Fish Structure and Function

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Fish Structure and Function

• WL 714

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SKIN
Theme of the lecture -fish skin is a compound
organ, compound in its structure, origin, and in
function.
The epidermis has mucus, and functions in wound
healing and covering (osmotic barrier), the
dermis has scales, pigments, and holds the cover,
and the hypodermis has fat and holds the dermis
to muscle.

• Germ layers- from 2 layers (epidermal and
  mesodermal)
• Structure - 3 layers
• Epidermis - cuticle and epithelial cells
• Dermis -2 layers of CT
• Hypodermis - 1 layer, contains fat, holds skin to
  muscle
• Components and functions - several
• Nerves, glands, light organs (reading)
• Scales - dermal armor, formation, use
• Mucus -the cell, distribution, uses
• Pigments
• Club Cells Schreckstoff

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Skin - Compound Organ

• Like the wrapping of a parcel, it is the
  integument which is first encountered in
  examination. But the skin is more that just a
  wrapping, it is an ORGAN, which has its own
  structure and important derivatives such as
  scales (light organs, poison glands).
• The skin is a compound organ
• morphologically - 3-layered, epidermis, dermis,
  hypodermis
• embryologically - two germ layers mesodermal
  becomes the dermis epidermal becomes the
  epidermis
• functionally - variety of functions, primarily
  protection and osmotic barrier external tendon
  (Moyle and Cech)
• The skin is not elastic (as frog and bird)
• fits snugly to reduce friction (link to swimming)
• microvilli hold skin to the underlying muscles

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Variable Thickness

• with age - one cell think in larvae (for
  respiration) ? top figure
• gills not formed, JFRBC 341554 in perch, gill
  filaments develop at 18 mm long ? bottom figure
• on body - head skin thick (no scales there)
• by sex - greater epidermal thickness and more
  mucous cells in the sex that builds the nest
• female thick skin in salmon, suckers, minnows
  (Copeia 1970332, and 5079)
• male thick and mucus in stickleback (Copeia
  1979533)
• by species - (epidermis only) in adult guppy 2
  cells think in eel 12 cells
• by season - skin thicker in cold (the cell
  hyperplasia or coldwater disease of plaice (cited
  on p 718, JFB 33)
• through stress - thicker in carp in fertilized
  water and in parasitized fish (JFB 33)

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Skin Cross-Sections
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Epidermis

• Cuticle (Whitear, 1970, recent EM work)
• an external surface coat called the glycocalyx
  (JFB27395) is a product of the epithelial cells,
  usually lost in histological preparations, one
  millimicron thick, attached to surface by
  microvilli (micro-ridges) of epithelial cells,
  make up unknown formerly thought that fish were
  covered with living cells, no dead cell layer
  like human skin, impedes establishment of
  bacterial colonies on skin
• Epithelial cells (filament containing cells of
  Henrickson and Matoltsy 1968), epithelial cells
  are columnar at basement membrane and horizontal
  at surface. Columnar cells very active
  mitotically, especially in wound healing and in
  cold water. They contain tiny filaments of with
  an unknown make up, maybe source of cuticle
• This layer of epithelial cells is also called the
  stratum germinativum
• EM work has shown epithelial cells to have
  micro-ridges that look like a thumb print and
  which may be species specific??
• I have a EM picture of this on gills, show p 19
  in salmon histology book.
• Basement membrane, single band of CT with light
  microscope, but with EM
  much convoluted
• Functions of epidermis (mucus cells, wound
  healing, cover)

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Dermis (Roberts et. al. 1970) and Hypodermis

• DERMIS
• Connective tissue can be two layers (trout) or
  one layer (catfish book)
• UPPER LAYER is STRATUM SPONGIOSUM or PAPILLARY
  LAYER
• Stratum spongiosum - because this layer of
  connective tissue (CT) is loosely and irregularly
  arranged fine CT (collagen) in salmon it forms a
  matrix for melanocytes.
• LOWER LAYER is STRATUM COMPACTUM
• Stratum compactum - dense CT, collagen, only cell
  found is fibroblast (few melanocytes, lymphatics,
  nerves, etc). A blast cell produces other cells,
  a precursor - to scales and lepidotricha, has a
  plywood nature, CT cells at right angles are
  called orthogonally arranged (p18 salmon
  histology book)
• Function (produce scales, holding cover,
  pigment), close wounds - contraction, pigment
• Hypodermis
• A third "layer" is the Hypodermis (subcutis) -
  area of loose connective tissue between dermis
  and muscle (hypo under), varies in thickness on
  body, thicker in parts of the head (gives form?)
  and fins, if adipose tissue is present, it is
  found in the hypodermis (from catfish book)
• Function (holding dermis, fat layers)

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OTHER COMPONENTS OF THE SKIN

