Epithelial Tissue

Tissues are collections of cells and the materials that surround them that work together to perform a certain job, similar to how words are combined to produce sentences. 

In the human body, there are just four basic types of tissue: 

  1. Epithelial tissue 
  2. Connective tissue 
  3. Muscle tissue 
  4. Nerve tissue
Epithelial tissue lines hollow organs, body cavities, and ducts and covers body surfaces. It also forms glands.

Connective tissue protects and supports the body and its organs. Various forms of connective tissue tie organs together, store energy reserves as fat, and aid in immunity.

Muscular tissue produces the physical force required to move body structures as well as body heat.

Nervous tissue detects changes in a wide range of internal and external situations and responds by generating action potentials (nerve impulses) that activate muscle contractions and/or glandular secretions.

The majority of epithelial cells, as well as some muscle and nerve cells, are closely packed together to form functional units. Cell junctions are places of interaction between the plasma membranes of tissue cells. 

The following are the five types of cell junctions present in the human body: 

  1. Tight junctions 
  2. Adherens junctions
  3. Desmosomes 
  4. Hemidesmosomes
  5. Gap junctions 
Fig. 1: Cell Junctions

Tight junctions are made up of weblike strands of transmembrane proteins that connect the outer surfaces of neighboring plasma membranes to close off routes between cells. The cells of epithelial tissues that line the stomach, intestines, and urine bladder have many tight junctions to slow the movement of substances between cells and prevent the contents of these organs from leaking into the blood or surrounding tissues.

Adherens junctions include plaques that adhere to both membrane proteins and cytoskeleton microfilaments. Cadherins are transmembrane glycoproteins that connect cells. Adherens junctions aid epithelial surfaces in resisting separation during contractile events such as food passage through the intestines.

Desmosomes, like adherens junctions, comprise plaque and transmembrane glycoproteins (cadherins) that bind cells together. However, unlike adherens junctions, the desmosome plaque does not adhere to microfilaments. Instead, a desmosome plaque connects to cytoskeleton elements known as intermediate filaments, which are made up of the protein keratin. Desmosomes keep epidermal cells from separating under stress and heart muscle cells from pushing apart during contraction.

Hemidesmosomes (half of a desmosome) connect cells to the basement membrane via transmembrane glycoproteins called integrins.
 
Gap junctions are made up of membrane proteins called connexins that have tiny fluid-filled tunnels called connexons that join neighbouring cells. Ions and tiny molecules can move from one cell's cytosol to another via connexons. Nutrients and maybe wastes are transferred through gap junctions in avascular tissues such as the lens and cornea of the eye. Gap junctions also allow nerve or muscle impulses to pass quickly among cardiac cells.

Epithelial Tissue

An epithelial tissue or epithelium is made up of cells that are arranged in continuous sheets in single or multiple layers. There is little intercellular space between adjacent plasma membranes and the numerous cell junctions hold them tightly together. All over the body, epithelial tissue develops into coverings and linings. It always has a free surface. 

Three primary purposes are served by epithelial tissues:

  1. It acts as a selected barrier that prevent or facilitate the passage of substances into and out of the body 
  2. It has a secretory surface that enable the release of cell products onto their free surfaces
  3. It has protective surface that fend off the environment's abrasive effects..
The overlying epithelial tissue uses the basement membrane as an attachment point and a support. The basement membrane is a thin extracellular layer made up of two layers: the basal lamina and the reticular lamina. 

The basal lamina is located nearer to the epithelial cells and is secreted by them. It includes glycoproteins, proteoglycans, and proteins like collagen and laminin.

The reticular lamina, which is located closer to the underlying connective tissue, contains collagen and other proteins made by fibroblasts, which are connective tissue cells.

Although epithelial tissue has its own nerve supply, it does not have a separate blood supply. The nearby connective tissue is home to the blood vessels that transport nutrients and waste away.

When layer arrangements and cell shapes are combined, the following categories of covering and lining epithelia result:
  1. Simple epithelium
    1. Simple squamous epithelium
    2. Simple cuboidal epithelium
    3. Simple columnar epithelium (nonciliated and ciliated)
    4. Pseudostratified columnar epithelium (nonciliated and ciliated)
  2. Stratified epithelium
    1. Stratified squamous epithelium (keratinized and nonkeratinized)*
    2. Stratified cuboidal epithelium*
    3. Stratified columnar epithelium*
    4. Transitional epithelium

Simple Epithelium

Simple Squamous Epithelium

Fig. 2: Simple Squamous Epithelium

Description: Single layer of flat cells with centrally located nucleus.

Location: Lines heart, blood vessels, lymphatic vessels, and air sacs of lungs

Function: Filtration, diffusion, osmosis, and secretion in serous membranes.

Simple Cuboidal Epithelium

Fig. 3: Simple Cuboidal Epithelium

Description: Single layer of cube-shaped cells; centrally located nucleus.

Location: Covers surface of ovary,lines kidney tubules and smaller ducts of many glands, and makes up the secreting portion of some glands such as the thyroid gland and the ducts of some glands such as the pancreas.

