Placoid scales are characteristic of fossil fishes that became extinct dozens, and some hundreds of thousands of years ago. However, in our time there are representatives of the underwater world who still have a similar skin. You can find out about which fish the placoid scales still remain, about its structure, as well as about other interesting facts, from this article.
General information
Fish scales are the outer cover formed by bone plates made up of several layers, including specialized substances and tissues. Their structure determines the classification existing in our time. There are four main types of scales - cosmoid, elasmoid, ganoid, and placoid. We will talk about the last of them in more detail.
Shapes and sizes of scales
Placoid denticles are isolated conical-like formations with an expanded base or the so-called basal plate, which is immersed in the dermis. A solid spike begins to grow from it at a certain angle. As it develops, it breaks through the epidermis and protrudes outward. The teeth with their peaks are always directed from head to tail.
Typically, the average size of such a flake is not more than 0.3 mm. In some species of sharks and stingrays, it can grow up to 4 mm. In this case, the scales will have a more complex structure, since it is already a multi-vertex formation - the result of the merger of several cloves at once. It was this structure that was inherent in the bony plates of most fossil fish.
In appearance, this type of scale in different fish is very different from each other. The dissimilarity can lie both in the form of a spike and in its base. In some species of cartilaginous fish, scales do not have a pointed tip. It looks like a fairly wide plate with several incisors along the edge and three or five longitudinal ridges. The base structure of placoid scales is quite diverse, in addition, its shape is inherent in other classifications. Its edge is sometimes smooth or with processes can be elongated or rounded.
Internal structure
The appearance of bone plates with spikes can be very different from each other. As for the internal structure of placoid scales, it is almost the same in all species. The outer coating can be formed by a durable substance, durodentin or vitrodentin, as well as real enamel.
The base of the clove is a plate formed by a cell bone. His body is made up of dentin. Underneath is a pulp cavity. A whole network of branched tubules containing nerve fibers and blood capillaries leaves deep into dentin. In the layers of the dermis, each clove is held by strands of collagen fibers, which originate in the bone tissue. Interestingly, in a person in this way all the teeth on the jaw are held. These cords are called Sharpei fibers (after the scientist who discovered and examined them).
Development
The formation of placoid scales begins with the formation of denticles. It occurs in the process of close interaction of two components - the epidermis and dermis. First, in the soft tissues, the clove precursor is born. It is still impossible to determine where the enamel and where is the dentin layer. Tissues become hard only when they grow to the size of a future serrated plate.
Such a development process as the formation and its further hardening means that the scales of this type (and, in particular, its teeth), having fully matured, can no longer increase in size. It is known that fish growth continues throughout its life. After a certain period of time, the scales begin to wear out, and a new one appears in its place. This process can be repeated several times throughout life. With each subsequent generation, the cloves become larger until they reach their maximum size. If the body still continues to grow, the laying of additional bone plates begins. It is safe to say that such processes in the skin are characteristic of representatives of all types of cartilaginous fish.
Location
Placoid scales on the body of fish are unevenly distributed. It is not grouped by some distinct rows, but rather it happens randomly, because the teeth can be placed both on a separate area of the skin and on its entire surface.
Most often, there is a certain distance between them, so a continuous scaly cover is not common. As a rule, such fish look “naked”, but at the same time their skin has a special roughness. Sometimes you can observe a completely different picture, when some scales overlap with others, thus covering the whole body and reliably protecting their master.
Functions
Based on this information, scientists concluded that placoid scales do not always fulfill the tasks that other types of bone plates perform. If they play the role of protective structures that form a fairly hard and reliable shell around the fish’s soft body, then things are different with placoid denticles. Their main task is to cut the stream of water flowing along it at a time when the fish is swimming. In this case, micro-eddies arise, which significantly reduce the friction of the body, thereby simplifying its forward movement.
