Module 1.3:

N. Swailes, Ph.D. Department of Anatomy and Cell Biology Rm: B046A ML Tel: 5-7726 E-mail: [email protected]

Required reading

Mescher AL, Junqueira’s Basic Text and Atlas, 13th Edition, Chapter 5 (also via AccessMedicine)

Learning objectives

1) Name the three major classes of connective tissue and give examples of each.

2) Identify and describe the origin, organization and fate of embryonic connective tissue

3) Identify and discuss the functional properties imparted to tissue by the : a. fibers (, Type I, II, III, IV and VII) b. ground substance (, , )

4) Distinguish between different connective tissue cells and discuss their roles: a. b. c. d. mast cells e. lymphocytes f. plasma cells g. eosinophils h. neutrophils

5) Classify the different connective tissues proper and compare and contrast their functional roles within an organ.

Introduction

The human body is made up of only four basic tissues: 1. Epithelial tissue 2. Connective tissue 3. Muscle tissue 4. Nervous tissue

By adjusting the organization, composition and special features associated with each of these tissues is is possible to impart a wide variety of functions to the region or organ that they form.

During this lecture you will examine the basic histological structure and function of Connective Tissue.

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a loose meshwork Part A: General characteristics of connective tissues that cushions and allows diffusion A1. There are three major classes of connective tissue

i. Connective tissues proper - the most common class of connective tissue in the body. - its components can be organized in a variety of ways - its functions vary based upon this organization

organized dense bundles that resist high tensile forces

ii. Specialized connective tissue - these are types of connective tissue that have very specific functions in very specific locations:

1. (See Adipose tissue) 2. blood (MOHD 1) 3. hematopoietic tissue (MOHD 1) 4. (MOHD 2) 5. (MOHD 2) 6. lymphoid tissue (MOHD 2)

iii. Embryonic connective tissue - also known as mesenchymal tissue - it is a special connective tissue. found only in the embryo, umbilical cord and pulp cavity of the teeth.

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A2. General composition of connective tissue

All connective tissues are composed of cells and the extracellular matrix (ECM) that they secrete.

The extracellular matrix is composed of: ii. Ground substance i. Fibers

glycosaminoglycans collagen elastin fibers fibers

proteoglycan A3. Collagen fibers aggregate

i. Overview of structure and synthesis of collagen - collagen is synthesized by fibroblasts, secreted as pro-collagen which is then assembled into fibrils outside the cell. - the fibrils of some types of collagen can further assemble into fibers and/or fiber bundles.

6. collagen fiber Some types of collagen fibrils can assemble to form fibers

collagen fibril collagen bundle

7. 5. Some types of tropocollagen self- collagen can assembles to form assemble to collagen fibrils form fiber

bundles

Pro-collagen peptidase 3. pro-collagen is

1. secreted

fibroblasts 4. synthesize pre- pro-collagen peptidase 2. procollagen cleaves propeptides from pre-procollagen is pro-collagen forming assembled into pro- tropocollagen collagen in cell 3 | Page: Connective Tissue Swailes

fibril

tropocollagen ~300nm 67nm

67nm

- in the electron microscope collagen fibrils have regular 67nm striations. - formed by deposition of heavy metal stains in the ‘gaps’ between adjacent tropocollagen filaments in the fibril.

ii. Functional properties of collagen The organization of collagen in the extracellular matrix affects the mechanical function of a connective tissue. For example connective tissues that contain mainly collagen:

1. Fibrils - provide a delicate supportive meshwork to the structures they surround. - allow structures to easily pass through and permit their movement (think of a pulsing arterioles running through a tissue). - allow substances to also easily diffuse between fibrils.

2. Fibers - are flexible, inelastic and have high tensile strength.

3. Fiber bundles - are the strongest types of connective tissues capable of withstanding high tensile forces (e.g. ).

iii. Types of collagen There are 28 different types of collagen. The ones you will encounter most in histology are:

Collagen Type Location Organization Type I Most connective tissues Fibrils, fibers and fiber bundles Type II Cartilage specific Fibrils *Type III (reticular fibers) Distensible organs Fibrils Type IV Lamina densa Sheets Type VII Lamina reticularis Sheets *must be stained with silver stain to be visualized 4 | Page: Connective Tissue Swailes

A4. Elastin fibers

1. i. Structure and synthesis of elastin proelastin is

synthesized by proelastin fibroblasts

2. microfibrils tropoelastin is formed and secreted from the cell tropoelastin 3. tropoelastin becomes

surrounded by fibrillin microfibrils

4. the tropoelastin is

crosslinked to form an crosslinking proteins elastin fiber (e.g. desmosine)

elastin fiber

Elastin fibers: - are thinner than collagen fibers - become interwoven with collagen fibers - may branch - stain poorly with eosin, therefore require a special (Verhoeff’s) stain - when present in large numbers may be refractile which makes them visible

ii. Functional properties of elastin The amount of elastin fibers in a tissue increases its ability to stretch and, importantly, recoil to its original shape.

