Epithelium, Skin and Exocrine Glands

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.

Layers of the Skin 

Epidermis is the most superficial (outermost) layer of the skin. In most skin it has four layers of cells called keratinocytes (cells that make keratin, a tough, fibrous protein). The thick skin of the palms and soles has a fifth layer, the stratum lucidum (a clear layer between the granular and horny layers). Keratinocytes are produced in the deepest layer of the epidermis (basal layer), then migrate up to the outer surface of the skin in a process called cornification (the cells fill with keratin and lose their nuclei, forming the tough outer layer).

It takes a keratinocyte roughly 28-40 days to complete its journey from the basal layer (stratum basale) to the horny layer (stratum corneum). There are some conditions that reduce the time taken for this journey. For example, in psoriasis, keratinocyte turnover is greatly increased. This leads to epidermal thickening and silvery scales on the surface of the skin.

The four layers of the epidermis are: 

• Horny layer (stratum corneum) - the keratinocytes have lost their nuclei and all their other organelles and contain keratin. This layer is thicker in sites such as the palms of the hands and the soles of the feet. It is thin on the lips, where it has only three to five layers of cells.
• Granular layer (stratum granulosum) - keratinocytes secrete waterproofing molecules such as lipids. These are essential for forming a protective barrier and keep in moisture. 
• Prickle cell layer (stratum spinosum) - desmosomes (anchoring intercellular junctions) join the keratinocytes together tightly.
• Basal layer (stratum basale) - this layer is responsible for the mitosis of keratinocytes.

Dermis (middle)

The middle layer of the skin is called dermis and this layer is made of collagen, elastin, and other connective tissues. These components are integral for providing flexibility and strength to skin to protect against external forces. In comparison to the epidermis, the dermis is thicker, more elastic, and firm.

There are a number of different structures and cell types found in this layer of the skin:

• Blood vessels and cutaneous (skin) sensory nerves
• Mast cells - cells that contain histamine (a chemical released during allergic reactions and inflammation) which is important for the innate immune response.
• Fibroblasts - these cells are responsible for producing extracellular matrix, which includes collagen and elastin.

A pilosebaceous unit is the combination of a hair follicle and sebaceous gland. These units are only found on hairy skin (hirsute skin). A sebaceous gland produces a waxy or oily material into the hair follicle, which then travels onto the skin to lubricate the surface of the skin and hair.

There are two main types of sweat glands found in the dermis:

• Apocrine gland - large sweat gland typically found in the genital and axillary (armpit) region. The products synthesised by these glands can be metabolised by microbes on the surface of the skin, which produces an odour.
• Eccrine gland - these sweat glands are found all over the body and secrete an odourless, clear substance. The secretion contains mainly water and sodium chloride. It is important in the regulation of temperature.

Hypodermis (innermost)

An alternative name for the hypodermis is subcutaneous tissue. A major store of the body’s adipose (fat) tissue is found in this layer. The size of the layer varies between individuals, depending on the amount of fat tissue present. The layer is highly vascularised.

This layer is important for storing energy reserves, acting as an attachment site for muscles below plus providing a smooth and contoured appearance to skin.

Medications such as insulin and low molecular weight heparin (an anticoagulant, a medicine that reduces blood clotting) can be given by subcutaneous injection into this layer. Adrenaline for anaphylaxis is not given this way: it is injected into muscle (intramuscularly), usually into the thigh.

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 (the new cells made when a cell divides) 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 (water-repelling) barrier for the skin. The barrier depends mainly on lipids (fats) in the gaps between the cells, while the keratin gives the cells strength.
• Melanocytes
  
  • Also located in the basal layer of the epidermis
  • Synthesise melanin, which is a pigment that darkens skin and protects against damage from ultraviolet (UV) light.
  • Individuals with darker skin have the same number of melanocytes as individuals with lighter skin, but each melanocyte produces a higher amount of melanin. 
• Langerhans cells
  
  • Antigen presenting cells (cells that show antigens to other immune cells) that sit in the epidermis. They can migrate to the local lymph nodes, where they present antigen and help to direct immune responses.

Common Skin Conditions

Atopic Eczema

• A chronic inflammatory skin condition that causes the skin to become itchy, dry and cracked. It is called atopic because it is linked with other allergy-prone conditions such as asthma and hay fever. The affected skin can look red, white, purple or grey, or lighter or darker than the skin around it, depending on skin tone. It can affect any part of the body.
• Although it is chronic, it often improves with age. It clears in about 65% of children by the age of 7 and about 74% by the age of 16, although it can come back. 
• There is no single known cause: genes, the immune system and the environment all play a part. It often runs in families, along with asthma and hay fever. It can be made worse by irritants (such as soaps and detergents), allergens (substances that trigger an allergic reaction), 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 (a rapid increase in their number), inflammation and vascular changes (changes in the blood vessels) in the skin.
• Genetic and immune factors both play a part. Symptoms can be triggered or made worse by injury to the skin (such as cuts, scratches or grazes), infections (such as strep A), certain medicines, stress, smoking and alcohol.

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 (areas of damaged skin, such as blackheads and whiteheads), inflammatory lesions (such as papules, which are small raised spots, and pustules, which are spots containing pus) or a mixture of both. It affects mostly the face but also the back and chest.
• Acne is thought to involve several processes: the hair follicle becomes blocked by a plug of skin cells, androgens (hormones such as testosterone) increase the amount of sebum (an oily secretion) made around puberty, bacteria such as Cutibacterium acnes multiply in the sebum, and the follicle becomes inflamed.

Edited by: Dr. Maddie Swannack

Reviewed by: Dr. Thomas Burnell and Dr. Marcus Judge