Mechanism of hormone action

Two principal types of hormone – receptor interactions are recognized

I- In one type, lipid soluble substances, including steroid hormones, 1, 25- dihydroxy cholecalciferol, and thyroid hormones, pass freely through the plasma membrane and bind to a cytoplasmic receptor.

The hormone – receptor complex translocates to the chromatin of the cell nucleus to initiate the DNA- directed synthesis of mRNA.

II- The second type of hormone- receptor interaction involves water soluble substances, which can not permeate through the cell plasma membrane.

Their first contact with the cell is to bind to protein receptors on the outer membrane surface.

In order for the message to reach intracellular sites, the hormone receptor complex can be internalized (endosytosis) or the information could be transferred at the membrane site (transduction) and carried by other mediators or messengers within the cell compartments.

Cellular activity is enhanced by removing the membrane restraint by increasing its permeability to the flow of ions or organic substances.

Hormone binding to the cell membrane or by indirectly affecting sub cellular membrane is in effect removing a restraint to cell activity or subparticle interaction.

Water soluble hormones, neurotransmitters, or other agents (lectins and growth factors) bind to a cell surface receptor as a result of a non-covalent association with a relatively high affinity to initiate a set of membrane events.

As a result there is a release of mediators (secondary messengers) that react with cytoplamic components.

Most polypeptide hormones stimulate the production of cAMP as the secondary messenger. There are important exceptions to this i.e. Insulin and prolactin activity can be expressed without direct stimulation of adenylate cyclase.

Cyclic Nucleotides as second

messengers Activation of adenylate cyclase as major route of

hormone stimulation can be demonstrated by the addition of cAMP or dibutyryl cAMP to a target cell. The addition of the nucleotide mimics most of the actions of some hormones on cellular events.

This is consistent with the effect of methylxanthines, such as caffeine, as inhibitors of phosphodiesterase, the enzyme that converts cAMP tot eh inactive 5'-AMP.

The number of receptors on the cell membrane is relatively low (5000 – 10000 per cell). In most systems relatively few receptors, 10-20% of the total present, need to be occupied to produce a maximum response of the cell.

Pineal organ of teleosts The pineal organ is a part of the central nervous system

and is formed, like the retina, as an evagination from the embryonic primary forebrain (dorsal roof of the diencephalon). During vertebrate evolution the pineal organ has transformed from a photosensory organ into an endocrine gland.

In most poikilothermic vertebrates, the pineal complex has two components.

In fish, they are the pineal and parapineal organs.

The parapineal organ remains more or less rudimentary, close to the habenular nuclei of the diencephalon while the pineal organ grows to form a relatively large vesicle located dorsal to the forebrain, immediately below or within the skull roof.

The pineal organ is often differentiated into a proximal slender pineal stalk and a distal expanded end vesicle.

In some species, the adult pineal organ is very large and covers the entire telencephalon.

The wall of the pineal organ i.e. the pineal epithelium, is in principle a unistratified epithelium, which may be strongly folded in many species and may almost obliterate the central lumen of the pineal organ.

The pineal epithelium of teleost fish consists of photoreceptor cells, neurons and ependymal interstitial cells which are often called supportive cells, interstitial cells or glial cells.

In addition, oligodendrocytes form myelin sheaths surrounding some neural axons while macrophages are found mainly in the central lumen.

The photoreceptor cells are the source of the melatonin produced by the pineal organ.

The lumen of the pineal organ remains as an extension of the 3rd ventricle of the brain in adult teleosts.

The cerebropinal fluid (CSF) of the ventricular system is thus potentially a means of transport for various chemical compounds between the pineal organ and the brain.

The pineal organ is well vascularized. The blood vessels do not penetrate into parenchyma but remain outside the basal lamina in the perivascular space.

The pineal organ generally lacks a blood-brain barrier, as reflected by the presence of fenestrated capillaries.

