Contents

1. Introduction to Epithelia
2. Types of Epithelia
3. Glands
4. Skin
   
   • Cell Types in the Skin
5. Common Skin Conditions
6. Quiz

Abstract

• Epithelial tissue has a number of functions including secretion, absorption, protection and transport.
• Epithelium can be categorised by shape (squamous, cuboidal and columnar) and layering (simple, stratified, pseudostratified and transitional).
• Epithelial cells make up glands, being either exocrine which possess ducts, or endocrine glands which are ductless.
• The skin is the body’s largest organ and is made up of three layers - epidermis, dermis, hypodermis.

Core

Epithelium is one of four basic tissue types in the human body. Epithelial sheets are made up of contiguous cells that cover the external surface of the body, line cavities and form glands.

Epithelium has several functions, including:

• Secretion
  
  • Epithelium lines areas that have a high level of secretion, such as the gastrointestinal tract.
• Selective Absorption
  
  • Epithelium is the first layer of cells in direct contact with materials which are absorbed by the body, e.g. intestinal epithelium. Epithelium can have specialised channels and transporters in the cell surface membrane to allow selective absorption.
• Protection
  
  • Specialised epithelium can protect internal tissues from the external environment by acting as a barrier e.g. skin.
• Specialised Transport
  
  • Some epithelial cells have a specialised cell membrane usually on the apical domain (the side facing into the lumen) that assists in the transport of substances e.g.
    
    • Microvilli - extend cell surface to allow more absorption to take place e.g. in the small intestine
    • Stereovilli - long microvilli e.g. in the inner ear
    • Cilia - motile processes that beat in synchrony to move particles e.g. in the bronchi

Basic Structure of the Epithelial Sheet

Epithelial cells are tightly packed together, joined to each other by specialised cell-to-cell junctions. The types of cell connections can assist with the function of the epithelium. The types of junctions joining epithelial cells are:

• Tight Junctions
  
  • Firmly adhere cells to form and impermeable barrier that limits passage of molecules and ions and block the movement of integral membrane proteins.
  • Allows tight control of the movement of substances across the epithelial layer e.g. in the nephron.
• Desmosomes
  
  • Firmly locks cells together
  • Allows resistance to stretching & twisting due to interlocking proteins.
• Gap Junctions
  
  • Connexon proteins run between cells and form a channel between them
  • Allows substance to pass between cells e.g. ions.

Epithelial cells rest atop a structure called the basement membrane and are joined to it by proteins called hemidesmosomes. The basement membrane is a thin protein-polysaccharide membrane which separates the epithelium from the underlying connective tissue and acts as an anchor for the epithelial cells.

Epithelium is avascular as it is not in contact with any blood supply directly. Epithelial cells receive nutrients and oxygen by diffusion through the environment or connective tissue.

Types of Epithelia

Epithelial tissue can be categorised by the epithelial cell shape and the type of layering which is present.

• Cell shape
  
  • Squamous (flat)
  • Cuboidal
  • Columnar
• Layers
  
  • Simple (one layer)
  • Stratified (multiple layers)
  • Pseudostratified (literally “false layers”)
    
    • The cells appear to be layered but are all in contact with the basement membrane
  • Transitional
    • Allows the cells to slide across one another. Transitional epithelium is found in the bladder as it allows the bladder to stretch as it fills with urine.

Table - The different types of epithelial tissue

Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Glands

A gland is an epithelial cell or collection of cells specialised for secretion. Secretion can be into the bloodstream or on to an epithelial surface. Glands can be classed by the presence or absence of ducts.

• Exocrine glands possess ducts e.g. the pancreas
• Endocrine glands don’t have ducts e.g. the pituitary gland

There are different structures of exocrine glands as shown below.

Table - The different types of glands

Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

Skin

The skin is the largest organ of the body and has a number of functions. It acts as a physical barrier to the external environment, protecting against infection, physical damage and chemical damage. It also prevents excessive fluids from leaving the body and assists in temperature regulation. Skin can be split into three different layers:

• Epidermis (outermost) which itself is made up of four layers:
  
  • Horny layer (stratum corneum)
  • Granular layer (stratum granulosum)
  • Prickle cell layer (stratum spinosum)
  • Basal layer (stratum basale)
• Dermis (middle)
  • Contains blood vessels and excretory glands
• Hypodermis (innermost)
  
  • Contains adipose tissue and macrophages

Diagram - The structure of the skin

Creative commons source by OpenStax College [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]

There are a multitude of different cell types within the skin:

• Keratinocytes
  
  • Synthesise keratin, a fibrous protein that gives the epidermis strength and is the main constituent of hair and nails.
  • They undergo mitosis in the basal layer and then the daughter cells move up through the prickle cell layer. As they migrate, keratinocytes lose their ability to divide and lose their nuclei, eventually forming the horny layer. This layer provides a hydrophobic barrier for the skin due to the amount of keratin.
• Melanocytes
  
  • Also located in the basal layer of the epidermis
  • Synthesise melanin, which is a pigment that darkens skin and protects against UV damage.
• Langerhans cells
  
  • Antigen presenting cells that circulate between the epidermis and local lymph mediate immune responses.

Common Skin Conditions

Atopic Eczema

• A chronic hypersensitivity reaction that causes the skin to become itchy, red, dry and cracked. It can affect any part of the body.
• Although it is chronic, it can improve or disappear in young children. 
• Primarily genetic and can be aggravated by allergens, heat and sweating.

Psoriasis

• Psoriasis is a skin condition that causes red, flaky, crusty patches of skin covered with silvery scales which normally affecting the elbows, knees, scalp, and lower back.
• The main features are proliferation of keratinocytes, inflammation and vascular changes in the skin.
• There are genetic immune factors at play, however, symptoms can follow trauma, infection and alcohol consumption.

Acne Vulgaris

• Acne vulgaris is a chronic skin disease involving the blockage of hair follicles and their accompanying sebaceous gland.
• Acne can present as non-inflammatory lesions, inflammatory lesions or a mixture of both, affecting mostly the face but also the back and chest.
• Acne can be caused by hormonal changes, inflammation and by bacterial colonisation of the affected areas.

Edited by: Bethany Turner

Reviewed by: Thomas Burnell

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