Functions
• The skin is one of the largest organs with up to 16% of the body weight and accomplishing uncountable functions:
- It protects the organism against water loss and abrasion thanks to the stratum corneum of the epidermis
- It constantly receives information on the surrounding environment and sends this to the CNS through sensitive nervous ends.
- It cooperates with the thermal regulation of the body through blood vessels, glands and adipous tissue
- The melanin (pigment produced and accumulated in the epidermis) protects against ultraviolet rays
- In the skin, vitamin D3 is formed by the sun’s ultraviolet rays on the precursors synthesized in the organism.

>> The junction between the epidermis and dermis is irregular, the dermis presents projections, the dermal papillae, that fit in the incurvatures of the epidermis, increasing the cohesion between these two layers.

Epidermis

• Formed by keratinized stratified squamous epithelium

Cell types

Keratinocytes
• The most abundant, each cell that forms the epithelium is a keratinocyte

Melanocytes
• Produce the melanin pigment

Langerhans cells
• Antigen-presenting cells, located throughout the entire epidermis, between the keratinocytes.

Merkel’s cells
• Mechanoreceptors located in the deepest region of the epidermis, supported by the basal lamina and bound to the keratinocytes through the desmosomes

Layers of the Epidermis (Thick skin)

Stratum Granulosum
• With only 3-5 lines of polygonal flattened cells with a central nucleus and a cytoplasm rich in basophilic granules called keratohyalin granules.
• The keratohyalin granules are not surrounded by a membrane
• They present lamellar granules, visualized only under electronic microscopy, that contain lamellar discs formed by lipid bilayers and are surrounded by a membrane. These granules fuse themselves with the plasma membrane and release their contents into the intercellular space of the granular layer, where the lipid material is deposited and will contribute to the formation of a barrier against the penetration of substances and turning the skin impermeable to water and avoiding dehydration of the organism.
• The waterproofing agent appeared in reptiles first and is important under an evolutionary point of view so to allow life outside the water.

Stratum Lucidum
• The most evident layer of the thick skin, formed by a thin layer of flattened, eosinophilic and translucid cells whose nuclei and cytoplasmic organelles were digested by enzymes from the lysosomes and disappeared.
• Its cytoplasm presents numerous compacted keratin filaments and it is also possible to see desmosomes between the cells.

Stratum Corneum
• It has a variable thickness, formed by flattened, dead and nonnucleated cells
• The cytoplasm of these cells is filled with keratin
• The tonofilaments agglutinate along with the matrix formed by the keratohyalin granules. At this stage of differentiation, the keratinocytes are transformed into lifeless plates and continuously shed.

Layers of the Epidermis (Thin skin): the epidermis is simple, frequently lacking the stratum granulosum and lucidum, and present a much smaller stratum corneum.

Melanocytes
• The color of the skin results from various factors of which the most important are: content of melanin and carotene, and the amount of capillaries in the dermis and the color of the blood in theses capillaries.
• They present a globular cytoplasm that originates extensions that penetrate the incurvatures of cells of the stratum basale and stratum spinosum and transfer melanin granules to the cells of these strata.
• The melanocytes do not form demosomes with the keratinocytes, but attach themselves to the basement membrane through hemidesmosomes.
• Once formed, the melanin granules migrate through the extensions of the melanocytes and are injected through yet unclear mechanisms into the cytoplasm of the keratinocytes that function as deposits of melanin and contain a greater amount of this pigment than the melanocytes.
• The melanin granules fuse with the lysosomes of the keratinocytes and that is why the more superficial cells of the epidermis do not contain melanin.
• In the epithelial cells the melanin granules are in a supranuclear position, offering maximum protection to the DNA against the hazardous effects of sun radiation.
• The genetic load influences the rate of activity of the melanocytes since almost everyone has the same amount.
• The darkening of the skin due to solar exposition occurs initially due to the darkening of the pre-existent melanin and the acceleration of the melanin transference to the keratinocytes. On a second stage, the synthesis of melanin is increased.

