It helps in defense , temperature regulation and production of new substances. It consists of three layers, the epidermis, the dermis and the hypodermis, from the outside to the inside.
Epidermis
The epidermis is composed of the covering epithelium , which is a stratified, squamous and keratinized tissue, that is, formed by several layers of cells with different shapes and functions.
The superficial cells are flattened like scales and contain keratin . The epidermis can be more or less thick, such as the soles and palms of the hands and eyelids, respectively. Normally, there are no blood vessels in this portion of the skin, nor nerves.
Learn more: epithelial tissue .
The cells, called keratinocytes or keratinocytes , produced in the basal layer are “pushed” upwards and change their structure. They join together by junctions ( desmosomes , which are specializations of the surface) and extensions, flatten themselves and produce keratin. The keratinocytes lose their nucleus and die. On the surface of the body they are eliminated by desquamation.
- Basal or Germinal Layer : This layer is always producing new cells, which divide by mitosis . Melanocytes are present , cells specialized in producing melanin , which is the pigment that gives color to the skin and hair. The extensions of the melanocytes penetrate the cells of this layer and the spinous layer, spreading melanin inside them. Merkel cells are mechanoreceptors, that is, they perceive mechanical stimuli from the outside and forward them to the nerve fibers.
- Stratum spinosum : contains cells with desmosomes and extensions that help to keep them tightly joined together, giving them a spiny appearance. Langerhans cells are found throughout the stratum and help to detect invading agents, sending an alert to the immune system to defend the body;
- Granular Layer : as they rise, the keratinocytes are flattened. In the granular layer they have a cubic shape and are full of keratin granules, which occupy the intercellular spaces;
- Horny Layer : The stratum corneum is located on the surface of the body. It is made up of dead, keratinized, flattened, nucleated cells. Its outermost part flakes off and is constantly replaced (every 1 to 3 months).
Dermis
The dermis is made up of dense connective tissue . Its composition is essentially collagen (about 70%) and other glycoproteins and fibers of the elastic system. The elastic fibers form a network around the collagen fibers that give the skin flexibility.
The layer immediately below the epidermis is called the papillary layer, as it has numerous dermal papillae embedded in the indentations of the irregular surface of the epidermis.
Next is the reticular layer , which contains more elastic fibers, as well as blood and lymph vessels and nerve endings. Sebaceous and sweat glands and hair roots are also found.
Hypodermis
Located just below the dermis is the subcutaneous tissue or hypodermis , which is a layer of loose connective tissue rich in fibers and adipose cells . The fat that accumulates in these cells functions as an energy reserve and thermal insulation .
Skin Attachment Structures
There are several structures related to the epithelial and connective tissues that form the epidermis and dermis, respectively, each with a specific function.
Glands secrete sweat or sebum that helps control body temperature and lubricate the skin. Nails protect the fingertips and help you grip objects.
Hair plays a sensory role, as it has nerve endings connected to the base of the follicle; there are also other endings spread throughout the skin, which allow the perception of stimuli such as: temperature, pressure, touch and mechanics.
See also: Layers of the skin
Sebaceous Glands
The activity of these glands is mainly controlled by male hormones, and they are most active during puberty. They release the sebum they produce in the hair follicle canal. They are not distributed equally throughout all areas of the body, with large glands in the skin around the mouth, nose, forehead and cheeks, which makes these areas quite oily. It is believed that their main function is to form a superficial fatty barrier, preventing water loss.
Sweat Glands
These glands are spiral-shaped and are formed by epidermal cells, but are located in the dermis. There are two types of sweat glands :
The eccrine glands, which release sweat directly into openings on the surface of the skin, the pores . Through perspiration, these glands regulate body temperature, because when sweat evaporates, it dissipates heat with it.
And the apocrine glands , which secrete their secretions inside the follicle canal. In the embryonic phase, rudimentary forms of these glands are spread throughout the body, but after birth they develop only in areas such as the armpits, in the ear canal, in the nipples, around the navel and in the region around the genitals and anus. This seems to have some ancestral relationship with the production of smell and sexual attraction.
Hair
They are composed of compacted and keratinized dead epidermal cells. Body hair and head hair are formed in the hair follicle , which is an epidermal tube surrounded by sensory nerves , which provide sensitivity to pressure exerted on the hair.
The base of the follicle, called the bulb, is located in the dermis and always produces new cells, which as they emerge receive melanin (which gives the hair its color; the more melanin, the darker it will be) and keratin.
Other structures linked to the follicle are: the arrector pili muscle (smooth muscle that moves the hair, making the skin stand on end), the sebaceous glands (which lubricate the hair) and the sweat glands.
Nails
They are similar in structure to hair, however, nails never stop growing, while the hair follicle sometimes goes into a state of rest, reducing hair growth.
The nail begins to form at the root , which is buried in the skin, where the cells multiply and emerge. The cells then synthesize keratin in the cuticle or eponychium region , which is a fold of skin, and continue their movement.
When they are exposed, the cells are already dead, quite flattened and keratinized, forming the nail as we see it.
Nails offer a good indication of a person’s health. They can become thin, brittle, or deformed due to stress, long periods of fever, or the use of drugs or strong medications.
They help protect the ends of the fingers, an extremely sensitive area, and also help with gripping objects.
Sensory Receptors
They are endings of myelinated nerve fibers, some are free and associated with epithelial cells, others are encapsulated.
There are 7 types of receptors that capture stimuli from the environment, send them to the nervous system and return sensory responses; they are:
- Merkel’s discs : ramifications of the ends of sensory nerve fibers, whose tips are disc-shaped and are connected to the cells of the epidermis. They perceive continuous stimuli of pressure and touch;
- Meissner corpuscles : these are encapsulated receptors, which adapt rapidly (respond to stimuli at the end), and perceive vibration, pressure and touch stimuli, located on the surface of the dermis;
- Paccinian corpuscles : encapsulated, rapidly adapting, sense rapid vibratory stimuli and pressure, located in the deep dermis;
- Ruffini corpuscle : encapsulated, slowly adapting (continuously responds to stimulus), feels pressure and is located in the deep dermis;
- Krause bulbs : encapsulated, they are little known, but associated with pressure stimuli, they are located on the edges of the epidermis;
- Hair Follicle Ends : these are sensory fibers coiled around the follicles, they can be slow or fast adapting;
- Free Nerve Endings : These are branches of unencapsulated myelinated or unmyelinated fibers. They adapt slowly and transmit information about touch, pain, temperature and proprioception. They are located throughout the skin and in almost all tissues of the body.
Learn more about:
- Human Body Systems
- Organs of the human body