Structure and Function of the Skin

Structure and Function of the Skin
By H Frayne

The 3 Functions of the Skin - Protection, Sensation & Regulation

The skin or integumentary system is the largest organ of the human body and is approximately 16% of a person's body weight. The skin's function is to keep our insides in and the outside world out (Protection). It protects us from heat, cold and injury. It is also a sensory organ that tells us when things are too cold or too hot, too sharp or too close and allows us to feel things with our fingers (Sensation).

The skin has mechanisms that help us to cool down and warm up. For example when the weather is cold, the blood vessels in our skin constrict to redirect blood to our vital organs, to keep them warm. Goosebumps help keep us warm by forming a layer of erect hair to retain heat. When the weather is warm, the blood vessels dilate or expand to send blood to the surface of the skin to cool down. Sweating, another function of the skin, also helps to cool us down. All of these mechanisms are a form of thermoregulation.

The skin consists of 3 major regions.

1. Epidermis

2. Dermis

3. Hypodermis or subcutaneous

The Epidermis

The Epidermis is made up of epithelial cells in 4 distinct layers over most of the body except the hands and feet which have an extra layer. The layers of the skin are:

Basal layer or Stratum Basale is the deepest layer of the epidermis. It is a single row of epithelial cells called Keratinocytes which are constantly dividing and sending new cells up into the next layer. This layer contains melanocytes and merkel cells.

The next layer is the spinous layer or Stratum Spinosum. It is the thickest layer of the epidermis and here the keratinocytes spread out and lock in to all the other keratinocytes creating a kind of patchwork quilt of oddly shaped cells. As many of the joins are at sharp angles, this is nicknamed the spiny layer. Melanin granules and Langerhan's cells exist in this layer.

The Stratum Granulosum or Granular layer is the next layer towards the surface. This layer is less dense than the Spiny Layer as the cells flatten out and become more compact. The Granular layer is where fibres called keratin filaments start to gather together and lipids (fats) accumulate to prepare the layer to fulfil its job of protecting the body. It is at this point that the cells is no longer living.

The Stratum Lucidum or clear layer is only present on the palms of the hands and soles of the feet. It is made up of dead cells that look clear under a microscope. It gives an extra layer of protection and flexibility to areas of most friction.

The Stratum Corneum is the thick, outermost layer of the skin and consists of dead, keratinatised cells called corneocytes. The cells are bound together with connectors called desmosomes and their function is to protect the deeper layers from water and injury. The Stratum Corneum is constantly sloughing off its outer cells while being replenished from below.

The Dermis

The dermis is connective tissue. Connective tissue is a supporting framework. Its main components are Collagen, Elastin and Ground Substance.

The Dermis supports and strengthens the epidermis and is made up of connective tissue. It has 2 layers. They are not clearly defined as in the epidermis but rather a continuum, from the papillary dermis near the epidermis, to the reticular dermis below it, which then merges with the subcutaneous tissue.

Layers of the Dermis

The Papillary Dermis
The Reticular Dermis

The Papillary Dermis

The Papillary Dermis is the thin upper layer closest to the epidermis. It is called the papillary dermis because it projects papillae (nipple-like structures) into the epidermis. This fixes the dermis to the epidermis so they don't slide over each other. The Papillary Dermis contains Capillaries - which provide nutrients to and remove waste from the cells of the epidermis.

The Reticular Dermis

The Reticular Dermis is the largest part of the dermis and it contains many structures such as hair follicles, sebaceous glands, sweat glands, nerves, blood vessels, lymph vessels, muscles and other glands. This layer gives strength and resilience to the skin because of the presence of a scaffolding of Collagen and Elastin fibres in a kind of syrup called Ground Substance.

The Hypodermis or subcutaneous tissue

The Hypodermis is the tissue that lies under the dermis. It is mainly made of adipose tissue (fat), connective tissue and blood vessels but many of the structures of the skin such as hair follicles, glands, nerves and muscles extend to this area. The hypodermis anchors the dermis to the underlying fascia (layers that surround body structures such as bones and muscles). The reticular dermis blends into the hypodermis rather than the two being well defined separate layers.

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