Histology Connective Tissue Chpt 5

Fibrillar collagens

* notably collagen types I, II, and III - polypeptide subunits that aggregate to form large fibrils clearly visible in the electron or light microscope - Collagen type I, the most abundant and widely distributed collagen, forms large, eosinophilic bundles usually called collagen fibers: often densely fill the connective tissue, forming structures such as tendons, organ capsules, and dermis

mesenchymal cells

*In addition to producing all types of connective tissue proper and the specialized connective tissues bone and cartilage, the embryonic mesenchyme includes stem cells for other tissues such as *blood, the vascular endothelium, and muscle.*

The major constituent of connective tissue is

*the extracellular matrix (ECM)*. Extra- cellular matrices: - protein fibers (collagen and elastic fibers) - ground substance

inflammation

- "redness and swelling with heat and pain" (rubor et tumor cum calore et dolore). - Increased blood flow and vascular permeability - local tissue swelling (edema), with increased redness and warmth. - Pain is due to the action of the chemical mediators on local sensory nerve endings *All these activities help protect and repair the inflamed tissue. - Chemotaxis (Gr. chemeia, alchemy + taxis, orderly arrangement), the phenomenon by which specific cell types are attracted by specific molecules, draws much larger numbers of leukocytes into inflamed tissues.

Plasma Cells

- B lymphocyte-derived, antibody-producing cells. - Each antibody is specific for the one antigen that stimulated the clone of B cells and reacts only with that antigen or molecules resembling it: antibody-antigen reaction → neutralize harmful effects caused by antigens. → antigen-antibody complexes are quickly removed from tissues by phagocytosis. - At least a few plasma cells are present in most connective tissues. Their average lifespan is only 10-20 days.

monocytes

- Bone marrow precursor cells that circulate in the blood - Immature macrophages - cross the epithelial wall of small venules - enter connective tissue, differentiate, mature, & acquire the morphologic features of phagocytic cells- - *monocytes to macrophages* in connective tissue involves ↑ cell size, ↑ protein synthesis, and ↑ in the number of Golgi complexes and lysosomes

Levels of Fibroblast Activity

- Can be observed histologically. - Synthetically active cells (fibroblasts): more abundant and irregularly branched cytoplasm, with lots of rough endoplasmic reticulum (RER) and a well-developed Golgi apparatus, with a large, ovoid, euchromatic nucleus and a prominent nucleolus. -*quiescent fibroblasts*(fibrocytes) inactive - scattered within the matrix they have already synthesized: smaller & usually spindle-shaped with fewer processes, much less RER, and a darker, more heterochromatic nucleus.

leukocytes

- Derived from circulating blood cells, they leave blood by migrating between the endothelial cells of venules to enter connective tissue ( ↑ migration during inflammation - signaled by release of chemical mediators from various cells, the ECM, and blood plasma proteins) - act on local blood vessels, mast cells, macrophages, and other cells to induce events characteristic - function in connective tissue: only for a few hours or days and then undergo apoptosis (some lymphocytes and phagocytic antigen-presenting cells normally leave the interstitial fluid of connective tissue, enter blood or lymph, and move to selected lymphoid organs)

Reticular Fibers

- Found in delicate connective tissue of many organs, notably in the immune system - consist mainly of collagen *type III*, which forms an extensive network (reticulum) of thin fibers for the support of many different cells - *argyrophilic* - seldom visible in hematoxylin and eosin (H&E) preparations but are characteristically stained black after *impregnation with silver salts* - periodic acid-Schiff (PAS) positive - due to the high content of sugar chains bound to type III collagen α chains - contain up to 10% carbo- hydrate as opposed to 1% in most other collagen fibers. - produced by fibroblasts found in the reticular lamina of basement membranes and typically also surround adipocytes, smooth muscle and nerve fibers, and small blood vessels - supportive stroma for the parenchymal secretory cells and rich microvasculature of the liver and endocrine glands. - Abundant in stroma of hemopoietic tissue (bone marrow), the spleen, and lymph nodes where they support rapidly changing populations of proliferating cells and phagocytic cells.

mononuclear phagocyte system

- Monocyte family cells: macrophage-like cells, all derived from monocytes, but different names/ various organs. - Long-living cells- may survive in the tissues for months. - debris removal, these cells are highly important for the uptake, processing, and presentation of antigens for lymphocyte activation

Where will you find high concentrations of mast cells?

