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.
During life, the pulp cavity is filled with vascular connective tissue called the pulp containing numerous odontoblasts, blood vessels, nerves and lymph channels. The spine is flat, trient, and projecting out of the skin. The surface of the spines is not smooth but presents a stratified appearance under the microscope.
The spine is composed of a hard calcarcous substance, the dentine. The dentine is travesed by minute nearly parallel canaliculi with delicate branches and is coated externally with a hard dense substance, the enamel.
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.
Development of Placoid Scales:
A group of cells collected in the dermis to form a dermal papilla whose cells are called odontoblasts. The dermal papilla grows upwards pushing the epidermis, and then it takes the shape of a basal plate and spine. The odontoblasts screte dentio all around forming the basal plate and spine of the palcoid sean.
The dentine of the basal plate becomes calcified. The maplighian layer of the epidermis in contact with the dermal papilla is known as enamel organ, the anamel organ brings about formation of virtpdentine over the dentine of the spine. The dermal papilla forms pulp in the pulp cavity of the scale. The overlying epidermis moves away and also wears off so that the spine projects above, while the basal remains embedded in the dermis.
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.
But there are objections to this supposition, and a more recent view is that both placoid scales and teeth are modified remnants of the bony dermal plates found in the ancestral ostracoderms and placoderms, so that teeth and placoid scales are homologous structures.
Moreover, it has been shown that there is no epidermal enamel layer in placoid scales, but it is a layer of vitrodermal formed from dermal cells. Enamel is the hardest substance in the body, where as vitrodentine is only a hardened outer to layer of dentine. The enamel organ does not secrete enamel; it only plays a role in shaping the spine, so that the entire placoid scale is mesodermal like the scales of bony fishes, whereas a tooth has a enamel covering derived from ectoderm. The claim that a gradation from placoid scales to teeth is observed in the mouth of a shark is interpreted as a divergence shown between vitrodentine covered placoid scales, on the one hand, and enamel-covered teeth on the other.