Connective Tissue: Types, Distribution and Cells

Connective Tissue Overview

• Connective tissue is present in a range of tissue types including epithelium, muscle, and nerve tissue, the bulk of which are cells.
• A major component is the extracellular matrix (ECM).
• The ECM consists of protein fibers (collagen and elastic)
• The ECM also contains ground substance.

Connective Tissue Proper Distribution and Function

• Found throughout the body.
• Connects epithelia to underlying body structures.
• Links muscles to bones.
• Holds joints together.
• Plays roles in defense, repair, storage, and nutrition.
• Provides a medium for oxygen and nutrient diffusion to cells from capillaries.
• Allows carbon dioxide and waste to diffuse back in to circulation.
• Allows organs to resist stretching and tearing forces.

Resident and Wandering Connective Tissue Cells

• Connective tissue cells are either resident or wandering.
• Cells of the resident cell population are stable and exhibit little movement.
• Resident cell types: fibroblasts, myofibroblasts, macrophages, adipocytes, mast cells, and adult stem cells.
• Wandering cells are primarily cells that have migrated into the tissue from the blood in response to stimuli.
• Wandering cell types: lymphocytes, plasma cells (differentiated B cells), neutrophils, eosinophils, basophils, and monocytes.

Fibroblasts: Main Cells of Connective Tissue Proper

• Represent the main cell population within connective tissue proper.
• These are large, flat, elongated (spindle-shaped) with processes extending from the cell body ends.
• The cell nucleus is flat and oval.
• They produce and maintain most of the tissue's extracellular components.
• Fibroblasts rarely divide in adults.
• Cell cycling and mitotic activity can resume when stimulated by growth factors, and when additional fibroblasts are needed.

Collagen and ECM Production by Fibroblasts

• Fibroblasts secrete collagen (most abundant protein), elastin, GAGs, proteoglycans, and multiadhesive glycoproteins.
• Most secreted ECM components undergo further modification outside the cell before matrix assembly.

Myofibroblasts and Wound Healing

• Fibroblasts are involved in wound healing, and known as myofibroblasts.
• Myofibroblasts have a well-developed contractile function.
• Myofibroblasts are enriched with a form of actin also found in smooth muscle cells.

Fibroblast Activity Levels

• Two activity levels can be observed histologically.
• Cells with synthetic activity are morphologically distinct from the quiescent ones.
• Quiescent fibroblasts may be scattered within the matrix that they've synthesized.
• Active cells are sometimes called "fibroblasts".
• Quiescent cells are sometimes called "fibrocytes".

Active vs Quiescent Fibroblasts

• Active fibroblasts have more abundant and irregularly branched cytoplasm.
• The active fibroblast nucleus is large, ovoid, euchromatic, and has a prominent nucleolus.
• Active fibroblasts have abundant rough endoplasmic reticulum (RER) and a well-developed Golgi.
• Quiescent fibroblasts are smaller than active ones and spindle-shaped
• Quiescent fibroblasts have fewer processes and less RER, and have darker, more heterochromatic nucleus.

Adipocytes: Fat Storage

• Adipocytes, or fat cells, are found in the connective tissue of many organs.
• Large, mesenchymally-derived cells are specialized for cytoplasmic storage of lipid as neutral fats.
• Fat deposits also cushion and insulate the skin and other organs.
• Adipocytes have major metabolic and medical significance.

Leukocytes and Diapedesis

• Besides macrophages and plasma cells, connective tissue contains leukocytes from cells circulating in the blood.
• Leukocytes, or white blood cells, create a population of wandering cells in the connective tissue.
• Leukocytes leave the blood by migrating between endothelial cells lining venules, in a process called diapedesis.
• Diapedesis increases greatly during inflammation, a vascular and cellular defensive response to injury or foreign substances.

Macrophages: Phagocytic Cells

• Macrophages derive from monocytes.
• Macrophages are characterized by phagocytic ability and turnover of protein fibers.
• Macrophages are responsible for removing dead cells, tissue debris, and other particulate material.
• Varying morphologic features exist according to their state of functional activity and the tissue they inhabit.

Macrophage Characteristics

• A typical macrophage has a 10-30 μm diameter and an oval or kidney-shaped nucleus.
• Macrophages are present in connective tissue of organs; pathologists often refer to macrophages as "histiocytes."
• In TEM, possess a characteristic irregular surface with pleats, protrusions, and indentations.
• These surfaces denote active pinocytotic and phagocytic activities.
• Macrophages have a well-developed Golgi apparatus and many lysosomes.

Macrophage Development

• Macrophages derive from bone marrow precursor cells that divide, producing monocytes that circulate in the blood.
• Monocytes cross the epithelial wall of venules and penetrate connective tissue.
• In connective tissue, monocytes differentiate, mature, and acquire morphologic features of phagocytic cells.
• Monocytes and macrophages are the same cell at different maturation stages.

