Histology Connective Tissue

Connective Tissue Cells

Specific to its cell type

Ex. Chondrocytes (cartilage), Osteocyte (bone), fibroblast (cartilage)

Ground Substance

Amorphous

Composed of GAGs, multiadhesive glycoproteins, proteoglycans

Colorless, viscous, homogenous.

Lost in H&E

Functions of Connective Tissue

Reflective of cell and fiber types present.

Support, maintain form, bind, anchorage, exchange (blood, loose CT), defense (WBCs), blood formation, tissue repair and development.

Extracellular Matrix

Protein fibers + ground substance

Produced by cells of tissue

Mechanical and structural support, biochemical barrier/filter, anchors cells within CT, regulation on embryonic development and differentiation.

With the help of CAMs, the ECM influences transmission of information across CT cells

Molecular Collagen

Glycoprotein

3 subunit polypeptides (alpha chains) form a triple helix. Every third amino acid is a glycine. A proline frequently follows the glycine.

46 different alpha chains form 29 different collagen molecules

Homotrimeric/Heterotrimeric - same/different chains

Combinations make different collagen molecules

Type 1 = heterotrimeric

Type 2 = homotrimeric

Categorized based on polymerization pattern

Fibril-Forming Collagen

Collagen 1-3

Aggregate to form fibrils

Basement Membrane Forming

Collagen 4 and 7

Form collagen superstructure in basement membrane of epithelial cells

Fibril-Associated Collagens

Flexible molecules, located on surface of fibril-forming collagen

Broad Categories of Collagen

Fibril-Forming

Basement Membrane Forming

Fibril-associated

Hexagonal network-forming

Transmembrane

Formation of Collagen Fiber

1. Alpha chains are created in RER with registration peptides

2. Hydroxylated proline and lysine is glycosylated, requires vitamin C

3. Procollagen is assembled (triple helix) in ER and transported to golgi and secretory vacuoles

4. Procollagen peptidase cleaves off registration peptide to form collagen

5. Molecular collagen forms collagen fibrils

(Constitutive Process)

Collagen Fiber Organization

1. Macromolecules (ex. collagen 1)

2. Fibril (linear structure of macromolecules)

3. Fibers (several fibrils, can see in microscopy)

4. Bundle

Collagen I

Fibril-Forming

Most abundant

Forms large collagen fibers and fiber bundles

Function: Provides resistance to force, tension, and stretch

Location: CT of skin, bone, tendons, ligaments (90% of all collagen) (Dense Irregular in dermis, loose CT in nerves/bone)

Fibroblasts,osteoblasts

Collagen II

Fibril-Forming

Found in cartilage

Fibrils only, cannot be seen in light microscopy

Function: Resistance to pressure

Chondroblasts

Collagen III

Fibril-Forming (doesn’t bundle)

Forms reticular fibers

Thin, branching network

Hematopoietic tissue (spleen, bone marrow, liver), CT of smooth muscle, blood vessels, basement membrane

Function: provide structural support and elasticity

Fibroblasts, reticular cells, smooth muscle cells

Collagen IV

Basement Membrane Forming

Component of Basal Lamina

Function: support and filtration barrier

Two-dimensional cross-linked network (will form pentamers and hexamers)

Epithelial cells, muscle cells, neurons

Collagen VII

Basement membrane forming

Present in anchoring fibrils of basal lamina

Function: secures basal lamina to connective tissue

Epithelial Cells

Osteogenesis Imperfecta

Deficit in collagen I, causes brittle bones, thin skin and weak tendons

Achondrogenesis (and other cartilage disorders)

Deficit in collagen II, causes short stature, restricted or abnormal joint movement

Ehler-Danlos syndrome

Deficit in collagen III, causes hypermobile joints, thin skin, and rupturing of internal vessels and organs

Collagen VII Deficiency

Causes severe blistering of skin from minor trauma

Collagen Fibers

Molecular collagen type I

“white CT”

1-20um diameter

Form bundles

Soft, flexible, and high tensile strength

Eosinophilic

Distinct striations

Reticular Fibers

Extensive network, no bundles

Molecular collagen type III

Agyrophilic, PAS positive

0.5-2um diameter

Indistinct striations

Elastic Fibers

“yellow CT”

1-10um diameter

Elastin, fibrillin

Branch and anastomose

Allows stretching

No striations

Verhoeff’s stain

Glycoaminoglycans (GAGs)

Linear, rigid polysaccharides (300 units)

Basophilic

Hydrophilic

Provides gel consistency

Allow diffusion of water, but inhibit bacteria and large molecules

Structural framework

Covalent attachment to proteins forms proteoglycans (exception - hyaluronic acid)

Hyaluronic Acid (Hyaluronan)

GAG, not sulfated

Does not bind to protein to form proteoglycan

Located in cartilage and synovial fluid, helps resist compression by hydrating tissue

Keratan Sulfate

GAG

Located in cartilage and bone

K

Chondroitin Sulfate

GAG

Located in cartilage and bone

C

Heparan Sulfate

GAG

found in basal lamina and surface of most cells

Proteoglycans

GAGs attach to core proteins

Appear like a pipe-cleaner/test tube brush

Functions: Resist compression, slow movement of microorganisms, molecular filter

Made by cells that also make the ECM

Glycoproteins (Multiadhesive)

