Histology: Cartilage and Bone

4 basic steps of intramembranous bone formation

1. Osteoprogenitors become osteoblasts and secrete matrix 2. Matrix is mineralized, and spicules form 3. Other osteoblasts deposit bone on spicules ("seeding") 4. Osteoclasts resorb bone for reshaping

How vitamin C contributes to bone maintenance

Formation of stable collagen triple helices.

4 basic steps of Endochondral bone formation

1. Cartilage growth, chondrocyte proliferation and hypertrophy 2. Chondrocyte apoptosis and calcification of cartilage matrix 3. Ossification 4. Periosteal collar formation around cartilage

3 steps of osteoclastgenesis

1. Osteoblasts make MCS-F, recruits monocytes. 2. Monocytes differentiate into osteoclast precursor cells and express RANK. 3. Osteoblasts make RANKL, which binds RANK on the precursor cells and stimulates osteoclast maturation.

3 basic steps of bone formation

1. Osteoid deposition: osteoblast secretes organic matrix constituents (woven bone). 2. Mineralization: osteoblast secretes factors that attract Ca2+ and PO4 for crystal formation. 3. Remodeling: Bone is remodeled by osteoclasts and blasts.

3 basic steps of fracture repair

1. Reactive: inflammatory reaction, granulomatous tissue forms 2. Reparative: periosteal stem cells become chondroblasts that make hyaline, and osteoblasts that make woven bone. Cartilage/woven bone come together to form soft callus. 3. Remodeling: Hyaline undergoes endochondral bone formation, woven bone matures. Clasts remodel bone.

4 steps of bone resorption by osteoclasts

1. Use chemotaxis to find the right place on bone, CAMs to adhere to the bone. 2. Seals to bone creating a Howship's lacuna. 3. Release H+ into cavity to dissolve bone matrix. 4. Releases lysosomal enzymes that degrade the bone matrix.

Most common form of dwarfism, what mutation causes?

Achondroplastic dwarfism, FGF receptor mutation.

At what stage do chondrocytes undergo apoptosis?

After the periosteal buds invade the matrix, right before ossification.

Bone growth - appositional or interstitial?

All bone growth is appositional, but there are 2 mechanisms (intermembranous and endochondral)

Synovial joint components and characteristics Articular cartilage (type of cartilage) Joint capsule (type of CT) Synovial membrane (purpose) Synovial fluid (components)

Articular cartilage: hyaline with unique overlapping collagen fibers Joint capsule: Outer layer dense CT, inner synovial membrane Synovial membrane: secretes fluid with hyaluronic acid and glycoproteins Synovial fluid: combination of secretions from synovial membrane and highly-filtrated capillary blood

Main difference between primary (diaphysis) and secondary (epiphysis) ossification centers

At the secondary ossification centers, hyaline cartilage remains on ends (articulation surfaces)

Rheumatoid arthritis: main cause, when does inflammation occur?

Autoimmune disorder, inflammatory cells get into synovial membrane, which thickens and folds (pannus). Cells release metalloproteinases, collagenase, etc. which destroys articular cartilage. Inflammation is primary, happens first.

Osteoclasts - how can you identify them?

Big cells on bone surface

How fibroblast growth factor (FGF) regulates bone growth, what disease is the effect of mutation?

Binds to FGF receptors on chondrocytes, inhibiting cartilage growth. Receptor mutations are usually "gain of function" so cartilage continues to proliferate but bone cannot form, causing dwarfism.

Role of Parathyroid hormone-related peptide (PTHrp) in endochondrial bone formation. What does it bind to in order to stimulate proliferation?

Binds to receptors on chondrocytes at the ends of long bones to stimulate proliferation.

What color does hyaline cartilage stain, why is this?

Bluish-purple, because it is base-ophilic (attracts hematoxylin which is blue)

Purpose of osteocyte, what cell type are they derived from?

Bone maintenance and Ca2+ homeostasis. Osteoblasts become osteocytes when they get surrounded by matrix.

Function of cathepsin K and metalloproteinases in bone resorption

Cathepsin K - degrades collagen Metalloproteinases - degrades other matrix components.

Osteoarthritis: main cause, when does inflammation occur?

Caused by joint wear and tear, cartilage erosion exposes bone surface, which degenerates. Inflammation is secondary, occurs after degeneration

What mediates osteoclast attachment to bone?

Chemotaxis to find the right place on bone, CAMs to adhere to the bone.

