CHAPTER 4 TISSUE LEVEL OF ORGANIZATION

Tissue--a group of similar cells and the substance surrounding them that work together to perform a particular function. The cells usually have a common origin in the embryo.

Histology is the study of tissues.

Pathologist is a doctor who studies cells and tissues for changes that might indicate disease. One responsibility of a pathologist is study of biopsies—living tissue removed to aid in diagnosis.

The body is made up of 4 major tissue types:

1. EPITHELIAL TISSUE (EPITHELIUM)

a. Covers body surfaces

b. Lines hollow organs, body cavities and ducts

c. Specialized epithelial tissue forms glands

2. CONNECTIVE TISSUE

a. Protects and supports

b. Binds organs together

c. Stores energy (fat)

d. Provides immunity (blood and lymphoid)

3. MUSCLE TISSUE

a. Movement

b. Generation of force

4. NERVE TISSUE

a. Detects changes inside and outside the body

b. Initiates and transmits nerve impulses that help maintain homeostasis

In Chapter 4 we study epithelium (ET) and connective tissue (CT) in detail except for blood and bone. Muscle and nerve we will cover in chapters of their own.

CELL JUNCTIONS

Some types of cells are tightly connected to each other. Cell junctions are points of contact between adjacent plasma membranes.

1. Tight junctions—weblike strands of transmembrane proteins form fluid-tight seals between cells (like Ziploc bags zippers) --common in epithelial cells lining stomach, intestines and urinary bladder to prevent inside contents from leaking out.

2. Adherens junctions—dense layers of proteins called plaque are found on the inside of

the plasma membranes of the connected cells. The plaque proteins attach to the microfilaments of the cytoskeleton and to membrane proteins. Cadherins (transmembrane glycoproteins) insert into the plaque and connect to cadherins of the opposite cells. In epithelial cells, adherens junctions form extensive zones called adhesion belts because they wrap around the cell. Found in sheets of ET, fastening cells firmly to each other.

3. Desmosomes—also have plaque and cadherins but desmosome plaque attaches to intermediate filaments. The intermediate filaments extend from desmosomes on one side of the cell through the cytosol to desmosomes on the opposite side of the cell. Found in sheets of ET and connecting cardiac and skeletal muscle fibers to each other.

4. Hemidesmosomes—connect cells to extracellular material (the basement membrane).

Transmembrane proteins called integrins attach to intermediate filaments inside the cell and to the protein laminin in the basement membrane.

5. Gap junctions--permit electrical or chemical signals to pass from cell to cell through tiny tunnels called connexons. The proteins forming these tunnels are connexins. The purpose is to allow rapid spread of nerve impulses or muscle action potentials from 1 cell to another--found in nerve tissue, heart muscle, and muscle of digestive tract. Nutrients may be transferred from cell to cell in avascular tissues.

EPITHELIAL TISSUE (ET)

2 MAJOR TYPES:

1. COVERING AND LINING EPITHELIUM

a. Forms the outer layer of skin and outer covering of some internal organs

b. Forms inner lining of blood vessels, ducts, body cavities and respiratory, digestive, urinary and reproductive systems

2. GLANDULAR EPITHELIUM--constitutes the secreting portion of all glands

General features of ET:

1. Consists of closely packed cells with little extracellular material between cells

2. Arranged in continuous sheets of one layer or more

3. Cells have 2 “sides”:

a. Free (apical) surface faces a body cavity or the outside

b. Basal surface is attached to the basement membrane

4. Cell junctions attach cells securely to each other

5. No blood vessels (avascular). BV are located in associated CT and substances move in and out of ET by diffusion

6. ET is always associated with CT that holds the ET in position and prevents tearing. The ET and CT are attached by a thin extracellular layer called the basement membrane. The basement membrane is secreted by both the ET and the CT. (Fig. 4.2 p. 110) It has 2 layers:

a. The basal lamina is made of proteins such as collagen & laminin, plus glycoproteins and proteoglycans. This layer is secreted by the epithelial tissue.

b. The reticular lamina is secreted by the connective tissue and contains fibrous proteins.

