What is Epithelial Tissue ? Histology | Anjani Mishra

Epithelial Tissue

Written by Anjani Mishra

It's not clear who invented the first microscope, but the Dutch spectacle maker Zacharias Janssen (b.1585) is credited with making one of the earliest compound microscopes (ones that used two lenses) around 1600.

The Dutch spectacle maker Hans Janssen and his son Zacharias are generally credited with creating these compound microscopes.

It was only until 1819 that Mayer coined the term “Histology”. He combined two Greek root words that are histos, for tissues, and logos, for study.

Due to the importance and originality of his contributions, the name of this new discipline devoted to the study of tissues was changed from 'tissue Anatomy' to 'Histology': this term had been coined in the early 19th century by Marie Francoise Xavier Bichat and reprised by Karl Mayer in 1819.

 The word Histology is derived from Greek word Histos + Logus/Logia where histos means tissu   (French) or tissue (English) means weave/texture and logus/logia means logy (English) means   study/knowledge. So, simply the meaning of Histology is the study/knowledge of tissue.                                                                    

Definition

Histology is the branch of Anatomy which deals with the microscopic form & structure (tissue) of an organisms.

Tissue

The world "Tissue" is derived from the French word "Tissu" which means weave or texture.

Tissue is an association of cells with intercellular substance and fluids. The cells are alike in form and function and are specialized for the performance of definite task in the body.

There are 4 types of basic tissues in the body :- 

1.      Epithelial tissue

2.      Connective tissue

3.      Muscular tissue

4.      Nervous tissue

These tissues are different from each other and one another structurally and functionally.

These differences are due to differences in the physiological properties of these cells and other composition with which the particular tissues are constituted.

Epithelial tissue 

The word epithelium is derived from Greek word epi+thelion where epi means above and thelion means nipple. This tissue was discovered at first by Marie Francois Xavier Bichat (French anatomist and pathologist) in between 1771-1802 in the nipple of mammary gland. So, the name has been given as epithelial tissue. The Histologists described epithelium as a tissue which covers the surface of the body as epidermis and lines all passages leading to the interior (Eg; lining the wall of digestive, respiratory, and uro-genital system. etc.)

Functions:

•           Providing protection- Skin
•           Providing covering & lining
•           Providing as mean for external & internal secretion
•           Excretion- sweat, urine
•           Absorption- intestinal mucosa absorb nutrients
•           Sensation- tongue epithelium- taste bud and (Some epithelial cells get modified neuroepithelial cells)

Classification

       On the basis of lining/covering/membrane/surface

      On the basis of secretory glandular division

A.   On the basis of Surface/lining division, it is of 3 types: namely;

1.      Simple epithelium - tissue with single layer of cells

2.      Stratified epithelium - tissue with more than one layer of cells

3.      Pseudostratified - tissue with single layer of cells but appears to be stratified which is  

                             actually a false impression of stratification

(1)   Simple epithelium

One the basis of morphology of cells it is of 3 types: namely;

•   Simple squamous 
•   Simple cuboidal
•  Simple columnar

(i) Simple squamous: it consists of single layer of cells. The cells are flattened, fish scale like, plate like spindle shape, polygonal cell. The nucleus is situated in the centre of a cell. The nucleus will follow the shape of the cell (flat nucleus)
 Lining :- 

Endothelium - Blood & Lymph vessels

Mesothelium - Body cavity and Serous membrane

         

[It also lines the parietal layer of Bowman's capsule, Visceral layer- modified sim. Sq. epi. cells, called podocytes]

Fig: Simple squamous epithelium, mesothelium, liver, cat

1. Squamous cell, nucleus
2. Connective tissue
3. Hepatocyte

(ii) Simple cuboidal: It consists of single layer of cells which are equal in length and width. The nucleus is situated in the centre of a cell and they are round in shape.

              [Note: The term cuboidal is applied when the height of each cell is approximately equivalent to its width.]

Lining the wall of smaller duct- Pancreas, kidney, liver

                                                  - Surface lining of ovary.

                                                  - Thyroid gland (Follicles)

                                                  - Parotid and Mandibular salivary gland

Fig: Simple cuboidal epithelium, kidney, cow

1. Cuboidal cell

(iii) Simple columnar: It consists of single layer of cells. The cells are higher in length than that of width. The nucleus is situated at the centre or towards the basement membrane and they are oval in shape.