• NERVES (Whitear 1889)
• Nerve bundles with myelin sheath course through
  the dermis
• Loose sheath and break-up when they enter
  epidermis
• End among epidermal cells as free nerve endings
• Tactile, temperature sensors, possibly pain
• POISON GLANDS AND LIGHT ORGANS
• Over 200 species are armed or lighted in some way
  but they are the marine forms - the stingrays,
  marine catfish, weever fish, and stargazers.
• Of importance to freshwater biologists are only
  the weak poisons of the catfishes. Little work
  done but the epithelium and spine of channel
  catfish are toxic to mammals, no effect on
  Gambusia, black bullhead great effect
• (see Comparative toxicology of stings, Comp.
  Biochem. Physiol. 22101-111, injected into
  Gambusia and measured amount of fish darkened,
  tissue sloughed, or death)
• Madtoms (Schilbeodes sp) have the worst
  reputation. They have a gland at the base of the
  pectoral spine(catfish also) but there is no duct
  up the spine and the nature of the gland contents
  is unknown. Other spines are as toxic as the
  pectoral in catfish. When spine is stuck in your
  finger, the skin tears liberating toxic
  substances that get in the wound rather than
  being "injected", (picture of axillary gland on p
  42 of catfish book)

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Club Cells or Fright Substance Cells

• Schreckstoff Cells
• Alarm Substances - substances which communicate
  the presence of danger and will illicit an odor
  dependent escape response in another fish.
• 91 species in 44 families have these pheromones.
• Little known about chemistry of alarm substances.
  
• Odor can be both intra- and interspecific
• One species can react to the alarm substances of
  another

Question What type of reactions might prey fish
make when sensing the fright substance pheromone?
Answer types of reactions vary - Phoxinus minnow
motionless, Tinca tench swim to bottom and stir
up mud Esomus jump at surface.
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Scales

• Dermal armor -discuss, emphasizing bony ridge
• Scale type
• placoid.
• ex._________
• ganoid..
• ex._________
• cycloid.
• ex._________
• ctenoid.
• ex._________
• scutes..
• ex._________

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Scale Compositions

• Bone is fibrous CT arranged in plates, salts 20
  with live bone cells (osteocytes) within lamellae
• Dentine - fibrous CT matrix, harder than bone
  with cells outside with processes entering a
  dentinal tubule
• Enamel - no CT, salts 90, no cells, salts
  arranged in lamellar like prisms, hardest
• Bony ridge scales - Formation -In dermis, certain
  CT cells "round up" and begin manufacturing the
  basal plate below and the calcarious deposits
  above (Are these cells osteoblasts?)

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Scale Growth

• Growth - scales grow by fibroblast cells that are
  numerous at the scale edge. They enlarge the
  scale by adding collagen rings called circuli.
  When growth is slow, circuli are close together.
• cold decreases appetite and metabolism
• fasting during spawning
• need for calcium, decalcification (reabsorption)
  in females developing eggs.

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Juvenile Scales
Question Scale pressing -Have you pressed scale
impressions into acetate and found only the
outline of the scale??
Answer You got it upside down, a trick played on
you by a few dermal cells.

• Location in juveniles
• (transparency series showing that in general
  scales are first formed at caudal peduncle
  (walleye, pike, drum, crappie) sometimes on belly
  or anterior lateral line (carp)

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Back Calculation

• body - scale relationship
• relates to back calculation of length from scale
  data. Line comes to 30 -40 mm on y axis -total
  length axis.

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Mucus

• mucus -the clear, viscous secretion, a noun
• mucous - relating to mucus, an adjective
• mucous cell - a one cell gland
• unicellular, not found in other vertebrates
• progress from germinativum to surface where they
  release mucus by dehiscence (a
  bursting open)
• within cell are vesicles which rupture (not
  necessarily, but may be found in mucous layer of
  fish

Question Why only a one-celled gland??
Answer More organized glans are difficult to
accommodate in the scaly surface.
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Mucus

• Cell distribution -more cells on anterior portion
  more on body than fins (fungus first)?
• (transparency of salmon head and tail skin)
• The mucus
• synthesized by golgi apparatus (prominent in
  cells with secretory activity)
• glycoprotein - (glycogen amino), and sialic
  acid a polysaccharide, same family as starch,
  glycogen, mucilage, cellulose, etc.
• synthesis under endocrine control

QUESTION By now you are probably wondering , how
can I get some mucus??
ANSWER Check PFC 4l18 for the answer. (suction
through Erlenmeyer like sperm suction apparatus,
authors cover disadvantages of other methods if
fish is to remain healthy)
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Mucus Microscopy
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Mucus Functions
Thought question Form a working hypothesis on
the mechanism that explains each function of the
skin before going further in the lecture.

