the centre of the model
this stimulates osteogenic cells in the perichondrium to differentiate into osteoblasts
when perichondrium produces bone instead of cartilage it is called the periosteum
periosteal capillaries in the model grow into the matrix and form the primary ossification centre where most cartilage will be replaced by bone tissue
spongy bone trabeculae form when osteoblasts are deposited and secrete bone matrix over the calcified cartilage
What happens in the fourth stage of endochondral ossification?
spongy bone is broken down by osteoclasts to form the medullary cavity filled with red bone marrow
primary ossification forms the diaphysis which is made of an outer layer of compact bone, lined with spongy bone that surrounds the medullary cavity
What happens in the fifth stage of endochondral ossification?
epiphyseal artery entering the epiphyses leads to development of secondary ossification centres (usually around the time of birth)
bone formation in secondary centres is similar to that in the primary centre
the difference is that spongy bone remains in the centre of the epiphysis and no medullary cavities are formed
secondary ossification proceeds outwards from the centre of the epiphysis to the external bone surface
What happens in the sixth stage of endochondral ossification?
hyaline cartilage covering the epiphysis becomes the articular cartilage and is retained throughout life
epiphyseal plate is hyaline cartilage - responsible for growth in length of bones in childhood, replaced by bone in adulthood
What is osteoporosis?
a disease of the skeletal system associated with fragile, porous bones with significant reduction in bone mass
What is osteopenia?
a mild to moderate reduction in bone mass below average levels
it puts a person at a higher risk of developing osteoporosis
What are some of the factors that may be involved in the development of osteoporosis?
genetic factors, hormonal factors, nutrition, level of activity, increased levels of glucocorticoids (natural or artificial), excessive alcohol consumption, caffeine, smoking
Why does osteoporosis affect more women than men?
loss of oestrogen at the menopause means that osteoblast activity and synthesis of bone matrix is greatly reduced
women’s skeletons are lighter with lower bone mass
What are some of the effects of osteoporosis?
increased likelihood of fractures, height loss, bone pain, fractures of the vertebrae which may lead to kyphosis (excessive outward curvature of the spine, causing hunching of the back)
What can older women, in particular, do to reduce the risk of osteoporosis development and fractures?
take enough vitamin D and calcium and enough exercise
What are the two distinct events involved in bone length growth from the epiphyseal plate?
interstitial growth of cartilage on the epiphyseal side of the epiphyseal plate
endochondral ossification of the hyaline cartilage on the diaphyseal side of the epiphyseal plate
How many zones are within the epiphyseal plate?
four
What is the structure of zone 1 (resting cartilage)?
layer closest to the epiphysis
contains small chondrocytes that are not actively involved in bone growth
they attach the epiphyseal plate to the epiphyses, deliver nutrients to the developing cartilage, and store materials (e.g. lipids, glycogen) needed for growth
What is the structure of zone 2 (proliferating cartilage)?
chondrocytes are slightly larger and arranged into columns
chondrocytes divide and replace dead cells on the diaphyseal side of the epiphyseal plate
they secrete the matrix and produce new cartilage through interstitial growth
What is the structure of zone 3 (hypertrophic cartilage)?
chondrocytes mature and enlarge (hypertrophy)
they are arranged into columns with a gradient of maturation
chondrocytes nearest the epiphysis are younger and less mature than cells nearer the diaphysis
What is the structure of zone 4 (calcified cartilage)?
very thin and contains large dead chondrocytes and calcified matrix
osteoclasts help dissolve the matrix and blood vessels
osteoblasts from the diaphysis enter the zone
this zone now becomes part of the diaphysis
How many stages are involved in bone thickness growth below the periosteum?
five
What happens in the first stage of bone thickness growth?
periosteal cells at the bone surface differentiate into osteoblasts
osteoblasts secrete collagen fibres and organic materials to form the bone extracellular matrix