Skin of dogfish is very rough like a sand paper due to the presence of exoskeleton of dermal denticles called odontoids or placoid scales.
Placoid scales:
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The presence of placoid scales is characteristic of the skin of dogfish. A typical placoid scale consists of two parts, a wide rhomboidal basal plate and flat trident spine arising from the center.
The basal plate is embedded in dermis firmly held by sharpey’s and other connective tissue fibers.
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It is formed by a trabecular calcified tissue called cement. In 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.
Numerous very fine tubules or canaliculi containing long protoplasmic processes of odontoblasts ramify through the dentine. Spine is coated externally with a layer of hard, 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 dermis lying just beneath epidermis and forming a dermal papilla.
As the epidermis buldges up, the outermost cells of dermal papilla become odontoblasts (scelroblasts) and deposit dentine between themselves and epidermis.
The epidermis over the dermal papilla now called malpighian layer or ameloblasts, forms the so called enamel organ which deposits vitrodentine over dentine.
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Cement (basal plate) is formed from surrounding cells of dermis. In a growing scale, the dermal papilla forms nutritive pulp containing pulp cavity.
In a fully formed scale epidermis wears off around the spine which projects above the skin, while the basal plate remains embedded.
Homology of placoid scales:
Placoid scales are considered as ancestral structures of the teeth of vertebrates. Both the structures have essentially similar form, structure and embryonic development, indicating that they are homologous structures. In dogfish, the teeth are actually specialized placoid scales.
The placoid scales of the skin at the jaw region in the mouth become especially large to serve as teeth. They serve for holding and tearing off the prey.