Structure, Development & Homology of Placoid Scales | Shark | Scoliodon

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What is the Placoid Scale?

Placoid scales, also known as dermal denticles, are a type of scale that is found on the skin of cartilaginous fishes such as sharks, rays, and skates. These scales have a flattened, plate-like shape and are composed of dentin, a hard, mineralized substance similar to the material found in our teeth, and enamel, a hard, mineralized substance that covers the dentin.

Structure of a Placoid Scale

• Placoid scales are characteristic of the skin of dogfishes, sharks, rays, etc.
• A typical placoid scale consists of two parts: a basal plate and a spine.
• The basal plate is wide and rhomboidal and the spine is flat and trident.
• The spine arises from the center of the basal plate.
• The basal plate is embedded in the dermis and firmly held by Sharpey’s and other connective tissue fibers.
• The basal plate is formed by a bone-like loose trabecular calcified tissue called cement.
• The trident spine projecting out of the skin is backwardly directed.
• The spine is formed by one median and two lateral spines.
• In the vertical section, the spine consists chiefly of a hard substance called dentine with a central pulp cavity containing blood vessels, nerve endings, lymph channels and dentine-forming cells called odontoblasts.
• Through dentine ramify numerous very fine tubules or canaliculi containing long protoplasmic processes of odontoblasts.
• The spine is coated externally with a layer of still harder, shiny, enamel-like vitro-dentine.

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Development of a Placoid Scale

• The first sign of a developing scale is an aggregation of mesodermal cells of the dermis, lying just beneath the epidermis and forming a dermal papilla.
• As the epidermis is pushed up, the outermost cells of the dermal papilla become odontoblasts (or scleroblasts) and deposit dentine between themselves and the epidermis.
• Meanwhile, the overlying epidermal cells of stratum germinativum or Malpighian layer, now called ameloblasts, form the so-called enamel organ which deposits vitrodentine over dentine.
• Cement (basal plate) is formed from the surrounding cells of the dermis.
• In a growing scale, the dermal papilla forms nutritive pulp occupying the pulp cavity.
• In a fully formed scale, the epidermis wears off around the spine which projects above the skin, while the basal plate remains embedded.

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Homology of Placoid Scales

• The homology of placoid scales refers to the evolutionary origin and relationships of these scales to other structures in different species.
• It has been suggested that placoid scales are modified teeth, with a similar developmental origin and genetic basis.
• In fact, some scientists believe that placoid scales and teeth are homologous structures, based on similarities in their embryonic development and gene expression.
• Studies have shown that the genes involved in tooth development, such as BMP4 and SHH, are also expressed in the development of placoid scales.
• This suggests that both placoid scales and teeth structures may have a common evolutionary origin and may have diverged from a common ancestor.
• The placoid scales and teeth have essentially similar forms, structures, and embryonic development, indicating that they are homologous structures.
• The placoid scales become especially large to serve as teeth in the mouth.
• The scales are used for holding and tearing prey with their backwardly directed spines.

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