The Tissue Level of Organization


The Tissue Level of Organization

Chapter 4





Four Basic Tissue Types



Muscle neural

Epithelial Tissue

Includes epithelia and glands

Epithelia – layers of cells that cover internal or external surfaces

Glands – secreting cells derived from epithelia

Important characteristics:

Cells bound closely together

A free (apical) surface exposed to the environment or to some internal chamber or passageway

Attachment to underlying connective tissue

By a basement membrane

Epithelial cells that are damaged or lost at the exposed surface are continuously replaced

Functions of Epithelia


Provide physical protection

Protect exposed and internal surfaces



Control permeability

Provide sensation

detect changes in the environment and relay the information to the nervous system


Produce specialized secretions

Gland cells

Glandular epithelium

Exocrine – secretions discharged onto the surface of the epithelium

Endocrine – secretions into the surrounding tissues and blood (hormones)

Intercellular Connections

Must remain firmly attached to a basement membrane and each other

Tight junction

Lipid layers of adjacent cell membranes are tightly bound together by interlocking membrane proteins

Gap junction

Two cells are held together by embedded membrane proteins.

Form a narrow passageway to let small molecules and ions to pass through


Cell membranes of two cells are locked together by intercellular cement and by membrane proteins connected to a network of intermediate filaments

The Epithelial Surface

Have specialized structures that distinguish them from other body cells

Many that line internal passageways have microvilli on their exposed surface

Increases surface area about 20 times

A cell with a ciliated surface has about 250 cilia that beat in a coordinated fashion to move materials across the surface of the cell

Example: the respiratory tract

The Basement Membrane

A network of protein fibers

Lies between the epithelium and underlying connective tissues

Is a place of attachment for epithelial cells

Provides strength and resists distortion

Is a barrier between underlying connective tissues and the epithelium

Epithelial Renewal and Repair

Epithelium must continually repair and renew itself

Are lost or destroyed by chemical or physical wear and tear

Stem cells

Unspecialized cells

Found deep in the epithelium near the basement membrane

Continuously divide to replace dead or damaged cells in the epithelium

Classifying Epithelia

Two types of layering:








Three Cell Shapes:




Table 4 – 1 page 93

Cell Layers

Simple epithelium

Single layer of cells covering the basement membrane

Line ventral body cavities, heart chambers and blood vessels

Where secretions or absorption occurs – thinness reduces diffusion time

Stratified epithelium

Has several layers of cells over the basement membrane

Provides more protection

Found in areas subject to more mechanical or chemical stress

Cell Shape


Squamous epithelium

Cells are thin and flat

Looks like fried eggs laid side by side


Cuboidal epithelium

Hexagonal boxes in 3D view, but in sectional view look like boxes


Columnar epithelium

Also hexagonal but taller and more slender

Nuclei are crowded in a narrow band close to basement membrane

Simple Squamous Epithelia

Found in protected regions where absorption takes place or where a slippery surface reduces friction

Simple Cuboidal Epithelium

Provides limited protection

Secrete enzymes and buffers in the pancreas and salivary glands

Involved in the production of urine

Simple Columnar

Provides some protection and allow for absorption and secretion

Lines the stomach, intestinal tract and many excretory ducts

Pseudostratified Epithelia

Nuclei are at varying distances from surface, it has a layered appearance

Pseudostratified columnar epithelium

It is not stratified, all cells touch basement membrane

Typically possess cilia

Lines most of the respiratory tract

Transitional and Stratified

Transitional epithelium

Withstands considerable stretching

Lines ureters and urinary bladder

Stratified squamous epithelium

Found where mechanical stresses are severe

Surface of the skin, lining of mouth, tongue and esophogus

Figure 4 – 4 and 4 – 5, pgs. 95&96

Glandular Epithelia

Produce exocrine or endocrine secretions


Discharge products through a duct onto some internal or external surface

Can be characterized as unicellular glands (goblet cells) or multicellular glands (secretory sheet)

Endocrine (hormones)

Produced by ductless glands and released into blood or tissues

Mode of Secretion

Each glandular cell releases secretions by one of three mechanisms:


Merocrine secretion

Most common mode of secretion

Product released in vesicles by exocytosis



Apocrine secretion

Involves loss of both cytoplasm and secretory product

Outermost portion of cytoplasm becomes packed with secretory vesicles and is the shed


Holocrine secretion

Entire sell becomes packed with secretions and then bursts apart and dies

Sebaceous glands

Modes of Secretion

Type of Secretion

Exocrine glands can be categorized as serous, mucous or mixed

Serous Glands

Watery solution containing enzymes

Mucous glands

Secrete mucins that form a thick, slippery mucus

Mixed glands

Contain both serous and mucous glands together

Connective Tissues

All have three basic components:




Specialized cells

Protein fibers

Fluid known as ground substance

Protein fibers and ground substance form the matrix that surrounds the cells

Accounts for most of the volume of connective tissue

Connective Tissue Functions

Support and Protection

Bony structural framework

Protect delicate organs

Transportation of materials

Fluid connective tissues

Storage of energy reserves

Adipose cells (fat)

Defense of the body


Classifying Connective Tissues


Connective tissue proper

Many types of cells and matrix fibers in a ground substance

Tissue under skin, fatty tissue, and tendons and ligaments


Fluid connective tissues

Distinct cells in a watery ground substance with dissolved proteins

Blood and lymph


Supporting connective tissues

Matrix of dense ground substance and closely packed fibers

Cartilage and bone

Connective Tissue Proper

Some cells are “permanent residents,” and others are not always present because they leave to defend and repair areas of injured tissue