Function: Secretion and absorption

Nonciliated Simple Columnar Epithelium

Fig. 4: Non-ciliated Simple Columnar Epithelium

Description: Single layer of nonciliated column-like cells with nuclei near base of cells; contains goblet cells and cells with microvilli in some locations.

Location: Lines the gastrointestinal tract (from the stomach to the anus), ducts of many glands, and gallbladder.

Function: Secretion and absorption.

Ciliated Simple Columnar Epithelium 

Fig. 5: Ciliated Simple Columnar Epithelium

Description: Single layer of ciliated column-like cells with nuclei near base; contains goblet cells in some locations.

Location: Lines some bronchioles (small tubes) of respiratory tract, uterine (fallopian) tubes, uterus, efferent ducts of the testes, some paranasal sinuses, central canal of spinal cord, and ventricles of the brain.

Function: Moves mucus and other substances by ciliary action.

Pseudostratified Columnar Epithelium

Fig. 6: Pseudostratified Columnar Epithelium

Description: Not a true stratified tissue; nuclei of cells are at different levels; all cells are attached to basement membrane, but not all reach the apical surface.

Location: Pseudostratified ciliated columnar epithelium lines the airways of most of upper respiratory tract; pseudostratified nonciliated columnar epithelium lines larger ducts of many glands, epididymis, and part of male urethra.

Function: Secretion and movement of mucus by ciliary action

Stratified Epithelium

Stratified Squamous epithelium

Fig. 7: Stratified Squamous Epithelium

Description: Several layers of cells; cuboidal to columnar shape in deep layers; squamous cells form the apical layer and several layers deep to it; cells from the basal layer replace surface cells as they are lost.

Location: Keratinized variety forms superficial layer of skin; nonkeratinized variety lines wet surfaces, such as lining of the mouth, esophagus, part of larynx, part of pharynx, and vagina, and covers the tongue.

Function: Protection

Stratified Cuboidal Epithelium

Fig. 8: Stratified Cuboidal Epithelium

Description: Two or more layers of cells in which the cells in the apical layer are cube-shaped.

Location: Ducts of adult sweat glands and esophageal glands and part of male urethra.

Function: Protection and limited secretion and absorption.

Stratified Columnar Epithelium

Fig. 9: Stratified Columnar Epithelium
Description: Several layers of irregularly shaped cells; only the apical layer has columnar cells.
Location: Lines part of urethra, large excretory ducts of some glands, such as esophageal glands, small areas in anal mucous membrane, and part of the conjunctiva of the eye.
Function: Protection and secretion.

Transitional Epithelium

Fig. 10: Transitional Epithelium

Description: Appearance is variable (transitional); shape of cells in apical layer ranges from squamous (when stretched) to cuboidal (when relaxed).

Location: Lines urinary bladder and portions of ureters and urethra.

Function: Permits distension.

Glandular Epithelium

A gland is made up of a single cell or a collection of cells that secrete chemicals into tubes (ducts), onto surfaces, or into the blood. There are two types of glands in the body: endocrine and exocrine. 

Structural Classification of Exocrine Glands

Exocrine glands can be either unicellular or multicellular in nature. 

Unicellular glands are single-celled, as the name suggests. For example, goblet cells (secrete mucus). 

Multicellular glands consist of numerous cells that come together to form an individual microscopic structure or macroscopic organ. For example, Salivary, sebaceous, and sudoriferous glands.

Two criteria are used to categorise multicellular glands:
  1. Duct branching
  2. Structure of the gland's secretory parts

Simple Gland have unbranched duct whereas compound glands have duct branches.

Tubular glands have tubular secretory portion.

Acinar glands (alveolar glands) have rounded secretory portions. 

Tubuloacinar glands have both tubular and rounded secretory components.

Fig. 11: Exocrine Glands

The following structural classification scheme for multicellular exocrine glands is based on combinations of the above features:

I. Simple glands
    A. Simple tubular

    Tubular secretory part is straight and attaches to a single unbranched duct.        Example: glands in the large intestine.

    B. Simple branched tubular 

    Tubular secretory part is branched and attaches to a single unbranched duct.
    Example: gastric glands.

    C. Simple coiled tubular 

    Tubular secretory part is coiled and attaches to a single unbranched duct.         Example: sweat glands. 

    D. Simple acinar 

    Secretory portion is rounded and attaches to a single unbranched duct.              Example: glands of the penile urethra.

 E. Simple branched acinar 

    Rounded secretory part is branched and attaches to a single unbranched             duct.
    Example: sebaceous glands.

II. Compound glands
    A. Compound tubular

    Secretory portion is tubular and attaches to a branched duct. Example:                 bulbourethral (Cowper’s) glands.

    B. Compound acinar 

    Secretory portion is rounded and attaches to a branched duct. Example:             mammary glands.

    C. Compound tubuloacinar

    Secretory portion is both tubular and rounded and attaches to a branched         duct.
    Example: acinar glands of the pancreas.

Functional Classification of Exocrine Glands

This classification is based on the mechanism of release of secretion

Merocrine glands release secretion in vesicles via exocytosis. Examples include the salivary glands and pancreas.

Apocrine glands store the secretion at the apical surface and that portion is pinches off to release the secretion. For example, mammary glands

Holocrine glands rupture itself to release the secretion. For example, sebaceous gland


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