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what is epidermal placoid scales
Placoid scales are the forerunners of vertebrate teeth because the two have essentially the same from and structure, and a gradation from placoid scales to teeth is seen in the mouth of a shark, shark teeth are enlarged placoid scales formed in the skin jaws.
 Placoid scales or denticles are small outgrowths that cover the skin of many cartilaginous fish. The cartilaginous fishes or Chondrichthyes are a major class of jawed fish that includes the sharks, rays, and skates, and whose skeleton is characterized by rubbery cartilage, which is very light and flexible, rather than bone, as in the bony fishes (class Osteichthyes]]. The placoid scales are similar in structure to teeth, and teeth may have evolved from denticle-like structures in primitive fish. The placoid scales in sharks are formed of dentine with dermal papillae located in the core. These scales develop from the dermal layer of the skin, which is in contrast to the epidermal development seen in the elasmoid scales of tetrapods. The shape of denticle varies from species to species and can be used in identification. Shark skin is covered in denticles, and can therefore be as rough as sandpaper.
The placoid scales are derived partly from the dermis and partly from the epidermis. The basal plate and the dentine of the spine are derived from mesoderm, while the enamel is secreted by the ecotoderm.A typical placoid scale consists of mainly two parts the basal plate and spine. The basal plate is shaped and lies embedded in the skin. The basal plate is formed of a trabecular-calcified tissue closely allied to the cement. The inner surface of the basal plate bears an opening, which leads into the pulp cavity.


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