- Occurring in connective tissue of many organs, mast cells are especially numerous *near small blood vessels in skin* and *mesenteries (perivascular mast cells)* and in the tissue that lines *digestive and respiratory tracts* (mucosal mast cells); - granule content of the two populations differs somewhat. - These major locations suggest that mast cells place themselves strategically to function as sentinels detecting invasion by microorganisms.

multipotent stem cells

- Property of some cells in mesenchyme - potentially useful in *regenerative medicine* after grafting to replace damaged tissue in certain patients - remain present in some adult connective tissues, including that of tooth pulp and some adipose tissue, and are being investigated as possible sources of stem cells for therapeutic repair and organ regeneration

interstitial fluid

- Water in the ground substance of connective tissue -ion composition similar to that of blood plasma. - Interstitial fluid also contains plasma proteins of low molecular weight that pass through the thin walls of the smallest blood vessels, the capillaries.

collagens

- a family of proteins with ability to form various extracellular fibers, sheets, and networks, all of which extremely strong and resistant to normal shearing and tearing forces. - Key element of all connective tissues, as well as epithelial basement membranes and the external laminae of muscle and nerve cells. - Collagen is the most abundant protein in the human body (30% of its dry weight) - A major product of fibroblasts, also secreted by several other cell types

keloid

- a local swelling caused by abnormally large amounts of collagen that form in scars of the skin. - Keloids occur most often in individuals of African descent and can be a troublesome clinical problem to manage. Not only can they be disfiguring, but excision is almost always followed by recurrence.

hyaluronan

- a very long polymer of the disaccharide glucosamine-glucuronate. - Uniquely among GAGs, hyaluronan is synthesized directly into the ECM by an enzyme complex, hyaluronan synthase, located in the cell membrane of many cells. - Hyaluronan forms a viscous, pericellular network which binds a considerable amount of water, giving it an important role in allowing molecular diffusion through connective tissue and in lubricat- ing various organs and joints.

Reticular tissue

- abundant fibers of type III collagen (Figure 5-12) forming a delicate network that supports various types of cells - reticulin - produced by modified fibroblasts often called reticular cells that remain associated with and partially cover the fibers - loose disposition of glycosylated reticular fibers provides a framework with specialized microenvironments for cells in hemopoietic tissue and some lymphoid organs (bone marrow, lymph nodes, and spleen). - The resulting cell-lined system creates a meshwork for the passage of leukocytes and lymph. - Macrophages and dendritic cells (also in the mononuclear phagocyte family) are also dispersed within these reticular tissues to monitor cells formed there or passing through and to remove debris.

immediate hypersensitivity reactions

- allergic reaction caused by release of certain chemical mediators stored in mast cells - occur within a few minutes after the appearance of an antigen in an individual previously sensitized to that antigen - There are many examples of immediate hypersensitivity reaction; e.g. anaphylactic shock, a potentially fatal condition.

Proteoglycans

- consist of a core protein to which are covalently attached various numbers and combinations of the sulfated GAGs. - Like glycoproteins, they are synthesized on RER, mature in the Golgi apparatus, where the GAG side-chains are added, and secreted from cells by exocytosis - As mentioned with epithelia, per- lecan is the key proteoglycan in all basal laminae

The Fibrous Components of Connective Tissue

- elongated structures formed from proteins that polymerize after secretion from fibroblasts - main types of fibers include 1. collagen 2. reticular 3. elastic fibers. - distributed unequally among the different types of connective tissue, with the predominant fiber type conferring most specific tissue properties.

