Placoid Scale

Placoid Scale

 

a scale characteristic of cartilaginous fishes. It consists of a flat base (the basal plate), a neck, and a crown. Inside each scale there are several pulp canals or a cavity filled with pulp. A placoid scale is formed of dentin; its tip is covered with a harder, enamel-like vitrodentin. The scale originates between ectoderm and mesoderm, and, as it develops, it breaks through the ectoderm and emerges as a spine. Its formation is complete when the basal plate forms in the mesoderm. The placoid scale is constantly replaced during the life of a fish. In the evolution of vertebrates, the placoid scale precedes the more complexly structured ganoid scale. The teeth of vertebrates are derivatives of placoid scales.

References in periodicals archive ?
1955), and is thought to be a modified placoid scale (Reif 1982; Kemp 1999).
Placoid scale types and their distribution in Squall's aranthias.
All are cartilaginous, have five to seven pairs of gills, rigid dorsal fins and small, hard, tooth-like placoid scales that protect their exterior surface.
These placoid scales have a drop-like, inclined, elongate crown, well-developed neck and small base with concave basal surface (Fig.
Shark skins bear thousands of rigid denticles, or placoid scales, comprised of hard materials, such as dentin and enamel, which are high in strength and stiffness (Raschi and Tabit, 1992).
Secondly we assign isolated placoid scales from synchronous levels of two stratigraphical sections (Middle Triassic) of the Iberian Chain (Spain), to these "functional" morphologies.
These placoid scales in sharks are formed of dentine with dermal papillae located at heir core.
The teeth of sharks are merely modified and enlarged dermal denticles, so it is not surprising that the teeth are almost identical in their structure with the placoid scales. The teeth are composed of a pulp, dentine and enamel-like vitrodentine over a bony base.
Sharks have placoid scales, consisting of a rectangular base embedded in the skin with tiny spines that poke upwards from the surface, making them feel rough.
The shark has skin made of teeth-sharp scales called placoid scales. 5.
Even though many sharks have an ancient ancestry, most modern sharks have very small scales, called dermal denticles or placoid scales. Placoid scales are like our teeth in that they are made of dentine.
A shark's skin is covered by small (usually microscopic) toothed structures known as dermal denticles or placoid scales, which help reduce friction while swimming.