In order to know the meaning of the term scale, it is necessary, first of all, to discover its etymological origin. In this case, we can emphasize that it is a word that derives from Latin, exactly from "squama", which can be translated as "scab" or "scale".
The term scale refers to an epidermal or dermal sheet that covers, either partially or totally, the body of various species of animals. The scales overlap each other to insulate and protect the organism.
Fish are animals that have scales. In this case, the origin of these flattened sheets is dermal, arising in the mesoderm layer. The characteristics of fish scales vary both in structure and in shape and size.
The process of formation of the scales of these aquatic animals begins after the consolidation of the mesenchyme, a tissue of the embryo . Then the induction of morphogenesis takes place (when the epidermal papilla is formed) and finally the so-called late metamorphosis.
In fish we can find different types of scales. Cycloid (smooth-edged) scales and ctenoid (toothed) scales cover the body of silversides, spiny eels, and other animals . The ctenoid scales, for their part, are divided into true ctenoids , spinoids and crenates .
Ganoid scales can be found in paddlefish and sturgeons, for example. In sarcopterygians, cosmoid scales appear. Rays and sharks , meanwhile, have placoid scales .
Of the ganoid scales, it can be highlighted that they appear in lungfish and that they have a layer called isopedina, in which there is another layer of spongy bone.
Of the so-called placoid scales we can highlight several characteristics such as, in the same way, they are known as dermal denticles, which do not grow but are added to the body of the fish as it grows and that they are composed of a dental pulp that has blood vessels.
On the other hand, reptiles also have scales, but of epidermal origin and made of keratin . These scales contribute to the conservation of moisture in the body, promote camouflage, facilitate locomotion and act as a shield. In snakes , to cite one case, they minimize friction and therefore reduce the loss of energy in movement.
In the same way, we cannot forget that scales also exist in other animals, such as birds. In this case they have them on their legs, specifically on their fingers and they can even be found on their ankle.
Of course, it should not be overlooked that there are certain mammals that are determined to have scales. It is true that it is unusual for this to occur, but they do appear in animals of this type such as the muskrat kangaroo, which has them in the area of its legs, or pangolins, which have them for their body, which are of keratin and that serve as protection.