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A5. Ground substance 1. Glycosaminoglycans (GAG’s)

- long unbranched polymers of i. Overview of ground substance structure repeating disaccharides - ground substance is a viscous, clear - common GAGs include keratin substance with high water content sulfate, and - it fills the gaps between fibers heparin sulfate - in connective tissues proper it is synthesized by fibroblasts - it is composed of:

2. Proteoglycan monomer - a protein core to which GAGs are linked. These vary in size, in cartilage they are very large molecules called aggrecan

Collagen fiber 3. Hyaluronan/ - in some tissues the proteoglycan monomers are also bound to a long molecule called hyaluronan

4. Glycoproteins - macromolecules that bind all components of the ECM together and to the cells that secrete them e.g. fibronectin, laminin, chondronectin

ii. Function of ground substance - GAGs are hydrophilic molecules and pull water into connective tissues. - the high water content allows the connective tissue to resist compression. - tissues that have lots of ground substance are therefore optimized for resisting compressive forces (e.g. articular cartilage, nucleus pulposus of the intervertebral discs).

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A6. Connective tissue cells Connective tissue cells can be classified as resident or wandering.

i. Resident cells Stable, permanent residents of connective tissue that include:

1. Mesenchymal cells Features - irregular, elongated cells in embryonic connective tissue Function - multipotent stem cells that can differentiate into many adult cells Find - cells surrounded by ground substance with spindle-like cytoplasmic processes

2. Fibroblasts Features - the major connective tissue cell Function - synthesizes collagen, elastin and ground substance Find - spindle shaped cells associated with collagen fibers

3. Adipocytes Features - the major component of adipose tissue (fat) Function - support, cushioning, insulation and an energy store Find - large, ‘empty’ cells with peripheral nuclei

ii. Wandering cells Consist of cells that have migrated into connective tissue from the blood in response to specific stimuli. They include:

1. Macrophages Features - very large, irregular shaped cells. Function: - phagocytic cells derived from monocytes (white blood cells) Find - phagocytosed debris in the cell, difficult to identify otherwise

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2. Mast cells Features - large oval cells packed full of granules (heparin and histamine). Function - granules released in response to toxins or trauma and trigger inflammatory events a. histamine  increases permeability of vessels causing edema b. heparin  acts as an anticoagulant Find - don’t bother (unless granules are stained for specifically)

3. Lymphocytes Features - small, white blood cells with very large nuclei Function - B and T-cells both play a role in the immune response Find - a very common cell that is almost all nucleus and barely any cytoplasm

4. Plasma cells Features - derived from B-lymphocytes that have been stimulated by antigen Function: - produce antibodies Find: - a cell with a ‘clockface’ nucleus and large perinuclear halo

5. Eosinophils Features - white blood cells, eosinophilic granules and bi-lobed nuclei Function - phagocytic cells that migrate into connective tissue during allergic reactions and parasitic infections Find - a cell with eosinophilic granules and a bilobed nucleus

6. Neutrophils (polymorphonucleocytes; PMNs) Features - white blood cells with neutrophilic granules and multi-lobed nuclei Function - phagocytic cells that recognize and digest bacteria during the inflammatory response Find - a cell with multilobed nuclei

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Part B: How to classify connective tissues proper

Connective tissues proper can be classified into one of three classes based upon the density and organization of the fibers in the extracellular matrix using a combination of the following terms:

Loose: collagen fibers form fibrils and fibers but not fiber bundles Dense: collagen fibers form fiber bundles Irregular: collagen fibers are arranged in a meshwork Regular: collagen fibers are uniformly organized

B1. Classifying Connective Tissues Proper

i. Loose irregular connective tissue (areolar)

Structure - composed of thin, sparse collagen fibers and fibrils (loose) - fibers run in all directions forming a meshwork (irregular) - abundance of ground substance - contains many wandering connective tissue cells

Function

- allows O2, nutrients and waste to easily diffuse - is a supportive, cushioning tissue - provides a scaffold for cell migration - does not resist tensile forces well

Location - always found beneath epithelia - surrounds small blood vessels and glands

ii. Dense irregular connective tissue

Structure - contains mostly Type I collagen fibers & bundles (dense) - fibers and bundles run in all directions (irregular) - contains little ground substance - main cell type is the

Function - resists tensile forces applied in all directions

Location - of hollow organs where it resists tension due to stretching (e.g stomach wall) - in the of the skin where it resist tensile forces applied during movement

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iii. Dense regular connective tissue

Structure - thick Type I collagen bundles (dense) - bundles run in direction of applied force (irregular) - contains little or no ground substance - fibroblast are packed between bundles

Function - resists tensile forces applied in one direction

Location - only found in tendons, and aponeuroses

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