Connective Tissue

• Mesoderm
• (middle embryonic germ layer)

Loose CT

Tendons / ligaments

• Dense Irregular CT

• - reticular layer of dermis
• - submucosa of digestive organs

• Insulation
• Energy storage
• Provides physical contours that distinguish men and women

• 1) Ground Substance
• 2) Fibers
• 3) Cells

amorphous / colorless / homogenous



Proteoglycans - synthesized / secreted by resident cells of CT. Protein backbone covalently bound to glycosaminoglycans. Proteoglycans are very hydrophilic (stain with eosin). Typically surrounded by a thick layer of solvation water.



• Glycoproteins - laminin (major component of basal lamina) & fibronectin


• Tissue fluid - similar to blood plasma (accumulation = edema)

• Collagen fibers
• Elastic fibers
• Reticular fibers

Types I - IV

(90% = Type I)

Type I Collagen

• Acidophilic due to charged amino groups
• (stains pink with Eosin in H&E)

Connective tissues (skin, bone, tendon, ligaments, fascia, etc...90% of body collagen) 



Provides resistance to force, tension, stretch

• Cartilage (hyaline, elastic)
• Notochord
• Intervertebral Disks

Loose connective tissue (organ tissue, blood vessels, smooth muscle, etc...)



Reticular fibers (forms loose meshwork - i.e. lamina propria, lymphatic organs, etc...)

Basal lamina of epithelia

Secures basal lamina to connective tissue fibers


(Anchoring fibrils of skin, eye, uterus, and esophagus)

Found in cartilage associated with type II collagen fibrils

(stabilize type II fibers by interaction w/ proteoglycan molecules)

Produced by chondrocytes in zone of hypertrophy of normal growth plate

Contributes to the mineralization process by forming hex. lattices to arrange types II, IX, XI collagen within the cartilage

• Produced by chondrocytes
• (associated w/ type II collagen fibrils, forms core of type I collagen fibrils)

Regulates size of type II collagen fibrils; it is essential for cohesive properties of cartilage matrix

Composed of elastin and fibrillin

Elastic fibers

Walls of blood vessels (tunica media of aorta)


ligamentum flavum (stretch)

Formed by type III collagen

Framework of specialized organs (lymphoid, kidney)

Loose connective tissue

Reticular lamina / lamina propria

Reticular fibers (argyrophilic)

Fibroblast

• Mast cells
• Plasma cells
• Leukocytes
• Macrophages

(Mainly found in loose CT)

Synthesize ECM (ground substance & fibers)

• White fat - unilocular

• - mostly in adults

• - energy storage, insulation, cushioning, hormone secretion



• Brown fat - multilocular

• - large amount in newborns (adults = adrenals, aorta, neck, mediastinum)

• - lipid mobilizaiton, heat generation (sympathetic stimulation)

Undifferentiated Mesenchymal Cells

Polypeptide from gastric epithelium



• Acts as an appetite stimulant via hypothalamus 

• (short term regulation)

Gastrointestinal peptide 



• Signals satiety via hypothalamus
• 
(short term regulation)

• Stimulates lipid synthesis and blocks lipolysis
• (long term regulation)

Produced by fat cells



• Acts on hypothalamus to suppress appetite

• (long term regulation)



Obese people are believed to be leptin resistant

• Norepinephrine (nerve endings) stimulates cAMP system
• -> activates hormone-sensitive lipase
• -> hydrolyzes stored triglycerides to free fatty acids and glycerol

-> diffuse into capillaries where fatty acids bind to albumin (hydrophobic moiety)

-> transported for use as energy

Triglycerides transported from intestine and liver by lipoproteins (chylo & vLDL) into blood

-> in adipose tissue capillaries lipoproteins are broken down by lipoprotein lipase

-> free fatty acids and glycerol are released and diffuse from capillary into adipocytes

-> re-esterified into glycerol phosphate -> forming triglycerides (stored until needed)

Large, ovoid cells w/ numerous cytoplasmic granules.

• Originate in bone marrow
• Differentiate in connective tissue

• Granules contain vasoactive and immunoreactive substances.
• - Histamine and slow reacting substance of anaphylaxis (SRS-A)
• - Eosinophil and neutrophil chemotactic factors (ECF and NCF)
• - Heparin (anticoagulant)

• Antibody producing cells derived from B-lymphocytes.
• (Common in loose connective tissue of the gut and glands)

• Basophilic cytoplasm due to RER
• Eccentric nucleus with “clockface” chromatin
• Prominent Golgi apparatus seen as clear, unstained region near nucleus.

Mast cells have receptors for IgE



Plasma cells make IgE in response to an antigen (i.e. bee venom)



• Upon second exposure, antigen will bind to IgE molecules on the mast cell

• -> This triggers mast cell degranulation

Derived from blood-borne monocytes.

In tissue, best seen if they are actively phagocytic (large vacuolated cytoplasm)

• Part of the Mononuclear Phagocytotic System.
• (Pathologists refer to them as histiocytes)

• Macrophages fuse to for very large, multinucleated cells
• in response to a foreign body (suture material / hair) that has gotten into an incision

Connective tissue immune surveillance system

• B-lymphocytes give rise to plasma cells that synthesize immunoglobulin.
• T-lymphocytes are involved in cell-mediated mmunity.
• Natural Killer (NK) Cells destroy virus infected cells and some tumor cells

Granulocytes with segmented nuclei

First cells to arrive at a site of acute inflammation (process called diapedesis)

• Active phagocytes (sometimes called microphages)
• Can phagocytose some bacteria. Other foreign substances must be opsinized.

(Pus is an accumulation of dead neutrophils)

Bilobed nucleus & large red granules

• Associated with 3 conditions:
• - Allergic reactions
• - Parasitic infections
• - Chronic inflammatory processes

• Often found in the lamina propria of the gut
• (increase in number with chronic inflammation)

Loose CT - lots of cells, fewer fibers

• Dense CT - fewer cells, more fibers
• - Dense irregular (dermis – reticular layer)
• - Dense regular (tendons, ligaments)