Mononuclear Phagocyte System

• Macrophage-like cells have different names in different organs.
• Kupffer cells are present in the liver.
• Microglial cells are present in the central nervous system.
• Langerhans cells are present in the skin.
• Osteoclasts are present in bone tissue.
• All the macrophage-like cells are derived from monocytes.
• All macrophage-like cells are long-living and survive for months in tissues.
• They are important for debris removal, uptake, processing, and presentation of antigens for lymphocyte activation, a function of the immune system.

Mast Cells: Immune Regulators

• Mast cells are a type of white blood cell.
• Consist of oval, irregularly shaped connective tissue cells, which measure 7-20 μm.
• The cytoplasm is filled with basophilic secretory granules.
• Nuclei are centrally situated and obscured by secretory granules.
• Granules are heterogeneous in structure and measure 0.3-2.0 μm.
• Granules are poorly preserved by common fixatives, so that mast cells are difficult to identify.
• Mast cells regulate of the immune system.

Mast Cell Development and Function

• Mast cells originate from progenitor cells in bone marrow.
• Progenitor cells circulate in the blood, crossing the venule and capillary walls.
• Progenitor cells penetrate connective tissues and differentiate.
• They release chemical mediators stored in mast cells and promote allergic reactions / immediate hypersensitivity reactions.
• Immediate hypersensitivity reactions happen within minutes of antigen appearance in an individual previously sensitized to the same or similar antigen.
• Mast cells function in localized release of bioactive substances with roles in the local inflammatory response, innate immunity, and tissue repair.

Molecules Secreted by Mast Cells

• Heparin is a sulfated GAG that acts locally as an anticoagulant.
• Histamine promotes increased vascular permeability and smooth muscle contraction.
• Serine proteases activate various mediators of inflammation.
• Eosinophil and neutrophil chemotactic factors attract leukocytes.
• Cytokines are the polypeptides that direct leukocytes and other cells in the immune system.
• Phospholipid precursors convert to prostaglandins, leukotrienes, and other lipid mediators of the inflammatory response.

Plasma Cells: Antibody Production

• Plasma cells are B-lymphocyte-derived, antibody-producing cells.
• They have large, ovoid basophilic cytoplasm, a result of richness in RER.
• The Golgi apparatus and centrioles occupy a pale region near the nucleus in routine histologic preparations.
• Nucleus is spherical but eccentrically placed.
• Plasma cells are present in most connective tissues.
• Plasma cells have an average lifespan of 10-20 days

Fiber Composition

• Fibrous components of connective tissue consist of elongated structures.
• Fibrous components of connective tissue are formed from proteins that polymerize after secretion from fibroblasts.
• 3 main types of fibers: collagen, reticular, and elastic.
• Collagen and reticular fibers originate from proteins of the collagen family.
• Elastic fibers consist mainly of elastin protein.
• These fibers distribute unequally among the different types of connective tissue

Collagen

• Collagens constitute a protein family able to form a variety of extracellular structures.
• Collagen fibers, sheets and networks are strong and resistant to normal shearing and tearing forces.
• It is the most abundant protein in the human body (30% of its dry weight).
• Major components of fibroblasts as collagens secrete from several other cell types.
• Collagen turnover and renewal in connective tissue is a slow, ongoing process.

Categories of Collagen

• Can be grouped according to their structures which are formed by their subunits.
• Fibrillar collagens: collagen types I, II, and III have subunits that aggregate to form large fibrils are visible in the electron or light microscope.
• Collagen type I: the most abundant and widely distributed collagen.
  • Forms large, eosinophilic bundles and is known as collagen fibers.
  • Collagen type I densely fills connective tissue, forming structures including tendons, organ capsules, and dermis.
• Sheet-forming collagens, including type IV collagen.
  • Sheet-forming collagens have subunits created by epithelial cells.
  • They are the major structural proteins of external laminae and the basal lamina in epithelia.
• Linking/anchoring collagens are short collagens linking fibrillar collagens to components of the ECM.
  • Type VII collagen binds type IV collagen.
  • Type VII collagen anchors the basal lamina to the underlying reticular lamina in basement membranes.

Collagen Synthesis

• Occurs in fibroblasts and several other cell types.
• The initial procollagen α chains are created first.
• Three α chains selected, aligned, and stabilized by disulfide bonds in the ER.
• Folded as a triple helix.
• The triple helix undergoes exocytosis and is cleaved to procollagen molecule.
• The procollagen molecule is the basic subunit from which the fibers or sheets are assembled.
• Subunits may be homotrimeric (all 3 chains identical) or heterotrimeric (2-3 chains having different sequences).
• Different combinations of procollagen α chains produce the various collagen types with different structures and functions.
• Collagen biosynthesis is complex and has many steps, meaning there are many points at which the process can be interrupted / changed by defective enzymes / disease processes.

