Continued from post Epithelial tissue- Part I

Combining these two characteristics : arrangement of cell layers with cell shapes of epithelial tissue; we finally get following types of epithelial tissues (covering and lining) present in human body. Below is the table:

Classification of Epithelial tissue

Let’s discuss these types of tissues one by one:

(1) SIMPLE SQUAMOUS EPITHELIUM

Simple squamous epithelium

• It is single layer of flat cells, like a floor of tiles when viewed from top (apical surface).
• Nucleus is flat oval or flat spherical in shape and located in center of cell.

Location– (1) forms lining of cardiovascular and lymphatic system, (i.e. heart, blood vessels, lymphatic vessels ) known as Endothelium in these places.

(2) forms lining of esophagus and mouth and epithelial lining of serous membranes (peritoneum, pleura, pericardium) known as Mesothelium in these places.

(3) Also in air sacs of lungs, bowman’s capsule of kidneys, inner surface of eardrum.

Function- found at sites of filtration (kidenys), diffusion (blood vessels, lungs), and secretion (serous membrane). Not found at sites which are subjected to get damaged or injured by mechanical stress.

(2) SIMPLE CUBOIDAL EPITHELIUM

Simple cuboidal epithelium

• It is single layer of cubical cells.
• Nucleus is centrally located and round in shape.

Location:(1) covers surface of ovary.

(2) forms lining of kidney tubules and ducts of salivary glands, where it provides mechanical support.

(3) forms pigmented epithelium at posterior surface of retina and lines the anterior surface of lens of eye.

(4) makes secretory portion of thyroid gland and pancreas.

Function–  secretion and absorption.

(3) SIMPLE COLUMNAR EPITHELIUM (NON-CILIATED)

Simple Columnar epithelium (Non-ciliated)

• It is a single layer of columnar (pillar-like) cells. Cells are non-ciliated.
• Nucleus is oval in shape and located near base of cells.
• This epithelium contains microvilli at apical surface of cells and goblet cells (mucus secreting cells). Goblet cells are basically modified columnar epithelial cells. Microvilli are finger-like cytoplasmic projections which help in increasing surface area for absorption.

Location- lines G.I. tract (Gastro-intestinal tract: from stomach to anus), gall bladder, ducts of many glands.

Function– Secretion and absorption. These cells have higher level of secretion and absorption than cuboidal cells because these contain more organelles. Secrets mucus lubcricating linings of digestive, respiratory, reproductive tracts, urinary tract. Helps to prevent stomach lining from damage by acidic gastric juice.

(4) SIMPLE COLUMNAR EPITHELIUM (CILIATED)

Simple Columnar epithelium (Ciliated)

• It is a single layer of ciliated column-like cells.
• Nucleus is oval in shape and located near base of cells.
• Goblet cells are interspersed.

Location- forms lining of bronchioles of respiratory tract, fallopian tubes, uterus, central canal of spinal cord, ventricles of brain.

Function– cilia beat in unison and helps in movement of mucus and foreign particles towards throat to clear it. Cilia in fallopian tube help to move oocytes relased from ovary to uterus.

(5) PSEUDOSTRATIFIED COLUMNAR  EPITHELIUM

Pseudostratified columar epithelium (ciliated)

• Pseudostratified meaning discussed earlier in this post.
• This epithelium is also ciliated and non-ciliated. Ciliated has goblet cells.
• Ciliated epithelium lines parts of upper respiratory tract, secret mucus and thus trap foreign particles and eliminate them.
• Non-ciliated epithelium lines epididymis, part of male urethra and larger ducts of many glands. It helps in absorption and protection.

(6) STRATIFIED SQUAMOUS EPITHELIUM

Stratified squamous epithelium (Non-keratinized)

• It is two or more layered of flat cells.
• Few layers from apical surface have flat/ squamous cells, while more deeper layer cells vary from cuboidal to columnar.
• Basal cells divide and daughter cells are pushed upwards towards apical layer. On moving upwards away from underlying connective tissue, blood supply gets reduced so these cells develop hard and tough proteins (Keratin protein) in themselves and become dehydrated and metabolically less active and eventually die. These dead cells are sloughed off from apical surface and replaced by new ones from below. It is a continuous process.
• On the basis of these, this epithelium is of two types: Keratinized stratified squamous epithelium and Non- keratinized stratified squamous epithelium.
• Keratinized stratified squamous epithelium has tough layer of keratin protein in few upper layers. It is present in superficial layer of skin. Helps to protect skin against abrasion, water loss, U.V. rays, foreign particles.
• Non-keratinized stratified squamous epithelium does not contain large amount of keratin in apical layer. Found in wet surfaces like lining of mouth, esophagus, tongue, epiglottis, pharynx, vagina. It is continuously moistened by mucus from salivary glands and mucus glands.
• Both of these types form first line of defense of body.

(7) STRATIFIED CUBOIDAL EPITHELIUM

Stratified cuboidal epithelium

• It has cube-shaped cells arranged in multiple layers.
• Located in adult sweat glands, part of male urethra.
• Helps in protection and secretion.

(8) STRATIFIED COLUMNAR EPITHELIUM

Stratified columnar epithelium

• Multiple layers of columnar cells. While basal layers are usually shortened and irregularly shaped cells.
• Conjunctiva of eye, anal mucous membrane.
• Protection and secretion.

(9) TRANSITIONAL EPITHELIUM

• Ideal for lining of urinary bladder, portions of ureters and urethra.
• In relaxed position, it appears like stratified cuboidal epithelium but apical layer tend to be large and rounded. While in stretched condition it appears like stratified squamous epithelium as cells become flatter.
• Allow urinary organs to hold variable amount of fluid without rupturing.