Integumentary System 1: Introduction and the epidermis

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What is the integumentary system?

Basically, the skin! The integumentary system comprises a cutaneous membrane (skin), as well as accessory structures including hair, nails and exocrine glands. The skin has several functions:
  • Protects underlying tissues and organs
  • Excretes salts, water and organic wastes
  • Maintains body temperature (insulation and evaporation)
  • Synthesises vitamin D3
  • Detects touch, pressure, pain and temperature
The skin is made up of three general layers:
  1. The epidermis: outer, thinner layer composed of epithelium (4-5 layers)
  2. The dermis: inner, thicker layer composed of connective tissue (2 layers)
  3. The hypodermis (subcutaneous) layer: fat store with blood vessels and Pacinian corpuscles
Below is a summary of the layers of the epidermis:

Stratum corneum: dead keratinocytes arranged in multiple layers which are continually shed and replaced from deeper strata cells, water resistant

Stratum lucidum: exists in thick skin only (subjected to friction) and is composed of dead keratinocytes

Stratum granulosum: keratin formation through keratinocyte apoptosis, lamellar granules release waterproofing secretions

Stratum spinosum: keratinocytes held together by desmosomes, dendritic cells (Langerhans), each time a cell divides a daughter cell is pushed superficial to the stratum spinosusm

Stratum basale/germinativum: stem cells that undergo continuous cell division (deepest layer)


The cells of the epidermis are described next:

Keratinocytes: produce keratin, which protects and adheres cells, and is a water proofing sealant

Melanocytes: produce melanin (skin pigment)

Langerhans cells: macrophages involved in immune response

Merkel cells: assist in touch and feel senses

How does our skin vary in colour?

Skin pigmentation is a result of carotene and melanin, as well as the circulation of red blood cells. 

Carotene is an orange-yellow pigment that accumulates within epithelial cells and fatty tissues of the dermis. It can be converted into vitamin A (retinal). It is found in carrots and effects eyesight. 

Melanin is a yellow-black pigment produced by melanosomes in melanocytes in the stratum basale. Melanin is transported in vesicles to keratinocytes, which then surround the nucleus of keratinocytes and protect from sun damage.

How is vitamin D synthesised?

Vitamin D is ingested through sunlight and food.
  1. Steroid compound in the skin helps convert 7-dehydrocholesterol into cholecalciferol
  2. Cholecalciferol is transported to the liver
  3. Cholecalciferol becomes an intermediary product at the liver → calcitriol
  4. Calcitriol stimulates calcium and phosphate ion absorption due to low blood calcium levels

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