Index
The skin covers the entire external surface of the human body and is the main site of interaction with the surrounding world.
Function
It serves as a protective barrier that prevents internal tissues from being exposed to trauma, ultraviolet (UV) radiation, extreme temperatures, toxins, and bacteria.
Other important functions include sensory perception, immune surveillance, thermoregulation, and control of loss of insensitive fluid.
The dermis is a layer of connective tissue that contains collagen and elastin fibers, fibroblasts, macrophages, and adipocytes, as well as nerves, glands, and hair follicles .
The integument consists of 2 mutually dependent layers, the epidermis and the dermis, which rest on a subcutaneous fatty layer, the adipose panniculus.
The dermis is the middle layer of the skin and is located just below the five layers of the epidermis.
The area of the basement membrane is the communication channel between the epidermis and the dermis.
The dermis supports the epidermis, provides it with nutrients and protects it.
The dermis constitutes about 90 percent of the skin, it is 1.5 to 4 mm thick.
Among its functions is to regulate the body’s temperature and provide the epidermis with blood with nutrients.
A large part of the water contained in the body is stored in the dermis.
For that reason the skin will show signs of dehydration long before the blood or any of the internal organs do.
When, on the contrary, the skin is too hydrated, this is called cutaneous edema and it is not caused by drinking too much water.
It is caused by water retention as a result of systemic conditions such as pregnancy in some women, kidney failure, heart failure, and even some diseases; or it may be the result of local conditions such as varicose veins, thrombophlebitis, insect bites, and dermatitis.
Cutaneous edema is known as “pitted” edema if, after pressure is applied to a small area of swelling, the cleft persists for a time after pressure is released.
Layers of the dermis
The dermis is made up of two layers:
The papillary dermis which is the portion of the dermis just below the epidermis and the reticular dermis that extends from the papillary dermis to the fat.
Underneath is the subcutaneous tissue, the shock absorber, and the energy storage and insulating layer.
The papillary layer
The papillary dermis is the region around the dermal papillae, which makes up about 20% of the dermis.
This layer contains loose connective tissue and has many capillaries.
The papillary layer is the top layer of the dermis and lies directly below the epidermis and connects to it through finger-like projections called papillae.
The papillary dermis is the upper portion below the epidermis, characterized by having a thin layer of randomly arranged collagen fibers.
When capillaries contract and expand, this controls the amount of blood that flows through the skin and determines whether the skin loses body heat when it is hot or whether it retains body heat when it is cold.
A double row of papillae on finger pads produces the striated fingerprints on the fingertips, as well as similar patterns on the palms of the hands and soles of the feet.
Fingerprints prevent skin from tearing and help grip objects.
Meissner’s corpuscles are found in the papillary layer, which are receptors for nerve endings. These nerve endings respond to touch and mechanical pressure.
They are responsible for the ability to feel a light touch and can perceive vibrations below 50Hz. They do not detect deep pressure or pain.
The number of Meissner corpuscles is reduced to a quarter of its original count between the ages of 12 and 50 years.
The reticular layer
The reticular layer is the lower layer of the dermis and lies directly below the papillary layer.
It is the deepest region, this is a layer of dense and irregular connective tissue, which contains collagen and elastin, fibroblasts, macrophages and fat cells.
The sweat glands are found deep in this region and in the hypodermis.
This layer is thicker than the papillary layer and more dense. The collagen bundles are woven into a thick network.
These dense collagen fibers are arranged parallel to the surface of the skin.
The papillary layer strengthens the skin, giving it structure and elasticity. It is, in fact, the primary location for dermal elastic fibers (elastin).
Elastic fibers are what give your skin the ability to return to its original shape after stretching or deforming.
Elastic fibers are capable of extending up to approximately twice their resting length.
Unlike collagen bundles, elastin fibers are wavy, as well as branched.
Elastin is notably absent from healing processes such as scars, keloids, and dermatofibromas, which explains its lack of flexibility.
Surprisingly, there is little loss in the number or function of the elastic fibers as a result of aging.
The loss of elasticity of the skin with aging is most likely the result of significant loss of thickness of the dermis, the loss of fibroblasts.
Cellular senescence (decreased biosynthetic activity of fibroblasts) and negative modifications of macromolecules in the extracellular matrix.
The cell matrix is the material that surrounds all cells in the reticular layer.
