Histology - Cartilage + Bone

Histology - Cartilage + Bone Direct From Lecture Slides.
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Cartilage
 
Connective tissue w/rigid extracellular matrix Chondrocytes (cells) with low metabolic activity Collagenous (tensile strength) and elastic fibers Chondroitin sulfate (ground substance) Development and growth of long bones Early fetal skeleton forms Supports soft tissues (ear + nose, respiratory system, moveable joints, intervertebral disks) Avascular (vasculature in adjacent perichondrium) No innervations No lymph vessels 75% water, forms a gel with ground substance that allows for diffusion of gases and nutrients
From where does cartilage develop?
 
Mesenchyme
Cartilage development/growth
 
Develops from mesenchyme Chondroblasts for collagen fibrils Differentiation of cells to chondrocytes and separation by the matrix Perichondrium envelops cartilage Interstitial growth Appositional growth
Perichondrium
 
Sheath of dense irregular connective tissue – with fibroblasts and vasculature, envelops cartilage Contains many undifferentiated cells which can differentiate into chondroblasts
Interstitial growth
 
Mitotic division of existing chondrocytes and production of new matrix (young cartilage)
Appositional growth
 
Differentiation of new chondrocytes from stem cells (inner layer of perichondrium) and production of matrix at surface
Name the 3 forms of cartilage
 
Hyaline Elastic Fibrocartilage
Hyaline (glass) cartilage
 
Most common Type II collagen Appears blue/white In embryo: serves as skeleton until replaced by bone – acts as a template Epiphyseal plate in long bone growth Joint surfaces, nose, larynx, trachea, bronchi, ends of ribs adjacent to sternum Hyaline cartilage is covered by perichondrium - except over articular surfaces
Hyaline cartilage matrix
 
Amorphous ground substance with proteoglycans – including chondroitin sulfate and kertain sulfate Basophilic Collagen fibers (20 nm) are very fine, appear structureless Territorial matrix adjacent to chondrocytes is rich in glycosaminoglycan and stains more basophilic than non-territorial matrix
Hyaline cartilage chondrocytes
 
Chondrocytes occupy small cavities in matrix (lacunae) Surface of cell irregular; holds onto matrix + allows for better nutrition Chondrocytes = basophilic – large amounts of ER, typical of protein producing cells Outer cells are elliptical w/long axis parallel to surface Large central nucleus Deeper chondocytes are round, may appear in groups of up to 8 – isogenous group (Cell nests), all offspring from one chondrocyte Lined up in rows in epiphyseal plate Often shrink in histological sections
name the two kinds of bone growth
 
intramembranous ossification
endochondrial ossification
Lacunae
 
Small cavities in cartilage/bone matrix
Isogenous group / cell nests
 
Deeper round chondrocytes grouped – up to 8 cells – all offspring from same chondrocyte
Elastic cartilage
 
Support w/flexibility Auricle of ear, external auditory canal, Eustachian tube, epiglottis, cuneiform cartilage of larynx Similar to hyaline cartilage – many elastic fibers visable Yellow in fresh sections – opaque Covered by perichondrium Matrix is much less homogeneous – extensive network around chrondrocytes
Fibrocartilage
 
Tough support + tensile strength Intervertebral disks, tendon and ligament attachment to bone, pubic symphysis Combination of hyaline cartilage and dense regular connective tissue Merges with adjacent dense connective tissue or hyaline cartilage Chondrocytes in rows or groups Matrix appears acidophilic – lots of collagen No perichondrium A form of connective tissue transitional between dense connective tissue and hyaline cartilage Merges into neighboring tissues – typically tendons or articular hyaline cartilage Develops like connective tissue – initially fibroblasts are separated by fibrillar material, then they differentiate into chondrocytes and produce cartilage matrix
Trabecular / cancellous /spongy bone
 
Slender irregular bars and sheets of bone, trabeculae, which branch and intersect to form a sponge-like network which is filled with bone marrow Ends of long bones (epiphyses) consist mainly of trabecular bone Trabeculae (plates) form a network – depending of mechanical requirements of the bone Formed from lamellae (osteocytes, lacunae, canaliclui) Spaces between trabeculae (marrow cavity of the diaphysis) filled by red bone marrow or yellow bone marrow
Compact bone
 
No spaces or hollow space visible to the eye Thick-walled tube of the shaft (diaphysis) of long bones, surrounds the marrow cavity (medullary cavity) Thin layer of compact bone covers epiphyses of long bones Consists mainly of extraceullar matrix Individual lamellae form concentric rings around larger longitudinal canals (50 micrometers in diameter) formed around blood vessels (Haversian canals)
Haversian canals
 
Individual lamellae form concentric rings around larger longitudinal canals (50 micrometers in diameter) formed around blood vessels Contains one or two capillaries and some nerve fibers Canaliculi communicate with central Haversian canal Run along the long axis of the bone Canals and surrounding lamellae (8 to 15) are called a Haversian system or an osteon From peristeal surfaces, canals enter the bone perpendicular to its long axis (Volkmann’s canals)
Osteon
 
Haversian canals and surrounding lamellae
Volkmann’s (perforating) canals
 
From peristeal surfaces, Haversian canals enter the bone perpendicular to its long axis Communicate with Haversian canals Contain: endosteum, blood vessels, nerves
Interstitial lamellae
 
Remnants of Haversian systems that have been partially destroyed Fills intervals between functional Haversian systems
Name the 2 lamella which run parallel to inner and outer surface of the bone, immediately beneath the periosteum and endosteum
 
