HISTO111 EPITHELIAL CELLS

Epithelial Tissue

• Consisting of closely apposed cells without intervening

intercellular substances.

• They are avascular, but all "grow" on an

underlying layer of vascular connective tissue.

• High cellularity

• only tissue type derived from 3 embryonic germ layers (ectoderm, mesoderm, endoderm)

• skin is made of this

• also exhibits polarity(particular cell orientation), upper portion: apical surface, edge: lateral surface, bottom portion: basal surface

• Classified into: surface epithelium and glandular epithelium

Basement membrane

The connective tissue that separates the epithelium

Mesothelium

Simple squamous epithelia that lines large internal body cavities(serous cavities: peritoneum, pleura, pericardium)

Endothelium

Simple Squamous epithelia that lines the internal surfaces of blood and lymph vessels

Protection

Skin protects from sunlight & bacteria & physical damage.

Absorption

Function of epithelial tissue in the lining of small intestine, absorbing nutrients into blood

Filtration

Function of epithelial tissue in the Lining of Kidney tubules filtering wastes from blood plasma

Secretion

Function of epithelial tissue in different glands that produce perspiration, oil, digestive enzymes and mucus

true

Epithelia are classified on the basis of the number of cell layers and the shape of the cells in the surface layer.

Simple

If there is only one layer of cells in the epithelium, it is designated ___

Stratified

If there are two or more layers of cells, it is termed _____

true

Cells in the surface layer are, as a rule, described according to their height as squamous (scale- or plate-like), cuboidal or columnar.

Simple squamous epithelia

• scale or plate-like cells

• present in sites of exchange(nephrons(kidney, bowman's capsule) and lungs)

• nuclei of epithelial cells are often flattened or ovoid, I e egg-shaped, located close to centre of cells

Simple Cuboidal Epithelium

• Cells appear cuboidal in sections perpendicular

to the surface of the epithelium.

• Nuclei are round and centrally located.

• Simple cuboidal epithelium occurs in the small

excretory ducts of many glands, the tubules of the kidney and on the surface of the ovaries.

Simple columnar epithelium

 The cells forming a simple columnar epithelium are

taller than they are wide.

 The nuclei of cells within the epithelium are usually

located at the same height within the cells - often

close to the base of the cells and are oval. An

example is the simple columnar epithelium which

lines the internal surface of the gastrointestinal

tract (GIT) from the cardiac of the stomach to the

rectum.

 Ciliated simple columnar epithelium lines the

bronchi, uterus and oviducts.

 Cilia are specialized for transport of fluid or mucus for propelling virus in bronchi and egg cell in uterus.

Goblet cell

Produces mucus

Enterocytes

simple columnar epithelial cells that are involved in the absorption of components of the digested food in the lumen of the intestines.

Glycoproteins and Glycolipids

projects from the surface of microvilli

Stratified Squamous epithelium

◦ Composed of several layers (2-20), vary in

thickness depending on the specific organ.

◦ The deepest cells, which are in contact with the

basement membrane, are cuboidal or columnar in

shape.

◦ Present in areas subjected to Friction & External Environments as Skin, Esophagus & Vagina.

◦ Basal cells often form a well defined layer at the border of the epithelium to the underlying connective tissue.

Keratinized stratified squamous epithelium

-very abundant in dermis, very smooth, sheds off layers

Non-keratinized Squamous Epithelium

-Not abundant in dermis and very dry with no lubrication, doesn’t shed layers

True

Next Layer is of cells with polyhedral outlines. Close to the surface of the epithelium, cells become more flattened. At the surface of the epithelium, cells appear like flat scales - similar to the epithelial cells of simple squamous epithelia.

Stratified Cuboidal Epithelium

A two-layered ___ is, for example, seen in the ducts of the sweat glands and major salivary glands.

Stratified Columnar Epithelia

Found in excretory ducts of the mammary gland

Transitional Epithelium

-Dome-shaped surface cells

-is found exclusively in the excretory urinary passages (the renal calyces and pelvis, the ureter, the urinary bladder, and part of the urethra).

-also called urothelium because primary seen in urinary system (in renal calyses, ureter, bladder, urethra)

-unique in mammals

-can assume two shapes depending on where they are aligned

True

In the "relaxed" state of the epithelium, it seems to be

formed by many cell layers. The most basal cells have a

cuboidal or columnar shape. There are several layers of

polyhedral cells, and, finally, a layer of superficial cells,

which have a convex, dome-shaped luminal surface.

True

In the distended state of the epithelium only one or two

layers of cuboidal cells are followed by a superficial layer

of large, low cuboidal or squamous cells. In the distended

state the epithelium will resemble a stratified squamous

epithelium.

Pseudostratified Epithelium

-Composed of one Layer of Cells

-All cells of this type of epithelium are in contact

with the basement membrane, but not all of them

reach the surface of the epithelium.

-The epithelium will look stratified but it is not -

hence its name "pseudostratified".

-not true stratified columnar epithelium, only appears as so because of cells with varying height and nuclei making it seem it has more than one layer

-to know if it is, look at the connection of basement membrane, if attached it is, if not it is pseudo(usually has cilia and can be found in respiratory system)

Ciliated Pseudostratified Columnar Epithelia

Type of epithelia found in the Respiratory System

Microvilli and Stereocilia

-are finger- or thread-shaped extensions of the epithelial cells. Their main function is to increase the surface area of epithelial cells.

-The only difference between these two are their length.

Microvilli

-contain actin filaments, which are in contact with the terminal web of the cell and are moving structures

-Much shorter

Stereocilia

-despite their name ("cilia"), not actively moving structures.

-Longer than microvilli

Zonula occludens

-Lateral Microvilli found in tight junctions

-between

epithelial cells mediates this aspect of epithelial

function.

◦ Proteins inserted into the cell membranes of

adjacent cells "stitch" the membranes of the

cells together and provide an effective barrier

to the diffusion of substances from the

outside of the epithelium (called luminal side if

the epithelium covers the surface of a tubular

structure).

Zonula adherens

-Lateral Microvilli found in GI tract

-is structurally not as well-characterized as the

desmosomes. The _____ typically appears

as a close and consistent apposition (15-20 NM) of

the cell membranes close to the apical cell surface

and surrounding the entire cell. Again, fibers of the

cytoskeleton insert into the cytoplasmic side of

this membrane specialization.

Desmosomes

-Lateral Microvilli that are abundant in skin

-◦are specializations of the lateral cell membranes which mediate cell adhesion.

-Fibers of the cytoskeleton attach to the cytoplasmic side of the desmosome to stabilize the area of contact.

-Example of it is the epidermis

Gap junctions

-Lateral Microvilli that keeps cells attached to eachother

-Cells are closely apposed but not fused,

separated by intercellular space.

-Plasma membrane consist of tiny tubes known as connexons.

“Zipper”

-Proteins inserted into the cell membrane of the adjacent cells form a protein- ____.

Hemidesmosomes

-mediate the attachment of the epithelial cells to the basal lamina.

glycoproteins (cadherins)

A group of _____ inserted into the opposing plasma membranes mediate cell-to-cell adhesion at desmosomes and also at the adhesion zones or patches mentioned below.

integrins

another group of proteins, allow the cell to attach to the matrix proteins of the basement membrane.

Basal infoldings of plasmalemma

-increase absorbing capacity.

-Ex. Renal tubule