Functions of the basement membrane:

a. Attachment of the 2 layers to each other (blood supply)

b. Support for ET

c. Acts as part of the filter in the kidney

d. Guides cells during tissue repair

7. ET does contain nerves.

8. High mitotic rate to replace cells lost to wear and tear or injury—important feature of ET

COVERING AND LINING ETs

There are lots of different types and we need to sort and classify them to understand their structure and function. In classification 2 things are considered:

1. Number of layers of epithelial cells

a. SIMPLE epithelium--one layer—found in areas that receive little wear and tear in normal use. Functions are secretion, absorption, diffusion, osmosis, and filtration---all of these involve moving substances across the ET and the thinner the better for this.

b. Stratified epithelium--cells stacked in 2 or more layers, found in areas of higher wear and tear

c. Pseudostratified epithelium--one layer of cells but some cells do not reach the apical surface (all DO touch the basement membrane) and the tissue has a stratified appearance.

2. Shape of cells--if more than one layer (stratified) the shape of cells on the outermost (free) surface is the one considered

a. Squamous--flattened and scalelike

b. Cuboidal--cube-shaped in cross-section

c. Columnar--tall cylindrical or somewhat rectangular

d. Transitional—cells readily change shape to allow a great deal of stretching without leakage between cells. Found in urinary bladder and parts of urethra.

OVERALL CLASSIFICATION OF COVERING AND LINING EPITHELIAL TISSUE

I. Simple

A. Squamous

B. Cuboidal

C. Columnar

D. Pseudostratified Columnar

II. Stratified

A. Squamous

B. Cuboidal

C. Columnar

D. Transitional

BE SURE TO LOOK AT THE ILLUSTRATIONS ON PAGES 113 – 117

I. SIMPLE--single layer of cells

A. SIMPLE SQUAMOUS--single layer of flat, scalelike cells with oval or spherical nuclei

1. Functions: Highly adapted for diffusion, osmosis and filtration in areas that receive little wear and tear

2. Location: Found in walls of air sacs of lungs and Bowman's membrane of kidney

3. Some of this tissue gets a special name because of its location:

a. ENDOTHELIUM is simple squamous epithelium that lines the heart and blood vessels and completely forms the walls of capillaries

b. MESOTHELIUM forms the epithelial layer of serous membranes such as the peritoneum

B. SIMPLE CUBOIDAL--cells are cuboidal in cross-section (cut at right angles to the surface). Nuclei are usually round.

1. Functions: Secretion and absorption; slightly more protection from wear and tear

a. Secretion--the production and release by cells of a fluid that may contain substances such as mucus, perspiration and enzymes

b. Absorption--intake of fluids or other substances by cells

2. Location: Found on the surface of the ovary, lining kidney tubules and small

ducts of many glands

C. SIMPLE COLUMNAR--cells somewhat rectangular with oval nuclei often located near the basal surface

1. NON-CILIATED--contains goblet cells (one-celled glands that secrete mucus) and in some areas microvilli (fingerlike projections of the plasma membrane that increase surface area for absorption)

a. Functions: Secretion and absorption; some protection of underlying tissue. Mucus from goblet cells also lubricates and protects tissue.

b. Location: Found lining the GI tract from stomach to anus and lining ducts of many glands

c. Cells may have microvilli, tiny projections of cytoplasm covered with plasma membrane, purpose is to increase surface area for absorption

2. CILIATED--has cilia on the free surface, may also contain goblet cells

a. Functions: Secretion and moving materials along the free surface of the cells

b. Location: Found in a few places in the respiratory tract and lining the Fallopian tubes

D. PSEUDOSTRATIFIED COLUMNAR--this is really one layer of cells that appears to be stratified. Nuclei are at varying depths. All cells touch the basement membrane but some do not reach the free (apical) surface.

a. Function: Secretion of mucus (by goblet cells) and movement of mucus (by cilia)

b. Location: Found lining most of the upper respiratory tract and ducts of some large gland

c. Cilia are often found associated with this tissue

II. STRATIFIED--at least 2 layers of cells and provides more protection than a simple epithelium

A. STRATIFIED SQUAMOUS--cells of the deeper layers may be cuboidal to columnar, but flatten out as they approach the surface. The basal (bottom) cells continuously divide and new cells push toward the surface. At the free surface the oldest cells are continuously sloughed off and replaced. This tissue has many layers.

1. NON-KERATINIZED--living cells (without keratin) up to the surface—kept moist a. Function: Protection

b. Location: Lining the mouth, esophagus, and vagina

2. KERATINIZED--the protein keratin is deposited in cells as they near the surface, which makes them tough and somewhat waterproof. Surface cells are dead and feel dry.

a. Function: Protection

b. Location: Skin

B. STRATIFIED CUBOIDAL--cells of the surface are cuboidal and there are generally 1 or 2 deeper layers, where cells may look somewhat roundish

1. Function: Protection

2. Location: Found in ducts of sweat glands and parts of the male urethra

C. STRATIFIED COLUMNAR--1 columnar layer with 1 or 2 deeper layers of irregularly-shaped cells. Nuclei of the columnar layer tend to be at the same level, forming a straight line.