CHO is branched

Have binding sites for components of ECM (GAGs, fibers, proteoglycans) and to specific cell membrane proteins to facilitate their attachment

Ex. Laminin - basal lamina

Ex. Fibronectin - most common found in adult CT

Connective Tissue Cells with Fixed Population

Fibroblasts

Macrophages

Adipocytes

Chondroblasts

Mast Cells

Osteoblasts

Connective Tissue Cells with Wandering (Transient) Populations

WBCs (Lymphocytes, plasma cells, neutrophils, eosinophils, basophils)

Active Fibroblast

Performs in synthesis activity

Irregular and branched cells

Large, ovoid nucleus well-developed RER and golgi

Inactive Fibroblast (Fibrocyte)

Quiet, mature fibroblasts

Spindle shaped, few cell processes

Smaller, dense nucleus

Myofibroblast

Properties of smooth muscle and fibroblast
Actin present

Wound contraction

No basal lamina, isolated cells

Macrophage

Also called histiocytes

Ovoid to fusiform shape

Central nucleus

Irregular cell surface (filipodia), abundant lysosomes, help phagocytosis and digestion

Function: phagocytosis, antigen presentation (MHC II)

Mononuclear Phagocytotic System

Macrophages of CT, spleen, lymph nodes, bone marrow and thymus

Kupffer cells of liver

Microglia of CNS

Dust cells in lung

Osteoclasts of bone tissue

Langerhans cells in skin

Monocytes of blood and bone marrow

Mast Cell

Oval shaped

Basophilic granules

Contains chemical mediators of inflammatory response (histamine, heparin)

Adipocytes

Unilocular (signet ring) - storage of neutral fats

Multilocular - color due to mitochondria and vascular supply, central nucleus

Reticular Cell

Large stellate cells, large pale nuclei

Produce reticular fibers

Phagocyte, antigen presentation

Found in lymphoid and myeloid tissue

Plasma Cell

Ovoid cell

Basophilic (RER)

Negative golgi

Clockface heterochromatin

antibody production

Blood-derived CT cells

Lymphocyte

Eosinophil

Neutrophil

Basophil

Mesenchymal Connective Tissue

Cell Type: Mesenchymal Cells

Fibers: Few collagen and reticular

Ground Substance: Watery, viscous

Organization: Loose array of stellate cells

Functions: Embryonic CT, forms adult CT

Locations: Throughout vertebrate embryos

Mucous Connective Tissue

Cell Type: Mesenchymal cells and Fibroblasts

Fibers: small number, mainly collagen

Ground Substance: syrupy, jelly-like

Organization: random distribution

Functions: forms elastic cushion to protect structures from pressure

Locations: Wharton’s Jelly in umbilical cord

Loose (Areolar) Connective Tissue

Cell Type: Fibroblasts, mesenchymal cells, mast cells, macrophages, adipocytes, plasma cells, leukocytes

Fibers: collagen, elastic, and reticular

Ground Substance: Moderately viscous, occupies more volume than fibers

Organization: more cellular than dense CT, cells suspended in loose network of loosely interwoven fibers, highly vascular

Functions: Suspends, supports, and protects vessels, nerves, and epithelia. Barrier to bacteria, diffusion of oxygen and nutrients, wandering cells for immune response.

Locations: dermis, hypodermis, glands and mucous/serous membranes, lamina propria

Dense Irregular Connective Tissue

Cell Type: Mostly fibrocytes

Fibers: Almost all collagen, traces of other fibers

Ground Substance: small amount

Organization: Fiber bundles of differing sizes woven into a dense collagenous sheet

Functions: Resists tensile stress, protects organs

Locations: Reticular layer of dermis, organ capsules, and submucosa

Dense Regular Collagenous Connective Tissue

Cell Type: Mostly fibrocytes

Fibers: Almost all collagen, traces of other fibers

Ground Substance: small amount

Organization: fibers are packed into parallel collagen bundles, fibrocytes lie parallel to bundles.

Functions: Transmits mechanical force of muscles, bind bone to bone, protective cover for organs

Locations: Tendons, ligaments, periosteum, perichondrium, joint capsules, aponeurosis, flat tendon

Dense Regular Elastic Connective Tissue

Cell Type: mostly fibroblasts, traces of other fibers

Fibers: Elastic fibers dominate, some collagen

Ground Substance: Rare

Organization: Fibers collect in parallel wavy bundles separated by collagen and fibroblasts. Resembles dense regular CT, but fibers are more squiggly.

Functions: Flexible support

Locations: vocal ligament, ligamentum flava

Reticular Connective Tissue

Cell Type: Reticular cells, traces of other cells

Fibers: Reticular fibers

Ground Substance: very little

Organization: Delicate 3D fiber network, long cell processes.

Functions: Support for motile cells, important in infiltration of blood and lymph fluid.

Locations: Myeloid and lymphoid tissue

Adipose Tissue

Fibers: Reticular fibers around individual cells, septa formed by collagen fibers

Ground Substance: rare

Organization: clusters of adipocytes divided into lobules by septa of collagenous CT.