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April 2017
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SKIN APPENDAGES


Skin appendages are:


  • hair follicles
  • sebaceous glands
  • sweat glands
  • blood vessels
  • nervation
  • nerve endings
  • melanophores.


Hair follicles

Hair follicles grow upwards through the epidermis and dermis via a construction in the shape of a bottle, called a follicle. There are 3 parts to the follicle: the root, which is the bottom part of the follicle which blood capillaries are connected to; the hair shaft, which is the middle part where the sebaceous glands are located and, in some areas of the body, the apocrine sweat glands; the opening, where secretions and the hair come out of. Hair follows a particular growth rhythm which alternates between rest periods and activity periods and involves various stages of development. On the scalp, where what we refer to generally as ‘hair’ grows, this growth is more evident, whilst body hairs are quite evident in the armpit and pelvic areas and, for men, on the chest and face. If the skin is atrophic, the follicle opens onto the glabrous skin surface, which is what commonly happens on foreheads, the sides of the nose and, for women, the chin. The follicle is connected to the hair muscle, which is a particular erector muscle situated under the sebaceous gland, and when it contracts, it causes us to have so-called goose bumps. Hair is made of keratin which does not develop horizontally as it does in the stratum corneum and in the nails, rather it grows vertically along the follicle axis. Hair and body hairs are made up of a scaly sheath, called the cuticle, an intermediate zone made up of a long plates, called cortex, and medulla, which is the internal part made from rounded bodies. The colour of the hair is determined by the melanin pigment present in the cortex, and the different tones are determined by the percentage of pigment present and how uniform it is.

The sebaceous glands

These are exocrine glands, that is, the secrete externally, and they are always connected to the follicle. These glands work constantly during our first year of life, then they become inactive until puberty. They can be different sizes and can be found all over the body, except for on the palms of the hand and soles of the feet. Sebaceous glands are structured in clusters and are made up of cells that produce a mixture of fluids called sebum. Sebum is realised on the surface of the skin and hair, passing through the shaft and follicle orifice. Sebaceous glands can be one of the ways that substances applied to the skin enter the body.

Sweat glands

These glands are nestled in the dermis, deeper than the sebaceous glands however, and their function is to bring sweat to the surface. Sweat is essentially made up of a diluted saline solution that is contained in various organic and inorganic substances. Secretions from sweat glands are stimulated by heat, the consumption of certain substances and hormonal and psychological stimuli. The excretion tube can open directly on to the skin’s surface via a pore that sits obliquely or, in certain parts of the body, it sits in the shaft of the follicle. Eccrine sweat glands are the former and they are present all over the body from birth, whilst the latter is called an apocrine sweat gland and is present from puberty in the armpit, pelvic and perianal areas.

Nervation

This is made up of sensory filaments situated on the dermis and, in particular, around hair follicles. These filaments pass through the dermis all the way back to their offshoots in the papillary zone. The nerve plexus originates from the vegetative and cerebrospinal nervous systems. Fibres of vegetative origin make up some of the innervation of the epidermis, follicles, blood vessels and sweat and sebaceous glands.

Fibres of cerebrospinal origin are mainly sensory fibres and they sometimes have special end-organs. The nervous system of the hypodermis is also made up of free corpuscles such as Pacini’s corpuscles (which are sensitive to vibration and pressure) and Ruffini’s corpuscles (sensitive to heat), whilst the Meissner’s corpuscles (sensitive to touch) and Krause end bulbs (which are sensitive to the cold) are present on the dermis.

Vascularisation of the dermis

There are two arterial vascular plexuses, one in the subpapillary space and the other between the dermis and the subcutaneous tissue. This network of blood vessels nourishes the skin and skin appendages. The arterial plexus located between the hypodermis and dermis form candelabra arterioles which run through the dermis like a mesh and then branch into capillaries, and the venous network runs through the skin in the same way but in the opposite direction. All capillaries end up in the papillary zone and form the sub-epidermic microcircle.

Melanophore cells

These are special cells in the dermis inside of which melanin is accumulated in the form of granules. The function of these cells is the same as that of melanocytes, which produce melanin, the substance that is responsible for skin pigmentation.

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