Epithelial Tissue: Types, Gerd, Barrets Esophagus

The epithelium tissue is a type of human tissue that borders hollow organs and body cavities and covers all internal and exterior body surfaces, making up the majority of glandular tissue. Epithelial tissue performs a variety of functions in many parts of the body, including absorption, secretion, and defense.

The four tissue types that make up your body's organs are as follows:

• Epithelial.
• Connective.
• Muscular.
• Nervous.

Epithelial tissue is made up of epithelial cells. The cells can take on a variety of shapes and can be arranged in a single layer or multiple layers depending on where they are located in your body and the functions they carry out. 

Types of Epithelial tissue  

• Simple epithelium: Single layers of cells are present.
• Stratified epithelium: Multi-layered cells are present and it is not always stratified. It may be pseudostratified or transitional.

Simple Epithelium

Types of Simple Epithelium	General Description
Simple squamous epithelium:	Single layer on the basement membraneIt has flat cellsNucleus: FlatSometimes nucleus can be globularMinimal cytoplasmResting thyroid follicle (less secretion)
Simple cuboidal epithelium:	Nucleus: Spherical-shaped (central) height is equal to the breadthThe cytoplasm of the nucleus is moderateOval thyroid follicle
Simple columnar epithelium:	Height is greater than breadthNucleus: Oval and basalCytoplasm is largeIt has increased the endoplasmic reticulum to secrete the thyroxine (secreting active thyroid follicles)In order to store the ER these cells increased their heights

Q. Thyroid follicles are lined by which type of epithelium 

1. Simple squamous 
2. Simple cuboidal 
3. Simple columnar 
4. Stratified cuboidal 

Answer: B

Explanation

Thyroid follicles are lined by the central spherical nucleus. The height of the cells is equal to the breadth. The type depends on the release of thyroxine. If the thyroid hormone is scanty then it is Squamous epithelium.  If Medium level of thyroid hormone then it is  Cuboidal and if High levels of thyroid hormone then it is Columnar epithelium.

Q. Secreting active thyroid follicles are lined by which type of epithelium 

1. Simple squamous 
2. Simple cuboidal 
3. Simple columnar 
4. Stratified cuboidal 

Answer: C

Explanation

If active thyroid follicles secrete, it may increase the amount of endoplasmic reticulum and there is an increase in the height of the cell. Oval basal nucleus and Increased secretions of thyroxine hormone suggest that it is the simple columnar epithelium.

Simple Squamous Epithelium

Location where simple squamous epithelium is present:

• Mesothelium lining ventral body cavities
• Endothelial lining ventral heart and blood vessels
• Portion of kidney tubules (thin sections of nephron Henle loops)
• Inner lining of cornea
• Alveoli of lungs

In this the nucleus is flat and globular and Cytoplasm is scanty.

Simple Cuboidal Epithelium

In this the height is equal to breadth and there is a Central spherical nucleus. Location where simple cuboidal epithelium is present is:

• Glands
• Ducts
• Proximal convoluted tubule and Distal convoluted tubule of Kidney
• Thyroid glands

Functions of simple cuboidal epithelium:

• Limited protection
• Secretion 
• Absorption

Simple Columnar Epithelium     

In this Height of the cells  is greater than breadth and there is a present Oval basal nucleus. Height is more due to increased cytoplasm above the nucleus.

Location where simple columnar epithelium is present:

• Lining of stomach, small intestine, gallbladder, uterine tubes (cilia), and collecting ducts of kidneys
• Actively secreting thyroid follicles
• Intestine - In the small intestine, microvilli increases the surface area for absorption. In this cell are regularly arranged, Hence, known as striated border

Stratified Epithelium                     

It is Multi-layered and is Present in the areas where cells need protection from abrasions. It has Transitional Epithelium that means It is not exactly stratified because each cell is touching the basement membrane so they are not qualified as stratified. This type of epithelium  is present in renal tube (Urothelium)

Stratified Squamous Epithelium

It is present in the regions which need protection. It is of two types:

• Stratified squamous non-keratinized epithelium
• Stratified squamous keratinized epithelium
• Location:
• Surface of skin (dry surface-keratinized epithelium)
• Lining of Oral cavity
• Throat
• Deeper oral cavity and Esophagus (wet surface: non-keratinized epithelium)
• Rectum
• Anus
• Vagina, urethra
• Any opening cavity is lined by Stratified squamous epithelium such as:
• Ear
• Nasal cavity
• Eyes 
• Oral cavity
• Dorsum of tongue- It is stratified squamous because superficial cells are squamous, Basal cells are columnar and Middle layers are cuboidal

Q. Epithelium lining of lingual surface of epiglottis is

1. Simple Columnar 
2. Pseudostratified ciliated columnar 
3. Simple Cuboidal 
4. Stratified squamous epithelium

Answer: D

Explanation

• Lingual surface (oral cavity)
• Epiglottis has a double surfaces
• Oral surface
• Laryngeal surface made of (Pseudostratified ciliated columnar)
• If they ask about the laryngeal surface then the answer is Pseudostratified ciliated columnar epithelium.

Pseudostratified Ciliated Columnar                 

It is present in the laryngeal surface (respiratory tube). Location where pseudostratified ciliated columnar 

• Lining of nasal cavity
• Trachea and bronchi
• Portions of male reproductive tract (epididymis)

Cilia is present to move the mucus. It is pseudostratified because every cell is touching the basement membrane but nuclei are in various levels

Pseudostratified Columnar with Goblet Cells

It is not exactly stratified. It is present in the respiratory tube. Posterior or laryngeal surface of epiglottis will have pseudostratified ciliated columnar epithelium with goblet cells

Transitional Epithelium               

It is a stretchable epithelium. In this the cells towards the lumen are very large called umbrella shaped cells. If urine is stored, then the epithelium will get stretched and there is Relaxation of the epithelium, if no urine is present.
Sometimes it appears as 8 to 10 layers when it is in Relaxation phase Whereas, sometimes it appears as 2-3 layers when it is Stretched. Each cell is attached to the basement membrane by a cytoplasmic extension. So, they are not qualified as stratified epithelium

Identification of Images
Pseudostratified columnar epithelium	Pseudostratification with cilia (respiratory epithelium)
Keratinized stratified squamous epithelium	Flat cells towards the surfaceBasement membrane not straight, having papillary appearanceLocated in skin
Transitional epithelium (Urothelium)	Cells towards the lumen are umbrella shaped (Globular)Basement membrane is straightFound in urinary bladder

Rule for Stratified Squamous Epithelium          

Any external opening of the body is always lined by the stratified squamous epithelium. Embryology describes an external opening lined by surface ectoderm (rule of embryology). Surface ectoderm is considered as stratified squamous epithelium. Stratified squamous epithelium is not only present on surface ectoderm, it can be present on the mesoderm and endoderm. For example  Vagina (Upper part- mesoderm, lower part- endoderm), But the entire epithelium is stratified squamous epithelium. Labia majora and minora lined by surface ectoderm

It may also be present in the internal opening for example Vocal cord. Abrasions of the vocal cord are caused due to the fast moving of air in and out during sneezing or coughing. So, in order to prevent this type of damage, it is made of stratified squamous epithelium. 

Larynx is lined by respiratory epithelium (pseudostratified ciliated epithelium). Vocal cords don't have the pseudostratified ciliated epithelium.

Openings of the Stratified Squamous Epithelium

There are seven openings that are divided into two categories:

• Four on the face
• Nasal cavity
• Oral cavity
• Eye
• Ear
• Three on the perineum
• Urethra 
• Vagina
• Anal canal

“The above all are lined by stratified squamous epithelium because they are prone to abrasions”.

