Skin - Dermis and Hypodermis
Dermis
The dermis is located beneath the epidermis and is the thickest layer, ranging from 1.5 to 4 mm. It is composed of an interconnected mesh of elastin and collagen fibres, which are manufactured by cells called fibroblasts.
The main functions of the dermis is to regulate the temperature of the body , to supply the epidermis with nutrients and to store a large amount of the body’s water supply.
It is made up of two sublayers of connective tissue called :
1. The Papillary Layer
This is made up of loose connective tissue supported by the collagen and elastin fibres that form a mesh and projects into the stratum basale (bottom layer) of the epidermis and forms dermal papillae or finger like projections. They contain fibroblasts, fat cells or adipocytes, blood and lymphatic vessels and touch receptors responsible for nervous system sensations called Meissner corpuscles.
The papillae also contain cells called phagocytes which are body defensive cells to fight against invading bacteria or other infections that breach the upper skin layers.
Dermal papillae form ridges on the surface of the skin, especially on the palms, digits, soles and toes. These ridges occur in a pattern called dermatoglyphs, forming whorls, arch or loop patterns, which form an individual’s unique fingerprint, moulded by both genetic and environmental factors. This is used in forensics for identification of people, as no two fingerprints are alike.
2. The Reticular Layer
This is the thicker layer of the dermis and is made up of collagen fibres that are parallel to the skin’s surface and provide strength, structure and elasticity to the skin, while supporting integuments or functioning components of the skin.
The dermis contains specialised skin structures like:
a. Blood vessels : The epidermis is an avascular layer and is dependent on the blood vessels arising from the dermis for oxygen and nutrients. It also transports waste material produced by the skin cells away from the area for disposal and acts as a conduit for metabolised vitamin d3 from the epidermis to the various ither parts of the body.
b. Lymph vessels : These carry lymph fluid into the connective tissue of the dermis in order to provide immune mediated cellular responses to invading bacteria that may cause infections in the skin.
c. Hair follicles : These surround the bottom part of each individual shaft of hair like a sheath. It nourishes the root of the hair. They extend from the deep dermal tissue to the border of the skin and are attached to the skin by the arrector pili muscles and the sebaceous glands.
d. Sweat glands: An average person has approximately 3 million sweat glands which are classified as Apocrine and Eccrine glands. Apocrine glands are found in the armpits and the genital area. They secrete sweat that encourages the growth of bacteria responsible for body odour. Eccrine glands are considered to be the true sweat glands, that can be found in all body regions. They regulate body temperature by bringing water or sweat through the pores on the skin surface, which evaporates in hot surroundings, thereby decreasing the temperature of the skin. The sweat released by the eccrine glands contain a large amount of water, with some glands producing approximately one to two litres of sweat an hour. Since the composition is mainly water it does not encourage the growth of bacteria responsible for odours or infections.
e. Sebaceous glands are called oil glands because they are attached to hair follicles and secrete oil that keeps the skin supple and waterproof, reducing the chances of bacterial or fungal infections . They are found everywhere except on the soles of the feet and palms of the hands.
f. Nerve endings for pain and touch receptors found in the dermis transmit stimuli received on the skin like pain, temperature, pressure or itching sensations to the brain, as sensory stimuli. If the body temperature is too low, it produces a motor response from the brain to initiate shivering, which due to the involuntary contraction and relaxation of the muscles, increases body heat. In addition, the arrector pili muscle which is attached to hair follicles on one end and dermal tissue on the other end, contract all together causing the hair to stand up straight. This appears as gooseflesh, due to the bumps that appear in the skin. The phenomenon often accompanies shivering
An interesting fact about the dermis of the skin is that permanent tattoos do not stay on the skin forever due to the quality of the ink used. Instead, it is because of the immune response of the skin. Getting a tattoo involves delivering multiple small pricks with a needle into the dermis of the skin. This creates wounds, which the body tries to repair by sending white blood cells called macrophages, that rush to the body’s defence. These cells engulf the tattoo ink in an effort to protect the body from the foreign invading material. The ink overpowers the macrophages, so these cells with the engulfed pigments get trapped in the dermis, through its lifetime. When they die they pass on the pigment to new macrophages, which results in the tattoo sticking around for a long time, unless laser surgery is done to remove the trapped pigment laden macrophages.
Subcutaneous Layer
The innermost layer of the skin is the subcutis, also called the hypodermis and it contains a network of collagen and fat. This acts as an insulating material that preserves body temperature as well as storing fat for energy reserves and protection of the inner organs through shock absorption. Additional skin integuments like blood vessels, nerves, lymphatics and hair follicles also cross through this layer. The thickness of the subcutis differs between individuals.
References
1. Holick MF, Smith E, Pincus S. Skin as the site of vitamin D synthesis and target tissue for 1,25-dihydroxyvitamin D3. Use of calcitriol (1,25-dihydroxyvitamin D3) for treatment of psoriasis. Arch Dermatol. 1987;123(12):1677-1683a.
2. https://www.news-medical.net/health/What-is-the-Dermis.aspx
3. https://emedicine.medscape.com/article/1294744-overview#a1
4. https://www.britannica.com/science/dermal-papilla
https://www.sciencedaily.com/releases/2018/03/180306101708.htm