The photoreceptor cells and the ependymal cells are tightly coupled by special intercellular junctions, forming a barrier for intercellular diffusion between the CSF of the pineal lumen and the perivascular space.

Thus the pineal parenchyma is exposed to an extracellular environment reflecting haemal composition basally, and CSF- composition apically i.e. towards the pineal lumen.

Photoreceptor cells Pineal photoreceptor cells may be distinguished into apical

(receptor) and the basal (effector) poles.

The apical pole contains the outer segment, which contains the light – sensitive photopigment, and the inner segment which contains numerous mitochondria and (sometimes visible) longitudinal cytoskletal elements.

The outer segment develops from 9×2+0 stereocilium, forming a stack of membrane lamellae that contain the photopigment.

The basal pole is the photoreceptor axon, which forms contacts with postsynaptic neurons or with the basal lamina that surrounds the pineal parenchyma.

The apical pole may vary in shape and size.

The outer segment lamellae may form a regular cone shaped stack, similar to that of retinal cone photoreceptors, but more often they form a dome-or cup-shaped stack.

Often, the outer segments are irregular in shape and may consist of a mixture of lamellar and tubular membrane formations.

The inner segment is typically a short, rounded structure, packed with mitochondria.

It is elongated in some photoreceptor cells when it is usually attached to a short cone-shaped outer segment.

The basal pole is also variable.

The large majority of the photoreceptor cells that are clearly pre-synaptic to intrapineal neurous possess short axons.

However, a small number of photoreceptor cells possess long axons that have been shown to project directly to the brain.

There are numerous photoreceptor cells that are not clearly presynaptic to neurons but possess basal processes that terminate without synaptic specializations on the basal lamina.

Photoreceptor axon terminals impinging mainly on the dendrites but also on cell bodies and axons of intrapineal neurons, typically contain so-called synaptic ribbons.

This type of photoreceptor-to-neuron contact has been observed in most teleosts.

Interstitial cells The large portion of the cells of the pineal parenchyma

consists of so-called supportive or interstitial cells.

They are located between the photoreceptor cells to which they are attached by tight junctions which thus form a diffusion barrier between CSF of the pineal lumen and the extra-cellular fluid surrounding the photoreceptor cell bodies and axons, and the intrapineal neurons.

In most species they appear to have widely extending basal poles that line the basal lamina forming a sheet that separates neural elements and the basal pedicles of most photoreceptor cells, from the perivascular space.

In view of their location (i.e. lining the ventricle) and their cytoskeletal constituents, this cell type fulfils criteria for both ependymal and glial elements.

NEURONS

The pineal organ of some teleost species contains an impressive neuronal population.

It appears that the vast majority of intrapineal neurons send axonal projections to the brain i.e. they are analogous to the ganglion cells of the retina.

Intrapineal neurons are postsynaptic to photoreceptor cells.

The neurons also form conventional synapses between each- other as well as with photoreceptor cells.

Axo-axonal synapses have been observed in the pineal tract.

The vast majority of the axons in the pineal tract (axonal projections from intrapineal neuronal population and some photoreceptor cells) are unmyelinated, but oligodendrocytes form myelin sheaths around a small traction of the axons that project to the brain and a small number of myelinated axons have been found in fishes.

Macrophages Macrophages have been noted to be present in the pineal

lumen in several species, often in close contact with photoreceptor outer segments.

They are believed to be involved in the degradation of degenerating photoreceptor cells and / or photoreceptor outer segments.

Also macrophages have been observed in the pineal epithelium and perivascular space where they are believed to take up and digest substances penetrating from the bloodstream.

Melatonin Synthesis (Photoperiod vs endogenous rhythm)

The synthesis of melatonin by the pineal organ is regulated by the intensity of the ambient illumination and reaches the highest level in complete darkness i.e. under natural conditions, during the night.

Melatonin synthesis by the pineal organ is regulated by the ambient light-dark cycle, being high during the dark period (scotophase) and low during the light period (photophase).