Haversian systems and lamellae are found in what type of bone?

Compact (cancellous) bone

What is the purpose of Volkman's canals in compact bone?

Connect Haversian systems together.

What type of preparation allows you to see the living components of bone?

Decalcified, where all the minerals are removed (usually with acetic acid)

Major cause of osteoporosis, why?

Declining estrogen levels. Estrogen stimulates apoptosis of osteoclasts, low levels allows them to continue degrading bone.

Deficiencies in vitamins E and K are associated with what?

Decreased bone mass and elevated fracture risk

Rickets and osteomalacia, what process is defective, who does it occur in, what are 2 deficiencies that cause?

Defect in bone mineralization. Rickets = children, osteomalacia = adults. Usually caused by issue with vitamin D metabolism/absorption, but can also be caused by calcium deficiency.

How Haversian systems and Volkmann's canals form (collar)

During collar formation, bone surrounds vessels from periosteum (formerly perichondrium), creating Haversian systems and volkmann's canals.

Cartilage growth types: early vs late growth

Early growth: interstitial (new growth) Late growth: appositional (on top of existing cartilage)

Perichondrium: which 2 types of cartilage have it, which does not?

Elastic and hyaline. Not found in fibrocartilage.

Where are secondary ossification centers located? When do they stop growing?

Ends of long bones, in epiphyseal plates. Very thick in early development, get smaller and close in late teens/early 20s.

Long bones: epiphysis vs diaphysis

Epiphysis - end of bone where growth occurs Diaphysis - long part of bone

What type of cartilage is a cross between dense connective tissue and hyaline cartilage?

Fibrocartilage

What type of cartilage are intervertebral discs and the pubic symphysis made of?

Fibrocartilage, because it is very tough.

What type of lacunae are osteoclasts found in?

Howship's lacunae, which are acidic depressions in the bone.

3 types of cartilage, characteristics, examples of where found: Hyaline Elastic Fibrocartilage

Hyaline: Slippery. respiratory passages, joints, ventral ribs Elastic: Stretchy. epiglottis, larynx, ear, eustachian tube Fibrocartilage: Strong. Intervertebral discs (both annulus fibrosus and nucleus pulposus), pubic symphysis

What cells secrete VEG-F, what does it cause growth of? What will they eventually become?

Hypertrophic chondrocytes, it stimulates blood-vessel budding from perichondrium. The periosteal buds invade calcified matrix, bringing osteoprogenitor and hematopoeitic stem cells with them. The vessels will become the Haversian systems and Volkman's canals in compact bone.

Histogenesis of hyaline cartilage. What type of growth is represented here?

Interstitial

Interstitial vs appositional cartilage growth

Interstitial: "growth from within" new cartilage. Chondroblasts in lacunae multiply, secrete matrix around lacunae. Appositional: "growth on top" of existing cartilage. Progenitor cells from perichondrium differentiate and lay down new matrix on existing cartilage.

2 mechanisms of bone development, what types of bones does it occur in? Intramembranous Endochondral

Intramembranous: bone develops directly from mesenchyme/osteoprogenitor cells. Flat bones (eg mandible, infant skull). Endochondral: Cartilage is formed first, gets calcified, and is replaced by bone. Long bones (eg growing femur).

How B vitamins contribute to bone maintenance, effect of deficiency

Involved in homocysteine metabolism. Elevated homocysteine impairs collagen formation and increases osteoclast activity

Why does synovial fluid contain WBCs?

It contains ultrafiltered blood, so some WBCs come along for the ride.

Hyaline cartilage: what spaces are the chondrocytes found in?

Lacunae

Symptoms of rickets

Large forehead, abnormally curved spine, deformed ribs, bowed legs, wide points in elbows/wrist

Fetal skeleton cartilage type

Made of hyaline cartilage, surrounded by dense CT perichondrium.

What cell types are progenitor cells derived from?

Mesenchyme (what all CT cell types are derived from).

What color is new bone vs mineralized bone under H&E stain?

New bone is eosinophilic, stains pink/red. Mineralized bone is base-ophilic, stains purplish.

What is woven bone?

New bone, has irregular arrangement of collagen fibers. Is remodeled to make mature bone.

Bone sialoprotein: what cell types secrete and what is purpose?

Osteoblasts, mediates binding of osteoblasts to matrix.

Osteoprotegrin, RANKL and MCS-F: what cell types secrete, and what is purpose?

