Histology                                                                                                             Stanley Shostak
BioSc 1450                                                                                                          Spring 05

Lecture 3. Connective Tissue (CT) Proper, and Special Connective Tissue, Cartilage, Bone, and Adipose Tissue
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 most criteria for ct, but frequently arises from fibroblasts or adventital cells resembling CT fibroblasts, and adipocytes are invested in capillaries, hence, in ct.
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.

Supporting tissue

Loose CT: biological packing material; supports epithelia lining gut, respiratory & urinary tracts, etc.; open = areolar.
Dense CT: physical support, regular: ligaments, tendons and capsules
irregular: dermis
Adipose tissue (adipocytes) Brown: multilocular
White: unilocular
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 osteoblasts

perforating 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.

Cells: fusiform (spindle); stellate (star) fibroblasts nuclei condensed and elongated in direction of collagen fibers; cytoplasm reduced adipocytes

tissue macrophages, mast cells, leucocytes (formerly reticular endothelial system)

Reticular cells (lymph nodes and hematopoietic cords of bone marrow) synthesize reticulum; may exhibit phagocytic activity; collagen type III
Mast cells: resemble basophils; long lived; can proliferate in situ; degranulation results in release of histamine and other vasoactive mediators; metachromatic; lack glycogen

leucocytes in situ

neutrophils: rare; multilobed nuclei poorly stained cytoplasm

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

nuclei irregular; heterochromatin clumped around envelope.
residual bodies may be exocytized or sequestered
antigen presenting cells (APC) also called dendritic cells
combined or coated with antibodies and complement: opsonins; opsonization: enhancement of phagocytosis lymphokines: act to increase metabolic and phagocytic activity.
lymphocytes: small, densely-stained nuclei plasma cells: synthesize antibodies; clockface (cartwheel) nuclei; negatively stained golgi apparatus (paranuclear); extensive basophilic cytoplasm
mesenchyme: embryonic loose connective tissue cells: irregular, stellate (star) or spindle (fusiform) shape; cytoplasmic network; oral nuclei; dispersed chromatin with nucleoli ground substance lacks fibers
mucous connective tissue: (Wharton's jelly of umbillical cord): has conspicuous fibers.
myofibroblasts = pericytes: share basal lamina with an epithelium; possibly of neural crest (ectodermal) origin

 Extracellular matrix (materials: ECM) ground substance: periodic acid Schiff (PAS) positive = carbohydrate glycosaminoglycans (GAGs = mucopolysaccharides) 7: expanded form; hydrophilic; bind sodium in extracellular fluid (water of hydration); determine permeability; bind fibers repeating disaccharide units: N-acetyl glucosamine or N-acetyl galactosamine + uronic acid acidic: frequently with sulphate side groups unsulfated, not covalently bound to protein; may form noncovalent links to proteoglycans Hyaluronic acid: predominant GAG in loose ct; no sulphate sulfated, covalently bound to protein = proteoglycans (= mucoproteins) chondroitin-4-sulfate
dermatan sulphate
heparan sulphate
heparin sulphate
keratan sulphate
1 to 2 GAGs per core protein: less hydrated (cornea)
80 to 100 GAGs per core protein: highly hydrated (hyalin cartilage)
fibers collagen secreted as tropocollagen
polymerizes in staggering pattern Type I: most abundant: dermis, tendon, ligaments, bone; triple alpha helix: 300 nm x 1.5 nm; 67 nm x 4 (quarter staggering) = 268 + 35 (gap) nm = 303 nm fibers: aggregates (bundles) 2-10 micro m Type II: hyaline cartilage; fine fibrils dispersed in ground substance
Type III: reticulin; non-banded; absorb metallic silver (stain black with silver impregnation); present in reticulum and coat of small blood vessels
Type IV: nonfibrilar; mesh-like structure of basement membranes; secreted by epithelia not ct.
elastin fibers or discontinuous sheets
secreted as tropoelastin; polymerizes; deposition requires presence of fibrillin (incorporated around and within elastic fibers)
structural glycoproteins produced by fibroblasts: fibril-forming fibrillin forms microfibrils 8 to 12 micro m; appear to enhance aggregation of other ECMs fibronectin influences deposition and orientation of collagen; has collagen and GAG binding cites; binding of cells (integrin [CAM] has fibronectin [receptor] binding cites) not produced by fibroblasts: nonfilamentous (non-fibrillary); produced by epithelium and glial cells laminin component basement membranes; binds specific CAM entactin binds laminin to type IV collagen tenascin bound by integrins; controls nerve outgrowth in embryo

Cartilage & Bone

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.

osteoid: becomes calcified soon after deposition; normally minimal amounts present; exaggerated in rickets, chronic renal failure

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

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

long bone: dumbbell medullary cavity: spongy bone; trabeculae: fine, irregular plates form network; red marrow: cite of hematopoiesis; yellow marrow: largely (adventitial) adipose tissue

epiphysis (ends of dumbbell): expanded ends Gk. phyesthai to grow or phyein to bring forth; epi upon.

articular cartilage: = joint (articular) surface; diaphysis: Gk. to grow between (shaft of dumbbell)
surface: osteocytes and osteoblasts may form bone membrane separating bone fluid from other extracellular fluid 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)
perforating fibers: = Sharpey's fibers; penetrate whole thickness of cortical bone;
endosteum: osteogenic cells lining entire inner surface of bone

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)

marrow: intertrabecular spaces sources of blood cells and B lymphocytes nutrient artery: reaches marrow through nutrient canal (pierces cortical bone)

periosteal capillaries: penetrate outer cortex and anastomose with nutrient artery-derived vessels; reenter marrow cavity to form sinusoidal plexus extending throughout marrow;

drainage: marrow sinusoids drain into large central sinus; drains from bone as emissary vein through nutrient canal
bone matrix & mineralization: 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.

  last revised: 01-09-05