Epithelium l General Histology & Biology MCQ for dental students

 

Epithelial tissue

Introduction

The basic tissues of the body are: epithelial tissue, connective tissue, muscular tissue and nervous tissue.

Epithelial tissue may be:

• simple epithelium, stratified epithelium, glandular epithelium, neuroepithelium or myo-epithelium.

1. Simple and stratified types are named "surface epithelium" as they cover surfaces or line cavities.
2. Epithelial tissue is sensitive, avascular (supplied by diffusion) and regenerative. Its cells are joined together by cellular junctions and based on basement membrane.
3. Basement membrane is a thin layer (between epithelial tissue and underlying connective tissue). It is formed of:

• Basal lamina (glycoprotein of cell membrane) from basal layer of epithelial tissue.
• Reticular lamina (type 4 collagen) from connective tissue. 

Function of epithelial tissue:

1. Protection (by covering surfaces or lining cavities).
2. Absorption (as intestine…).
3. Secretion (as salivary glands…).
4. Excretion (as kidney…).
5. Reproduction (as testis and ovary).
6. Sensation (as neuro-epithelium).

Simple epithelium:

It is formed of a single layer of cells ® 6 types:

1. Simple squamous epithelium:

• It is formed of a single layer of flat cells with flat nuclei.
• Sites:  endothelium, mesothelium - alveoli, Bowman’s capsule – anterior chamber of eye, adult ovary…
• Function:  concerned with blood flow, intestinal movement, gas diffusion

2. Simple cuboidal epithelium:

• It is formed of a single layer of cube-like cells with rounded nuclei.
• Sites: salivary acini, thyroid follicles - proximal and distal convoluted tubules - eye lens, infantile ovary.
• Function: concerned with secretion, reabsorption

3. Simple columnar epithelium:

•  It is formed of a single layer of tall cells with oval nuclei.
•  Sites: stomach, intestine, gall bladder, CBD - goblet cells, cervical canal…
•  Function: concerned with secretion, absorption.

4. Simple columnar ciliated epithelium:

• It is formed of a single layer of columnar cells with cilia (containing 9×2 + 2 microtubules).
• Sites: bony eustachian tube, bronchioles - uterus, fallopian tube – central canal.
• Function: concerned with movement of mucous, ova.

5. Pseudo-stratified columnar epithelium:

• It is formed of a single layer of columnar cells with some basal cells.
• Sites: large salivary ducts - upper vas deferens, membranous urethra.
• Function: concerned with protection.

6. Pseudo-stratified columnar ciliated epithelium:

• It is formed of a single layer of col. ciliated cells with some basal cells.
• Sites: nose, nasopharynx, cartilaginous eustachian tube - trachea, bronchi lacrimal sac…
• Function: concerned with movement of mucous.

NB:

Cilia may be:

• Motile (containing microtubules and microfilaments) as trachea.
• Non-motile (containing microfilaments only) as epididymis.

Stratified epithelium:

It is formed of many layers of cells ® 4 types:

1. Stratified squamous epithelium:

• It is formed of many layers with superficial squamous cells (with or without keratin layer).
• Sites:

1. Keratinized type: skin, orifices…
2. non-keratinized type: oral cavity, esophagus, vagina, cornea.

 

NB: stratified epithelium is formed of closely packed 3-30 layers of cells divided into:

• Many superficial layers which are variable in shape and shed off with regeneration.
• Many intermediate layers which are polygonal and joined by cellular junctions.
• One basal layer on the basement membrane which is columnar and germinative.

 

2. Stratified cuboidal and transitional epithelium:

• It is formed of many layers with superficial cuboidal cells (fixed or transitional).
• Sites: sweat ducts, seminiferous tubules -kidney calyces, ureter, urinary bladder, prostatic urethra.

 

NB: transitional uro-epithelium of urinary tract contains mucous-like substance which:

• Cover superficial cells leading to:

1. Protection of epithelium (from acidity or urine).
2. Barrier between urine and tissue fluid "osmotic barrier".

• Separate intermediate cells and facilitate their gliding leading to:

1. Thick transitional epithelium (6-8 layers) in empty urinary bladder. Now superficial layer is cuboidal "stratified cuboidal epithelium".
2. Thin transitional epithelium (3-4 layers) in full urinary bladder. Now superficial layer is squamous "stratified squamous epithelium".

 

3. Stratified columnar epithelium:

• It is formed of many layers with superficial columnar cells.
• Sites: conjunctival fornicies, large ducts - recto-anal junction.

 

4. Stratified columnar ciliated epithelium:

• It is formed of many layers with superficial columnar ciliated cells.
• Sites: nasal soft palate, laryngeal epiglottis - fetal esophagus.

Glandular epithelium:

the secretory type of epithelium , Most glands are formed of collections of epithelial cells , The glands are classified according to their duct system to:

Exocrine: 

• giving secretions to duct system , Example: salivary glands.

Endocrine: 

• giving hormones to blood sinusoids , Example: pituitary gland.

Mixed: 

• has exocrine part and endocrine part, Example: pancreas.

Neuro-epithelium:

• the sensory type of epithelium.
• In this type epithelial cells act as sensory receptors for special stimuli.
• Each neuro-epithelial cell has apical hairlets (to receive sensation) and basal nerve (to transmit sensation).

Function and site of neuro-epithelium:

1. Tasting:

• Taste buds (in tongue epithelium).

2. Hearing:

• Organ of Corti (in cochlea of internal ear).

3. Equilibrium:

• Crista ampularis (in semicircular canals of internal ear) for circular movements.
• Macula utriculi and macula saculi (in vestibule of internal ear) for linear movements.

Myo-epithelium:

• the contractile type of epithelium , Myo-epithelial cells (Basket cells) are contractile branched cells , They are present between secretory cells and their basement membrane ® their evacuation.