Objective

To become familiar with epithelial cells, their shapes and layering. To understand the role of the skin, its structure and some of its pathologies.

Abstract

Epithelial tissue has a number of functions including secretion, absorption, protection and transport. Epithelium can be categorised by shape (squamous, cuboidal, columnar) and layering (simple, stratified, pseudostratified).

Epithelial cells make up glands, being either exocrine (possess ducts) or endocrine glands (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 contiguous cells of varied embryonic origin 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.
• Protection
  
  • Specialised epithelium is able to protect internal tissues from the external environment e.g. skin.
• Transcellular Transport
  
  • Specialised epithelial cells can assist in transport of substances. This specialisation is usually on the apical domain (anterior surface) of the epithelial cells e.g.
    
    • Microvilli - Extend cell surface
    • Stereovilli - Long microvilli
    • Cilia - Motile processes that beat in synchrony to move particles

Epithelial cells are usually tightly packed together, joined by specialised cell-to-cell junctions. They contain little intercellular material between them when compared to other types of cells. The types of junctions joining epithelial cells are:

• Tight Junctions
  
  • Firmly adhere cells to form barrier that limits passage of molecules and ions. Block the movement of integral membrane proteins.
• Desmosomes
  
  • Firmly adhere and strengthen bonds – allow resistance to stretching & twisting due to interlocking proteins.
• Gap Junctions
  
  • Allow cells to communicate via connexon proteins – allow substance to pass between cells e.g. ions.

Epithelial cells rest atop a structure called the basement membrane. The basement membrane is a protein-polysaccharide membrane which separates the epithelium from the underlying connective tissue.

Epithelium is avascular as it is not in contact with any blood supply directly. Epithelial cells receive nutrients 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

Diagram: Table of 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
• Endocrine glands don’t have ducts

There are different structures of glands also, as shown below.

Diagram: Table of 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 and macrophages

Diagram: 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 through the prickle cell layer, losing their ability to divide, eventually forming the horny layer.
• Melanocytes
  
  • Also located in the basal layer.
  • Synthesise melanin, which is a pigment that darkens skin and protects against UV damage.
• Langerhans cells
  
  • o Antigen presenting cells that mediate immune responses.

Common Skin Conditions

Eczema

• A chronic condition 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.
• These scales normally appears on the elbows, knees, scalp, and lower back.
• 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: Thomas Burnell

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