Adipocytes

- found in the connective tissue of many organs - mesenchymally derived - specialized for cytoplasmic storage of lipid as neutral fats, or less commonly for the pro- duction of heat. - *adipose connective tissue* serves to cushion and insulate the skin and other organs. - major metabolic significance

Mast Cells

- high content of acidic radicals in their sulfated GAGs, mast cell granules: display *metachromasia,* which means that they can change the color of some basic dyes (eg, toluidine blue) from blue to purple or red. The granules are poorly preserved - difficult to identify. - function in localized release of many bioactive substances important in the local inflammatory response, innate immunity, and tissue repair

Macrophages

- highly developed phagocytic ability & specialize in turnover of protein fibers and removal of dead cells, tissue debris, or other particulate material, - abundant at sites of inflammation. - Size and shape vary considerably, corresponding to their state of functional activity - present in the connective tissue of most organs and are sometimes referred to by pathologists as "histiocytes."

ground substance

- highly hydrated (with much bound water), transparent, complex mixture of three major kinds of macromolecules: glycosaminoglycans (GAGs), proteoglycans, and multiadhesive glycoproteins - Fills the space between cells and fibers in connective tissue - allows diffusion of small molecules and, because it is viscous, acts as both a lubricant and a barrier to the penetration of invaders - In histologic analysis its components aggregate as fine, poorly resolved material - Appears in TEM preparations as electron-dense filaments or granules

Mucoid (or mucous) connective tissue

- is the principal component of the fetal umbilical cord, *Wharton's jelly* - With abundant ground substance composed chiefly of hyaluronan, mucoid tissue is gelatinous, with sparse collagen fibers and scattered fibroblasts - Included among the fibroblastic cells are many mesenchymal stem cells, which are being studied for their potential in regenerative medicine. - Mucoid connective tissue is similar to the tissue found in the vitreous chambers of eyes and pulp cavities of young teeth.

Fibroblasts

- produce & maintain the tissue's extracellular components - synthesize and secrete *collagen* (the most abundant protein of the body) and *elastin,* which both form *large fibers, GAGs, pro- teoglycans, and multiadhesive glycoproteins* that comprise the ground substance.

Elastic Fibers

- thinner than the type I collagen fibers and form sparse networks interspersed with collagen bundles in many organs subject to regular stretching or bending - rubberlike properties that allow tissue containing these fibers, such as the stroma of the lungs, to be stretched or distended & return to their original shape - elastin also occurs as fenestrated sheets called elastic *lamellae* (arteries) - not strongly acidophilic and stain poorly with H&E; they are stained more darkly than collagen with other stains such as *orcein and aldehyde fuchsin* - composite of *fibrillin*forms a network of *microfibrils*, cross-linked *elastin* is embedded. Both components secreted from fibroblasts (and smooth muscle cells in vascular walls)-

Network or sheet-forming collagens

- type IV collagen - subunits produced by epithelial cells and are major structural proteins of external laminae and all epithelial basal laminae.

Cells of Connective Tissue

1. Fibroblasts are the key cells in connective tissue proper -originate locally from mesenchymal cells and are permanent residents 2. Other cells = Transient Cells: circulate in the blood, and then move into connective tissue where they function then apoptose.

molecules released from Mast Cells' secretory granules

1. Heparin, a sulfated GAG that acts locally as an anticoagulant 2. Histamine, which promotes increased vascular permeability and smooth muscle contraction 2. Serine proteases, which activate various mediators of inflammation 3. Eosinophil and neutrophil chemotactic factors, which attract those leukocytes 4. Phospholipid precursors, which are converted to prostaglandins, leukotrienes, and other important lipid mediators of the inflammatory response. 5. Cytokines, polypeptides directing activities of leuko- cytes and other cells of the immune system

mesenchyme

All connective tissues originate from embryonic mesen- chyme, a tissue developing mainly from the *middle layer of the embryo*, the *mesoderm.* Mesenchyme consists largely of viscous ground substance with few collagen fibers

