Histology Of The Skeletal System Flashcards
Cartilage
Tough durable form of CT.
Serves as the precursor to bone in long bones
Two components of cartilage
Perichondrium and cartilage tissue
*three types of cartilage tissues
Perichondrium
Outer most layer that surrounds elastic and hyaline cartilages.
Surrounds the diaphysis and metaphysis of bones (not the epiphyses though).
Contains high levels of fibroblasts and chondrogenic progenitor cells
Fibroblasts
Produce and maintain ECM and type 1 collagen fibers in devolving bone and cartilage.
- found only in fibrocartilage
Chondroblasts
Naive chondrocytes found at the periphery of hyaline and elastic cartilage that are not surrounded by ECM.
Usually arrive from the progenitor cells located in the perichondrium.
Chondrocytes
Mature chondrocytes that undergo mitotic divisions and gives rise to isogenous groups of chondrocytes.
Always have lacunae surrounding them
Produce type 2 collagen fibers and elastic fibers in ECM
Found in all types of cartilage
Hyaline cartilage
Most common type of cartilage
Contains mostly type 2 collagen fibers and ground substance rich with aggrecan.
Stains basophilic (Purple with darkest near the lacunae)
Has perichondrium all around it except at epiphyses in long bones
- function is to provide low friction surfaces in joints and structural support for respiratory tract.
Common sites of hyaline cartilage
Synovial joints, epiphyseal plates of all bones, coastal cartilages nasal cartilages and rings of cartilage found in trachea, bronchi and larynx
Elastic cartilage
Contain type 2 collagen fibers and elastic fibers throughout the ECM
Stains normal
- only cartilage with elastic fibers
- possess perichondrium
- function is to provide flexible shape and support of soft tissues
Common Locations for elastic cartilage
External ear, auditory tube, epiglottis and larynx
Fibrocartilage
A mix of hyaline cartilage and dense CT. Has type 2 and type 1 collagen fibers
Stains partially basophillic and
Acidophilic
- only cartilage with type 1 collagen
- no perichondrium
- function is to provide cushioning and resistance to tearing and compression
Has cartilaginous and fibrous areas
- cartilaginous contains lacunae and chondrocytes and type 2 collagen
-fibrous contains chondroblasts and type 1 collagen
General locations of fibrocartilage
IVDs, pubic symphysis, meniscus, tendon insertions and some joints.
What embryological derivative does cartilage come from?
Paraxial somatic and lateral Mesoderm or neural crest cells (pending on wear it goes)
- head cartilage = neural crest
- limb cartilage = lateral plate mesoderm
- all other cartilage = paraxial somatic mesoderm
Functions of cartilage
Facilitate movement at joints
Provide flexible support and mechanical protection
General Characteristics of cartilage
Avascular and lacks innervation
Located within lacunae when in bone.
Solid w/ non-mineralized ground substance
Has varying protein fibers based on type of cartilage
Aggrecan complex
Multiple proteoglycans attached to hyaluronan molecules in the ground substance of hyaline cartilage
Interstitial growth (growth from inside)
Chondrocyte proliferation produces isogenous groups of chondrocytes
- gradually secrete ECM and generate growth from the inside out
Appositional growth
Progenitor cells differentiate into chondroblasts in the perichondrium.
- most common type of postnatal growth of cartilage
growth from the outside out.
Repair of cartilage
Slow process as age increases due to avascular and low metabolic nature of cartilage, immobility of chondrocytes and limited proliferation of mature chondrocytes.
Dependent upon cells of perichondrium invading damaged area and producing new cartilage
Usually forms scars or bone when repaired.
Calcification of hyaline cartilage
Occurs via CaPO4 crystals accumulating in ECM
- occurs naturally while aging and especially in areas of contact with bone tissue
Periosteum
Well-vascularized and innervated outer layer of bone
Has two specific layers
- Fibrous layer: outer layer of periosteum made up of dense irregular CT and fibroblasts
- Cellular layer: deep layer containing osteoprogenitor cells.