1- Circumferential lamellae 2- Endosteal lamellae
Formation of compact bone
 
Osteoblasts produce lamellae of bone matrix inward on the surface of longitudinal cavities Cavities reduced to narrow canals containing blood vessels – called “primitive Haversian canals”
Intramembranous ossification
 
type of BONE GROWTH Primitive connective tissues layer that becomes vascularized Cells differentiate into osetoblasts – which enlarge the connective tissue fiber (osteoid) Osteoblasts deposit the minerals thereby calcifying the matrix Lacunae and canaliculi are formed around connected cytoplasmic processes Then osteoblasts appear on surface – bone increases in thickness. Osteoblasts become trapped. Initially bone forms woven network without a preferred orientation, and lamellae are not present. During reorganization and growth, woven bone is replaced with lamellar bone Connective tissue surrounding the bone becomes the perioteum
Endochondral ossification
 
type of BONE GROWTH transformation of cartilage (bone model) to osseous tissue periosteal bud invades cartilage model and allows osteoprogenitor cells to enter cartilage. Cartilage is in stage of hypertrophy (very large lacunae and chondrocytes) and partial calcification which eventually leads to death of chondrocytes lamellar bone deposited onto cartilage scaffold initial site of bone deposition of called a primary ossification center secondary ossification centers occue in the epiphyses of the bone periosteal collar (thin sheet of bone) deposited around shaft of the cartilage model periosteal collar thickens cartilage at the site of ossification can be divided into distinct zones: reserve cartilage / zone of chondrocyte production / zone of cartilage maturation / zone of cartilage calcification / zone of retreogression / zone of ossification/ zone of resorption zone of reserve cartilage becomes reduced in length as ossification takes place
epiphyseal plate
 
between the diaphysis and the epiphyses – a thin sheet of cartilage is maintained until adulthood. It continues cartilage production and thereby provides the basis for rapid growth in the length of the bone
periosteal collar
 
consists of a thin sheet of woven bone deposited around shaft of the cartilage model formed during endochondral ossification
Decalcification of bone – tissue preparation
 
Cellular and organic components preserved and inorganic components removed by acid Then embedded, sectioned, stained Cells appear shrunken and matrix blurred
Ground bone – tissue preparation
 
Grining thin piece of bone with abrasives Matrix well preserved but cells are removes
Periosteum
 
Layer of dense connective tissue Covers all bones except over articular surfaces
Endosteum
 
Thin layer of cell-rich connective tissue Lines the surface of marrow cavity/spaces
Osteoprogenitor cells
 
Stem cells of bone Located in periosteum and endosteum Hard to distinguish from surrounding connective tissue cells Differentiate into bone cells
What components are responsible for bone’s hardness?
 
Cells (osteocytes) and intercellular matrix which contains organic compounds (collagen fibers) and inorganic components (calcium phosphate, calcium carbonate)
Lamellar structure
 
Bone matrix organized in layers
Lacunae
 
Cavities which contain cells
Canaliculi
 
Radiating from each lacuna, narrow channels that penetrate adjacent lamellae
Osteoblasts
 
Associated with bone formation Small cuboidal or pyramidal cells, form continuous layer Large nucleus, basophilic cytoplasm (ribosome, protein formation) Found adjacent to one another upon lamellae they have just secreted Deposit bone matrix in lamellae (3 to 7 micrometers) and become trapped in lacunae Communicate via canaliculi
Collagen fibers
 
Within lamellae Run parallel and oblique in adjacent lamellae
Osteocytes
 
Small inactive cells Isolated from each other in individual lacunae Cell processes extend out through canaliculi and provide gap-junctional contact among neighboring osteocytes for (i) communication and (ii) nutrient supply Amorphous layer separates cells from matrix Fainly basophilic cytoplasm, darkly stained nucleus
Osteoclasts
 
Bone-restoring cells Multinucleated (5 to 10 per section) Giant cells (up to 100 micrometers) Each one sits alone within a small hollow excavation in matrix (Howship’s lacuna) Faintly basophilic cytoplasm with vacuoles, lysosomal in nature Surface facing matrix has numerous cytoplasmic projections and microvilli – ruffled border Closely related to monocytes – form distinct cell population derives from same precursor cells as macrophages
Howship’s lacuna
 
small hollow excavation in matrix contains osteocytes
list the zones of cartilage at the zone of ossification that occur during endochondral ossification
 
reserve cartilage zone of chondrocyte proliferation zone of cartilage maturation + hypertrophy zone of cartilage calcification zone of retrogression zone of ossification zone of resorption
reserve cartilage
 
Farthest away from the zone of ossification looks like immature hyaline cartilage
zone of chondrocyte proliferation
 
longitudinal columns of mitotically active chondrocytes
zone of cartilage maturation and hypertrophy
 
cells and lacunae grow in size
zone of cartilage calcification
 
forms border between cartilage and zone of bone deposition matrix stains deeply basophilic deposition of minerals
zone of retrogression
 
cartilage cells die matrix destroyed and vascular primary marrow extends into spaces
zone of ossification
 
osteoblasts differentiate from cells in the marrow tissue and gather on calcified cartilage
zone of resorption
 
marrow cavities increase in size resorption of bone in diaphysis cavity forms secondary marrow cavity
bone remodeling
 
osteoclasts “drill” circular tunnel within existing bone matrix osteoblasts deposit new lamellae of bone matrix on the walls of these tunnels resulting in the formation of a new Haversian system within the matrix of compact bone parts of older Haversian systems remain between newer ones + become the interstitial lamellae in mature bone capillaries and nerves sprout into new Haversian canals
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