Structural Organization

Tissue I

Tissue—a group of closely associated cells performing a restricted range of functions.


Overview of Tissues



Tissue Type





Primary Function

Information Processing

Contraction to Generate Force

Cover Exposed Surfaces

Structure and Support

Cell Types













Mast Cells

Plasma Cells






Basement Membrane





Nutrient-Rich, Aqueous



Depends on Type of Connective Tissue


I. Classes of Tissue

A. Nervous Tissue


1. Neurons

            a. Chemical and electrical transmission of information

2. Glia


            a. Support and repair


B. Muscle

1. Function: Contracts to generate force

2. Types

            a. Skeletal

                        i. Striated

                        ii. Multinucleated

                        iii. Voluntary control

            b. Smooth

                        i. Non-striated

                        ii. Uninucleated

                        iii. No voluntary control

            c. Cardiac

                        i. Striated

                        ii. Intercalated disks

                        iii. No voluntary control


C. Epithelial


1. Functions:

            a. Protection

            b. Absorption

            c. Filtration

            d. Secretion

2. Characteristics

            a. Cellularity: close-packed cells with limited extracellular material

            b. Cellular connections

                        i. Tight junctions

                        ii. Desmosomes

            c. Cellular organization

                        i. Apical surface in contact with fluid or air

                        ii. Basal cell layer in contact with basement membrane (lamina)

            d. Connective tissue support

                        i. All epithelial sheets are supported by connective tissue

ii. Deep to the basement lamina is a layer of connective tissue—reticular lamina

iii. Basement lamina + reticular lamina = basement membrane

            e. Innervated—receives nervous innervation

            f. Avascular—contains no blood vessels

            g. Highly regenerative

                        i. Cells are replaced rapidly by cell division

                        ii. Cell loss due to friction and contact with hostile environments

3. Classification


4. Nomenclature

            a. Two names

                        i. First indicates number of cell layers

                        ii. Second indicates cell shape

            b. All cells in a given layer will have the same shape

5. Epithelial layers

            a. Simple

                        i. Single cell layer

                        ii. Areas of absorption and filtration

            b. Stratified

                        i. Two or more cell layers

                        ii. Areas of high abrasion

6. Cell shape: all cells have six irregular sides that differ in height

            a. Squamous

                        i. Flattened

                        ii. Scale-like

            b. Cuboidal

                        i. Boxlike

                        ii. As tall as wide

            c. Columnar

                        i. Tall

7. Shape of nucleus

            a. Conforms to cell shape

                        i. Squamous—disc shaped

                        ii. Cuboidal—spherical

                        iii. Columnar—elongated from top to bottom

8. Types of Simple Epithelia

            a. Simple squamous


            b. Simple cuboidal


            c. Simple columnar


            d. Pseudostratified columnar


9. Types of Stratified Epithelia

            a. Stratified squamous


                        i. Cell shape varies according to layer

                        ii. Name is based on shape of apical surface

            b. Stratified cuboidal


            c. Stratified columnar


            d. Transitional epithelia



Nonglandular Epithelia

I. Simple

A. Simple squamous

1. Function

            a. Diffusion and filtration

2. Location

            a. Endothelium

                        i. Lining of lymphatic system

                        ii. Lining of all organs in cardiovascular system

            b. Mesothelium

                        i. Serous membrane linings of ventral body cavity

B. Simple cuboidal

1. Function

            a. Secretion and absorption

C. Simple columnar

1. Function

            a. Absorption and secretion

2. Location

            a. Digestive tract

3. Modifications

            a. Dense microvilli on apical surface

            b. Goblet cells that secrete protective lubricant

D. Pseudostratified columnar

1. Single layer of cells that vary in height

2. Only tallest reach apical surface

3. Nuclei are located at different heights

4. Function

a. Absorption and secretion

5. Modifications

            a. Ciliated with mucous cells

                        i. Mucous traps particulate matter

                        ii. Cilia propel trapped matter out


II. Stratified epithelia

A. Characteristics

1. Two or more cell layers

2. Regenerate from below via mitotic division

            a. Basal cell divide

            b. Move apically to replace older surface cells

3. Durable

4. Protection

B. Stratified squamous

1. Surface cells are squamous

2. Deep layers consist most often of cuboidal

3. Location

            a. Areas of abrasion

            b. Forms external surface of the body

                        i. Extends into all body openings

                        ii. Outer layer (epidermis) is keratinized

4. Surface cells are flattened and atrophied

C. Stratified cuboidal and columnar are rare

D. Transitional

1. Location

            a. Lining of urinary organs

                        i. Need to stretch (undergo a transition)

2. Cell organization

            a. Basal surface—cuboidal or columnar

            b. Apical surface

                        i. Unstretched—rounded and dome-like

                        ii. Stretched—flattened; squamous-like

            c. Cell layers

                        i. Unstretched—six layers

                        ii. Stretched—three layers


Glandular Epithelia 

A. Terms:

1. Gland: consist of one or more cells that make and secrete a particular product

2. Secretion: refers to both  the aqueous product of glandular cells and the process of making that product

a. Formation involves active processes

i. Made in ER, packaged in Golgi (secretory vesicles), secreted by exocytosis

B. Classification


1. Route of secretion

a. Exocrine

            i. Secrete via ducts

            Secrete onto body surface or cavities

b. Endocrine (Discussed later)

i. Ductless

            ii. Secrete directly into extracellular space

2. Cell number

            a. Unicellular

            b. Multicellular

C. Multicellular exocrine glands

1. Common elements

            a. Duct derived from epithelium

            b. Secretory unit consisting of secretory cells

            c. Supportive connective tissue

                        i. Supplies blood and nervous fibers

            d. Fibrous capsule

                        i. May penetrate gland and divide it into lobes

2. Classification based on duct structures

            a. Simple

                        i. Single unbranched duct

            b. Compound

                        i. Branched duct

3. Classification based on secretory parts

            a. Tubular

                        i. Secretory cells form a tube

            b. Alveolar

                        i. Secretory cells form a flask-like sac

            c. Tubuloalveolar

                        i. Contain both

4. Classification based on how product is secreted


            a. Merocrine glands

                        i. Secrete via exocytosis without altering secretory cell

            b. Holocrine glands

i. Accumulate products until cell bursts, releasing secretory products, then dies

            c. Apocine glands

                        i. Accumulate products just beneath free surface

                        ii. Top of cell is removed and products are released

                        iii. Cell is repaired

D. Unicellular exocrine glands

1. Single cells scattered in epithelial sheet

2. Ductless

3. Goblet cells

            a. Produce mucin

            b. Protects and lubricates surfaces