1. Function: Protection

2. Location: Found in large ducts of certain glands and parts of the male urethra

D. TRANSITIONAL--appearance varies according to the degree of stretch or relaxation

1. Relaxed (empty bladder)--contains a number of layers with irregularly- shaped cells in deeper layers and large rounded cells in the superficial layers (unlike stratified squamous which has flattened cells at the free surface). Our slides in lab will be taken from an empty bladder.

2. Stretched (full bladder)--cells on the free surface are flattened like stratified squamous and there appear to be fewer layers

3. Function: to allow great distention (stretch) without leakage or rupture

4. Location: Found in the urinary bladder and parts of the ureters and urethra

GLANDULAR EPITHELIUM

Function is secretion. Glandular cells often cluster deep to covering and lining ET.

Gland--one cell or a group of highly specialized epithelial cells that secrete substances into ducts, onto a surface, or into the blood.

1. Exocrine glands--secrete their products into ducts (little tubes) which may empty on the skin surface or into the lumen of a hollow organ.

Sweat glands--perspiration

Salivary glands--saliva, which contains mucus and a digestive enzyme

2. Endocrine glands--ductless glands--secretory products (hormones) are released into the blood

Pituitary gland

Adrenal glands

Thyroid gland

CLASSIFICATION OF EXOCRINE GLANDS

A. STRUCTURAL CLASSIFICATION

1. Unicellular--single-celled gland--goblet cells which produce mucus for lubrication are an example

2. Multicellular--many-celled glands (most of the exocrine glands)

B. FUNCTIONAL CLASSIFICATION

1. MEROCRINE GLANDS--form the secretory product and export it by exocytosis, leaving the cell unharmed. (Most common type)

Salivary glands

Pancreas

Most sweat glands, probably including those in the armpits

Probably mammary glands

2. APOCRINE GLANDS--accumulate their secretory product at the free (apical) surface of the secreting cells. This portion of the cell then pinches off and forms the secretion. The cell repairs itself and repeats.

3. HOLOCRINE GLANDS--accumulate the secretory product in the cytoplasm, the cell dies and the whole cell is discharged with its contents as the glandular secretion. The cell is replaced.

Sebaceous (oil) glands of skin

There is currently an unresolved issue—do humans have apocrine glands at all?

CONNECTIVE TISSUE (CT)

Most abundant tissue of the body. Great variety of forms & functions.

Functions:

1. Binds together, supports and strengthens other tissues of the body

2. Protects and insulates internal organs

3. Compartmentalizes structures such as skeletal muscle

4. Transport system and immunity (blood)

5. Major stored energy reserve (fat)

General features of CT:

1. Consists of 2 basic elements:

a. Cells--which rarely touch each other because they are separated by extracellular material

b. Matrix (this is the extracellular material) and consists of:

1) Ground substance--secreted by CT cells

2) Fibers—also secreted by CT cells

2. Do not occur on free surfaces but do line joint cavities

3. All except cartilage have a nerve supply

4. Usually highly vascular (rich blood supply) EXCEPT:

a. Cartilage--avascular

b. Tendons--scanty

5. Matrix is secreted by CT cells and determines the tissues qualities. It may be:

Fluid

Semifluid

Gelatinous

Fibrous

Calcified

CT CELLS

ALL cells derive from mesenchyme, an embryonic CT. In many CTs there are 2 stages of cells:

1. Immature

a. Called -------blast (chondroblast in cartilage, osteoblast in bone, etc.)

b. Capable of cell division

c. Secretes whatever matrix characteristic of its tissue

d. May differentiate and become:

2. Mature

a. Called --------cyte (chondrocyte, osteocyte)

b. Reduced capacity for cell division

c. Important in maintaining matrix

Cells found in many connective tissues include:

1. Fibroblasts--large, flat spindle-shaped cells with branching processes--secrete the matrix (both fibers and ground substance)

2. Macrophages--these develop from monocytes, a circulating WBC. They are capable of phagocytosis and help in defense of the body against invaders by engulfing bacteria and cellular debris.

a. Wandering macrophages--leave blood and migrate to trouble spot

b. Fixed macrophages--locate in certain tissues and remove bacteria, etc. that happen to come by

3. Plasma cells--develop from another WBC, the B lymphocyte. Plasma cells secrete antibodies. Many are in various CTs of the body, esp. of the GI tract and mammary glands

4. Mast cells--found along blood vessels and produce:

a. Histamine--vasodilator

b. Heparin--natural anticoagulant--prevents inappropriate clotting in vessels

5. Adipocytes--fat storage cells

6. Other WBC (leukocytes)—these would be in small numbers in normal CT

MATRIX

Gives specific CTs their properties. Consists of ground substance with protein fibers embedded--produced and deposited by CT cells.

