Question

What are placoid and ctenoid scales?

Answer 1

Placoid scales:

1. They are tri-radiate scales that are tiny and pointy.
2. Denticles are found embedded in their skin's dermal layer.
3. It is made up of a diamond-shaped or rhomboidal-shaped base plate with a pulp cavity opening and a flat trident spine.
4. Their base plate stays entrenched in the dermis, which is kept together by Sharpey's and other connective tissue fibers and is composed of trabecular calcified tissue, the cement.
5. Their spine is comprised of a hard calcareous substance, and their dentine is covered on the outside with hard and dense enamel, such as vitrodentine.
6. Dentine-forming cells, the odontoblast, blood vessels, nerves, and lymph channels are found in the pulp cavity of their basal plate and spines.
7. Scales like these can be seen on the skin of cartilaginous fishes like sharks, skates, and rays.

Ctenoid Scales:

1. These are soft, flat dermal plates with a securely attached groove-like portion and free hind teeth bearing portion.
2. It is made up of a center-thicker section termed the 'nucleus' and several concentric lines of growth that are used to determine the age of the fish.
3. Numerous longitudinal channels are present on their posterior end, which are employed for sucking nutrients from the fish skin.
4. They lack a pulp cavity and dentine.
5. These are ganoid scale derivatives that lack ganoin layers, cosmine layers, and bone cells.
6. This can be seen on perch and sunfish.