Normal human skin structure

The nail is mainly composed of the hard nail plate arising from a matrix. The shape of the nail is very variable in different persons; it is roughly rectangular and flat in shape; The nail bed has a pink color that can be seen because of the transparency of the plate and its extensive vascular network.

A whitish crescent -shaped lunula projecting from under the proximal nail folds is observed in the thumbs, uncommonly in the other fingers and in the large toenails. The lunula is the more distal portion of the matrix and determines the shape of the free edge of the nail plate. Its color is due in part to the effect of light scattered by the nucleated cells of the matrix and in part to the thick layer of the epithelial cells making up the matrix.

As the nail plate emerges from the matrix, its lateral and proximal borders are enveloped by folds of skin termed lateral and proximal nail folds. The skin underlying the free end of the nail is referred to as the hyponichium and is contiguous with the skin on the tip of the finger.

In humans, nails grow at an average rate of 3 mm a month. The growths of fingernails and toenails have different speeds. Complete regrowth of fingernails require 3 to 6 months while complete toenail regrowth needs 12 to 18 months. The growth rate is related to the length of the terminal phalanges; the nail of the little finger grows slower than the nail of the index finger. They grow faster in summer than in the other season. Contrary to popular belief, they do not continue to grow after death; it is the severe postmortem drying and shrinking of the soft tissue around the nail plate that give the illusion of a nail growth.

The main functions of the nails are to protect the end of the digit and to help in grasping objects.

In the skin, only the dermis and hypodermis are vascularized, the epidermis being a non-vascular tissue. The skin is richly supplied with a vascular network, largely exceeding its nutritive requirements, being also involved in thermoregulation, wound healing, immune reactions and the control of blood pressure. Cutaneous vessels belong to the arterial, venous or the lymphatic system; they are small branches from underlying vessels of the muscles that penetrate the subcutaneous fat, enter the deep reticular dermis and ascend vertically to the epidermis.

The dermis is one of the constitutive layers of the skin between the epidermis and subcutaneous tissues; it is composed of two layers, the papillary dermis lying immediately below the epidermis and the reticular dermis accounting for the largest part of the dermis. It is a 2 to 4 mm-thick layer of connective tissue mainly composed of extracellular matrix (ECM) produced by fibroblasts.

The dermis houses the neural, vascular, lymphatic systems, and epidermal appendages including excretory and secretory glands (sebaceous, eccrine and apocrine glands), hair follicles, and nails. Neural structures include sensory nerve receptors of Merkel and Meissner’s corpuscles (for touch), Pacinian corpuscles (for pressure), and Ruffini corpuscles (mechano-receptors). The dermis also hosts multifunctional cells of the immune system such as dendritic dermal cells, macrophages and mast cells .

The dermal-epidermal junction also named cutaneous basement membrane zone is the acellular zone that is between the dermis and the epidermis.

The skin is the envelope of our body and is the most visible and the largest organ, with a surface area of 1.8 m2 and a weight of 3 kg.

The skin fulfils many functions, the main one being a barrier function. Being in direct contact with the outside environment, skin is a dynamic barrier that allows and limits inward and outward passage of water, electrolytes and various other substances and that protects against microorganism invasion, toxic agents, ultraviolet radiation   and mechanical insults. Skin is also a temperature regulator, an immune organ that can detect infections, and a sensory organ to detect temperature, touch, pain, itch and mechanical stimuli at every point of the body. Healthy skin being a major component of our physical appearance, skin play a major role in our social and sexual communication.

The epidermis is the outermost portion of the skin. It is a continually renewing stratified (= several layers), squamous epithelium. The keratinocytes (= keratin-producing cells) are the main epidermal cell type (90 to 95 %). The other epidermal cells include melanocytes, Langerhans cells and Merkel cells.

Normal skin pigmentation   is a complex process that, in the epidermis   as in the hair follicles, begins with the synthesis of melanin within melanosomes in the melanocytes, followed by melanosome transfer to neighboring basal and suprabasal keratinocytes. In basal cells, melanin granules are translocated to the upper pole of the nucleus, forming a melanin cap that protect DNA from UV   rays. Melanin granules are eventually degraded as the keratinocyte undergoes terminal differentiation.

In humans, the melanocytes are localised either in the basal layers of the epidermis or in the hair follicles. Whatever is their localisation in skin, the melanocytes are derived from precursor cells (called melanoblasts) that originate from the neural crest.

Mammalian melanocytes produce in their melanosomes, two chemically distinct types of melanin pigments: black-brown eumelanin and yellow-reddish pheomelanin. In the melanocytes, eumelanosomes and pheomelanosomes cohabit.

Tyrosinase is the key-enzyme which regulates the first steps of eumelanin and pheomelanin synthesis: the transformation of L-tyrosines into L-3,4 dihydroxyphénylalanine (DOPAs) and then into DOPAquinones. From DOPAquinones, the synthesis pathways are different for the pheomelanin or eumelanin.

In human, as in the others mammals, the skin and hair color is mainly determined by the number, the size, the type, and the mode of repartition of the melanosomes. It is particularly interesting to note that in normal conditions, the racial differences in skin pigmentation in human do not depend of the melanocyte number in the epidermis. For a specific area, the number of epidermal melanocytes is nearly the same in Caucasoids, Negroids and Mongoloids.

Summary

Merkel cell were first identified by German anatomist Friedrich Sigmund Merkel in 1875. Merkel cells are epidermal cells localised in the basal layer of the epidermis   and the epithelial sheath of hair follicles. The vast majority of Merkel cells are intimately associated with a nerve terminal but some are not.

Whether Merkel cells originate from embryonic epidermal or neural crest progenitors has been a matter of intense controversy but recent data demonstrate an epidermal origin of mammalian Merkel cells.

Merkel cells are sensory receptor cells, that transmit signals through synaptic contacts with somatosensory neurons. Merkel cell-neurite complex are among the most sensitive touch receptors mediating one form of light touch important for tactile two-point discrimination and for detection of shapes, curvature and textures.

The Merkel cells that are without contact to nerve terminals form part of the diffuse neuroendocrine system involved with modulation of peripheral neural responses. It is these cells, rather than those acting as mechanoreceptors, that are believed to be at the origin of a highly malignant skin cancer   called Merkel cell carcinoma.

Desmosomes are highly specialized anchoring junctions that link intermediate filaments to sites of intercellular adhesion, thus facilitating the formation of a supracellular scaffolding that distributes mechanical forces
throughout a tissue.

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