Reticular Fibers

• Found in the delicate connective tissue of organs.
• Consist mainly of collagen type III and form an extensive network (reticulum) of thin, heavily glycosylated fibers.
• Seldom visible in hematoxylin and eosin (H&E) preparations.
• Stained black by impregnation with silver salts.
• Periodic acid-Schiff (PAS) positive.
• Reticular fibers contain up to 10% carbohydrate, as opposed to 1% in most other collagen fibers.
• Reticular fibers (produced by fibroblasts) occur in the reticular lamina of basement membranes.
• Surrounded adipocytes, smooth muscle and nerve fibers, and small blood vessels.
• Delicate reticular networks support the stroma for parenchymal secretory cells and rich microvasculature of the liver and endocrine glands.
• Abundant reticular fibers characterize the stroma of hemopoietic tissue (bone marrow) and some lymphoid organs (spleen / lymph nodes).

Elastic Fibers

• Elastic fibers are thin compared to type I collagen fibers.
• Form sparse networks in highly bent or stretched organs.
• Have physical properties similar to rubber.
• Allow tissues to stretch / distend and return to original shape.
• In the walls of large blood vessels, especially arteries, elastin occurs as fenestrated sheets called elastic lamellae.
• Elastic fibers and lamellae stain poorly with H&E and are more darkly stained by orcein and aldehyde fuchsin.
• Elastic fibers and lamellae are a composite of fibrillin microfibrils embedded in a larger mass of cross-linked elastin.
• Both components secrete from fibroblasts (and smooth muscle cells in vascular walls) and produce elastic fibers in a stepwise manner.
• Initially, microfibrils consist of fibrillin which forms protein fibrillin.
• Microfibrils scaffold elastin, on which elastin is deposited.
• Elastin accumulates between microfibrils, embedding most of these with the mass eventually comprising most of the elastic fiber.
• The properties of elastic fibers and lamellae result from the structure of elastin subunits, and unique cross-links holding them together.
• Elastin molecules are rich in glycine, proline and lysine, giving protein a random-coil conformation.
• Elastin resists digestion by most proteases and is hydrolyzed by pancreatic elastase.

Ground Substance

• The ECM ground substance is a highly hydrated, transparent mixture of macromolecules.
• Macromolecules are made of glycosaminoglycans (GAGs), proteoglycans & multiadhesive glycoproteins (laminin and fibronectin).
• Ground substance fills spaces between cells and fibers in connective tissue.
• Acts as both a lubricant and barrier to the penetration of invaders.
• The hydrated nature of connective tissue provides a medium for nutrient exchange and metabolic waste between cells / the blood supply.
• Glycoproteins stabilize the ECM by binding to matrix components / integrins in cell membranes.

Glycosaminoglycans (GAGs)

• GAGs (mucopolysaccharides) are long polysaccharides consisting of repeating disaccharide units.
• Hyaluronic acid (hyaluronan or HA) is the largest and most important GAG.
• GAGs bind large amounts of water, allowing polyanions to swell and occupy large tissue spaces.
• 4 major GAGs found in proteoglycans dermatan sulfate, chondroitin sulfates, keratan sulfate, heparan sulfate
• All have different disaccharide units and tissue distributions

Proteoglycans

• Proteoglycans consist of a protein core covalently attached to sulfated GAGs in varied numbers/combinations.
• Like glycoproteins, synthesized on RER.
• Mature in the Golgi, where GAG side chains are added and secreted from cells by exocytosis.
• Contain a greater mass of polysaccharide chains than polypeptide.
• The major cartilage constituent aggrecan is a proteoglycan.
• ECM regions contain various proteoglycan core proteins, each one carrying GAGs of different lengths/compositions.

Proteoglycan Types

• Decorin is a small proteoglycan with few GAG side chains that binds fibrils of type I collagen.
• Cell surface proteoglycans (syndecan) have transmembrane core proteins and serve as additional cell attachments to ECM.
• Aggrecan is a large proteoglycan with a core protein carrying chondroitin sulfate and keratan sulfate chains.

Multiadhesive Glycoproteins and Substrates

• Multiadhesive glycoproteins provide multiple binding sites for cell surface receptors (integrins) and matrix macromolecules.
• The adhesive glycoproteins are large branched molecules and important in adhesion of cells to their substrate.
• Laminin provides adhesion for epithelial and other cells, with integrin, type IV collagen, and proteoglycan binding sites.
• Basal and external laminae are rich in laminin.
• Laminin is essential for structure assembly / maintenance.
• Fibronectin (synthesized by fibroblasts), has binding sites for collagens and forms insoluble fibrillar networks.
• Provides specific binding sites for integrins and is important for cell adhesion and migration.
• In addition to the hydrated ground substance, connective tissue has some free interstitial fluid, with ion composition like that of blood plasma.
• It’s plasma protein content is low in molecular weight that pass through the thin walls of capillaries.