Pineal serotonin content also shows a diurnal rhythm, with the highest level during scotophase in the pike, during dark-light transition in the eel and with higher levels in continuous darkness in continuous light.

The melatonin synthesis profile of the pineal organ is reflected in the plasma levels of melatonin : melatonin levels are highest at night and low during day.

Removal of pineal organ may or may not eliminate the nocturnal rise in plasma melatonin.

While melatonin synthesis by the mammalians gland is indirectly controlled by the environmental photoperiod via the cir cadian oscillator in the hypothalamus but it was early shown that in the chicken pineal organ melatonin synthesis is directly controlled by photoperiod and an intrapineal circadian oscillator.

There is evidence for intrapineal oscillators also in birds in some lizards and inn teleost fishes.

In all salmonids studied to date, melatonin synthesis is directly controlled by the light-dark cycle.

There is no evidence for endogenous rhythm in melatonin synthesis and there is thus strong evidence that the pineal organ does not contain a circadian oscillator in salmonids.

The pineal organ does not contain a circadian oscillator in salmonids.

In certain other fishes cultured pineal organs display an endogenous circadian rhythm in melatonin synthesis.

This rhythm persists in continuous darkness for atleast a few cycles in pineal tissue culture as well as in pineal photoreceptor cell culture.

It shows that circadian oscillator controlling melatonin synthesis is located in the pineal organ itself, most likely in the photoreceptor cells.

Melatonin Biosynthesis

Melatonin is also synthesized also in the retina.

So far there is no evidence for an endogenous melatonin rhythm in the retina of teleosts.

Melatonin produced by the retina is believed to have mainly intraretinal actions, particularly light- and dark adaptation.

Melatonin is rapidly metabolized within the eye by the enzyme melatonin deacetylase.

Still retinal melatonin may contribute to circulating melatonin levels in a species specific manner.

Still another possible source of circulating melatonin has to be considered.

In mammals and birds, the gastrointestinal tract has been demonstrated to synthesize melatonin.

So for a no such report for teleosts is there.

Melatonin production by the retina reportedly is higher during the day than during the night in the rainbow trout, pinealctomy does not abolish the elevation of plasma melatonin during the night, although plasma levels during both day and night as well as the magnitude of the rhythm are decreased.

In the goldfish pinealectomy does not abolish plasma levels of melatonin completely.

The gastrointestinal tract is a possible source of this ‘residual’ plasma melatonin.

Physiological role of the pineal organ and melatonin Circadian locomotor rhythm, behavional thermoregulation,

body growth and metabolism, pigment cells, during ontogeny and other physiological and behavioral parameters apart form reproduction are influenced by pineal organ.

Reproduction

Experimental approach- Effects of pinealctomy and / or melatonin administration under different photothermal regimes, at specific stages of the reproductive cycle.

Effect have been shown to vary with gender, photothermal regimes and reproductive phase moreover different species show different sensitivity to pinealctomy and / or melatonin administration.

The effects of pinealectomy vary with species and with reason within any species. Pinealectomy apparently affects the timing of reproduction, as demonstrated by effects on gonadal maturation regression or recrudescence.

In the goldfish pinealctomy may lead to a presvulatory gonadatropin surge and ovulation at the ‘Wrong’ phase of dark-light cycle.

In ♀ Clarias batrachus pinealctomy initiates gonadal recrudescence in the resting phase, and there after accelerates gonadal development when performed during the preparatory phase. No effect in prespawning, spawning or postspawning phase.

Administration of melatonin during prespawining phase reduces plasma levels of oestradiol – 17B etc.

A general observation is that daily melatonin injections attenuate ovaprim development induced by long-day conditions.

In the atlantic samon and rainbow trout, administration of melatonin implants does not affect timing of spawning although pinelctomy does affect timing, indicating the importance of the cyclicity with low day time melatonin levels.

There is no evidence indicating that the pineal organ is necessary for reproductive responses to photoperiod.

