Epithelial tissue and its types

Introduction

  • It is one of the four basic types of animal tissues.
  • It is a covering tissue.
  • It forms thin sheets which cover body structures and also line the cavities of all the hollow organs of the body.
  • It is arranged in single or multilayered sheets.
  • It is supported and attached to underlying connective tissue by a basement membrane.
  • A basement membrane is a continuous sheet consisting of collagen and other proteins.
  • Cells are compactly arranged and lack intercellular space.
  • It is not supplied with blood vessels.
  • It gets oxygen and nutrients by diffusion from lymph vessels.
  • It is derived from ectoderm and endoderm.

Functions:

  • Provides cover to most of the structures and covering in the body.
  • Forms barriers that keep different body systems separate.
  • Provides protection to the body organ.
  • Some are concerned with the secretion of useful substances.
  • Helps in excretion and absorption.
  • Helps in receiving stimuli.
  • Gives rise to germ cells.

Types of epithelial tissue

A) Simple epithelial tissue

  • It consists of a single layer of cells.
  • The cells rest on a basement membrane.
  • It is of various types. They are as follows:

Image result for simple epithelial tissue types

i) Squamous epithelial tissue

  • The cells are thin, flattened, and contain little cytoplasm enclosing a centrally placed nucleus.
  • The margins are irregular and cells are hexagonal or polygonal arranged like the tiles on the floor.
  • It is also known as pavement epithelium.
  • It forms the peritoneal lining of the coelom, buccal cavity, nasal cavity, blood vessels, bowman’s capsule of the kidney, and alveolar lining of the lungs.
  • It provides protection, absorption, filtration, and exchange of gases.

ii) Cubical(cuboidal) epithelium

  • The cells are cube-shaped, having equal height and width.
  • Have a central spherical nucleus.
  • Form the lining of many ducts such as the salivary, pancreatic, and collecting ducts of the kidney.
  • Also found in many glands such as the mucus, sweat, and thyroid glands.
  • Play an active role in secretion, absorption, and excretion.

iii) Columnar epithelium

  • Cells are elongated, having a spherical nucleus at their bases.
  • Are placed side by side like columns.
  • Form the lining layer of the mucous membrane of the stomach, intestine, and the urinogenital organs.
  • Also lines and protects many kidney ducts and is a component of the thyroid and gland bladder.
  • Are mainly protective, secretory, and absorptive in function.

iv) Pseudo-stratified epithelial

  • Are columnar and ciliated.
  • Found in the epithelial lining of the respiratory tract, urinary tract, vas deferens, epididymis, etc.
  • All cells do not reach the surface.
  • Longer cells have cilia and shorter cells do not have cilia.
  • Nuclei are arranged at different levels as their placement is in such a way.
  • Gives it the appearance of multi-layered and stratified.

B) Compound or stratified epithelial tissues

  • Are made up of several layers of cells.
  • They are mainly protective in nature.
  • They are of two types. They are:

 

Image result for compound stratified epithelium

i) Stratified epithelium

  • Made up of many layers of cells.
  • Different layer cells have different natures.
  • The lowermost layer is a columnar epithelium resting on a basement membrane.
  • Germinal in nature and produces new cells which are pushed upward.
  • They are of following types:

a) Stratified squamous epithelium

  • The cells of the upper layer are flattened polygonal known as squamous.
  • These are of two types:

1. Keratinized stratified epithelium

  • The outermost layer contains keratin (a fibrous protein).
  • The keratin makes the outer layer impervious to water and the cells become dead.
  • The outermost layer is called the stratum corneum (horny layer).
  • These are found in hairs, claws, and nails.

2. Non-keratinized stratified epithelium

  • Layers of living squamous cells are formed by lower cuboidal cells.
  • They are found on the wet surface like the buccal cavity, esophagus, and vagina.

b) Stratified cuboidal epithelium

  • The cells of the outermost layer are cuboidal.
  • They form the conjunctiva of the eyes, the lining of sweat glands, mammary glands, and the female urethra.

c) Stratified columnar epithelium.

  • The cells in the upper layer are columnar types.
  • The middle layers are of cuboidal type.
  • They form the lining of the vasa-differentia, respiratory tract.

ii) Transitional epithelium

  • Made up of several layers of cells.
  • They are living, large, and more stretchable than the stratified epithelium.
  • Allows the expansion of the organs by changing it’s appearance.
  • Found in the ureter, urinary bladder, and renal pelvis.

C) Specialized epithelial tissue

  • Some epithelial tissues are specialized to perform various other functions.
  • They are of the following types:

Simple Columnar Epithelium - an overview | ScienceDirect Topics

Image source: sciencedirect

i) Ciliated epithelium

  • Cells are elongated like columnar epithelium but bear numerous cilia at their free surfaces.
  • Some goblet cells are found in between the ciliated epithelial cells which secrete mucus.
  • The inner surface of the respiratory passages, oviducts, ureter, spinal cord, etc. is formed of ciliated epithelial cells.
  • Serve to move materials from one location to another.

Image result for specialized epithelial cells

ii) Sensory epithelium

  • Modified forms of columnar epithelium.
  • Found in between the columnar epithelial cells.
  • Cells are provided with sensory hairs (cilia) at their free surfaces, which are sensitive in nature.
  • Perceive stimuli of different kinds.
  • Form the lining of the tongue and nasal cavities.

iii) Germinal epithelium

  • Found in gonads.
  • Form lining of the ovary and seminiferous tubules of the testis.
  • Cells divide by meiosis division to form sperms in males and ova in females.
  • Either cuboidal or columnar cells.

Image result for gonads epithelium

 

iv) Glandular epithelium

  • Modified columnar and cuboidal cells.
  • Specialized in the secretion of chemical substances needed for different activities of life.
  • They form glands.
  • The glands secrete hormones, enzymes, saliva, and mucus.

References: 

i) https://training.seer.cancer.gov/anatomy/cells_tissues_membranes/tissues/epithelial.html

ii) https://byjus.com/neet/epithelial-tissue/#:~:text=There%20are%208%20types%20of,and%20Transitional%20epithelia%20or%20urothelium.

Epithelial tissue and its types