Perforating (Sharpey) fibers
Collagen fibers Penetrate the periosteum into underlying bone tissue anchoring the periosteum, ligaments and tendons to the bone
Endosteum
Single layer of CT lining the interior bone (medullary canal)
- contains endosteal cells and osteoprogenitor cells
Osteoblasts
Cells that secrete osteoid matrix which becomes ossified by calcium salt deposits.
Undergo apoptosis or differentiate into osteoclasts when done secretion of the ECM.
- cuboidal shaped cells found only at the surfaces of bone matrix
Osteoid matrix
Consists of Type 1 collagen, glycoproteins, proteoglycans and matrix vesicles
Osteocytes
Monitor surrounding bone matrix and communicate with other osteocytes
- primary job is modulating bone remodeling, possess mechanoreceptors and cell-to-cell communication
- differentiate into osteoblasts when entrapped in bone matrix
Osteoclasts
Large multinucleated cells that reabsorb old or damaged bone matrix
- originate from monocytes
- develops howships lacunae when reabsorbing bone.
General characteristics of bone
Solid w/ mineralized ground substance
Protein fibers vary based on type of bone
Cells are located within lacunae
Bone tissue types
Compact: dense outer layer of bone
Cancellous (spongy): soft hollow-like bone w/ interior spongy meshwork of bone trabeculae
All bone is well-vascularized and innervated
Osteocalcin
Vitamin K-dependent poly peptide released by osteoblasts.
Bind Ca2 in blood and bring to matrix where matrix vesicles are increasing concentrations of PO4
PO4 + Ca2 = Hydroxyapatite crystals which aggregate together and form calcifications.
Osteoclast breakdown of bone
Osteoclast ruffled border binds to bone
Vesicular zone of osteoclast releases vesicles with highly acidic protons in them
Protons release in bone matrix and break down hydroxyapatite molecules and collagen fibers
Ca2 molecules produced from break down are released into blood stream
Forms “HowShip Lacuna”
Shapes of bones
Long
Short
Flat
Irregular
Seasmoid: form within tendons
Parts of the bone
Diaphysis
Metaphyses
Epiphyseal line: reminant of growth plate
Epiphyses: covered by articular cartilage
Intramembranous ossification vs Endochondral ossification
Intramembranous ossification:
- Direct ossification. Is the least common form, bone the earliest form.
- Primarily occurs in bones of skull and face, clavicle/sternum and scapula
Endochondral Ossification:
- Indirect ossification. Most common and occurs second
- primarily occurs in long, short and rib bones.
Intramembranous osteogenesis steps
- Ossification centers within mesenchyme give rise to osteoprogenitor cells and proliferate/differentiate into osteoblasts
- Osteoblasts produce osteoid matrix which in turn forms woven bone and entrap osteoblasts within the lacunae of the woven bone.
- step 2 continues enlarging the bone.
- woven bone eventually remodels and is replaced with lamellar bone.
- non-ossified mesenchyme develops into periosteum and endosteum.
Endochondral ossification osteogenesis steps
Growth occurs at epiphyseal plates via interstitial growth.
- hyaline cartilage, shaped similar to the bone to form, develops a “bone collar” of periosteum beneath perichondrium via.
- chondrocyte hypertrophy occurs in underlying cartilage causing calcification around them and ultimately intimating apoptosis.
- Perichondrium is invaded by osteoprogenitor cells and capillaries to form the Primary ossification center in the diaphysis.
- osteoid is deposited in 1st ossification center and calcified into woven bone and compact bone.
- at birth, secondary ossification centers develop in a similar fashion at the bone epiphyses.
Centers are divided by the epiphyseal line
Zones of epiphyseal plate
1st- Zone of reserve cartilage: Primary zone that is made up of hyaline cartilage
2nd - Proliferative zone: below zone of reserve, contain rapidly dividing cartilage cells that secrete type 2 collagen and proteoglycans and become organized in columns parallel to long axis of bone
3rd-zone of hypertrophy: swollen terminally different chondrocytes which compress matrix and secrete type X collagen. Promotes vascularization in primary ossification center.
4th- zone of calcified cartilage: chondrocytes undergo apoptosis and release osteocalcin and matrix vesicles to form hydroxyapatite crystals.