There is no evidence that removal of the pineal organ abolishes photo periodic responses, however, it does have an effect.

Although melatonin often attenuales progonadal effects of long days, there is currently little evidence in support of a major role for melatonin in the control of reproduction in teleosts.

In principle in ‘long-day breeders’ the pineal organ appears to exert an inhibitory influence on reproduction in short photoperiod and stimulatory influence the pineal organ exerts a stimulatory influence in short photoperiod.

Body growth and Metabolism

Photoperiod and temperature control seasonal changes in lipid metabolism and growth.

The pineal organ appears to be involved in the mediation of photoperiod and temperature signals.

In the goldfish, pinealectomy induces a reduction in growth (body weight and linear growth) in fish (sex not specified) kept under artificial short days except during summer.

In another cyprinid, the goldfish shiner, Notemigonus crysoleucas, body lipids increases in fish of both sexes kept at low temperature during the preparatory, prespawning and spawning periods but decrease when the fish are kept at high temperature during the same periods.

The changes are larger in artificial short day conditions than in artificial long days.

Pinealectomy attenuates or counteracts the changes seen in short day conditions, but enhances the changes seen in long day conditions.

Interestingly the effect of pinealectomy appears to be greater as day length increases.

It is well established that melatonin induces melanophore contraction in teleosts.

The precursors serotonin and N-acetyl serotonin were without effect but the metabolites like 5-methoxy tryptamine and 6-hydroxy melatonin were more effective in pigment aggregation than melatonin.

Melatonin may also induce melanosome dispersion in some populations of melanophore.

Melatonin may weaken the melanosome-aggregating effect of melanin-concentrating hormone and enhance the aggregating effects of α2- adrenergic agonists.

The pineal organ and melatonin do not appear to play any role in changes in body colouration during background adaptation.

Pigment cells

Environmental influences on fish Environmental influences on fish reproduction reproduction

Main three ecological (environmental) regions

1- Temperate – Low temperature with more difference in night and day length. Fishes inhabiting temperate regions are referred as temperate fishes. European and American sub-continent

2- Sub tropical – Moderate fluctuations in temperature and night and day length with prominent rainy reason between summer and winter seasons. Indian Subcontinent

3- Tropical- Usually high temperature and long day length throughout the year with intermittently lowering of temperature associated with rainfall. African sub-continent.

Main reproductive processes of fish influenced by environmental

changes 1- Gonadal development (gametogenesis)

2- Spawning Main Environmental factors associated with

gemetogenesis/ spawning 1- Photoperiod 2- Temperature 3- Substrate / nesting 4- Rainfall, flood, water flow 5- Oxygen, pH, Salinity 6- Barometric pressure, lunar cycle, social factors etc.

Environmental influences on Gonadal Development (Gametogenesis)

Temperate species –

♦ Much research done in temperate species. Most of studies on freshwater species and some on marine or estuarine ones.

♦ Of the environmental factors, photoperiod and temperature are generally recognised as the most important cues in the timing of gametogenesis in temperate species.

Photoperiod –

♦ In species which spawn in spring or early summer gonadal recrudescence is often stimulated by long photoperiod, particularly in combination with warmer temperatures.

Examples –

Gasterosteus aculeatus, Gambusia affinis, Notemigonus crysoleucas, Oryzias latipes, Carassius auratus.

As abrupt increase in photoperiod, which is commonly used in studies is not ecologically meaningful because fish are normally exposed to a gradually increasing photoperiod except some as Phoxinus phoxinus which comes out suddenly from its hide out.

In contrast, in species which spawn in autumn or early winter, gonadal recrudescence is often favoured by short or decreasing photoperiods as in Salmonids and ayu.

In the rainbow trout, Salmo gairdneri, a decreasing photoperiod is much more effective than a constant short photoperiod in stimulating gametogenesis.

Light intensity is often not considered in photoperiod studies. The photoperiod effects in the ayu are dependent on light intensity.