• Melanin is a dark-brown pigment produced by the melanocytes that are found in the junction between the dermis and epidermis or between the keratinocytes of the stratum basale of the epidermis.

Dermis
• It is the connective tissue where the epidermis rests upon, and that binds the skin to the subcutaneous tissue or hypodermis.
• The dermis presents a varying thickness and its outer surface is irregular, presenting saliencies, the dermal papillae, that accompany the corresponding invaginations of the epidermis.
• The papillae increase the contact area of the dermis with the epidermis strengthening the link between these two layers, the papillae are most frequently found in the zones subject to aggressions and abrasions.
• The dermis is composed by two layers with slightly distinct limits : The superficial Papillary Dermis and the deeper Reticular Dermis.
• The papillary layer is thin, composed of loose connective tissue that forms the dermal papillae. In this layer, special collagen fibrils were described, that insert themselves on one side into the basement membrane and on the other side penetrate deeply into the dermis, forming the dense connective tissue.
• The reticular layer is thick, composed of dense connective tissue.
• Both layers contain many fibers of the elastic system, partly responsible for the elasticity of the skin.
• Asides the blood and lymph vessels, and the nerves, the following structures derived from the epidermis are found in the dermis: hair follicles, sebaceous glands and sweat glands.

Sensory receptors of the Skin

Pacinian Corpuscles
• They capture special vibratory and tactile stimuli
• They are formed by a nervous fiber whose unmyelinated end portion is surrounded by many layers that correspond to various supporting cells.
• The end-layer is capable of sensing the application of pressure, which is transmitted to the other layers and sent to the corresponding nervous centers.

Merkel Discs
• They detect specially pressure and tactile sensibility.
• An afferent fiber is normally branched with many end-discs of this kind of nervous branching.
• These discs are comprised in a specialized cell whose distal surface is fixed on epidermal cells through an extension of its protoplasma.
• This way the movements of pressure and traction on the epidermis result in a stimulus.

Free Nerve Ends
• Sensitive to mechanical, thermal and specially pain stimuli
• Formed by a branched axon surrounded by Schwann cells, and these are both surrounded by a basement membrane.

>> Glabrous (hairless) skin exclusive receptors

Meissner’s Corpuscles
• Sensitive to touch stimuli
• They are in the hairless saliencies of the skin.
• Formed by a myelinic axon whose end-branches are entwined with accessory cells.

Krause End-Bulbs
• Cold-sensing thermal receptors
• They are formed by a nervous fiber whose ends have a club shape.
• They are placed in the boundary region of the skin and mucous membranes ( e.g.: around the lips and genitals)

Sebaceous Gland

• Placed in the Dermis and its ducts generally open in the Hair Follicle
• However, in certain areas( lips, glans penis and labia minora of the vagina) the ducts open directly onto the surface of the skin. The glabrous skin of the palms and soles do not have sebaceous glands.
• The sebaceous glands are alveolar and generally a series of alveoli debouch into a short duct.
• The alveoli are formed by an outer layer of squamous epithelial cells that rest upon a basement membrane. These cells proliferate and differentiate into round cells that accumulate in their cytoplasm their secreting product of lipid nature.
• The nuclei gradually condense and disappear. The more central cells of the alveolus die and disintegrate, forming a sebaceous product.
• The sebaceous gland is an example of a holocrine gland, since its secretion results from the death of cells.

Sweat Gland

• The merocrine sweat glands are very numerous and throughout the skin, except in certain regions such as the glans penis.
• They are simple coiled tubular glands whose ducts open at the surface of the skin.
• The secreting portion is found in the dermis, the secreting cells are pyramidal and between them and the basement membrane are the myoepithelial cells that help release its secretion.
• The duct of the gland opens at the surface of the skin and follows a helical course as it crosses the epidermis.
• It is formed by stratified cuboidal epithelium(two layers of cells) that rest on a basement membrane.