GROUND SUBSTANCE—between CT cells and fibers

Materials found in ground substance:

1. Water

2. Hyaluronic acid—substance that is slippery and sticky all at once. It binds cells and lubricates joints, and aids in migration of phagocytes.

3. Chondroitin sulfate—jellylike and provides support and adhesiveness in cartilage, bone, skin, blood vessels

4. Dermatan sulfate—skin, tendons, blood vessels, heart valves

5. Keratan sulfate—cornea

6. Adhesion proteins—fibronectin is the main one—links components of ground substance to one another and to the surfaces of cells

7. Minerals—bones & teeth

FIBERS

Fibers in the matrix strengthen and support CTs. They are synthesized by fibroblasts and are of 3 types:

1. Collagen fibers--tough and resistant to pulling yet allow flexibility. Often occur in bundles of tiny fibrils lying parallel to each other, which gives great strength. Made of collagen, the most abundant protein in the body. Found especially in bone, cartilage, tendons and ligaments.

2. Elastic fibers--thinner, freely branching fibers that provide strength and stretch. Made of the protein elastin with a coating of a glycoprotein called fibrillin. These are yellow in color, found in skin, blood vessels, and lungs.

3. Reticular fibers--collagen with a coating of glycoprotein. Very thin and form branching networks--provide support in walls of blood vessels, networks around fat cells, nerve and muscle fibers. Provide support and strength--also form the stroma (framework) of many soft organs such as spleen and lymph nodes. Also found in the basement membrane.

OVERALL CLASSIFICATION OF CONNECTIVE TISSUE

I. Embryonic connective tissue

A. Mesenchyme

B. Mucous connective tissue PICTURES PAGES 124 - 130

II. Mature connective tissue

A. Loose CT

1. Areolar CT

2. Adipose tissue

3. Reticular CT

B. Dense CT

1. Dense regular CT

2. Dense irregular CT

3. Elastic CT

C. Cartilage

1. Hyaline cartilage

2. Fibrocartilage

3. Elastic cartilage

D. Bone (osseous tissue)

E. Liquid connective tissue

1. Blood tissue

2. Lymph

I. EMBRYONIC CT--present mainly in the embryo (first 2 months of development)

A. MESENCHYME--this is the tissue from which all other CTs arise--as the baby

develops, mesenchyme cells differentiate and become bone cells, cartilage cells, fibroblasts, etc. Contains mesenchymal cells, a semifluid ground substance and reticular fibers.

B. MUCOUS CT (WHARTON'S JELLY)--found in the umbilical cord--a form of mesenchyme with a viscous ground substance with collagen fibers.

II. MATURE CT--exists in the newborn, has cells differentiated from mesenchyme and does not change in type after birth.

A. LOOSE CT--fibers are loosely woven, many cells

1. AREOLAR CT--one of the most widely distributed CTs and contains many cells—

fibroblasts, macrophages, plasma cells, mast cells, adipocytes, and white blood cells. Also all 3 types of fibers are present. Ground substance is fluid, semifluid or gelatinous and contains large amounts of hyaluronic acid.

An enzyme, hyaluronidase, acts to dissolve the ground substance into a watery material and is sometimes added to subcutaneous injections. Phagocytic cells secrete this substance to ease their migration through tissues. Unfortunately some bacteria also produce this enzyme and use it to spread infections through tissues. (

Function: To provide strength, elasticity and support.

Location: Found in the subcutaneous layer of skin, the dermis of the skin, in mucous membranes, blood vessels, nerves and organs.

2. ADIPOSE TISSUE--cells called adipocytes are derived from fibroblasts and

specialized for the storage of fats. Cytoplasm and nucleus are pushed to one side of the cell as a large fat droplet accumulates. Adipose tissue found wherever areolar CT is found.

