Histology of the Bone Flashcards
general features of bone
Cells, fibers, ground substance
contains extracellular matrix consisting of hydroxyapatite
dynamic: undergoes constant remodeling in response to chronic stress
general features of bone
mechanical support, locomotion, support, storage of calcium and phosphate ions
bone architecture: compact bone versus spongy bone
compact bone is called lamellar; encloses inner spongy layer
spongy bone is called trabecular or cancellous bone
lamellar or woven with spaces for bone marrow
have trabeculae
Long bones are ___ and include these two bones:
tubular, humerus, femur
Short bones are ____ and include these bones
cuboidal, ankle and wrist
Flat bones are ____ and include
protective, cranium
Irregular bones are ____ and include
various shapes, include the pelvis
sesamoid develop in ____ and an example is the
develop in tendons
patella
Diaphysis
the “shaft” compact bone surrounding marrow cavity
Epiphysis
ends of the bones
spongy bone surrounded by compact bone
contains growth plate or epiphyseal plate
Metaphysis
spongy bone between the end of the diaphysis and the epiphysis
periosteum
connective tissue covering the bone
endosteum
connective tissue lining the marrow cavity of the bone
which area contains the epiphyseal plates?
epiphysis
which bone area connects tissue lining of the marrow cavity?
endosteum
Connective tissue of the periosteum and endosteum
cell types, protein
fiber types
innervation/vascularization
limitations of periosteum
fibroblasts and collagen in the outer layer
Sharpey’s fibers = periosteal fibers that penetrate the bone matrix
osteoprogenitors cells line the inner layer
osteoprogenitor cells are also contained in the endosteum
note: osteoprogenitor cells become osteoblasts)
periosteum = highly vascularized + nerve supply
periosteum does not cover articular surfaces
what is a common injury associated with periosteal inflammation?
shin splints
“medial tibial stress syndrome”
it is an overuse injury
Lamellar bone
contains a regular, circumferential arrangement of collagen fibers
Haversian systems (osteons) structural unit
Haversian systems
Osteons
unit of structure in lamellar bone
circumferential arrangement of collagen fibers
lacunae are the spaces for cells (osteocytes) within the lamellae
Lacunae are the spaces for cells (osteocytes) within the lamellae
osteocytes are located in
what are osteocytes
lacunae within osteons
mature or differentiated osteoblasts
Canaliculli =
small canals that radiate from the lacuna and “house” cell processes
blood vessels occupy the central canal of a Haversian system
Osteons =
concentrically arranged lamella around a central haversian canal
what are in the haversian canal?
blood vessels
How do blood vessels get into the lamella?
Blood vessels from periosteum enter the lamella through Volkmann’s canal, perpendicular to the diaphysis, and branch to enter into Haversian canals
in the adult, what cells and where are these cells located when bone repair is required?
osteoprogenitor cells in the endosteum which differentiate into osteoblasts
from outside in
periosteium outer later (collagen, blood vessels, Volksmann canals) –> periosteum inner layer (osteoprogenitor cells) –> endosteum (osteoprogenitor cells and Volksmann canals) –> lamella with Volksmann canals and Haversian systems
Osteocytes
maintain bone matrix
solitary cells, usually flattened or spindle shaped
surrounded by mineralized matrix
visually, think spidery looking cells surrounded by extracellular matrix
Osteocyte processes (where are they, how are they connected, and what purpose)
in canaliculi and are connected by GAP JUNCTIONS
allows communication and transport of nutrients
interstitial lamella is the result of
bone remodeling
Bone matrix =
organic component = Type I collagen, proteoglycans, chondroitin sulfate, karatan sulfate
inorganic component = calcium phosphate and hydroxyapatite
woven bone
usually temporary, weaker than lamellar
difference between them is that collagen is not aligned
also found in developing bone
osteoprogenitor cells
resemble fibroblasts, flattened, usually alone
differentiation regulated by BMP & core binding factor alpha
CFBA1
core factor binding alpha 1: one of the two differentiation factors required by osteoprogenitor cells for production
the other is BMP
osteoblasts
differentiated, bone forming cells
ACTIVE cells: basophilic and cuboidal (Squamous cells in inactive bone)
found in association with newly secreted matrix
differentiation controlled by CBFA-1 and RunX2 and osteocalcin
Sox9 directs the differentiation of osteoprogenitor cells into osteoblasts
factors that stimulate differentiation of osteoprogenitor cells versus factors that stimulate differentiate osteoprogenitor cell differentiation into osteoblasts
BMP, CBFA-1 = osteoprogenitor cells
CBFA-1+RunX2+osteocalcin = transcription factors for osteoprogenitor cell into osteoblasts; Sox9 directs differentiation
Osteoblast differentiation: gene required, 1st indicator, and deficiency outcome of 1st indicator
OB-specific genes direct differentiation
CBFA-1/Runx2 —> first indicator (tissue marker) of osteogensis
mice deficient of CBFA-1/RunX2 have cartilage skeletons instead of bone
what do osteoblasts do
they deposit osteoid
osteoid
osteoblast deposition consisting of:
non-mineralized collagen 1, osteocalcin, osteopontin, sialoprotein
osteoblasts control bone mineralization
osteoblasts deposit a lot of matrix and become disconnected from the outer surface of developing bone. what happens to them
they become flattened and a lacunae develops around them
how do basophils appear under the microscope?
