General Biology/Tissues and Systems/Connective Tissue

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General Biology | Getting Started | Cells | Genetics | Classification | Evolution | Tissues & Systems | Additional Material

Connective tissue

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This is a “grab bag” category of diverse tissue types. Functions include binding and supporting. Types include bone, cartilage, fibrous connective tissue, blood and adipose (fat) tissue.

If you took away everything in the body except the connective tissue, you’d still be able to see the basic form of the body.

Form: distinctive cells surrounded by a cell matrix made of extra-cellular fiber grounded in a ground substance (excluding blood)

1. connectile connective tissues (can be 1. loose or 2. dense)
2. special connective tissue (includes blood, bones and cartilage).

Fibroblasts form connective tissue proper;
chondoroblasts form cartilage;
asdasdasdasdasdasdads osteoblasts form bone;
and blood is formed from various sources.

Ground substance: “unstructured” material that fills space between cells and contains fibers. Made of
1. interstitial fluid (bathes cells)
2. proteoglycans (protein core with attached polysaccharides, glycoaminoglycans or GAGs such as chondroitin sulfate, keratin sulfate, and hyalronic acid, whose consistency is syrupy to gelatin-like)
3. cell-adhesion proteins (connect connective tissue cells to the fibers).

Fibers of connective tissue:
1. Collagen (flexible protein resistant to stretching, tensile strength, most abundant protein in animals, white)
2. elastin (rubbery, resilient protein, in dermis, lungs, blood vessels, yellow when fresh)
3. andreticulin (like collagen).

Loose connective tissue: found beneath skin, anchors muscles,nerves etc. Include fibroblasts, macrophages, mast cells,and adipose cells. Fibers include collagen and elastic fibers. Ground substance is “syrupy”. Adipose included.

Dense connective tissue: largely densely packed fibers of collagen or elastin regularly or irregularly arranged. Forms tendons and ligaments, coverings of muscles, capsules around organs and joints, and dermis of skin.

Cartilage vs. bone

Feature Bone Cartilage
cell type osteocytes chondrocytes
ground substance calcium phosphate  chondroitin sulfate
vascularization vascular avascular
micro architecture  highly ordered less organized
units called osteons
fibrous sheath peristeum perichondrium

Cartilage: There are three cartilage types:
1. hyaline cartilage
2. fibrocartilage (fibrous cartilage)
3. elastic cartilage

Hyaline cartilage: most widespread cartilage type, in adults forms articular surfaces of long bones, rib tips, rings of trachea, and parts of skull. Mostly collagen, name refers to glassy appearance. In embryo, bones form first as hyaline cartilage, later ossifies. Found in tracheal rings. Few collagen fibers.

Fibrous cartilage: have lots of collagen fibers. Found in intervertebral discs, pubic symphesis. Grades into dense tendon and ligament tissue.

Elastic cartilage: springy and elastic. Found in internal support of external ear and in epiglottis, yellow when fresh.

Chondrocites (cartilage cells) rely on diffusion for nutrients, as cartilage has no direct blood supply, and no enervation (nerves). Can be loaded with calcium salts.

Bone: Specialized connective tissue, calcium phosphate arranged in highly ordered unit called osteon, or Hyvercian system. Concentric rings around central canal with blood vessels and enervation (nerves). Bone varied, not all vertebrate bone is even cellular. Our concern: simple pattern for mammals.

Lacuna (spaces in which osteocytes found); canaliculi (little canals) bigger diagonal cells, layers of bone called lamellae.

Three types of bone cells, ending in
-blast, (mend bone)
-cyte (fortify bone) 
-clast (tear down bone)

Classified by
1. appearance (spongy vs. hard)
2. where found (outside or inside)
3. how it is formed (endochondral cartilage model forms first and then is ossified, and entramembranous, bone forms directly without cartilage precursor)

Example of endochondral bone formation: long bone begins to ossify from center shaft, calcified region expands and cuts off diffusion of nutrients as bone replaces cartilage. In young mammals, secondary ossification centers then form at bone ends, growth has stopped by sexual maturity as all primary bone is ossified. In other animals, bones continue growing throughout their lifetime.

Three types of intramembrous bone:
1. dermal bone
2. sessamoid bone
3. perichondral bone.

Dermal bone forms skull, shoulder/pectoral girdle, and integument, descended from dermal armor of ancestor. Comes from mesoderm, in dermis of skin.

Sessamoid bones: form directly in tendons. Example: kneecap, also in wrist. Deals with stress.

Perichondral bone means “around cartilage,” forms around cartilage or bone. Functions in bone repair and in ossification of endochondral bone.

Bone remodeling and repair: bone has mineral structure, and develops tiny fractures, which, under stress, can lead to larger fractures. To combat this, bone is constantly replaced. Osteoclasts channel through existing bone, tear down and leave behind osteoblasts and lacuna, leaving osteocytes. Continually resets mineral structure of bone, and is preventative maintenance.

When bone broken, callus forms in open ends, periosteum gives rise to new bone with calcium and new bone matrix, leaves irregular mend. Later, osteoblasts continue fixing over time and slowly removing imperfection.