.Lecture [2]

ByProf. dr. ashraf mahmoud m

Epithelial tissue

• Cells are the smallest functional unit of the

organism.

Tissues: 4 main types of tissues are distinguished:

1. Epithelial tissue.

2. Connective tissue.

3. Muscular tissue.

4. Nervous tissue.

Epithelial tissue: is a

tissue that composed

mainly of cells with little

intercellular substance.

These tissues exist in

association with one

another forming

different organs and

systems.

Characterstics of the Epithelial Tissue:

1. Cells are close to each other.

2. Little intercellular substances.

3. Tends to form junctions.

4. Lines surfaces or cavities to form the

parenchyma of the organ.

Characterstics of the Epithelial Tissue:

4. Cells show polarity: apical (free) surfaces, lateral, basal

surfaces.

5. The cells lie on a basement membrane (basal lamina).

6. No blood vessels enter in between the cells (Avascular),

but nerves can pass inbetween the cells.

7. The epithelia are continuously renewed and replaced .

Types of Epithelium

Covering epithelium (surface epithelium)

sheet like tissues that cover or line the surfaces,

cavities and organs of the body

Glandular epithelium : perform secretory

functions

Neuroepithelium

Myoepithelium

Classification of epithelial membranes

Number of cell layers

Simple: one layer

Stratified: 2 layers or more.

Shape of apical surface cells

Squamous

Cuboidal

Columnar

I- Covering epithelium:

Simple Epithelium:

It is formed of one layer of cells resting on the basement

membrane.

Simple squamous

Simple cubical (cuboidal)

Simple columnar

Psudostratified columnar

Simple squamous

epithelium It is formed of one layer of flat cells. Each

cell is formed of a thin cytoplasm and flat

bulging nucleus.

Sites:

Lining the blood vessels endothelium

Lining the serous membranes :pleura,

pericardium and peritoneum

mesothelium

Lining the lung alveoli.

Parietal layer of Bowman’s capsule of the

kidney.

Simple cubical (cuboidal) epithelium

It is formed of one layer of cubical cells

(square cells), resting on the basement

membrane. Each cell has a single

rounded, central nucleus.

Sites:

Thyroid gland.

Anterior surface of the lens of the eye.

Surface of infantile ovary.

Simple columnar epithelium

It is formed of one layer of columnar cells (height of

the cell exceeds its width-rectangular).

Each cell has a single, basal oval nucleus

Types:

Unmodified: in excretory ducts of the glands

Modified:

Secretory: e.g. lining epithelium of the stomach.

Absorptive: e.g. absorptive columnar cells of the small

intestine

Ciliated: the free surface is supplied by cilia e.g.

uterus fallopian tube and lung bronchioles.

The cells are overcrowded, but they all rest on the basement

membrane. Some cells appear short and not reaching the surface. So

appear as if it is formed of more than one layer.

Sites :

Trachea, bronchi (Ciliated with goblet cells).

Large ducts of salivary glands (Non-ciliated).

Pseudostratified columnar epithelium

Stratified epithelium

The cells are arranged in two or more layers.

1. Stratified squamous non-keratinized

The basal cells are columnar and lie on

a basement membrane. This layer is

responsible for generation of other cells.

The intermediate cells are polygonal which gradually flattens to

become thin squamous cells at the surface.

-Sites:

It is found in wet surfaces of all openings leading to the skin:

Oral cavity, Tongue, Oesophagus, Cornea, Anal canal, Vagina,

External auditory meatus.

Types:

Stratified epithelium

Stratified squamous non-keratinized

2. Stratified squamous

keratinized

like stratified squamous non-

keratinized epithelium but

The flat cells of the top layer

are changed into non-living

layer of keratin.

This type is tough and resists

friction and is impermeable to

water.

-Sites: epidermis of the skin.

Stratified epithelium

3. Transitional epithelium

Stratified cells that appear cuboidal

when the organ or tube is not

stretched, and squamous when the

organ or tube is stretched by fluid.

Sites:

Lining the viscera which are

subjected to distension e.g. urinary

passages. The number of layers are

nearly 4-8 layers in the empty bladder

and become 2-3 layers in full bladder

Stratified epithelium

Transitional Epithelium with rounded binucleated surface cells

Stratified epithelium4. Stratified cubical epithelium

The cells in uppermost layer are cubical in shape.

Site: Cells lining the ducts of sweat

glands of the skin.

5. Stratified columnar epithelium:

The cells in the uppermost layer

are columnar cells.

Sites:

- Large ducts of salivary glands.

- Fornix of conjunctiva.

- Penile part of male urethera.

Stratified epithelium

II- Glandular Epithelium

Classification of the glandular epithelium:

According to:

Number of cells: Unicellular & multicellular.

Structure of the gland: Simple & branched.

Mode of secretion: Exocrine & endocrine.

Type of secretion: Serous & mucous …

Cellular changes after secretion (mode of secretion):

Merocrine, apocrine, holocrine.

Epithelium characterized by its secretory function.

Exocrine glands

Unicellular: (Goblet cells)

The gland is formed only of

one cell.

Sites: in tracheal and intestinal epithelium.

L/M: Flask in shape; slender base and expanded

apex due to presence of mucous

granules. These granules

are not stained by H&E.The nucleus is basal and flat.

Multicellular Exocrine glands

Classification:

According to the type of secretion:

Mucous glands: Secretes a viscous

thick secretion containing mucin.

Serous glands: Secretes a watery

secretion containing proteins and

rich with enzymes.

Mixed glands :(Seromucous

acinus): Secretes both mucous and

serous secretions

Mucous glands

Serous glands:

Mucous acini Serous acini

Size

Lumen

In a section

Cytoplasm

Nucleus

Secretion

Relatively large in diameter

Wide

Formed of a few number of cuboidal cells that have well-defined boundaries

Pale basophilic

Each cell contains a single flattened and basal nucleus

Mucous ( a viscous thick secretion)

Relatively smaller than mucous acinus

Narrow

Formed of a relatively larger number of pyramidal cells

Dark Basophilic

Each cell contains a single rounded nucleus , shifted towards the base

Watery whey-like containing enzymes

According to the mode of secretion:

1.Merocrine (eccrine): The secretory cells release their contents by

exocytosis with no loss of the cytoplasm or cell membrane e.g.

salivary glands.

2. Apocrine: Apical Part of the cytoplasm is released with the

secretion e.g. lactating mammary glands.

3. Holocrine: The secretory cell together with its accumulated

secretion is released into the duct e.g. the sebaceous glands of the

skin

III- Neuroepithelium It is a special type of epithelium which is modified to

receive sensations.

Neuroepithelium has three main types of cells;

1) Sensory cells for receiving the stimuli

2) Supporting cells

3) Basal cells ( acts as stem cells)

Sites:

• Taste buds for taste sensation

• Olfactory mucosa

• Organ of hearing

IV- Myoepithelium

• Epithelial cells which have the ability to contract. The cells contain myofilaments.

• They surround the secretory acini of glands

• They help in releasing the secretory products to the lumen of the acini.