Temperature

Temperature has often not been considered in photoperiod studies, therefore it is not clear whether the photoperiod effects reported are temperature dependent.

Temperature dependency of photoperiodsim has been noted in some species.

Long photoperiod stimulate gametogenesis only if combined with warm temperatures as in N. crysoleucas, Gambusia.

In some species long photoperiod are stimulator at both warm and cold temperatures as in C. auratus.

In salmonids decreasing or short photoperiods stimulate gametogenesis regardless of temperature.

Temperature may even play dominant role in sexual cycling in some species (longjaw goby, Gillichthys mirabilis).

Temperature may exert its effects by

1- A direct action on gametogenesis

2- An action on pituitary gonadatropin secretion

3- An action on metabolic clearance of hormones

4- An action on the responsiveness of the liver to estrogen in production of vitellogenesis

5- An action on the responsiveness of the gonad to hormonal stimulation

Sub-tropical or sub-temperate species

In these regions seasonal variations in photoperiod and temperature are relatively small.

Nevertheless several species have been found to respond to such change.

Both photoperiod and temperature affect gonadal recrudescence in the Indian catfish, H. fossilis but temperature is apparently the more important factor.

Maintenance of female H. fossilis under continuous darkness or light at 250C for 34 month did not eliminate the reproductive cycle but only modified it. Therefore an endogenous in the control of sexual cycling in H. fossilis is suggested.

Long photoperiod also stimulate gonadal development in two other Indian teleosts whose spawning seasons also fall during the summer months.

One is the catfish, Mystus tengara and the other carp, Cirrhinus reba. C. reba under long photoperiods also remian sexually mature for one month beyond the breeding season.

In those fishes which spawn during the winter months (gray mullet, Mugil cephalus) short photoperiod induces vitellogenesis but the magnitude of response depends on the temperature, being greater at lower temperature than at higher temperatures.

Tropical species In tropical regions (near the equator) photoperiod

hardly varies although temperature may change slightly in accordance with the wet and dry seasons.

Tropical species tend to have an extended spawning period, or even continuous breeding throughout the year but spawning peaks do occur.

Which are usually associates with seasonal rainfall and / or floods.

Little is known of environmental cues for such seasonal peals in reproductive activity.

Factors associates with rainfall or floods are more likely to be related to synchronization of final maturation and spawning rather than gametogenesis.

Environmental influences on spawning

Considered under spawning are several physiological processes. Oocyte matur lation (germinal vasicel breakdown GVBD), ovulation and oviposition in female and spermiation and sperm release in the male. These stages are expected to require precise environmental cues for synchronizatin. Failure at these (particularly ovulation) is often reported for captive fish in aquaculture.

A- Temperate species-

1- Temperature - In goldfish ovulation is influenced by warm temperature. In coldwater (12 ± 10C ), vitellogenesis can proceed to the tertiary yolk granule stage at the faster rate than in warm water but ovulation will not occur unless vegetation is present.

When the water temperature is raised to above 200C or higher, or the fish are transferred from cold to warm water, ovulation will occur in sexually mature goldfish within a few days even in the absence of vegetation.

Similarly in an Australian freshwater fish, Plectroplitus ambiguus, ovulation does not occur below 23.60C although spermiation can occur.

Warm temperatures have also been suggested to stimulate final maturation stages of spermatogenesis in the killifish, Fundulus heteroclitus and oogenesis in the marsh killifish, F. confluentus.

In Notemigonus crysoleucas, final oocyte maturation, ovulation and spermiation occurred only in fish exposed to a long photoperiod-high temperature regime, neither long photoperiod not high temperature alone was effective.

On the contray, autumn or winter breeders spawn at relatively low temperatures.

In the rainbow trout, low temperature are important to ovulation otherwise the ova survive only for a short time.

However, the changes in barometric pressure are considered more important.