[Note: Nuclei are situated in the middle of the cells when the cells are not secretory and at the base when they are secretory]

Ciliated- Uterus, fallopian tube

Non-ciliated- stomach (Glandular part), Intestine

        

Fig: , Ciliated Simple columnar epithelium, infundibulum, oviduct, cow

1. Secretory bleb 
2. Cilia
3. Extruded nucleus

Fig: Non-ciliated Simple columnar epithelium, jejunum, dog 

1. Columnar cell
2. Lymphocyte
3. Goblet cell
4. Striated border

(2)  Stratified epithelium

Consist of more than one layer of cells. On the basis of morphology of the surface cell layer, it is classified into 4 types: namely;

i)       Stratified squamous

ii)     Stratified cuboidal

iii)   Stratified columnar

iv)   Stratified transitional 

          I.            Stratified Squamous

•   Consists of more than one layer of cells (generally 3-5 layers, but it may be upto 30 layers)
•   The basal layer consists of a single layer of columnar cells with oval nuclei
•   The cells of middle layer are irregular, polyhedral is shape with round nuclei
•   The cells of superficial layer are flattened with flat nuclei

It is of two types:

         Keratinized (dead layer of protein):- lining- skin, rumen, reticulum, omasum

         Non-keratinized:- lining- oesophagus, pharynx, vagina, mouth & nasal cavity

Fig: Stratified squamous epithelium, keratinized, wattle, pig

1. Keratinized cells
2. Dermis

Fig: Stratified squamous epithelium, non keratinized, esophagus, cat

1. Basal cell
2. Esophagus, lumen

       II.            Stratified Cuboidal

•        Consists of more than one layer of cells
•        The basal layer consists of a single layer of columnar cells  
•      The cells of superficial layer are cuboidal in shape with round nucleus located centrally 

               Lining - Larger duct; Sweat gland, Antrum of ovarian follicle, testis, Conjunctiva

Fig: Bistratified cuboidal epithelium, esophagus, dog

1. Stratified cuboidal epithelium
2. Smooth muscle

    III.            Stratified Columnar

•  Consists of more than one layer of cells
• The cells of deeper layer are usually cuboidal in shape
• The cells of middle layer are irregular, polyhedral is shape with round nuclei
• The cells of superficial layer are columnar in shape  

                  Lining-  male urethra

                                nasal cavity(certain respiratory portion)

                                esophagus(certain portion)

                                anal canal(certain portion)

                                conjunctiva(certain parts)

Fig: Stratified columnar epithelium, urethra, goat

1. Stratified columnar epithelium

    IV.            Stratified Transitional 

• Consists of more than one layer of cells
• The cells of basal layer is columnar in shape
• The cells of middle layer are irregular polyhedral shape
• The superficial cells are large, rounded/flattened in shape

               Lining- urinary bladder, ureter, urethra

Fig: Stratified transitional epithelium, urinary bladder, cat

1. Transitional epithelium

(3) Pseudo-Stratified epithelium

                     All the cells are located on a common basement membrane with their nuclei at various level, but all the cells are not reaching upto the surface. Nuclei of the cells are arranged into many layers and this is due to the differences in the length of a cells and this gives an impression of false stratification

It is of two types:

         Ciliated pseudostratified

Lining- Respiratory tract (trachea, bronchi, bronchiole)

        Non-ciliated pseudostratified

Lining- Male genital system (epididymis, ductus deferens)

Fig: Ciliated pseudostratified columnar epithelium, trachea, cow

1. Columnar cell, ciliated
2. Goblet cell
3. Basal cell
4. Basement membrane

 B.    On the basis of secretory glandular division

          A/to the number of cells present in the gland, it is of 2 types;

i)       Unicellular- it has only one cell, eg; goblet cell, unicellular salivary gland

ii)     Multicellular- it has many cells, eg; liver, pancreas, testis, ovary

         A/to the type/nature of secretion, glands may be classified into 3 types;

i)       Serous gland- secreting serous/watery fluid

Eg; Sweat gland, Parotid Salivary gland

ii)     Mucous gland- Secreting mucous substance

Eg; Goblet cell

iii)   Sero-mucous gland- Secreting watery fluid and mucous both

Eg; Sub-lingual salivary gland, Mandibular salivary gland

         A/to the mode of secretion, glands may be classified into 3 types;

i)       Apocrine gland (Apo means 'Apex', crine means to produce)

The apex of a cell damage as a secretion. Eg; Sweat, mammary & uterine gland

ii)     Holocrine gland (Holo means 'All')

The entire cell damage as secretion eg; Testis, ovary, sabaceous gland

iii)   Merocrine gland (Mero means 'part')

The cells gives out their secretion without loosing any part. Eg; Liver, pancreas, salivary gland

       A/to the presence/absence of duct, glands may be classified into 2 types;

i)       Exocrine gland- glands with duct, eg; Sweat, salivary gland

ii)     Endocrine gland- ductless gland, eg; Pituitary, adrenal, thyroid

        A/to the duct system, glands may be classified into 2 types;

i)       Simple gland- with single duct system

Eg; sweat gland, intestinal gland

ii)     Compound gland- with branched duct system

Eg; Mammary, Salivary gland

        A/to the size, shape and structure of the excretory unit of exocrine gland, it may be classified  into 3 types;

i)       Tubular gland

Eg; Intestinal gland

ii)     Alveolar gland

Eg; Sweat gland, pancreas

iii)   Tubulo-alveolar gland

Eg; Salivary glands

        A/to the morphology of the secretory unit of endocrine gland, it may be classified into 3 types;

i)       Cord type of gland

Eg; Liver 

ii)     Clump type of gland

Eg; Adrenal, pancreatic islet's

iii)   Follicular type of gland

Eg; Pituitary, Thyroid

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