Lecture 8: Microanatomy of Bone Flashcards
what are the 3 parts of a long bone
epiphysis (E)
metaphysis (M)
diaphysis (D)
what is the function of bone
- structural framework for support and protection of organs
- levels for attached muscles
- house and protect hematopoetic tissues & store fat
- resevoir for minerals (Ca, P, Mg, K)
compositon of the bone matrix
Organic component: type I collagen
Inorganic component: Calcium hydroxyapatite
what are the cells of bone
- osteoblast
- osteocyte
- osteoclast
what are the two layers of bone
Periosteum (lines external surface)
- fibrous periosteum
- inner cellular layer
Endosteum (lines central cavity)
- monolayer of osteogenic cells and osteoblasts
*both are vascularized CT
layer of periosteum characterized by an outer layer of dense CT
fibrous periosteum
layer of periosteum characterized by the presence of osteogenic cells
inner cellular layer
characteristics of the endosteum layer of bone
has cells similar to the periosteum but with loose CT
has a monolayer of osteogenic cells and osteoblasts
type of bone that provides support to external surfaces of bones and makes up the shafts of long bones
compact (cortical bone)
what are the 4 lamellar systems of compact bone ?
- outer circumferential lamellae
- inner circumferential lamellae
- osteons
- interstitial lamellae
woven (immature) bone characteristics
- initial bone formation; fracture repair
- poorly organized type 1 collagen
- relatively weak
lamellar (mature) bone characteristics
- replaces woven bone
- very strong, well organized
- type I collagen
when does lamellar (mature) bone replace immature woven bone?
- during formation of primary bone (primary osteons)
- during remodeling to form secondary bone (secondary osteons)
bone type formed by interconnecting bony struts (trabeculae) that provide internal support to bone
Cancellous (trabecular, spongy) bone
characteristics of cancellous bone that makes it capable of calcium ion exchange b/w bone and blood
high surface area
what types of marrow (spaces) exist between the trabeculae of cancellous bone
- red marrow : produces blood cells
- yellow marrow : made of adipose tissue
what cell type is responsible for the formation of bone ECM
osteoblasts
what are osteoprogenitor cells
stem cells near bony surfaces that differentiate into osteoblasts during periods of growth, remodeling, and fracture repair
cells responsible for the resorption of bone ECM
osteoclasts
— derived from monocyte-macrophage lineage located in the bone marrow
cells responsible for the deposition of bone ECM (type I collagen, prosteoglycans, glycoproteins)
Osteoblasts
(when inactive, called bone-lining cells)
what bone cell type lowers blood Ca levels
osteoclasts
identifying characteristics of osteoblasts
- found along surface of bone in sheet like arrangement
- extensive rER and golgi
- polarized organelles
–> nucleus is away from region of secretory activity - communicate with each other via gap junctions
what is an osteoid
the initial organic component synthesized by osteoblasts
*calcifies
bone cell that forms gap junctions in response to mechanical stimuli by sending cell processes through channels in bone called canaliculi
osteocyte
how is bone ECM formed
one lamellla at a time by osteoblasts
what is the organic portion of bone ECM made of
mostly type I collagen fibers
what is the inorganic (mineral) portion of bone made of
hydroxyapatite, calcium ions bound by multiadhesive glycoproteins (MGP)
bone specific alkaline phosphatase (BALP) aids in hydroxyapatite formation
how do osteoclasts raise blood calicum levels
reabsorb bone, endocytose ECM byproducts, secrete them into blood
characteristics of osteoclasts
- large
- multinucleated
- ruffled border
- clear zone
what is the ruffled border of an osteoclast
- portion of osteoclast that is in direct contact w/ bone
- abundant lysosomes and mitochondria
what is the clear zone of an osteoclast
area b/w black arrows
- ring of cytoplasm around the resorptive compartment that immediately surrounds the ruffled border
- lacks organelles (‘clear’)
- actin ring inside clear zone helps seal off the subosteoclastic compartment
what is the purpose of the actin ring found within the clear zone of osteoclasts
- allows bone resportion to occur while preventing damage to surrounding tissue
- by: sealing off subosteoclastic compartment
how is the inorganic portion of bone ECM resorbed by osteoclasts
- by pumping H+ into the resorptive cavity under the cell
how is the organic portion of bone ECM resorbed by osteoclasts
- gets degraded by lysosomal enzymes
what happens to the byproducts of bone ECM resorption
- they get endocytosed by osteoclast and secreted into the blood
what hormones regulate the activity of resorbing osteoclasts and what is their effect
- Parathyroid hormone (PTH)
- stimulates osteoclasts to INCREASE bone resporbtion
- Calcitonin
- inhibits osteoclasts to DECREASE bone resorption
what is the effect of calcitonin on blood Ca levels
- decreases them by slowing resporption of bone ECM
- calcitonin is secreted by thyroid gland
what is the effect of PTH on blood Ca levels
- PTH causes an increase in blood Ca levels by promoting ECM resorption
- cause osteoblasts to produce osteoclast stimulating factor
as the mesoderm develops, what affects the differentiation of mesenchymal cells into cartilage or bone
oxygen tension (O2 levels) in the tissue
In a high O2 environment, what cell type do mesenchymal cells differentiate into
- osteoprogenitor cells and osteoblasts that form bone
- process is called: intramembranous ossification and requires vascularized mesenchyme to supply O2
In a low O2 environment, what cell type do mesenchymal cells differentiate into
- chondrogenic cells and chondrocytes that form hyaline cartilage
explain the process of endochondral ossification
- A low O2 environment causes mesenchymal cells to differentiate into chondrocytes that form hyaline cartilage
- As blood vessels grow into the cartilage, forming ossification centers where chondrocytes exposed to high O2 start to act like bone cells by calcifying cartilage ECM.
- Eventually, the cartilage ECM is replaced with bone.
- process is called endochondral ossification. It begins in avascular mesenchyme and results in highly vascularized bone
what are the steps of intramembranous ossification
- mesenchymal cells near blood vessels become osteoblasts
- osteoblasts form bone trabeculae
- early sites of bone formation = ossification centers
- overtime, trabeculae form bones w/ external compact bone and internal trabecular bone and marrow cavities
growth plate
- sheet of hyaline cartilage b/w epiphysis and diaphysis
- responsible for longitdudinal growth
- growth ends when growth plates become vascularized and are replaced w bone
what are the 5 zones of a growth plate
- Zone of reserve - directs proliferation
- zone of proliferation - chondrocyte mitosis
- zone of hypertrophy - chondrocytes enlarge, bring in O2
- zone of calcification - chondrocytes calicify the ECM
- zone of ossification - cartilage is replaced w/ bone