The perch, Perca fluviatilis, spawns at around 11.50C. The winterflounder Pseudopleuronectes americanus, may ovulate at temperatures as low as 60C but not lower.

Aquatic vegetation enhances the ovalatory response of goldfish to warm temperatures (it can even induce ovulation in coldwater).

Whether vegetation or other spawning nesting substrates play a similar role in other teleosts is not known, but it is a common practice to introduction such substrates to spawning ponds of cultured fishes such as giant gouramy (Osphronemus gouramy), catfish (channel catfish, Ictalurus panctatus, C. batrachus, and C.carpio).

Other Factors-Several other factors have been reported to influence

spawning 1) Water Current –

The minnow P. phoxinus, will not spawn in still water.

2) Oxygen-

Low dissolved oxygen levels reduce or prevent spawning in the fathead minnow, Pimphales promelas and the black crappie Pomoxis nigromaculatus. However, a high oxygen concentration enhances or triggers ovulation in the carp C. carpio.

3) pH –

Spawning of some species may be inhibited in acidic waters.

4) Salinity –

In the sea bass, D. labrax oocyte maturation and ovulation will not occur in freshwater, although spermiation may occur in salinities as low as 1-2%.

5) Barometric pressure –

In rainbow trout, spawning activity appeared to coincide with an increase or decrease in barometric pressure (but not with high or low pressure as such).

6) Rainfall, flood, lunar cycle and social factors –

Also play important role in spawning and will be discussed collectively with sub-tropical and tropical species.

Environmental factors and spawning

(spermiation) Spermiation is less dependent on environmental

modulation than are oocyte maturation and ovulation.

As mentioned earlier in P. ambiguus, spermiation

can occur below 23.60C, but ovulation can not.

In D. labrax spermiation can occur in low salinities but ovulation can not. In cyprinids, spermiation may occur almost all year round, but ovulation normally can only occur in the warm season.

However, in a few species, spermiation is affected by environmental factors.

In the lake chub, C. plumbeus, high temperatures promote spermiation.

In contrast, in rainbow trout, spermiation occurs at low temperatures under decreasing photoperiod.

In the stickleback, androgen secretion and perhaps spermiation are controlled by photoperiod and influenced by temperature.

Sub-tropical and Tropical species In tropical and sub-tropical species, peaking

spawning activity is often associated with rainfall, floods or the lunar cycle.

Some temperate species living in lower latitudes may also spawn during floods but only when the temperature is appropriate.

Rainfall and Floods

Species that have been reported to spawn in relation to rainfall and / or floods include African catfish Clarias gariepinus, H. fossilis, C. batrachus, Indian major carps, Puntius spp. etc.

Further it is not clear what specific factors or factors associated with rainfall or floods are involved in spawning stimulation.

Lake (1967) suggested a factors (possible an oil, petrichor) from the dried soil when water comes into contact with it.

Sinha et. al. (1974) and Breton (1979) suggested numerous related factors, among them lowering of water temperature, petrichor from newly wetted soil, dilution of electrolytes eg. chloride (decrease in conductivity), increase in oxygen content and a change of pH.

No single factor has been identified, perhaps a consortium of factors is involved.

Lunar cycle Many tropical and sub-tropical marine fishes

exhibit lunar or semilunar spawning periodicity.

These include rabbit fishes, siganid species, milkfish, threadfin etc.

Some temperate species also show this phenomenon Ex. California grunions, Leuresthes tenuis and L. sardina, Fundulus heteroclitus.

Most of the fish spawn on or around the new or full moon in synchrony with the spring tides.

Timing of spawning to concide with the ebbing spring tides may be have the adaptive value of maximizing tidal transport of eggs offshore.

The milkfish spawns during the first-and last quarter moon (neap tides) with corresponding peak appearance of fry (3 week age) during the new and full moon.

Circadian spawning rhythm

Many species spawn at a specific time or period of the day.

Goldfish in warm water (210C) always ovulate during the later part of the dark phase despite alterations of photoperiods.