Fibroblasts are connective tissue cells that produce proteins such as collagen and elastin, as well as other macromolecules.
The reticular layer supports components of the skin, such as sweat glands, sebaceous glands, and hair follicles.
Structure of the dermis
Despite its limited functions, the dermis contains most of the skin’s specialized cells and structures, including blood vessels, lymphatic vessels, and hair follicles (including the small pili arrector muscles that make your hair stand on end ).
The structures of most interest are:
Collagen
It is the main protein of the dermis and 90% of the dermal fibers.
Collagen provides resistance to stress or forces that can deform the skin. Collagen fibers are cross-linked triple helix of polypeptide chains.
It is continuously synthesized by fibroblasts and degraded by collagenase. The bundles are parallel to the surface in the reticular dermis.
The most abundant collagens in the dermis are type 1 and type 3. They are also found in the walls of blood vessels and are selectively stained with silver salts.
Collagen holds the dermis and elastin together, giving it its ability to stretch without losing its shape.
Both are formed in fibroblastic cells found in the dermis, and the protein in both is similar.
Collagen is found throughout the body and is primarily located in connective tissues. Collagen keeps muscles and organs in place.
Collagen is a tough, insoluble protein. In the skin, collagen has the function of supporting the epidermis, which provides it with stability.
Unlike elastin, which does not decrease as you age, collagen levels in the skin decline, which contributes to the appearance of aging skin.
Elastic fibers
Elastic fibers comprise 10% of the fibers in the dermis. They return deformed skin to its resting state.
They are made of amorphous elastin protein and surrounded by microfibrils. The fibers are finest in the papillary dermis and thickest in the reticular dermis.
The extracellular matrix or ground substance
This is the medium where the connective fibers meet.
It is an amorphous viscoelastic gel made of anionic polysaccharides (glycosaminoglycans): hyaluronic acid, dermatan sulfate, and chondroitin-6-sulfate. It is produced and degraded by fibroblasts and mast cells.
These molecules have the function of capturing water and controlling the flow of solutes at the dermis level.
Fibroblasts
Fibroblasts are responsible for producing collagen, elastin (fundamental substance), hyaluronic acid and fibronectin (a glycoprotein).
They are the functional cells of the dermis, responsible for the formation and remodeling of tissue. Fibroblasts are the main cells of the dermis.
They are found in large quantities and are essential to achieve firmness, elasticity and density in the skin. The destruction of fibroblasts occurs automatically as the body ages.
Blood vesels
The arteries in the subcutaneous tissue send arborizing branches to the dermis.
Blood vessels are lined by endothelial cells, sheathed in collagen and surrounded by pericytes, dendritic macrophages, and T-lymphocytes.
The hypothalamic control produces its constriction and dilation with heat, emotional stress (blush with shame, anger, chill of fear) and spicy foods.
Lymphatic vessels
Lymphatic vessels are found mainly in loose connective tissue forming a wide network throughout the dermis.
The nerves
It is a network of sensory and autonomic nerve fibers. Composed of different sensory nerve endings for touch, which are those that cause itching, heat, cold, pressure and pain.
Nerve endings, if necessary, involuntarily contract and relax muscles to generate body heat.
Characteristics of the erector pili muscles
They originate near the basement membrane area and join the hair follicle near its base.
The erector pili muscles are smooth muscles.
They cause the erection of the hairs when exposed to cold or fear commonly called goose bumps.
It is best developed in the areola of the nipple and in the tunic dartos of the scrotum.
Immune cell characteristics
In the connective tissues of the dermis there are lymphocytes, neutrophils, monocytes (activated histiocytes and macrophages) and mast cells, arranged around blood vessels or dispersed in the dermis.
Which are capable of activating and eliminating various microorganisms, can also cause inflammation through the secretion of cytokines .
They can modify the tissue environment of a given area by initially destroying the tissue and then replacing the destroyed tissue with connective tissue.
Sweat glands
The sweat glands are small tubular structures that produce sweat. Sweat glands are classified as:
The eccrine glands
It is found throughout the body, but its density varies from region to region.
The sweat they produce is “generally” clear and odorless, consisting mainly of water and salt.
Eccrine sweat can sometimes have a very strong odor, as it can pick up odors from the food you eat (such as garlic) and when it purges toxins, such as fever and night sweats.
The eccrine glands are the primary form of temperature regulation.