• FUNCTION
• wound healing
• combat disease by physical and antibodies
• precipitate, reduce, and remove heavy
  metals/sediment
• lubrication
• communication and smells
• osmoregulation -may act as a barrier
• stress indicator -hemastix

• MECHANISM
• ______________________
• ______________________
• ______________________
• ______________________
• ______________________
• ______________________
• ______________________

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Wound Healing (resulting from-)

• Wound healing - consequences of wounding from ?
• parasites 50 anchorworms 5.3 of body of 12
  inch trout
• scales removed from 25 of body
• none lethal but do allow bacteria in (see Berry
  et al, JWD 27206, Utah Acad??)

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Wound Healing (process)

• 2 steps in wound healing (JFB36421, JWD 27206
  TAF 119145)
• 1)quick
• mucus forms a plug, profuse mucus secretion,
  mucous cells in area reduced
• epidermal cells moved toward cavity without
  becoming disconnected from each other,
  re-epithelialization
• alkaline phosphatase a lysosomal enzyme produced
  by mucous and epithelial cells
• large holes - CT contracts wound margin (TAF 119)
• razor blade cut closed in carp in 6 hr at 20C
• transmitter hole in catfish heals in 93 D at
  22-25C
• 2) slow
• dermal capillaries moved into epidermis
• fibroblasts secret much collagen
• white blood cells abundant
• melanocytes darken area (melanin may be
  bactericidal)

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Wound Healing (Summary)

• First Intention Healing or epithelialization
  occurs when wound is small and edges close,
  healing takes about 5 d.
• Second Intention Healing occurs when wound edges
  are far apart, healing by granulation tissue plug
  that contracts wound margins, then covered by
  skin at 43 d, histologically normal at 93 d.

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Protection Against Disease
QUESTION If fish defend themselves with mucus,
where on the body are they most vulnerable?
ANSWER Zoologica, fish which demonstrated
immunity towards a monogenetic trematode had
greatest infection in the EYE

• Physical
• thickened whorls of mucous/epidermal cells seen
  in anchorworm infections
• mucus replacement prevents colonization by
  bacteria, parasites, fungi (leucocytes invade
  epidermis to help out)
• Antibodies
• Furunculosis - Aeromonas salmonicida. Rainbow
  trout recognized for years as being resistant,
  partially because of serum protein (antibodies)
  that neutralize the toxicity of the bacterial
  products.
• An example was Reed's Creek, W. VA where 25
  brown trout, 6 brook trout, and 0 rainbow trout
  died while each species had similar antibody
  titers in the blood
• So why the graded differences in mortality?
  ?Next Slide

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Differential Immunities (results)

• The resistant fish produced a mucus precipitin
  activity when reacted with cell-free bacterial
  extracts in immunodiffusion assays ?
• Results -the mucous precipitin activity against
  A. Salmonicida correlated with resistance of
  species
• breeding studies with brown trout showed that
  fish selected for high level of mucus precipitin
  produced progeny that were more resistant
• the mucus also combated Vibrio anguillarum and A.
  hydrophila.(TAF 11583).

QUESTION If immunity is genetic, can the fish
culturist breed a strain of disease resistant
fish?
ANSWER Breeding studies with brown trout showed
that fish selected for a higher level of mucous
precipitin produced progeny that were more
resistant
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Contaminants and Sediment

• sediment trapped and sloughed, thick mucous coat
  visible on fish, mucous cells disappear in 3 d in
  carp in ponds with manure (aquaculture in Israel)
  (JFB33711)
• chromium reduced from hexavalent forms by
  protein-bound sulfhydryl groups and decrease Cr
  penetration rate (Chemosphere 20397-402)

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Communication and Smells

• fish odor is in the mucus? (may be an old wives
  tale)
• bullheads that use mucous smell to identify
  individuals (The Chemical Language of Fishes
  (Sci. Amer. 224(5)98-108)
• Alarm substances -substances which communicate
  the presence of danger and will illicit an odor
  dependent escape response in another fish.
• Types of reactions
• Phoxinus minnow motionless
• Tinca tench swim to bottom and stir mud
• Esomus jump at surface
• hatchet fish form dense school
• cavefish has no predators but has the cell, when
  sensing it, begins to look for food
• cypriniformes order -minnows, carps, suckers
• tadpoles have it but fish do not react to tadpole
  injury
• 91 species in 44 families
• little known about chemistry of alarm substance,
  etc. odor is interspecific, one species will
  react to alarm substance of another

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PIGMENT CELLS(Sci. Amer. reading covers
structural color)good coverage in Bond and
Lagler et al.