This diurnal periodicity of ovulation is disrupted in coldwater (120C) but appears to occur also in goldfish in tropics (260 – 310C).

In medaka, Oryzias latipes oviposition normally occurs within one hour after the onset of light, and shifts in daily photoperiod will induce corresponding shifts in the time of oviposition.

Constant light disrupts the periodicity and reduce the spawning frequency.

This suggested that the spawning rhythm is synchronized by the onset of light or darkness.

Inter-renal Tissue In majority of the teleostean fishes the inter-renal

and chromaffin cells are found diffused in the head kidney near the post cardinal vein.

The inter-renal cells usually occur in one or several layers along the vein.

In Puntius ticto forms a thick glandular mass and in Channa punctatus arranged as small lobules along the postcardinal vein.

The inter-renal cells are eosinophilic and columnar having round nucleus at base. At least two types of secretions are produced by fish inter-renal.

(a) Mineral corticoids affecting osmoregulation.

(b) Gluco-corticoids influencing blood sugar level.

LYM

RBC

UTVEIN

CH.C.

INT.R.

T.S. head kidney CH.C., Chromaffin cell; INT.R., Interrenal; LYM, Lymphocyie; RBC.

Red blood cell; UT., Uriniferous tubule

Presence of steroid converting enzymes, steroid precursors and corticosteroids have been shown in the inter-renal cells of fishes.

The inter-renal gland of some teleosts can convert pregnenolone and progesterone to cortisol and cortisone. These cells undergo changes in response to stress.

Atrophy of inter-renal after hypophysectomy shows its hypophyseal regulation.

The hormonal secretions of inter-renal may be under the control of ACTH or GtH depending upon time and fish. Secretion of ACTH is probably under the control of corticotropin releasing factor (CRF).

However the hypothalamic-pituitary - inter-renal (HPI) system is not as well defined in fishes as for mammals. The inter-renal of teleosts is considered homologous with the mammalian adrenal cortex.

Chromaffin tissue In teleosts the chromaffin tissue may be associated with

the inter-renal tissue and scattered in the head kidney of several teleosts. They may occur singly or in groups among the inter-renal cells or associated with the post cardinal veins.

Aderenaline, noradrenaline, dihydroxyphenylalanine (DOPA) and 5-hydroxytryptamine (serotonin) have been found to be present in the chromoaffin cells of fishes.

Extracts from the chromaffin tissue of fishes stimulate the sympathetic nervous system on being injected into another fish.

The hormone produced by the chromaffin tissue concentrates pigment granules in melanophores, controls blood pressure, and performs functions similar to the adrenal medulla of mammals, hence, is considered homologous with it.

Corpuscles of stannius

The Corpuscles of Stannius were first described by Stannius in 1839 as discrete glandular bodies in the kidney of sturgeon.

These are nodular bodies lying partly or completely embedded in the kidney of bony fishes on its dorsal, dorso-lateral or ventrolateral sides. They are generally oval or round in shape and vary in number from one to six in different species.

Histologically each CS is composed of parenchymal cells that are arranged in the form of follicles or irregular cords, separated by connective tissue and surrounded by a fibrous capsule. Two types of cells i.e. AF positive and AF negative are reported to be present.

Change in granulation of cells has been observed with changes in the salinity of water. Hypophysectomy causes loss of granules.

Removal of CS caused reduction in the plasma Sodium, and an increase in the potassium and calcium concentration.

By some authors it was considered as homologous with the adrenal glands of vertebrates and preferred to call them as ‘posterior adrenal glands’ because of its capability in maintaining electrolyte homeostatis.

The hypercalcemia could be alleviated simply by injecting CS extracts back into the animal

A connection between CS and calcium homeostatis was established with inference that the glands were the source of an antihypercalcemic hormone now known as stanniocalcin.

The hypercalcemia might be due to either decreased urinary calcium excretion or to increased mobilization of calcium from bone.