Basement Membrane Constituents

• The basement membrane has 2 sublayers the basal lamina and the reticular lamina.
• The basal lamina (100 nm thick) consists of fine protein filaments embedded in an amorphous matrix produced by the epithelial cells.
• The reticular lamina consists of reticular fibers from cells of the underlying connective tissue.
• Basement membrane functions by supporting the epithelium and acting as a selectively permeable filter.
• ECM components of basal laminae is laminin self-assemble in a lacelike network below the basal poles of cells (anchored by trans-membrane integrins).
• The laminin and type IV collagen networks held by the adhesive glycoprotein entactin/nidogen, and the proteoglycan perlecan.
  • Type IV collagen is a sheet-forming collagen that consists of monomers containing 3 polypeptide chains, self-assembling to form feltlike layers.
• Type VII collagen, linking/anchoring collagen, binds to type IV collagen and anchors the basal lamina to the underlying reticular lamina in basement.

Connective Tissue

• Meshwork contains type III collagen
• collagen anchored to 2 basal laminae by anchoring fibrils composed of type VII collagen.
• Meshwork produced by cells of the connective tissue.

Connective Tissue Origin

• Originates from embryonic mesenchyme, a tissue developing from the middle layer of the embryo, or mesoderm.
• Mesenchyme has a viscous ground substance with few collagen fibers.
• Mesenchymal cells are undifferentiated and have large nuclei, nucleoli and fine chromatin.
• Mesenchymal cells are “spindle-shaped,” with cytoplasmic extensions.
• The cells produce all types of connective tissue proper, specialized connective tissue, bone, and cartilage.
• The embryonic mesenchyme consists of stem cells for other tissues such as blood.

Types of Connective Tissue

• Different combinations and densities of cells, fibers, and extracellular macromolecules produce variations in histological structure.
• Types used denote a major component or a structural tissue characteristic.

Connective Tissue Proper Classification

• Connective tissue proper has 2 classes: loose and dense.
• Loose and dense refers to the amount of collagen present in the tissue.
• Loose connective tissue (areolar tissue) is common and supports epithelial tissue.
• Loose connective tissue comprises lamina propria ( thick layer) under epithelial lining and fills spaces between muscle and nerve fibers.
• Usually well-vascularized thin layers of loose connective tissue surround small blood vessels.

Loose Connective Tissue Fibers, Cells, and Function

• Loose connective tissue mostly contains equal parts cells, fibers, and ground substance.
• Most numerous cells are fibroblasts, along with types of connective tissue cells in the body and blood vessels.
• Collagen fibers predominate, but also elastic and reticular fibers.
• Loose connective tissue has a flexible consistency and is not stress resistant.

Dense Connective Tissue Function and Composition

• Dense connective tissue is adapted for resistance and protection.
• Shares components with loose connective tissue, but has fewer cells.
• Dense connective predominates collagen fibers instead of ground substance and is much more resistant to stress than loose connective tissue.
• Has 2 subtypes: regular and irregular

Irregular Density Connective Tissue

• In dense irregular connective tissue bundles of collagen appear randomly interwoven, with no definite orientation.
• The collagen fibers form a 3D network, providing resistance to stress from all directions.
• Dense irregular connective tissue is closely associated with loose connective tissue

Regular Density Connective Tissue

• Type 1 collagen bundles are arranged according to pattern.
• Fibers and fibroblasts are aligned in parallel to resist stress from the same direction.
• Tendons and ligaments are types of connective tissue ( strong straps and chords that connects components of the system).
• Dense has packed collagen, and appears white in the fresh state and almost inextensible.
• Parallel, closely packed bundles of collagen are separated by very little ground substance.

Reticular Tissue Characteristics

• Type III collagen fibers create a delicate 3D network that supports various types of cells.
• Fibrous network produced by modified fibroblasts called reticular cells.
• The network remains associated with and partially covering the fibers.
• Loose reticular fibers provide specialized framework for microenvironments of hematopoietic tissue and lymphoid organs.
• Resulting system creates a meshwork for lymphocyte and lymph passage.
• Macrophages and other cells of the mononuclear phagocyte system are dispersed into these reticular tissues.

Mucoid Tissue

• Mucoid tissue is an embryonic connective tissue.
• Mucoid tissue is found mainly in the umbilical cord and fetal organs.
• A chief component is its abundant ground substance composed mostly of hyaluronic acid.
• mucoid tissue is jellylike with sparse collagen fibers, and embedded scattered fibroblasts
• referred to as Wharton’s jelly.
• Also in the pulp cavities of young teeth, provides a postnatal stem cell source.