• Iridocytes - mirror cells or schematochromes,
  also called leucophores - reflect light from
  intracellular pigment organelles which contain
  guanine
• Purines -mostly guanine are crystalline
  substances but are colorless and
  non-motile in chromatophores
• They are reflective producing the silvery sheen
  common in most fish and can be
  called either an iridocyte or iridophore
• stratum argenteum in pelagic fish is a sheet of
  guanine cells (iridophores)
• Examples are many
• the silver in a seine haul
• kokanee immature - lake fish
• kokanee in spawning (pair and group)

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Pigment Cells (continued)
Thayer's principle and Obliterative Shading

• Chromatophores -pigment cells that have true
  color pigments called biochromes
• irregular in shape
• branching permits fish to change color by moving
  pigment granules to the center pale spread
  out color
• under nervous and hormonal control, but some
  references claim stress hormones cause both
  pallor and darkening
• adrenalin causes aggregation pallor
• pigment cells found in dermis primarily but also
  in epidermis and hypodermis, in peritoneum,
  eyes, etc.

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Color in Fish
QUESTION How do taxonomists use peritoneum color
to distinguish between Notropis ortenburgeri and
N. telescopus?
ANSWER The peritoneum or lining of the body
cavity is silvery in ortenburgeri and brownish or
heavily covered with melanophores in telescopus
(Eddy, p 97)

• Melanin -dark red, brown, black, may be
  bacteriocidal. Cells containing these colors are
  called melanophores.
• Examples
• Black bullhead I. Melas
• black spots on channel catfish
• light and dark paddlefish, dark just caught
• bars on northern pike
• small mouth bass/largemouth bass
• bluegill with pumpkinseed (note carotenoids)

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Colors in Fish

• Carotenoids -reds and yellows. Cells containing
  these colors are called xanthophores. Yellow over
  a structural blue ???? Green
• Examples
• most reds in coldwater fish
• most famous is the rainbow of the RBT, but
  rainbow missing from lake fish
• the cut - throat of the cutthroat
• cutthroats in breeding color
• spawning males of mountain sucker, female at same
  time
• finescale dace
• red is attractive (attracts predators) but is the
  first color filtered out in water, a good
  compromise color for spawning fish since visible
  up close but not over distances
• Unusual colors
• albino and albinism reported in many fish
  -flatfish TAF 96400, madtoms
  101566, catfish Copeia 1944124
• blue trout
• golden (black eyed albino) and blue
• California golden - most beautiful fish
  

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Color Patterns

• RELATIONSHIP BETWEEN MELANOPHORES AND IRIDOPHORES
  Iridophores are clustered in the dermis in
  groups of 2 - 10 cells, The melanophores lie
  beneath but have their dendritic processes
  projecting upward and enclosing the reflecting
  cells. Melanophores act like blinds (see
  enhanced skin handout and transparency from
  Salmonid anatomy atlas)
• Color pattern (from Schreck and Moyle, editors,
  Methods in Fish Biology)
• Primary pattern
• aligned with scale rows, longitudinal,
  transverse, diagonal
• not aligned with scale rows blotches,
  reticulations
• Secondary patterns
• Ocelli -spottail bowfin
• concentric and radial patterns
• combinations

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Significance of Coloration to the fish

• Resemblance - fish to background
• Obliterative shading -Thayers principle
• Smolts change from using resemblance to
  obliterative shading when the migrate to the sea
• Disruptive coloration - reef fish
• Advertisement - reef fish
• Communication in Tilapia mossambica
• Quantification of patterns (Schreck and Moyle)
• for taxonomic purposes, remember, color is the
  most variable characteristic of a species, and
  may change with time of day, age, and environment

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Management Implications
QUESTION How do fish managers use fish color?
ANSWER For taxonomic purposes (see Eddy) or
identification of reproductive condition, stress,
disease (blackspot, whirling, etc.), and stocks.

• 1) Using scales to distinguish wild vs hatchery
  species (TAF 117) and genera (Lagler)

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Management Implications

• 2) Taking Scales

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Management Implications

• 3) Marking
• subcutaneous tags
• fluorescent pigment

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Management Implications

• 3) Marking
• branding

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Management Implications

• 4) Stress/Starvation Indicator
• dying fish often are schymotic (blotched) because
  Hb from destroyed RBCs is released into tissues.
  Free Hb infiltrates the skin into the mucous and
  is discharged by mucous cells at surface of fish.
  Hemastix impregnated with chemicals which give a
  colorometric change in the presence of Hb gave a
  positive result if fish held in net in air for 2
  min. JFB 9537
• The authors later tested for ketones which
  accumulate as a result of cellular starvation, a
  catabolic state, the data were very messy, even
  control group showed mild reaction on stix,
  nevertheless the authors concluded that they had
  demonstrated its value in detecting starvation.
  jfb 12105 (Good example of biologists trying to
  find rapid evaluation technique)
• 5) non-lethal health inspection - sampling fish
  mucus rather than kidney for A. Salmonicida
  detection, mucus was better than kidney for fish
  in an epizootic, subclinically infected, and
  asymptomatic carriers (See RIB 199249)