The Dermis
The structure of the dermis
The dermis is the largest/thickest layer of the skin also known as the ‘true skin’. It is composed of areolar connective tissue and contains the protein collagen, elastin fibres, fibroblasts, macrophages, mast cells and an enzyme, numerous blood vessels, lymph vessels, nerves, sweat glands, hair follicles, and arrector pili muscles and is bathed in water. The dermis is the thinnest on the eyelids, palms of hands, and soles of the feet and thicker in men than women.
Functions of the dermis
- giving protection to the body from bumps and knocks
- removing waste products of metabolism from the epidermis, which are also carried away in the blood
- providing shape and form to the body, by holding all its structures together
- contributing to skin colour, particularly in people with little melanin in the epidermis
- regulation of body temperature through control of blood flow and sweating
- skin sensations of touch, pain, heat and cold through nerve endings
The dermis is composed of two layers called the papillary and reticular layers
Papillary layer
Lies directly under the germinative layer and nourishes the epidermis. It contains small cone-shaped projections called papillae which contain looped blood capillaries. Others contain tactile corpuscles and nerve endings.
The reticular layer
This layer contains fat cells, blood and lymph vessels, sweat glands, hair follicles and nerve endings. The larger of the two layers contains cells called fibroblasts, which produce the yellow elastin fibres, which make up 4% of connective tissue and give the skin its flexibility and ability to recoil. White protein collagen fibres give the skin its strength and make up 75% of connective tissue. Reticular fibres run between the dermis fibres and structures , also known as appendages of the skin, helping to support them and hold them in situ.
There are also specialised cells within this layer, which are called macrophages and mast cells.
Macrophages destroy bacteria and tissue debris, whilst mast cells secrete histamine, a substance, which enlarges small blood vessels and heparin, an anti-coagulant that stops the blood clotting.
There are three kinds of fibres that intermingle with the cells of the dermis:
Collagen fibres: These make up almost 78% of the dry weight of the skin.
Reticulum fibres: These form fine branching patterns in the connective tissue helping to link the bundles of collagen fibres.
Elastin: This is contained within the collagen fibres. This protein has elastic qualities and helps to give the skin its resilience.
Skin appendages
Within the dermis are structures called skin appendages. These include:
- sebaceous glands
- sweat glands
- hair follicles
The sebaceous glands are attached to the hair follicles. They secrete sebum (oil) to keep skin and hair soft and moist.
The sudoriferous glands/sweat glands secrete sweat to regulate body temperature. This sweat controls the body’s temperature by evaporating and cooling the skin surface. Both sebum and sweat contribute to the skin’s protective ‘acid mantle’ which defends the skin against any bacterial invasion.
Hair in its follicle can be found on every part of the body (except palms and soles). This helps maintain body temperature through the arrector pili muscle, which traps air between the hair shaft and skin for insulation. The hair shaft itself offers additional protection to the skin surface by acting as a buffer, while the sebum secreted from the sebaceous gland via the follicle opening contributes to the skin’s protective acid mantle.
The dermis is a dense meshwork of connective tissue, collagen and elastin fibres. These two connecting proteins offer stability whilst giving the skin its plump elastic tendency. This meshwork supports tiny lymph and blood vessels that allow the skin to breathe and be nourished, as well removal of waste and toxins from the tissues.
