Integumentary System (Skin and its derivatives)

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

1) 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.

2) 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)

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 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.

3) OSTEICHTHYES (Bony Fishes)

Examples: Lung fishes, Polypterus, Notopterus, Labeo, Anabas, Tetradon, Diodon etc.

• Skin of most extant bony fishes especially dipnoans and teleosts have a relatively large number of mucous glands.
• Different kinds of dermal scales present except placoid type which are present in cartilaginous fishes.

Epidermis

• Epidermal Glands
  • Chiefly unicellular mucous glands are present – maintain a constant thin mucous coat on skin surface.  
  • Multicellular glands are relatively sparse – some of them secrete mucoid cocoon that envelopes aestivating lungfishes during dry season to prevent desiccation.
  • Granular glands are few in number – secrete irritating/poisonous alkaloid.
  • Photophores – Many deep sea inhabiting teleosts have photophores that serve as lures/warnings and also, facilitating species recognition.

Dermis

• Characterized by presence of scales – ancient ganoid/modern cycloid/ctenoid.
• Ganoid or modified ganoid scales – found on many relicit basal ray finned fishes like Polypterus or basal neopterygians like gars, as plate forms on head & scales on body and tail.
• Amia possess modern fish scales on body.
• Teleosts, except few naked ones, possess either ctenoid or cycloid scales or both.
• Some scaleless teleosts develop abortive scales during embryonic stage.

Representative Organism – Labeo

Epidermis

• Stratified epithelium – resting on basement membrane.
• Surface layer cells rubbed off by water friction & are replaced by new cells produced by Malpighian layer below.
• Outer layers have little flattening & keratinization, making cells alike throughout epidermis.
• Horny layer is absent [In contrast to land vertebrates where it is present].
• Mucous glands are present which secrete slimy mucus which spread all over body.
  • Functions of Mucus –
    • Reduces friction.
    • Facilitates escape from enemy.
    • Checks growth of moulds etc.
    • Assists in controlling osmosis.

Dermis

• Consists of connective tissue – whose fibres almost all are parallel to surface.
• Pigment cells – mostly present in outer part of dermis.
• Cycloid Scales
  • Nearly circular with smooth margin.
  • Lodged in pockets of dermis beneath epidermis.
  • Flat oval plates of bone.
  • As the fish grows, the scales also increase in size. Thus, a young fish have same number of scales as it would have as an adult.
  • Shows concentric rings of growth.
  • Faster growth in spring & summer.
  • Growth almost stops in winter.
  • Examination of scales can reveal fish age.
  • Imbricate arrangement – like shingles on roof that permit bending of body. A small three sided portion of scale lies just under epidermis & remaining portion is covered by scales immediately anterior.

To be continued…