Dermal denticle
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Denticles are body surface structures found on some fish and insects.
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[edit] Fish
Denticles or placoid scales are small outgrowths which cover the skin of many cartilaginous fish including sharks. They are similar in structure to teeth, and teeth may have evolved from denticle-like structures in primitive fish. The placoid scales in sharks are formed of dentine with dermal papillae located in the core. These scales develope from the dermal layer of the skin, which is in contrast to the epidermal development seen the in the elasmoid scales of tetrapods. The shape of denticle varies from species to species and can be used in identification.
Shark skin is covered in denticles, and can therefore be as rough as sandpaper; some societies have in fact used it as such for many years (see Oroshigane). Most point backward, so stroking a shark from head to tail produces a smooth feeling. However, stroking the skin in the opposite direction reveals the rough texture. An interesting exception is the Basking Shark [Cetorhinus maximus]. The crowns seem to point every which way, rather than towards the tail. Sharks have been observed to use their skin to inflict wounds on prey.
Studies have found that the denticles create tiny vortices that reduce drag to make swimming more efficient. Denticles also allow sharks to swim silently compared to other fish that generate considerable noise when they ply the water.
In Japan, traditional sword-makers used shark-skin for the hand-grips of swords, to make them less likely to slip in the hand.
[edit] Insects
B. Diagramatic representation of two adjacent denticle bands of two adjacent body segments.
C. Diagramatic representation of the denticle pattern on the ventral body surface of an early Drosophila larva.
Denticles on insects are cuticular spikes projecting from body surface epithelial cells. The denticles of Drosophila larvae are present in bands on each body segment. During embryonic development, cells are directed to either make or not make a denticle depending on the pattern of expression of morphogens. The Wingless protein is produced by a stripe of cells (w, in the figure to right) in each segment. It is released from those cells and acts on nearby cells where it prevents denticle production resulting is a band of "naked cuticle".