These glands move water through the pores to the surface of the skin.
The apocrine glands
Apocrine glands are larger than eccrine glands and have a different secretion mechanism.
They are found only in the hairy areas of the body, especially in the armpits and pubic region.
These glands secrete an odorless, oily, and opaque secretion that takes on its characteristic odor from sweat from bacterial decomposition.
In humans, part of that milky secretion they produce is a cornucopia of pheromones.
Additionally, being sensitive to adrenaline, apocrine sweat glands in humans are involved in emotional sweating induced by anxiety, stress , fear, sexual stimulation, and pain.
Both apocrine and eccrine sweat glands secrete muscle cells that then sit between the gland’s cells and the underlying extracellular material.
It is the contraction of these muscle cells that compress the gland and discharge accumulated secretions.
The secretory activities of gland cells and myoepithelial muscle cell contractions are controlled by the autonomic nervous system and circulating hormones.
The sebaceous glands
These glands are found in all parts of the body, except for the palms of the hands and the soles of the feet.
These glands secrete a waxy oil known as sebum (from the Latin for “fat”) that protects against the overgrowth of microorganisms on the skin, such as fungi and bacteria, and helps keep the skin plump and supple.
These glands are located on the face and scalp.
Sebum is made from triglyceride oils, wax, and squalene (an oily liquid related to cholesterol).
Sebum is produced in specialized cells in the sweat glands and is released when these cells burst, a very different method of delivery from that found in the endocrine and exocrine glands that we have discussed above.
They are considered by most anatomists to be a subset of exocrine glands, but due to their unique delivery system, which involves the disintegration of the entire cell, the sebaceous glands are sometimes considered unique and later referred to as the holocrine glands, from the Greek ‘ whole”.
The sebum keeps hair and skin supple. Sebum is odorless, but its bacterial decomposition can produce odors.
Excess sebum is the reason why some people have “oily” hair. Men secrete slightly more sebum than women.
It is a natural oil that works to moisturize the skin and hair and prevent them from drying out.
In fact, sebum is essential for healthy hair and skin, which is one of the reasons why men tend to wrinkle less than women.
It is also one of the reasons why men tend to have more body hair and acne.
And there is another problem for men. Sebum tends to come loaded with dihydrotestosterone.
For those who are genetically sensitive to dihydrotestosterone, that translates to hair loss.
For those who are sensitive, then, a key step in managing the onset of hair loss is keeping sebum levels low.
The sebaceous glands are very important for another reason: they store and secrete a form of cholesterol called 7-dehydrocholesterol or provitamin D3.
The production of vitamin D in the body begins when ultraviolet light from the sun interacts with provitamin D3 that has been released in the dermis and epidermis.
The combination of sunlight and heat from the sun and the body causes a structural change in provitamin D3 that converts it to vitamin D3.
The D3 produced in the skin is picked up by the rich network of blood vessels that penetrate the dermis and diffuses into the bloodstream, where it is transported to the liver.
Your liver then converts it to 25-hydroxy-vitamin D3, which in turn is converted in your kidneys to 1,25-dihydroxy-vitamin D3, or calcitriol.
And calcitriol is the form of vitamin D that the body actually uses.
As you age, the ability of your sebaceous glands to produce provitamin D3, and therefore to produce vitamin D, decreases by more than 50%.
Skin circulation
The hypodermis is located below the dermis, and it mainly contains adipose tissue.
The arteries supplying the skin are deep in hypermique. The branches of the arteries pass upward to form a deep and a superficial plexus.
The deep cutaneous plexus is located at the dermal and hypodermic junction.
It supplies the fatty tissue of the hypodermis and the deeper parts of the dermis, including the capillaries of the hair follicles, the deep sebaceous glands and the sweat glands.
The superficial subpapillary plexus lies just below the dermal papillae and supplies the capillaries to the dermal papillae.
The pink color of the skin is mainly due to the blood seen in the venules of this plexus. There are many arteriovenous anastomoses in the dermis, which can prevent blood from entering the superficial cutaneous plexus.
This strategy is used in response to cold as a way to conserve heat. The danger is that if the epidermis loses its blood supply for a long time, hypothermia and death occurs.
Alternatively, in hot weather, more blood is allowed into the superficial plexus and the skin becomes red.
The blood in the superficial capillaries is cooled by the evaporation of sweat from the surface of the skin.
