It functions as a membrane that isolates the organism, or part of it, from the external environment.
It is related to the coating and protection of external (eg skin) and internal (eg stomach) surfaces. It also acts on the absorption of substances, the secretion of various products, the removal of impurities and may contain various types of sensory receptors (notably on the skin).
Skin: Contact Organ
In vertebrates, the skin is an important organ of contact with the environment. The conquest of the terrestrial environment by vertebrates became possible, among other things, through the isolation and protection of the body and mechanisms of relationship between the living being and the environment.
Touch, sight, smell, taste, and hearing are helpful in the animal's relationship with the environment. The skin, the organ responsible for tactile sensations, has different types of "sensors", which record and inform the living being variations in temperature (heat or cold) and pressure (touches, shocks, shocks). The skin is also an important defense organ against various types of infectious agents.
Pavement Multi-stratified Epithelial Fabric Keratinized. Optical microscopy. (E) Epidermis, (D) Dermis, (SC) Keratinized cells and wax.
Considering the whole body, the skin of a person weighs 5 kg and has a total area of 18 m2. It is therefore the largest organ of our body.
The histology of the skin
In mammals, the skin is an organ composed of two layers: epidermis and dermis.
The epidermis is a multi-stratified epithelial tissue. It is formed by strata (or layers), of which stand out the basal stratum (also called germinative stratum), which is supported by the dermis and is formed by cubic cells. In this layer is intense the activity of mitotic cell division, which constantly restores the lost cells in the daily wear to which the surface of this tissue is subjected. As new cells are formed, they are "pushed" to form the remaining cells until they are exposed on the surface of the skin.
The dermis is a layer formed by dense connective tissue, whose fibers are oriented in several directions. Several cell types are found, especially fibroblasts and macrophages. Nerves, nerve endings, different types of sensory corpuscles, and a wide network of blood capillaries cross the dermis in various directions. It is an important maintenance and support fabric. The nutrients in the blood diffuse into the epidermal cells.
In mammals, the dermis is crossed by thin bands of muscle cells, the erector muscles of the hair, whose contraction is involuntary and allows to increase the air layer removed between the hair, which contributes to thermal insulation. Similar mechanism occurs in birds, with feathers.
Below the dermis is a layer of loose connective tissue, the subcutaneous cellular tissue (also known as the subcutaneous and hypodermic mesh), which is not part of the skin but connects with adjacent structures, allowing it to slip. In certain regions of the body, the hypodermis contains a varying number of layers of fat cells, forming the adipose panicle (the popular “pork fat”), important as an energy reserve, thermal insulator, and facilitator of water flotation.
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