Anatomy
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Structure of Skin

Structure of Skin

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The Skin: Functions, Gross Structure, and Ultrastructure

The skin is the largest organ in the human body and comprises approximately 8% of total body mass. It is a versatile structure with a wide range of functions, and its exact composition varies across different regions of the body’s surface. In this article, we will discuss the function, gross structure and ultrastructure of our skin.

Functions of Skin

The skin provides an essential barrier between the external environment and internal body contents. It protects against mechanical, chemical, osmotic, thermal, and UV damage, as well as microbial invasion. Its other functions include:

  • A role in the synthesis of vitamin D
  • Regulation of body temperature
  • Psychosexual communication
  • A major sensory organ for touch, temperature, pain, and other stimuli

Gross Structure

The composition of skin varies across the surface of the body. It can be thin and hairless, hair-covered soft or stiff, or any combination thereof. Glabrous skin is the thick skin found over the palms, soles of the feet and flexor surfaces of the fingers that is free from hair. Throughout the body, skin is composed of three layers; the epidermis, dermis and hypodermis.

Ultrastructure

Epidermis

The epidermis is the most superficial layer of the skin, and is largely formed by layers of keratinocytes undergoing terminal maturation. This involves increased keratin production and migration toward the external surface, a process termed cornification. There are also several non-keratinocyte cells that inhabit the epidermis: melanocytes, Langerhans cells, and Merkel cells.

The epidermis can be divided into layers (strata) of keratinocytes, which reflects their change in structure and properties as they migrate towards the surface. From deepest to most superficial, these layers are:

  • Stratum basale – mitosis of keratinocytes occurs in this layer
  • Stratum spinosum – keratinocytes are joined by tight intercellular junctions called desmosomes
  • Stratum granulosum – cells secrete lipids and other waterproofing molecules in this layer
  • Stratum lucidum – cells lose nuclei and drastically increase keratin production
  • Stratum corneum – cells lose all organelles, continue to produce keratin

A keratinocyte typically takes between 30 – 40 days to travel from the stratum basale to the stratum corneum.

Dermis

The dermis is immediately deep to the epidermis and is tightly connected to it through a highly-corrugated dermo-epidermal junction. The dermis has only two layers, which are less clearly defined than the layers of the epidermis. They are the superficial papillary layer, and the deeper reticular layer. The reticular layer is considerably thicker, and features thicker bundles of collagen fibres that provide more durability.

The following cell types and structures can be found in the dermis:

  • Fibroblasts – these cells synthesise the extracellular matrix, which is predominantly composed of collagen and elastin
  • Mast cells – these are histamine granule-containing cells of the innate immune system
  • Blood vessels and cutaneous sensory nerves
  • Skin appendages – e.g. hair follicles, nails, sebaceous and sweat glands

The skin is an organ that performs an incredible variety of functions and is composed of fascinating layers. Understanding our skin is key to maintaining its health and keeping it functioning properly. In this article, we discussed the function, gross structure, and ultrastructure of the skin.

Layers of the Epidermis

The epidermis is the outermost layer of the human skin that serves multiple functions and consists of two main layers, the epidermis and the dermis. The epidermis is divided into four layers called the stratum basale, stratum spinosum, stratum granulosum, and stratum lucidum, with the stratum corneum being the topmost layer, comprised of dead, keratinized cells. Each layer of the epidermis is composed of keratinocytes, which are cells that continually migrate and change in structure and properties as they move toward the surface. It takes around 30-40 days for a keratinocyte to travel from the stratum basale to the stratum corneum.

The dermis is the inner layer of human skin and is connected to the epidermis through the highly-corrugated dermo-epidermal junction. It is further divided into a superficial papillary layer and a deeper reticular layer, the latter being thicker, stronger due to thicker bundles of collagen fibres. The dermis contains many structures and cells that give it its durability such as fibroblasts, mast cells, blood vessels, cutaneous sensory nerves, hair follicles, sebaceous glands, and sweat glands. Just below the dermis lies the hypodermis, or subcutaneous tissue, which is a major body store of adipose tissue and helps connect the skin to underlying tissue and organs.

The epidermis is populated with cells such as melanocytes, keratinocytes, Langerhans cells, and Merkel cells which collectively contribute to the skin's structure and function. Clinically, disorders of the skin such as alopecia areata, vitiligo, and psoriasis can occur when the structure and function of the skin is compromised.

In conclusion, the human skin is a complex organ system with multiple layers, each layer possessing its own unique structure and function. The epidermis is composed of four layers, with the stratum corneum being the topmost layer and the dermis is composed of two layers, namely the papillary layer and the reticular layer. Finally, the hypodermis is the innermost layer of skin and provides a connection to underlying tissue and organs.

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