5th- zone of ossification: bone tissue appears here (specifically woven bone).
Why does the epiphyseal plate not change thickness?
Rates of chondrocyte proliferation and death are approximately equal.
Zones of the epiphyseal plate that directly contribute to bone growth
Proliferative zone and hypertrophic zone.
Reserve zones job is to ensure the epiphyseal plate does not prematurely seal or move.
Site of actual growth in longitudinal bone growth
Zone of calcified matrix
Appositional Bone growth
Changes in Thickness of bones and/or the diameter of the marrow cavity.
Begins with formation of periosteum bone collar on the diaphysis.
Osteoblasts proliferate and produce osteoid matrix at the surface of the periosteum bone collar while osteoclasts remove bone in the endosteum.
-This usually occurs at an equal rate.
When does the bone collar form on a child?
Late 1st trimester
When does the primary and secondary ossification centers become produced?
At time of birth
What causes the epiphyseal plate to fuse shut?
Primary and secondary ossification centers meet up and fuse together (usually late teens-20 years of age)
Osteon formation steps
Cutting Cone: Osteoclasts form a “cutting cone” to tunnel through comapct bone and fill this cavity with mesenchyme and blood vessels
Reversal zone: Osteoblasts line the cavity wall and lay down osteoid matrix, generating the first and subsequent lamella. After which they apoptosis or differentiate into osteoclasts.
Closing cone: Continues in subsequent cycles forming rings of lamallae until only the central canal remains.
Central canal houses neuorvasculature.
Lamellar bone is found in what types of bone?
Cancellous and compact bone
Lamellar bone formation
After woven bone is initially laid down, it is removed by osteoclasts and new osteoid is produced by osteoblasts in an organized layering model.
Each layer traps osteoblasts within a lacunae (ring layer) that either undergo apoptosis or differentiate into osteoclasts.
External and internal lamellar layers in the marrow Canal and outer surface form compact bone
Inner layers form spongy bone from endosteum
Haversian canals vs Volkman canals
Haversian canals are longitudinal through one osteon, whereas volkman canals are perpendicular and connect Haversian canals.
Stages of fracture repair
Hematoma stage: torn blood vessels from fracture release blood around fracture that clots
Procallus stage: macrophages clean up hematoma tissue and dead cells. Ultimately replaced with soft fibrocartilage.
Bony callus stage: regenerating blood vessels invade procallus and proliferating osteoblasts and ultimately produces woven bone.
Remodeling stage: woven bone is remodeled via osteoclasts and osteoblasts into lamellar bones. Fully functional vasculature is reestablished.
Metabolic role of bone
Serves as main reservoir for Ca2+
Done via exchange of hydroxyapatite -> interstitial fluid and vise verse.
Two polypeptide hormones that regulate calcium homeostasis
-PTH: works by stimulating osteocyte differentiation (Directly) into osteoclasts (indirectly) and ultimately releases Ca2+ into the blood stream
PTH also causes osteoblasts to secrete RANKL in a paracrine fashion which further stimulates reabsorption.
- Calcitonin: works by directly inhibiting osteoclast activity and opposing PTH levels. Ultimately lowers Ca2+ levels in blood stream.
Synathoses
little to no movement.
Three subtypes:
- synostoses: only a thin layer of dense CT. skull bones, interdigitating bones
- syndesmoses: held together by only dense CT (ligaments). Tibiofibular and sacroilliac joints.
- symphyses: contain a bad of fibrocartilage with articular surfaces of hyaline cartilage. pubic symphysis and Inter Vertebral Joints
Diarthoses
Synovial joints that allow free joint movement
Contain hyaline cartilage that lines all articular surfaces.
Possess a joint capsule that encloses the joints w/ a fibrous outer layer and synovial fluid inner layer.
Ligaments cross these joints.
- GH, limb Joints.
How do osteoblasts generate bone?
Release osteocalcin matrix vesicles, collagen fibers and proteoglycans
Osteocalcin binds Ca with the PO in the matrix vesicles forming CAPO (hydroxyapatite)
Matrix vesicles serve as the site for hydroxyapatite formation.