Openings on the face	Description
Ear	Lined by stratified squamous epithelium.Middle ear cavity and the external ear cavity are separated by the tympanic membrane.External ear cavity is lined by stratified squamous epitheliumMiddle ear cavity, lined by stratified cuboidal epithelium This tympanic membrane has two epitheliumsInner layer: Stratified cuboidal epitheliumOuter layer: Stratified squamous epitheliumIt is lined by the skin (sweat and sebaceous gland)Sweat gland in the ear: Ceruminous gland (Wax gland)It is a modified apocrine sweat gland
Eye	Lined by Stratified squamous epitheliumFirst layer of cornea is lined by stratified squamous epithelium
Nasal cavity	Lined by Stratified squamous epitheliumIt is lined by skinNasal vestibule is lined by the skin with hair: Stratified squamous epithelium
Oral cavity	Lined by stratified squamous epitheliumIt is discussed in two partsTongue, hard palateThese are keratinised called as orthokeratinization and parakeratinizationTonsils, epiglottis (lingual cavity- non keratinized)

Difference Between Parakeratinized and Keratinized Epithelium   

Parakeratinized	Keratinized
It is found in the hard palate, tongue (Oral cavity)	It is found on the skin
Nucleus is present in the superficial layers	Superficial cells of a keratinized epithelium has no nuclei

Openings of Perineum	Description
Urethra	It is present at the anterior regionIt is 4 cm in length (In female)Entirely, lined by stratified squamous epithelium It is 20 cm length in maleTip of the male urethra is made by stratified squamous epithelium
Vagina	It is present at the middle portionEntire vaginal epithelium is made of stratified squamous epithelium
Anal canal	It is present at the posterior regionLocated below the pectinate line (Dentate line) is made of stratified squamous epithelium

Squamo-Columnar Junctions         

These junctions are prone to metaplasia, dysplasia and it  finally leads to cancer. It includes different type of cancer:

• Anal canal
• Oesophagus
• Uterine cervix

GERD (Gastro Oesophageal Reflux Disorder) 

Due to GERD the cells of  epithelium are  moved back from the stomach into the lower esophagus by changing the epithelium that is from normal squamous  epitheliumIt is replaced into the columnar epithelium. It is dangerous because there is metaplasia that can further lead to dysplasia. This metaplasia is called Barrett's Esophagus, Finally, causing the cancer (adenocarcinoma) to the lower esophagus.

Barrett's Esophagus

In this esophagus there is a squamo-columnar junction known as the Z line. It is a zig zag line found in the lower end of the esophagus. Lower 2 cm of the esophagus is lined by columnar epithelium (like the stomach). Whitish mucosa is the stratified squamous epithelium. 

Histologically, at the upper mucosa whitish epithelium is Squamous and Lower pinkish mucosa is columnar epithelium.This squamo-columnar junction is prone to metaplasia, dysplasia, and cancer. It can occur in the uterine cervix and anal canal as well.

Z-line is 2 cm above the gastroesophageal junction. But in some patients it is 5 cm above as it is becoming more proximal. It is dangerous, because cancer may be developed.

Upper part of the gut tube is the oral cavity, Then it will continue as the pharynx. Further continues as the esophagus Oesophagus further continues as the stomach. Stomach forms small intestine (Proximal duodenum). Gastroesophageal junction is present between the distal Oesophagus and proximal duodenum. 2 cm above the GEJ has SCJ (Squamo-columnar junction).It is a Z-line or zig zag line. Epithelium of the oral cavity is stratified Squamous epithelium. 

It is encroaching into the pharynx, esophagus (but not entirely). It moves up to the lower esophagus. The lower most esophagus is made of columnar epithelium (lower 2 cm). Stomach and duodenum are lined by columnar epithelium, So it is called Squamo columnar junction present 2 cm above the GEJ. In the case of GERD, acid is moved back into the lower esophagus. Now this acid will burn the epithelium, so the epithelium will be replaced by columnar epithelium causing metaplasia

In Barrett's Oesophagus, we will find that squamous cells are replaced with columnar cells (in biopsy). In this condition, we will also find goblet cells. As it has columnar epithelium with goblet cells, it is called intestinal metaplasia. Hence Barrett's metaplasia is an intestinal metaplasia. 

According to the Pathology Robbins, it is called a misnomer because columnar epithelium is not for absorption. As columnar epithelium is not involved in absorption, it is not called intestinal metaplasia. But Harrison, Bailey, Robinson, and pathology call it as intestinal metaplasia.Gray's anatomy describes it as gastric metaplasia because during biopsy, he found parietal cells. Calling Barrett's metaplasia as gastric metaplasia is a misnomer Because stomach doesn't contain goblet cells. So the best answer is Barrett's metaplasia is intestinal metaplasia.

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