basophilic
Osteoclasts
bone MOs, derived from monocyte-mac progenitor cells
mononuclear phagocyte system
located in Howship’s lacunae
subosteoclastic compartments
Howship’s lacunae
location of MOs
“osteoclasia” translates
bone resorption
what do osteoclasts “have” intracellularly
lots of mitochondria and acidified vesicles
which part of the osteoclast has a ruffled boarder
the side closest to the bone
what does the osteoclast do
breaks down mineralized bone with acid
osteocytes are actively regulated by what two hormones?
calcitonin and PTH
Bone formation 1: intramembranous osteogensis occurs in the following sequence (5 steps)
Mesenchymal cells aggregate
mesenchymal cells form osteoblasts: bone blastema is formed
bine matrix deposited by cells of blastema, forming primary bone.
ossification centers fuse to form trabeculae (spongy bone)
three things to keep in mind about intramembranous ossification that distinguishes it from the other form of bone formation
1) develops from a well vascularized primitive connective tissue
2) bone formation is not preceded by cartilage
3) an aggregate of mesenchymal cells differentiates directly into osteoid producing osteoblasts
bones formed by intramembranous bone formation include
clavicle, frontal and parietal bones, parts of occipital, temporal, mandible, and maxilla bones
Endochondral osteogenesis
this is when bone replaces a cartiaginous template
chondrocytes in the cartilaginous template become isolated by the extracellular matrix (mostly type II collagen)
endochondral osteogenesis beginning with chondrocytes secreting _____ collagen
Chondrocytes secrete type X collagen, promoting angiogenesis
increased blood supply promotes calcification of matrix and chondrocytes undergo apoptosis
primary centers of ossification form in the diaphysis and secondary form in the epiphysis
How does endochondral ossification begin?
Chondrocytes in the cartilaginous template become isolated by the ECM (mostly type II collagen)
chondrocytes then hypertrophy and secrete Type X collagen which in turn promotes angiogensis
blood vessels deliver calcium and promotes general calcification of ECM.
then chondrocytes undergo apoptosis.
primary ossification forms in the diaphysis and secondary centers of ossification form in epiphysis
Primary ossification centers
occur where proliferation of chondrocytes have hypertrophied and secrete angiotactic factors
Secondary ossification centers
occur in the epiphyseal sites: vascularization of the epiphysis delivers bone forming
chondrocytes in the epiphyseal plate secretes IHH which promotes chondrocyte proliferation but inhibits hypertrophy
these cells add to the bone in both diaphysis and epiphysis. eventually epiphyseal plate becomes epiphyseal lines and cease growing
Reserve Zone
hyaline cartilage right under the epiphyseal cartilage
Proliferative Zone
chondrocytes prolif: VERTICAL LINES or stacks, share a common territorial matrix
Hypertrophic Zone
Chondrocytes enlarge, secrete Type X collagen and vascular endothelial growth factor
Matrix calcifies
transverse septa are thinner
angiogenesis is stimulated
Vascular invasion zone
blood vessels invade
osteoprogenitor cells move in
blood vessels transverse septa
osteoblasts beneath the sites of vascular invasion begin to deposit osteoid on the cartilaginous cores forming trabecular bone
“osteoclasts chase”
the ossification front begins in the vascular invasion zone: chondrocytes are destroyed
in which direction do chondrocytes proliferate in the proliferative zone?
away from the ossification front, which increases the length of the cartilage
proliferating chondrocytes express _____ in the prolifierative zone
CFBA-a/RunX2
in which direction does the osteoclast chasing/osteocyte running occur?
in the direction of the epiphyseal cartilages
Unlike intramembranous ossification, endochondral ossification occurs because of
chondrocytes at primary ossification centers have secreted type II collagen (hyaline cartilage) which is the template that is eventually replaced by bone
where do the osteoprogenitor cells come from in intramembraneous ossification versus endochondral ossification
intramembranous ossification center, mesenchymal cells become osteoblasts and directly deposit osteoid
endochondral ossification requires osteoprogenitors imported via vascular supply from the perichondria arrive and are responsible for secreting bone matrix
bone remodeling: compact bone remodeling occurs within the
osteon
a _______ indicates the edge of a remodeled osteon
cement line
in ________ more bone is resorbed than replaced
osteoporsis
Activation/Resorption/Reversal/Formation
Osteoblasts are recruited in the Haversian canals, differentiate into osteoclasts, which begin resorption of the inner lamella moving outward, leaving “interstitial lamella”
when osteoclasts stop, osteoblasts are recruited and begin depositing osteon (osteoclast/osteoblast reversal)
cement line indicates the boundary of newly organized lamella
when a bone breaks…
the vascular supply is disrupted: blood clot forms.
haversian system fails to receive blood, it dies.
MOs scavenge/clean area of debris
intense proliferation of osteoprogenitor cells from periosteum and endosteum
osteoblasts form cellular collar around fracture site
immature bone is formed (woven bone)
trabeculae unite to form bony callus that temporarily unite the bone fragments
woven bone is remodeled into lamellar bone
endosteum (for real this time) line
haversian canals and marrow cavities
Shin splits
Medial Tibial stress syndrome
periosteum