The Cell Population

Fibroblasts – most abundant cells

Produce and maintain connective tissue fibers and ground substance

Macrophages – “big eater” cells

Phagocytize damaged cells or pathogens

Mobilize immune system

Fat cells – adipocytes

Large droplet of lipid that pushes nucleus and organelles to one side of the cell

Mast cells – found near blood vessels

 release chemicals to begin body’s defensive activities after an injury or infection

Antibodies, white blood cells and stem cells

Connective Tissue Fibers


Collagen fibers

Long, straight and unbranched

Strong and flexible, most common


Elastic fibers – contain protein elastin

After stretching return to original length


Reticular fibers – least common

Commonly form a branching, interwoven framework in various organs

Ground Substance

Fills spaces between cells and surrounds fibers

Clear, colorless and has consistency of maple syrup

Slows movement of bacteria and other pathogens

Loose Connective Tissue

Areolar tissue

Contains all cells and fibers found in connective tissue proper

Extensive blood supply

Forms a layer that separates the skin from underlying muscles

Provides padding and allowing movement

Adipose Tissue; Figure 4 – 9 pg. 102

Loose connective tissue containing large numbers of fat cells

Provides more padding and shock absorption for body

Insulation and energy storage

Dense connective tissue

Mostly collagen fibers

Two types: dense regular and dense irregular

Dense regular connective tissue

Collagen fibers packed tightly, parallel to one another

Tendons – attach muscle to bone

Ligaments – attach bone to bone; often contain elastic fibers

Dense irregular connective tissue

Interwoven network of collagen fibers

Provides support to areas subjected to stresses from many directions

Fluid Connective Tissues

Blood and lymph

Proteins dissolved in watery matrix do not form fibers

Watery matrix is plasma in blood

Red blood cell – makes up ½ of volume of blood

Transport oxygen

White blood cell – component of immune system

Platelets – cell fragments that function in blood clotting

Lymph forms as interstitial fluid enters small passageways, or lymphatic vessels

Cells of immune system monitor the composition of lymph

Supporting Connective Tissues

Cartilage and bone


Matrix of a firm gel with embedded fibers

Chondrocytes – only cells found in matrix

Live in small pockets called lacunae

Avascular – limits repair capabilities


Structure that separates cartilage from surrounding tissues



Hyaline cartilage – most common

Closely packed collagen fibers

Tough but flexible

Connects ribs to sternum, supports respiratory tract, and covers opposing bone surfaces


Elastic cartilage – numerous elastic fibers

Elastic flap of ear and epiglottis


Fibrocartilage – little ground substance, lots of collagen

Extremely durable and tough

Spinal column, between pubic bones and pelvis and in a few joints

Resist compression, absorb shocks and prevent damaging bone – bone contact

Figure 4 – 10, page 104


(osseous tissue)

Lacunae contain osteocytes (bone cells) and surround blood vessels


Branching network within bony matrix


Covering of bone that has outer fibrous and inner cellular layers

Table 4 – 3 page 105


Form barriers or cover and protect structures and tissues

Four types:


Mucous membranes




Serous membranes

Cutaneous membranes

Synovial membranes

Mucous Membranes

 aka mucosae, line cavities that communicate with exterior

Kept moist at all times

Most lined by simple epithelia

Also simple columnar, stratified squamous and transitional squamous

Line the digestive, respiratory, reproductive and urinary tracts

Serous Membranes

Line the sealed, internal subdivisions of the ventral body cavity

Three kinds:


Pleura – covering the lungs


Peritoneum – lines organs of abdominal cavities


Pericardium – covers the heart

Has parietal and visceral portions

Friction is reduced by water, serous fluid

The Cutaneous Membrane


Covers surface of the body

Thick, relatively waterproof, and usually dry

Synovial Membranes

Joins or articulations are surrounded by a synovial


Ends of bones are covered by hyaline cartilage and separated by viscous synovial fluid

Helps lubricate the joint and permits smooth movement

Muscle Tissue

Specialized for contraction

Involves interaction between filaments of myosin and actin

Three types in the body:







Skeletal Muscle Tissue

Contains large, multinucleated cells

Are usually called muscle fibers

Produced through division of stem cells

Partial repairs can occur after injury

Marked by a series of bands called striations

Nervous system provides voluntary control over its activities

Striated voluntary muscle

Cardiac Muscle Tissue

Only found in the heart

Cells are interconnected at intercalated discs

Specialized attachment sites containing gap junctions and desmosomes

Very limited ability to repair itself

Do NOT rely on nerve activity

Specialized cells called pacemaker cells establish a regular rate of contraction

striated involuntary muscle

Smooth Muscle Tissue

Walls of blood vessels, around hollow organs, and in layers around the respiratory, circulatory, digestive and reproductive tracts

No striations

Cells are able to divide

Regenerate after injury

Nonstriated involuntary muscle

Neural Tissue

Specialized for conduction of electrical impulses from one region of the body to another

Two types of cells:

Neurons – communicate through electrical events that affect their cell membranes

Neuroglia - provide physical support for neural tissue, maintain the chemical composition of the tissue fluids, supply nutrients to neurons and defend the tissue from infection


Longest cells in your body (up to 39 in.)

Cannot divide – very limited ability to repair themselves

Contain three parts:


Cell body – w/ large nucleus


Dendrites – numerous branching projections


Axon – long and slender part, communicate with other cells

End at synaptic terminals

Tissue Injuries and Repair

Inflammation – area is isolated from healthy tissue while damaged cells are cleaned up.

Swelling, warmth, redness and pain

Regeneration – second phase of following injury

Fibroblasts produce dense network of collagen fibers known as

scar tissue or fibrous tissue

Over time, scar tissue is remodeled and resumes normal appearance

Is more successful in some tissues than others

Permanent replacement of normal tissues is called fibrosis