Bone 1 Flashcards
What are bones?
- specialized connective tissue composed of cells and calcified extracellular matrix (ECM)
- The cells and calcified ECM form a rigid organ
What is the function of bones?
- Hematopoiesis= the formation of blood cells from hematopoietic stem cells found in the bone marrow
- Lipid & mineral storage= bone is a reservoir holding adipose tissue within the bone marrow and calcium within the hydroxyapatite crystals
- Support= bone forms scaffolding for skeletal muscle and other soft tissues, it creates a framework that gives the general shape of the body
- Protection= particularly the axial skeleton which surrounds the major organs of the body
What are components of bone?
- Extracellular matrix
2. Cells of bone tissues
Describe the extracellular matrix bone
Refers to the molecules that provide biochemical and structural support to the cells that populate bone
Describe the organic portion of ECM in bone
-Organic portion is predominantly (90% of total weight of matrix proteins) composed of collagen one proteins
-Mainly type 1 collagen (also type V, III, XI, XIII)
-Non-collagenous proteins (about 10%) constitute the ground substance and include proteoglycans, glycoproteins, bone specific proteins, growth factors and cytokines
Essential to bone development, growth, remodel8ng, and repair
Both collagen and ground substance become mineralized to form bone tissue
Describe the inorganic portion of ECM in bone
Represents about 50% of the dry weight of the matrix and is composed mainly of calcium phosphate in the form of hydroxyapatite crystals
-Contains trace minerals such as bicarbonate, citrate, magnesium, potassium and sodium
Describe the cells that are components of bone
-Osteoprogenitor(osteogenetic): the stem cells of bone, they are derived from primitive mesenchymal cells
Forms a population of stem cells that can differentiate into osteoblasts which secrete the organic unmineralized matrix or osteoid
- Osteoblasts: synthesize uncalcified/unmineralized extracellular matrix called osteoid. This will later become calcified/mineralized to form bone.
- Osteocytes: As the osteoid mineralizes, the osteoblasts become entombed between lamellae in lacunae where they mature into osteocytes. They then monitor the minerals and proteins to regulate/maintain bone mass
-Osteoclasts: Multinucleated giant cells derived from monocyte progenitor cells responsible for resorbing and remodeling bone by releasing H+ ions and lysosomalenzymes
The balance of osteoblast to osteoclast activity is crucial in the maintenance of the tissue’s structural integrity. It also plays a role in conditions such as osteoporosis
Bone-lining cells: found on the inner and outer surface of bone; the endosteum & periosteum
What is the function of osteoclasts?
Functions in resorption, the breakdown of bone matrix
Describe osteocytes formation
Osteoggenic cell develops into an osteoblast
Osteoblast forms bone matrix and develops into osteocytes
Osteocyte maintains bone tissue
What are osteoblasts?
Cuboidal cells located on the periphery of bone. Displays euchromatic nuclei with a basophilic cytoplasm
What are osteocytes?
Mature osteocytes: mature osteoblasts entrapped in bones matrix
What are osteoclasts?
Large, multinucleated macrophages located along the endosteum
What are the shape and location of osteogenic cells?
- Are spindle shaped connective tissue cells (derived from embryonic mesenchyme stem cells) in the periosteum and endosteum that can differentiate into osteoblasts
- In mature bone in which there is no active new bone formation or remodeling, the osteoprogenitor cells become flattened spindle shaped cells closely applied to the bone surface, when they are sometimes called “inactive osteoblasts”
What are osteoblasts derived from?
Osteoprogenator cells; are basophilic
What are the shapes of osteoblasts?
Morphologically May appear as a cuboidal layer(high secreting activity) or flattened (declining secretions)
Cytoplasmic processes contact them with other osteoblasts and osteocytes
Describe the significant chemical composition and secretions of osteoblasts
- During synthesis osteoblasts have well developed rER and Golgi apparatus
- PAS positive granules in the cytoplasm are most likely the precursors of glycosaminoglycans (of the osteoid matrix)
- Eventually matrix secretion entraps the osteoblast in a lacunae with it’s cytoplasmic processes extending into canaliculi
- Can secrete osteoclast stimulating factor
- Osteoclast eventually ceases its secretory function, undergoes changes to its morphology and becomes an osteocyte
Explain the structure-function of osteocytes
- Mature bone cells, derived from osteoblasts
- Housed in their own lacunae, maintain communication and exchange nutrients and metabolites via gap junctions between their narrow cytoplasmic processes (filopodia) extending through the canaculi
- Increased amounts of condensed nuclear chromatin, reduced amounts of rER, and a smaller Golgi apparatus vs osteoblasts
Describe ground bone preparations
- Bone samples are ground to near translucency using mechanical attrition
- No organic components of bone remain after grinding
- The tissue is then impregnated with Indian ink
Describe the structure function of osteoclasts
Multinucleated giant cells (up to 50 nuclei) resulting from the fusion of hematopoietic monocyte progenitor cells
- Howship’s lacunae (resorption bay) are depressions on the bone surface that house osteoclasts
- These lacunae result from the osteolytic activities of osteoclasts
What are the 3 distinct regions of osteoclasts?
- Ruffled border
- Clear zone
- Basal region
What are the ruffle border for osteoclasts?