Elastin Digestion

Elastin resists digestion by most proteases, but it is hydrolyzed by pancreatic elastase

Marfan syndrome

Fibrillins comprise a family of proteins involved in making the scaffolding necessary for the deposition of elastin. *Mutations in the fibrillin* genes result in a disease characterized by a lack of resistance in tissues rich in elastic fibers. Because the walls of large arteries are rich in elastic components and because the blood pressure is high in the aorta, patients with this disease often experience aortic swellings called aneurysms, which are life-threatening conditions.

focal adhesions

Integrin-microfilament complexes are clustered in fibroblasts and other mesenchymal cells to form this type of adhesive junction is typically present at the ends of actin filaments bundled by α-actinin as cytoplasmic stress fibers and focal adhesion kinases provide a mechanism by which pulling forces or other physical properties of the ECM can change various cellular activities.

Macrophages Role in Repair

Macrophages play a very important role in the early stages of repair and inflammation after tissue damage: - accumulate in connective tissue by local proliferation of macrophages and recruitment of more monocytes from the blood & macrophages present in the stroma of most organs *mononuclear phagocyte system* important for the uptake, processing, and presentation of antigens for lymphocyte activation

tendonitis

Overuse of tendon-muscle units can result in tendonitis, characterized by inflammation of the tendons and their attachments to muscle. Common locations are the elbow, the Achilles tendon of the heel, and the shoulder rotator cuff. The swelling and pain produced by the localized inflammation restricts the affected area's normal range of motion and can be relieved by injections of anti-inflammatory agents such as cortisone. Fibroblasts eventually repair damaged collagen bundles of the area.

Linking/anchoring collagens

Short collagens that link fibrillar collagens to one another (forming larger fibers) and to other components of the ECM. Type VII collagen binds type IV collagen and anchors the basal lamina to the underlying reticular lamina in basement membranes

Hurler, Hunter, Sanfilippo, and Morquio syndromes.

The lack of specific hydrolases in the lysosomes has been found to be the cause of several disorders, including the Because of their high viscosity, hyaluronan and proteo- glycans tend to form a barrier against bacterial penetration of tissues. Bacteria that produce hyaluronidase, an enzyme that hydrolyzes hyaluronan and disassembles proteoglycans com- plexes, reduce the viscosity of the connective tissue ground substance and have greater invasive power.

procollagen molecule

These subunits may be homotrimeric, with all three chains identical, or heterotrimeric, with two or all three chains having different sequences. Different combinations of procollagen α chains produce the various types of collagen with different struc- tures and functional properties.

multinuclear giant cells

When macrophages are stimulated (by injection of foreign substances or by infection): *activated* macrophages exhibit enhanced metabolic & lysosomal enzyme activity. Macrophages are also secretory cells producing an array of substances, including various enzymes for ECM breakdown and various growth factors or cytokines that help regulate immune cells and reparative functions. When adequately stimulated, macrophages may increase in size and fuse to form multinuclear giant cells, usually found only in pathologic conditions.

Edema

is the excessive accumulation of interstitial fluid in connective tissue. This water comes from the blood, passing through the capillary walls that become more permeable during inflammation and normally produces at least slight swelling.

Multiadhesive glycoproteins

ll have multiple binding sites for cell surface integrins and for other matrix macromolecules -Laminin -Fibronectin

Hydrostatic Pressure

of the blood caused by the pumping action of the heart, which forces water out across the capillary wall

The Colloid Osmotic Pressure

produced by plasma proteins such as albumin, which draws water back into the capillaries

Connective Tissue Purpose

provides a matrix that supports and physically connects other tissues and cells together to form the organs of the body. The interstitial fluid of connective tissue gives metabolic support to cells as the medium for diffusion of nutrients and waste products.

Ground substance

watery complex of anionic, hydrophilic pro- teoglycans, glycosaminoglycans (GAGs), and multiadhesive glycoproteins (laminin, fibronectin, and others)