Scoliodon :- Placoid Scales

For Scoliodon fish slide (PLACOID SCALES) Take a few small pieces of the skin of Scoliodon and boil in 5 to 10% KOH solution in test tube till skin
dissolves. Cool and allow the scales to settle at bottom. Decant KOH and wash the material in water
several times to remove KOH. Stain in borax carmine or picro-indigo carmine, dehydrate, clear and mount. Placoid scales do not take stains properly. These scales can be mounted without staining. Dehydrate, clear and mount.
Comments :
(1) Placoid scales or odontoids are minute dermal denticles, closely arranged in regular oblique rows.
(2) They form entire exoskeleton of the shark and give a rough appearance to the skin. (3) Each placoid scale comprises of a diamond-shaped, basal plate embedded in the skin and is derived
from dennis.
(4) Anteriorly the scale has a flat trident spine projecting out of the skin.
(5) With very few exceptions, placoid scales are abundantly found in dermis of elasmobranch fishes.
(6) Placoid scales are arranged in regular oblique rows. They are dermal in origin and cover entire surface of the body, forming dermal exoskeleton of the sharks.
(7) Each scale is composed of a basal bony plate embedded in the dermis, from which projects trident
spines.
(8) Basal plate is formed of a trabecular calcified tissue.
(9) Spine is composed of dentine covered by a hard material, vitrodentine.
(10) Placoid scale contains a pulp cavity in spine.
(11) Pulp cavity contains odontoblasts dentine forming cells, blood capillaries, nerves and lymph channels.
(12) General similarity in structure of placoid scales to teeth of higher forms should be apparent. Both are considered to be remnants of bony armour of such primitive vertebrates as ostracoderms and certain placoderms.
Identification : Since this scale has trident spines, hence it is placoid scale of Scoliodon.

Scoliodon : Development of Placoid Scales

Comments :
(1) Development of placoid scale can be studied in a V.S. of integument of dogfish.
(2) V.S. integument shows 2 layers epidermis and dermis. Placoid scales originate from dermis. Various
development stages of placoid scales are as under.
(i) Mesodermal cells accumulate and form dermal papillae beneath stratum germinatum.
(ii) As epidermis is pushed dermal papilla develops odontoblasts which deposit deutine between
themselves and epiderms to form placoid scale.
(iii) Epidermal cells of papilla are called as ameloblast cells which form enamel organ which deposites
vitrodentive over dentino.
(iv) In developing placoid scale dermal papilla gives rise to pulp cavity and dentine canalculi.
(v) When fully formed placoid scales project covered by epidermis.
(vi) Other structures seen are loose connective tissue, compact connective tissue, vertical fibers, nerve
fibers, chromatophores and cement.
Identification: Since the section contains dermal papilla and above features, hence it is V.S. of integument showing placoid scales of Scoliodon.

Nandus : Ctenoid Scales

For this fish slide (NANDUS : CTENOID SCALES) Make permanent mount like cycloid scale. Each scale comprises of several concentric lines of growth, teeth or denticles and nucleus. Ctenoid scales are found in acanthopterygian fishes.

Comments :
(1) Ctenoid scales are commonly found in most teleost and actinopterygian fishes.
(2) They are thin, soft and dermal translucent plates.
(3) They are composed of underlying fibrous layer covered by bone-resembling layer.
(4) They contain concentric rings, representing lines of growth which vary in different specimens.
(5) Ctenoid scales do not contain ganoin.
(6) Each scales is embedded in a small dermal pocket.
(7) Scales are obliquely arranged so that posterior end of one overlaps the anterior end of the following
scale.
(8) Basal end is scalloped and free edge uears numerous comb-like projections.
(9) Ctenoid scales are derivatives of ganoid scales in which ganoin, cosmine layers and bone cells are lost.
Pulp cavity and dentine are entirely absent.
Identification: Since the above scale has concentric lines of growth with teeth like projections posteriorly, hence it is ctenoid scale of Nandus.

Polypterus :- GAnoid Scales

Comments :
(1) Ganoid scales are found in primitive ray-finned fishes such as Polypterus and gar pikes.
(2) Scales are covered with a hard, shiny and translucent material of mesodermal origin called as ganoin.
(3) Ganoid scales fit together like tiles and are arranged in diagonal rows.
(4) Scales are dermal in origin.
(5) Each scale consists of a bony base, coated by shining substance called as ganoin and openings of lateral line canal.
Identification : Since the above scale is overlapping and fitted like tiles, hence it is ganoid scale of Polypterus.