Skeletal Tissues
Skeletal tissues
Functions
1) support •Frame; muscles, organs and CT attach.
2) protection •Brain, spinal cord, thoracic organs; heart and
lungs.
3) movement
•Aids muscle contraction; generate leverage.
4) mineral stores Calcium
•~2500 gm; also used for cell signaling
Phosphorus
•For many molecules; ATP for example
5) blood producing
•Red and whites blood cells
•Within red marrow
6) energy stores
•Shafts of bones; filled with fatty material; yellow marrow
Histology of skeletal tissue
consists of several types of tissues
Cartilage
•Covers the ends of bones; part of the joint.
•Hyaline cartilage
dense CT: •periosteum; covers bones
•Anchors tendons and ligaments
bone •Hard matrix- consisting of hydroxyapatite
cells
•4 cell types
1-Osteoprogenitor
•Ancestral cells (stem cells)
•Change (differentiate) into...
2-Osteoblasts •Make bone- fibers and ground substance
3- Osteocytes •Mature bone cells- maintain bone
•reserve cells for bone repair
4- Osteoclasts •Erode bone- required for bone remodeling.
•Derived from hemocytoblasts
matrix •GS + fibers
Ground substance •Hard, rock-like material
•Brittle, can break easily
Hydroxyapatite •Pizoelectric crystal, formed from
calcium phosphate
•Dense hard minerals, form crystals
calcium carbonate
collagen fibers •Gives the matrix strength; prevents fractures; made by
osteoblasts
proximal epiphysis
Epiphysis
•Ends of bone
•Both a proximal and distal epiphysis
diaphysis
diaphysis
•Bone shaft; between the two epiphyses
•Has an inner marrow cavity; yellow marrow.
distal epiphysis
1) compact bone
•Hard and dense bone; most of the diaphysis
•Thin layer over each epiphysis
9
Osteon (Haversian system) •Fundamental living unit of bone; consists of cells,
layers of matrix and vessels.
concentric lamellae
•Layers of matrix around a central canal.
Haversian canal
•Central canal; blood vessels; supply energy and
nutritional needs of the osteon.
Lacunae
•Open spaces; “room”; between lamellae.
•Contains a single osteocyte
Canaliculi
•Tiny channels; interconnect central canal,
lacunae, osteocytes.
•Nutrient delivery and
waste removal.
2) Spongy or cancellous
bone
•Location: epiphysis.
•No distinct organization
Trabeculae •Functional unit
•Bone extensions project into red
marrow.
•Lacunae, osteocytes, canaliculi and
lamellae; no central canal.
red marrow
•Red and white blood cell
formation.
•Nutrients can enter the
trabeculae here.
Physiology of ossification
Ossification (osteogenesis)
Bone formation.
•Starts in either cartilage model or CT layers
•Involves 2 cell types
Chondroblasts •Form cartilage model for bone structure
Osteoblasts •Produce bone within cartilage model.
Two types of ossification
•Intramembranous
•Endochondral
Intramembranous ossification
•Within a CT layer during fetal development
Ossification center
•Initiated by blood vessel
•Penetrates middle of CT layer.
•Precursor cells become osteoblasts
•Form a collagenous matrix- non-calcified
•Matrix becomes calcified later forming trabeculae.
Trabeculae fuse •Forms the meshlike structure of
spongy bone.
Apposition
•Layers of osteoblasts on both sides of the CT.
•Make matrix on the surface of the CT model; bone increases
in thickness.
•Adds to pervious bone
Osteocytes
•Osteoblasts become
trapped in the growing
matrix
•Matrix becomes
calcified; these cells
become osteocytes
nucleating seeds
How are the hydroxyapatite
crystals organized within the
matrix?
•Collagen fibers act as
templates; guide
hydroxyapatite deposition
•gives matrix strength
along certain directions.
•Collagen fibers are
nucleating seeds
endochondral ossification •Endo= within; chondral=cartilage
•Bone formation within a cartilage model
Diaphysis
•Ossification starts within the shaft of the
bone.
hyaline cartilage model
•Covered by a CT layer called the perichondrium
•Def: “near cartilage membrane”
Endochondral Ossification
Step 1:
•Chondrocytes in hyaline cartilage
enlarge
•form struts --> calcify
•chondrocytes die, leave cavities
Step 2
•Blood vessels around cartilage
periphery
•Perichondrium; become
periosteum; find osteoblasts
•Produce bone around diaphysis;
subperiosteal collar
•Continue to grow --> compact bone
(appositional growth).
Endochondral Ossification
Step 3.
•Blood vessels enter cartilage
•Deliver osteoblasts.
•Spongy bone develop; primary
ossification center.
Step 4.
•Remodeling creates a marrow
cavity
•Bone replaces cartilage in the
epiphyseal plate.
Endochondral Ossification
Step 5.
•Capillaries,osteoblasts enter
epiphyses
•Secondary ossification
centers.
Step 6.
•Epiphyses fill with spongy
bone
•Articular cartilage:
•Within joint
•Epiphyseal plate
•Between epiphysis and
diaphysis.
Physiology of bone growth
•Bone continues to grow.
•Requires two processes:
•1) Epiphyseal plate; bone elongation.
•2) Appositional growth; increasing bone thickness.
Epiphyseal plate
•Aka, growth plate.
•Starts as hyaline cartilage
Before plate fusion
After plate fusion
4 zones of the epiphyseal plate
4 zones of the epiphyseal plate
Zone of reserve cartilage
•First zone
•Next to the epiphysis; hyaline cartilage.
•Provides cells for the next zone.
Anchor
•Anchors the plate to the epiphysis
Nutrients
•Provides nutrients and cartilage cells
to the other zones
Zone of proliferating cartilage
•Second zone
replacement
•Cells supplied by the first zone
multiply here.
•Cartilage proliferates
•Plate growth occurs here (small
amount).
Zone of hypertrophy
•Zone three
maturation
•Cartilage cells increase in size;
reach maturity.
Expansion
•Most growth continues here.
zone of calcified matrix
•Zone four
•Cartilage cells convert the cartilage matrix into a
calcified matrix (not bone)
Dead •Chondrocytes die
Erosion
•Matrix erodes
•Forms pockets and spaces; merge to form
marrow cavity; yellow marrow
Osteoclasts
•Erode matrix.
Rate of growth
Human growth hormone (hGH)
•One of several; regulates bone growth
•Lack of hGH during development; e.g., pituitary dwarfism
•Too much hGH before puberty; gigantism
Sex hormones
•Testosterone
and estrogens
also play a role in normal
bone development.
epiphyseal line
•Epiphyseal plate fuses late in adolescence; female, 16-18 years;
male, 18-20 years.
•Called closure.
•An epiphyseal line.
•Not cartilage; bone.
•Too much hGH after closure
•Bones thicken; appositional growth
• Acromegaly.
Rhondo Hatton
Film actor, early 20th Cen.
Homeostasis of bone remodeling •Bone; dynamic equilibrium between growth and erosion.
•Bones can remodel to adapt to new stresses.
•During development; bone growth > erosion
•Osteoblasts are more active than osteoclasts.
Osteoporosis
•Osteoclasts are more active than osteoblasts
•Osteoblasts; less active with age
•Bone looses mass; porous; easy to break.
•Due to aging; changes in normal hormone levels.
Osteoporotic bone
Normal bone