Functions:

a. Acts as an insulator, preventing heat loss through the skin

b. Major energy reserve

c. Supports and protects various organs

Location: Found in the subQ layer of the skin, around the heart and kidneys,

in yellow marrow of long bones, as padding around joints and behind the eyeballs.

A special type of adipose tissue, brown fat, helps maintain body temperature in newborns.

It is not found in adults.

3. RETICULAR CT--consists of fine interlacing reticular fibers and reticular

cells.

Functions: Helps bind together the cells of smooth muscle, forms a framework for soft organs such as the spleen and lymph nodes.

Location: Found in the liver, spleen, lymph nodes, bone marrow, reticular

layer of basement membrane

B. DENSE CT--contains more numerous and thicker fibers with fewer cells

1. DENSE REGULAR CT--contains bundles of collagen fibers in an orderly parallel arrangement that gives great strength and is designed to withstand pulling in one direction, along the long axis of the fibers. Fibroblasts produce the fibers and ground substance and are visible in rows between the fibers. The tissue is silvery white and tough but pliable.

Function: Provide strong attachment between structures

Location: Found in tendons (which attach muscle to bone), ligaments (which attach bone to bone), and aponeuroses (broad sheetlike tendons)

2. DENSE IRREGULAR CT--contains collagen fibers interwoven in all

directions and found in parts of the body where tension is exerted in various directions

Function: Strength

Location: Found in dermis of skin, periosteum, joint capsules, capsules around organs such as liver and kidney, heart valves

3. ELASTIC CT--fibers are elastic fibers which give the tissue a yellowish color. The tissue can be stretched and will spring back into shape.

Function: Stretch and strength

Location: Found in lung tissue, walls of elastic arteries, vocal cords,

certain respiratory passages

C. CARTILAGE—all types consists of a dense network of collagen and elastic fibers

embedded in a matrix mainly consisting of chondroitin sulfate, a rubbery ground substance material that gives cartilage resilience. The cells of mature cartilage are chondrocytes, which are found in spaces in the matrix called lacunae. The surface of most cartilage is surrounded by the perichondrium, a membrane of dense irregular CT which contains some blood vessels and nerves. Other than these, cartilage has no blood or nerve supply.

1. HYALINE CARTILAGE--appears as a bluish-white shiny substance. Fine

collagen fibers are present but not visible on our slides. This is the most abundant type of cartilage in the body

Functions: Provides movement at joints, flexibility, & support

Location: Found on the ends of all long bones, anterior ends of ribs, nose, larynx, trachea, bronchi and forms the embryonic skeleton

2. FIBROCARTILAGE--chondrocytes are scattered among bundles of collagen fibers

Function: Strength, rigidity and fusion

Location: Found in the symphysis pubis, intervertebral discs, menisci of

knees

3. ELASTIC CARTILAGE--chondrocytes are located in a threadlike network of

elastic fibers within the matrix

Functions: Support, strength and maintains shape of organs

Location: Found in the external ear, epiglottis and Eustachian tubes

Since cartilage is a somewhat more specialized tissue than some of the other CTs,

its growth and repair have some special aspects:

REPAIR OF CARTILAGE

Cartilage is a relatively inactive tissue, mainly because its poor blood supply. Repair of cartilage is slow at best. Cells which would do the clean-up (which must always come first) and then repairs must try to migrate from the perichondrium into the piece of cartilage. Substances brought to injured tissue by the blood must try to diffuse through the cartilage matrix from blood vessels of the perichondrium. The body is not always highly successful in cartilage repair.

GROWTH OF CARTILAGE

1. Interstitial growth--cartilage increases in size due to division of existing young chondrocytes and the continuous deposition of matrix. The chondrocytes deposit new matrix and are pushed away from each other, so the cartilage expands from within. This occurs during childhood and adolescence.

2. Appositional growth--activity of cells of the inner layer of the perichondrium leads to growth. Fibroblasts of the perichondrium divide and differentiate into chondroblasts which surround themselves with matrix and become chondrocytes. New matrix accumulates on the surface of the cartilage, increasing its size. This type starts later and continues through adolexcence.

Bone and blood--later in their own chapters

MEMBRANES—flat sheets of pliable tissue that cover or line a part of the body.

I. EPITHELIAL MEMBRANES--layer of ET and underlying CT

A. MUCOUS MEMBRANE (MUCOSA)--lines a body cavity that opens directly to the outside (digestive, respiratory etc.)

1. ET layer—exactly what type of ET varies in different mucous membranes

a. Protection—hard for microbes to penetrate

b. Tight junctions between cells prevent leakage