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Introduction

The common structural pattern of both aquatic & terrestrial vertebrates of integument is…

  1. Epidermis
    •  Derived from Ectoderm
    • Stratified i.e. consists of multi-layers of cells
  2. Dermis
    • Derived mainly from mesoderm
    • Formed of connective tissue
    • Contain blood & lymph vessels, nerve & smooth muscle fibres, receptors and also special irregular pigment cells (chromatophores)
    • Below dermis, subcutaneous loose connective tissue is present, which may consist of fat deposits at certain regions.

Extent of variations in epidermis and dermis may occur because of…

  1. Relative number + complexity of glands
  2. Epidermal differentiation & specialization
  3. Dermal bone development

Comparative from Agnathans till Mammals

AGNATHANS (JAWLESS FISHES)

(Example: Lamprey & Hagfishes etc.)

Epidermis:

  •  Scales are absent (exoskeleton absent)
  •  Multilayered epidermis (i.e. stratified)
  •  Abundant unicellular mucous glands, they secrete slime.
  •  All layers are living, mitotic (including surface cells too)
  •  Cornified structures are…
    • Horny denticles in buccal funnel (in Lamprey).
    • Cornified teeth (in other living agnathans).

Dermis:

  • Thinner than epidermis.
  •  Tough layer with interwoven collagenous bundles of connective tissue.
  • Many melanophores present.
  • Tightly adhered to underlying part of body wall musculature.

INTEGUMENTARY GLANDS IN AGNATHANS

  • Unicellular merocrine glands in epidermis present, of following cell types…
    • Goblet Cells
    • Granular Gland Cells
    • Beaker cells
  • Slime Glands in epidermis of Hagfishes have thread cells. Slime glands on stimulation throws out thread cells along mucous. Those thread cells uncoil & along with mucous forms protective coat around the animal.

REPRESENTATIVE ORGANISM- LAMPREY (Petromyzon)

Lamprey’s skin consists of both epidermis & dermis. Exoskeleton absent.

  1. Epidermis:
    • Multilayered
    • Small amount of flattening along with keratinization occurs, therefore, cells are alike throughout the epidermis.
    • Stratum corneum absent.
    • Abundant merocrine unicellular granular mucous glands present, making body slippery.
    • Club-shaped cells present.
    • Both club shaped cells & granular mucous cells appear as two forms of same cell type as granular cells developing from club cells.
  2. Dermis:
    • Thinner than epidermis.
    • Collagen & elastin fibres present interspersed with blood vessels, nerve fibres & smooth muscle fibres.
    • Star-shaped chromatophores present, whose density at places cause variation in pigmentation of the area, changing from pale to dark or dark to pale.
    • Beneath dermis is sub-cutaneous tissue present, containing fat.

CHONDRICHTHYES (CARTILAGINOUS FISHES)

(Example: Sharks, Rays, Skates, Chimaeras etc.)

Epidermis (including Glands):

  1. Numbers of cell layers more than Agnathans.
  2. Unicellular glands are less abundant (except in Chimaera)
  3. Multicellular glands are few & restricted. Example: In males, at base claspers.
  4. In Stingrays, modified goblet cells are present at base of stinger, which secrete toxins.
  5. Presence of Photophores- luminious organ formed by modified multicellular mucous epidermal gland. Example- in “Lantern Sharks” one of them is Etmopterus spinax (Velvet Belly Lantern Shark) which have bioluminescent velvet belly- thought to camouflage the shark. The photophores are in species specific pattern.
  6. Just below the epidermis are melanophores.
  7. Chimaeras’ body surface have becomes slippery due to presence of much epidermal mucous glands.

Dermis:

  1. Thicker than epidermis.
  2. Placoid scales present having bony basal plate in dermis & spine projecting out above the body surface, penetrating through epidermis.
  3. Chimaeras have lost scales over their most body surface, additionally epidermal glands are present, as mentioned above.

REPRESENTATIVE ORGANISM- Scoliodon (Dog fish)

Scoliodon V.S. Skin

Epidermis:

  1. Ectodermal origin & stratified.
  2. Stratum germinativum: Innermost layer of cuboidal cells. Rests on basement membrane secreted by epidermis & dermis. Cells of stratum germinativum divide by mitosis forming new cells, adding to above epidermis.
  3. Cells of the surface are rubbed off by friction, replaced by new cells.
  4. Stratum corneum absent.
  5. Horny layer of dead cells absent.
  6. Little flattening & keratinization occurs in outer layers.
  7. Unicellular Mucous glands present. Mucous reduces friction, prevent hold by enemies, settling of organisms & also controls osmosis.

Dermis:

  1. Mesodermal origin.
  2. Made of fibrous connective tissue with white collagenous fibres & various cells.
  3. Collagen fibres run parallel to surface. Number & compactness various in different regions of dermis-
    • Stratum laxum: Outer dermis. Few fibres present.
    • Stratum compactum: Inner dermis. Compactly arranged fibres.
  4. Dermis also contain pigment cells or chromatophores just below the epidermis.
  5. Also have blood capillaries, smooth muscle fibres & nerve fibres.
  6. Presence of nerve fibres make it sensitive to external stimuli.

SCALES:

Placoid Scales
  • Placoid Scales are present in Scoliodon, on almost entire body as exoskeleton.
  • They are microscopic & close set.
  • In contrast to scales of bony fishes, placoid scales stop growing after reaching a certain size.
  • Scale projects out reaching above the surface of body.
  • Scales are in directed in backward direction (while in forward direction over claspers). Additionally, scales have riblets over them which are also directed in flow direction of water. All this results in reduced drag during movement making swimming efficient as well as silent.
  • They also cause lotus effect (Self-cleaning property as in Nelumbo) over body surface of shark.
  • A placoid scale consists of-
    • Basal Plate:
      • Rhomboidal or round in shape.
      • Resemble a bone & is formed of loose, calcified, trabecular material.
      • Anchored in Stratum compactum of dermis by Sharpey’s fibres.
      • Has a perforation in the centre leading to pulp cavity in middle of spine. Pulp cavity consists of Pulp which further consists of-
        • Dentine forming cells- Odontoblasts.
        • Capillary network formed by a small artery and a vein.
        • A nerve fibre.
        • Areolar tissue.
    • Spine (Cusp):
      • Flat, trident.
      • Made of Dentine, canaliculi present.
      • Covered by vitrodentine.
      • Spine erupts through epidermis, projecting outwards.
      • Responsible for roughness on shark body.

SHARPEY’S FIBRES: (= Bone Fibres, Perforating Fibres)

Sharpey’s fibres are matrix of connective tissue consisting of bundles of Type I collagen fibres connecting periosteum (membrane that covers outer surface of all bones except those of long bone joints) to bone.

To be continued…

Harjeet Kaur

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