Lecture 8: Microanatomy of Bone Flashcards by Riley Hrasky

what are the 3 parts of a long bone

epiphysis (E)
metaphysis (M)
diaphysis (D)

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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)

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compositon of the bone matrix

Organic component: type I collagen
Inorganic component: Calcium hydroxyapatite

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what are the cells of bone

osteoblast
osteocyte
osteoclast

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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

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layer of periosteum characterized by an outer layer of dense CT

fibrous periosteum

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layer of periosteum characterized by the presence of osteogenic cells

inner cellular layer

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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

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type of bone that provides support to external surfaces of bones and makes up the shafts of long bones

compact (cortical bone)

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what are the 4 lamellar systems of compact bone ?

outer circumferential lamellae
inner circumferential lamellae
osteons
interstitial lamellae

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woven (immature) bone characteristics

initial bone formation; fracture repair
poorly organized type 1 collagen
relatively weak

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lamellar (mature) bone characteristics

replaces woven bone
very strong, well organized
type I collagen

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when does lamellar (mature) bone replace immature woven bone?

during formation of primary bone (primary osteons)
during remodeling to form secondary bone (secondary osteons)

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bone type formed by interconnecting bony struts (trabeculae) that provide internal support to bone

Cancellous (trabecular, spongy) bone

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characteristics of cancellous bone that makes it capable of calcium ion exchange b/w bone and blood

high surface area

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what types of marrow (spaces) exist between the trabeculae of cancellous bone

red marrow : produces blood cells
yellow marrow : made of adipose tissue

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what cell type is responsible for the formation of bone ECM

osteoblasts

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what are osteoprogenitor cells

stem cells near bony surfaces that differentiate into osteoblasts during periods of growth, remodeling, and fracture repair

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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

1. Zone of reserve - directs proliferation 2. zone of proliferation - chondrocyte mitosis 3. zone of hypertrophy - chondrocytes enlarge, bring in O2 4. zone of calcification - chondrocytes calicify the ECM 5. zone of ossification - cartilage is replaced w/ bone