3. Connective Tissue (CT) Proper, and Special
Connective Tissue, Cartilage, Bone, and Adipose
Classical definition: Supporting tissue, minimum cell-to-cell contact; maximum extracellular space; connects blood vessels to other tissues; repair and replacement (scarring).
Origin in mesenchyme: ectodermal and mesodermal origins
Note 1: Adipose tissue defies
for ct, but frequently arises from fibroblasts or adventital cells
CT fibroblasts, and adipocytes are invested in capillaries, hence, in
Note 2: Blood was once considered a type of ct with fluid (as opposed to solid) extracellular material. But blood's extracellular material is not secreted for the most part by blood cells, and blood's white cells are predominantly sequestered and not circulating. Moreover, blood cells are formed at colonized sites and source of cells in bone marrow justify a separate category for blood and lymphatic cells.
Cells: fusiform (spindle); stellate (star)Cancellous (spongy: lg. cavities):
network bony trabeculae separated by labyrinth of interconnecting spaces containing marrow; trabeculae thin; composed of irregular lamellae with lacunae containing osteocytes; does not contain osteons; osteocytes exchange metabolites via canaliculi ; lined by endosteum containing osteoprogenitor cells, osteoblasts and osteoclasts
Compact (hard; without cavities )
cortical bone: concentric lamellae beneath periosteum; similar lamellae at inner surface; merge with trabeculae of cancellous bone
osteon system: Haversian system = central canal + lamellae; parallel bony columns disposed along long axis of bone; lamellae: concentric bony layers make up columns
central canal: = Havers or Haversian canals contains blood vessels, lymphatics and nerves; note nuclei of osteoblastsperforating canal = Volkmann's canal; pierce columns at right angles (or obliquely) to central canals; connect neurovascular bundles with endosteum and periosteum
interstitial (intermediate) systems (lamellae): partially resorbed osteons between osteons
osteocytes: densely stained, irregular nuclei and pale, basophilic cytoplasm [note shrinkage artifact]; surrounded by bone fluid trapped in lacunae in matrix; arranged in concentric rings within lamellae; connected by canaliculi to central canal.
tissue macrophages, mast cells, leucocytes (formerly reticular endothelial system)
leucocytes in situ
eosinophils: abundant in loose ct; bilobed nuclei strongly eosinophilic cytoplasmic granules
basophils: resemble mast cells; poorly stained in H&E
monocytes = phagocytes = macrophages, fixed macrophages, histiocytes
mucous connective tissue: (Wharton's jelly of umbillical cord): has conspicuous fibers.
Cartilage: semi-rigid matrix: extracellular material (ECM) of proteoglycan aggregates, massive; sulphated glycosaminoglycans (chondroitin sulfate and keratan sulfate [GAG]) with hyaluronic acid (nonsulfated GAG) as central backbone of complex, and collagen of various types; cells: chondroblasts (secrete matrix) and chondrocytes (embedded in matrix) within lacunae and frequently containing large fat droplets. Collagen type II: (except articular cartilage) not banded arranged interlacing network of fine fibrils
hyaline cartilage: small aggregates of chondrocytes embedded in amorphous matrix typically containing type II collagen (except at articulate surfaces), and overlaid by perichondrium except at articulate surfaces.
fibrocartilage: intervertebral disks, some articular cartilages, pubic symphysis, association with dense collagenous tissue in joint capsules, ligaments and connections of some tendons to bone.; alternating layers hyaline-like cartilage matrix and dense collagen fibers oriented in direction of functional stress; type II collagen, no perichondrium (since always occurs between other forms of cartilage or bone). elastic (fibroelastic) cartilage: external ear and auditory canal, epiglottis, parts of laryngeal cartilages & wall auditory (Eustacean) tubes; numerous bundles branching elastic fibers in cartilage matrix, fiber concentration dense in immediate vicinity of chondrocytes; collagen (also major constituent of ECM) makes up bulk of perichondrium; growth both interstitial and appositional growth. Resembles hyaline cartilage by way of containing type II collagen and perichondrium.
Bone: ECM mineralized (hydroxyapatite); ground substance with less sulfated glycoproteins than cartilage; fibrous matrix collagen type I (banded), dynamic state of growth & resorption; cells: osteoblasts (secrete osteoid [immature bony matrix before mineralizing]) and osteocytes (embedded in bony matrix] within lacunae and attached to other osteocytes by gap junctions at tips of cytoplasmic extensions within canaliculli; osteoclasts dissolve bony matrix.
two main histological varieties (and stages of development):
primary, immature or woven: immature; random weave of coarse collagen fibers; first bone to develop; remodeled into lamellar bone; first bone laid down at site of fracture
long bone: dumbbell
secondary, mature or lamellar: most mature skeleton; successive layers with organized infrastructure [note: lamellar does not refer exclusively to osteon]; compact bone or spongy = cancellous bone
medullary cavity: spongy bone; trabeculae: fine, irregular plates form network; red marrow: cite of hematopoiesis; yellow marrow: largely (adventitial) adipose tissuesurface: osteocytes and osteoblasts may form bone membrane separating bone fluid from other extracellular fluid
epiphysis (ends of dumbbell): expanded ends Gk. phyesthai to grow or phyein to bring forth; epi upon.
diaphysis: Gk. to grow between (shaft of dumbbell) articular cartilage: = joint (articular) surface; periosteum: dense fibrous ct; insertion muscles, tendons, ligaments; not present on articular surfaces, sites of insertion of tendons & ligaments and discrete sites (e.g., subcapsular area of neck of femur); osteoprogenitor cells resemble fibroblasts; osteogenic cells = osteoblasts (2 to 3 layers thick = epithelioid)
endosteum: osteogenic cells lining entire inner surface of bonemarrow: intertrabecular spaces
muscular insertion (onto periosteum): broad (as opposed to narrow attachment of tendon); collagen fibers of endomycium mingles with those of periosteum, look for perforating fibers (Sharpey's fibers)
sources of blood cells and B lymphocytes bone matrix & mineralization: nutrient artery: reaches marrow through nutrient canal (pierces cortical bone)drainage: marrow sinusoids drain into large central sinus; drains from bone as emissary vein through nutrient canal
periosteal capillaries: penetrate outer cortex and anastomose with nutrient artery-derived vessels; reenter marrow cavity to form sinusoidal plexus extending throughout marrow;
hole zones: within collagen type I; initial site of mineral deposition;
osteocalcin (Gla protein): noncollagen organic material, binds calcium during mineralization
osteonectin: serves bridging function between collagen and mineral component
sialoproteins: rich in sialic acid; concentrated from plasma
hydroxyapatite: mineral component of bone; calcium and phosphate; conjugated to small proportion magnesium carbonate, sodium and potassium ions; affinity for heavy metal and radioactive environmental pollutants; note: 20% or more of mineral component remains amorphous
pyrophosphate: inhibitor calcium deposition; crucial for controlling bone mineralization
matrix vesicles: membrane bound vesicles derived from osteoblast plasma membrane; contain alkaline phosphatase and other phosphatases; neutralize effect of pyrophosphate.