Finger-like evaginations along Howship’s lacunae depict sites of active bone resorption
What is the clear zone of osteoclasts?
Region of cytoplasm that surrounds the ruffled border
-Microfilaments, anchor osteoclast to the bone surface and help isolate osteoclastic activity
What is the. Basal region of the osteoclasts?
Houses the organelles and numerous nuclei of the cell
How do osteoclasts undertake bone resorption?
Lysosomal enzymes(From Golgi complex) and hydrogen ions released into the confined space betweeen bone matrix and the osteoclast’s peripheral clear zone -Acidic environment provides optimal pH for activity of lysosomal hydrolases and facilitates the dissolution of calcium phosphate from bone
Decalcalcified bone matrix is then broken down by acid hydrolases, collagenous and proteolytic enzymes and the cell resorbs the organic and inorganic
What are the bone-lining cells?
- Osteoblast derived flat cells with little cytoplasm and scant organelles
- Found on the outer (periosteal cells) and inner (endosteal cells) surface of non-remodeling bone
- Function in maintenance and nutritional support of underlying cells
- Regulate movement of phosphate and calcium into and out of bone
What is the significance of the periosteum and endosteum ?
-Nom-calcified connective tissue layers covering external & internal surfaces of bone, except over Articular surfaces
Where is the periosteum?
-Attached to the outer circumferential lamellae by Sharpey’s fibers (type 1 collagen)
Describe the layers of the periosteum
- Outer fibrous layers: consists mostly of a dense irregular connective tissue with fibroblasts. Contains blood vessels and nociceptors
- Inner cellular/ osteogenic layer: contains osteoprogenitor cells and their derivatives (periosteal cells)
- cells actively secreting matrix increase the width of bone (appositional growth)
Where is endosteum contained?
-Lines any space or cavity in bone
(Marrow spaces, Haversian canals, volkmann’s canal, lacunae and canaculi)
What is contained in the endosteum?
-It contains mesenchymal stem cells which have the potential to differentiate into osteogenic progenitor cells and differentiated osteoblasts, fibroblasts, blood vessels, and sympathetic nerves
Contains osteoclasts (bone remodeling occurs from within)
Summarize bone growth and remodeling
Elongation of bones is a result of interstitial growth of cartilage at epiphyseal growth plate
-Interstitial (internal)growth doesn’t occur in bone
Bone increases in width by apossitional growth (osteprogenitor cells in the periosteum)
- Remodeling of bone is a coordinated action between osteoblasts and osteoclasts
What are the classifications of bones?
- Immature (primary or woven) bone
- Mature (secondary or lamellar) bone
3.
Describe immature (primary or woven) bone
- First compact bone elaborated; contains numerous osteocytes and irregular arranged collagen
- low mineral content
- remodeled and replaced by mature bone except in certain places such as in tooth sockets, near suture lines in the bones of the skull, and at insertion site of tendons
Describe mature bones (secondary or lamellar) bone
- Replaces primary bone and made up to Haversian systems, or osteons
- Calcified matrix is arranged in regular concentric layers of lamellae (3 to 7 nm thick) surrounding a Haversian canal that transmits blood vessels, nerves and some loose connective tissue
- Osteocytes in lacunae are located between the lamellae and occasionally within them
Contrast gross structure of mature and immature bone
Immature- non-lamellar, no Haversian systems
Mature- lamellar, Haversian systems compact outside, spongy inside
Contrast collagen fiber arrangement of immature & mature bone
Immature- random/disorganized
Mature- parallel, stress oriented (organized)
Contrast osteocyte population density of immature and mature bone
Immature- high (many)
Mature- relatively low (few)
Contrast constitution of immature and mature bone
Immature- low mineral density: high water content
Mature- high mineral density: low water content
Contrast location of immature and mature bone
Immature- embryo, newborn, fracture callus, growth plate
Mature- develops from 1-month post natal
What are the types of secondary (mature) bone?
- Compact bone- very dense and heavy
- Spongy (cancellous) bone
- filled with spaces that are interconnected
- In long space filled with red (containing hemopoietic cells) or yellow (containing primarily fat) bone marrow
What are the characteristics of Haversian canal?
Functional unit= Haversian system or osteon
-cylindrical and composed of 4 to 20 concentric lamellae
Because bone is constantly being remodeled, the Haversian systems are NOT all uniform and in parallel array
- Have numerous canaliculi that permit communication between lacunae and with the Haversian canals
- Each lamellae consists of multiple parallel arrays of collagen fibers
- adjacent lamellae, have alternating directions of collagen fibers providing great strength despite its light weight
Describe the outer & inner circumferential lamellae of the compact bone
- deep to the periosteum the lamellae are arranged in concentric rings- outer circumferential lamellae
- Similar arrangement of lamellae can be found next to the endosteum-inner circumferential lamellae
- interstitial lamellae are remnants of old remodeled osteons between newly laid down osteons
Describe volksmann canal of the compact bone
Connect adjacent Haversian canals to each other and to endosteum at the marrow cavity on the inside and periosteum on the outside
Allow for passage of neurovascular urel through compact bone
Where are inner and outer circumferential lamellae found?
These follow the entire inner and outer circumferences of the shaft of long bones
Both perforated by blood vessels and nerves as they access Volkmann’s canals and Haversian canals
OCL lined by periosteum
ICL lined by Endostium
