TISSUE TYPES / EPITHELIAL TISSUES
Tissue = collection of cells + associated intercellular materials specialized for a
particular function.
There are 4 basic tissues in the body:

1. Epithelium = characterized by a virtual lack of intercellular substances between
adjacent cells; forms skin and lines interior surfaces of body (gut, peritoneum, etc.);
epithelial cells can also be arranged in masses with a secretory function.
2. Connective Tissue = cells usually are widely separated by an abundant intercellular
matrix; includes blood, cartilage, bone, tendons, adipose (fat); 2 Major Divisions =
CT Proper andCT Supportive.
3. Muscle Tissue = elongate cells separated by fine, vascular (many blood vessels and
capillaries) connective tissue. Three types occur: skeletal, cardiac, and smooth -
each is specialized for particular functions.
4. Nervous Tissue = cells are grouped into masses or bundles, many with long
processes; specialized for sensory reception and transmission of impulse functions.

EPITHELIAL TISSUES - General Features

1) Closely packed cellular arrangement
2) Cover or line surfaces (interior or exterior)
3) Form ducts and secretory portions of glands (= parenchyma), connective tissue
forms glandular support (= stroma)

a) Exocrine Glands (have ducts) - secrete to some surface
b) Endocrine Glands (lack ducts) - secrete to bloodstream by diffusion
- Both types develop in the same manner, as outgrowths of epithelial membranes

4) All cells of epithelia are joined by cementing substances + mechanical processes

a) Neutral Mucopolysaccharides (glycosaminoglycans) = make up cementing
substance
- PAS stain specific for carbohydrate groups on GAGs, results in reddish-purple color.
b) Interdigitations exist between cells (mechanical cementing)

5) All epithelia are anchored to underlying tissues by a basement membrane
composed of an upper basal lamina and a lower layer of reticular fibers (a CT fiber
type).
6) Avascular - no blood vessels in epithelial tissues, cells derive nutrients by diffusion
from blood vessels in surrounding CT

EPITHELIAL FUNCTIONS

1. Protective = prevents desiccation (skin), abrasion
2. Absorption = digestive and excretory tracts
3) Secretory = oils, water, enzymes, hormones, salts (in some animals)
4) Excretion = removal of harmful wastes (e.g., Loop of Henle, Chloride cells in fish
gills)

EPITHELIAL TISSUE CLASSIFICATION

1) Cell Shape

a) flat = squamous
b) square = cuboidal
c) elongate = columnar

2) Layering or Stratification

a) Simple - 1 cell-layer thick
b) Stratified = > 2 cell-layers thick

I. Simple Epithelia Types (All 1 layer thick)

1) Simple Squamous = flat or spindle-shaped in cross-section. In surface view gives
appearance of "tiled floor" so it is also named "pavement epithelium." Found in thin
Loop of Henle (kidney medulla), endothelial vessel walls, lining of body cavity
(mesothelium), pulmonary alveoli, etc.
2) Simple Cuboidal = square cells found in covering of ovaries, convoluted and
collecting tubules in kidney, ducts of most glands.
3) Simple Columnar = column-shaped cells found in absorptive/secretory lining of
digestive tract, larger ducts, lining of uterine cervix, etc. Many have microvilli (= brush
border in intestinal lining) on apical surface, some have cilia (e.g., fallopian
tubes).
Goblet Cells = mucus-secreting cells within simple secretory epithelium, mucus aids
passage of contents down digestive tract, out of respiratory tract.

II. Pseudostratified Columnar Epithelium (PSC) = all cells are in contact with
basement membrane (EM necessary to distinguish this), but not all cells reach
epithelial surface, and nuclei reside at more than 1 level. This gives the appearance of
>1 layer, when in fact only one layer is present. PSC occurs lining larger excretory
ducts, larger ducts of male reproductive system, and respiratory tract. PSC lining
respiratory tract has numerous goblet cells and is ciliated.
III. Stratified Epithelium (classify by the outer layer of cells)

1) Stratified Squamous = thick membrane, only the outer layer(s) is squamous. Basal
layers often demonstrate considerable irregularity. In Amniotes (reptiles, birds,
mammals) this epithelium usually becomes keratinized to decrease water loss and
protect from abrasion (skin).
- Keratinized = surface cells die and transform into soft scales of keratin, but remain
strongly adherent to live cells below.
- Nonkeratinized = fairly rare, lines mouth, esophagus, and vagina.
2) Stratified Cuboidal/Columnar = usually only two layers thick, rare. Cuboidal
present only in the ducts of sweat glands in humans, skin of aquatic vertebrates.
Columnar found in parts of male urethra, larger excretory ducts, and conjunctiva of
eye.
3) Transitional Epithelium = composed of several layers of cells, thickness influenced
(as is cell shape) by the state of the organs which it lines. Present only in
distensible surfaces of excretory tract (bladder, ureters, pelvis of kidney).
- In contracted state, it is many layers thick. The basal layer is cuboidal to columnar,
intermediates layers are polyhedral, surface layer with large rounded cells.
- In distended state, upper cells become flat and the entire epithelium appears as a thin
(2-5 layers thick) stratified squamous epithelium.
- Rounded surface cells may be binucleate.

CELL ADHESION IN EPITHELIAL MEMBRANES
- Cell Junctions are of three general types:

1) Occluding Junctions (Tight Junctions) = cell membranes of adjacent cells are fused
by interlocking ridges, probably composed of integral membrane proteins,
providing an impermeable seal between cells. This type of junction is exclusive to
epithelial tissues.
2) Adhering Junctions = cell junctions acting to mechanically hold cells together.
3) Gap Junctions = cytoplasmic interconnections between cells allowing cell-to-cell
communication.

SEE HANDOUT ON CELL JUNCTIONS IN EPITHELIAL MEMBRANES

To Lecture 4