Osteoblasts, needed for osteoclastgenesis

What type of cells secrete MCS-F, what is purpose?

Osteoblasts, recruits monocyte/macrophage stem cells to bone and stimulates their differentiation into osteoclast precursor cells

Paget's disease mechanism

Osteoclast activity increases, osteoblasts compensate by rapidly laying down new bone (woven bone) which is weak and disorganized. Result is large bones that fracture easily.

Periosteal collar formation: intramembanous and appositional

Osteoprogenitor cells from perichondrium become osteoblasts and deposit thin layer of bone around diaphysis. Is initially intermembranous, then appositional.

Periosteum vs endosteum

Periosteum: dense outer connective tissue layer on outside of bone. Endosteum: thin vascular membrane of connective tissue that lines the inner surface of the bony tissue. Separates the marrow from the bone. Both contain osteoprogenitor cells.

What type of chondrocytes secrete Indian Hedgehog protein (Ihp) how does it regulate bone growth?

Pre-hypertrophic chondrocytes. Ihp increases PTHrp receptors on proliferating chondrocytes, which allows proliferation to continue and delays hypertrophy. Ihp also regulates the formation of the periosteal collar.

Long bones: where are the primary and secondary ossification centers?

Primary: center of diaphysis (middle of bone) Secondary: epiphysis (end of bone)

How does osteoprotegerin inhibit osteoclast differentiation?

Produced by osteoblasts, mimics the RANK receptor on the clasts. Floats around and binds RANKL on blasts, preventing it from binding to RANK on osteoclast precursor cells.

Osteocyte 3 stages, based on what activity they're promoting

Quiescent, resorptive, formative

Osteocalcin and osteopontin: what cell types secrete and what is purpose?

Secreted by Osteoblasts. Involved in bone mineralization.

Cartilage: what are lacunae, what cell types and what type of growth?

Spaces occupied by chondroblasts. They multiply within the lacunae and secrete matrix around it. Interstitial growth.

Spicules and lamellae are found in what type of bone?

Spongy (trabecular) bone

3 joint types and examples: Synathroses Amphiarthroses Diarthroses

Synarthroses: immovable (skull sutures) Amphiarthroses: slightly movable (pubic symphysis) Diarthroses: fully movable, also called synovial joints

What is a Howship's lacuna?

The resorptive cavity on bone occupied by an osteoclast.

Endochondral bone formation: what happens to chondrocytes as they mature after proliferation? What stimulatory factors do they secrete?

They hypertrophy. Once this happens, they can no longer contribute to cartilage growth. Secrete factors that mediate matrix calcification/degradation, blood vessel budding (VEG-F)

3 basic steps of ossification (role of osteoprogenitors and hemopoietic cells), what is the triggering event?

Triggered by vessel budding into the matrix, bringing in osteoprogenitors and hemopoietic cells. 1. Osteoprogenitors become osteoblasts, deposit bone matrix onto calcified cartilage. 2. Hemopoietic cells become osteo/chondroclasts, resorb calcified cartilage matrix and remodel new bone. 3. Hemopoietic cells develop in marrow spaces within developing spongy bone.

4 protein types found in bone matrix Collagen (type?) GAGs/proteoglycans (just them) Non-collagenous proteins (examples) Regulatory proteins (2)

Type 1 collagen (is tough) GAGs/proteoglycans Non-collagenous proteins (osteocalcin, osteopontin, bone sialoprotein, osteoprotegrin) Regulatory proteins (RANKL, MCS-F)

What type of collagen is found in the bone matrix?

Type I collagen

What 2 types of collagen does fibrocartilage contain?

Type II (all cartilage) and type I (most common type in the body)

What 2 types of fibers does elastic cartilage contain?

Type II collagen (all cartilage) and elastic fibers.

Cartilage ECM components (collagen type, GAGs, H2O)

Type II collagen, hyaluronic acid, rich in keratin sulfate and chondroitin sulfate GAGs, high water content

What type of collagen does all cartilage contain?

Type II.

Gout cause

Urate crystals in joints. Diet, kidney disease.

First type of bone laid down during growth and fracture repair

Woven bone, which has an irregular arrangement of collagen fibers. Is replaced by mature bone.

What is the purpose of canaliculi in compact bone?

allow communication between osteocytes, they insert cellular processes into the canaliculi to other cells.

Osteoblasts: how can you identify?

lined up in a row, are basophilic and stain purplish-blue