In many fish the removal of CS had no effect on the renal handling of calcium or for that matter, bone resorption.

However, hypercalcemia did not develop when fish was transferred to low calcium water following stanniectomy.

The gills were then pinpointed as the affected target organ as the rate of gill calcium transport increased dramatically following stanniectomy.

Then it was concluded that CS were the source of an inhibitor of gill calcium transport for the active principle is stanniocalcin (STC).

The CS plays a major role in regulating calcium homeostatis through the synthesis and secretion of stanniocalcin, a glycoprotein hormone.

Ultrastructural structures show an extensive network of rough endoplasmic and golgi and secretory granules in CS cells which is indicative of polypeptide synthesis.

Main role of stanniocalcin is prevention of hypercalcemia through a different mechanism unlike to calcitonin. Calcitonin inhibits osteoclastic bone resorption whereas stanniocalcin lowers the rate of gill calcium transport from the aquatic environment.

Mammals rely on bone as calcium reservoir while fish rely on the environment as their principal source of calcium and use the gills to draw from it according to metabolic needs.

Ultimobranchial Glands

These are located in the transverse septum between the abdominal cavity and sinus venous just ventral to the oesophagus.

These glands develop from pharyngeal epithelium near the 5th gill arch and resemble superficially with the parathyroid of higher vertebrates.

These glands are believed to be source of calcium regulating hormone, calcitonin of (thyrocalcitonin) as high concentration of calcitonin is found in the ultimobranchial glands of fishes.

Urohypophysis

A small swelling at the end of the spinal cord in the tail of teleosts is called Urohypophysis. The cells are similar to the neurosecretory cells of the hypothalamus its function is not definitely known.

Islets of Langerhans

The fish islets are fairly large in size, and are composed of three types of cells. Of these β cells are believed to secrete insulin and are stained dark purple with aldehyde fuchsin (AF) stain.

The AF negative cells are called α cells and are of two types α1 and α2. α1 are silver positive, becoming black with silver staining while α2 are silver negative.

In fishes α1 are more numerous than α2 cells.

The β cells are generally concentrated towards the centre of the islet but in some both cell types are scattered uniformly throughout the islet.

The β cells are generally polyhedral in shape with a large round central nucleus. The α cells are roughly triangular in shape with an eccentric nucleus.

T.S. Pancreatic Islet of a teleosts A., Alpha cell; B. Beta cell; BC., Blood capillary; CT., Connective

Tissue covering; EX. C., Exocrine cell., N., Nucleus

β cells appear to secrete insulin as they are selectively destroyed by β-cytolaxic drugs like alloxan and streptozotocin, resulting in hyperglycemia.

Injection of glucose causes rise in blood sugar and simultaneous degranulation and degradation of β-cells.

One of the two α cells is believed to secret glucagon.

It is also suggested that α-cells might be associated with secretion of a pancreatic hormone or a gastrin like substance.

Thyroid gland The basic histological unit of all vertebrate thyroid glands

is the follicle, a hollow ball consisting of a single layer of epithelial cells enclosing a fluid (colloid) filled space. The thyroid gland is a highly vascular assembly of follicles.

In each vertebrate group the shape of the gland is reasonably characteristic. Colloid is the site of storage of a protein bound form of the thyroid hormone. So this gland is unique in having out extracellular storage of hormone within the gland.

In teleosts there is usually no organized thyroid gland, instead the follicles are scattered singly or in small groups in the loose connective tissue under the pharynx. The pattern of distribution of these loose follicles usually follows the course of the ventral aorta and the roots of its branches into the gills. However there are some exceptional teleosts (Bermuda parrot fish, Swordfish, Tuna) in which compact thyroidal glands have been found. These may be bilobed, one lobe behind the other in the midline.

Perhaps the most unusual anatomic feature of the teleosts thyroid is its tendency to undergo widespread dispersion out of the pharyngeal area.

The most common non-pharyngeal site in which thyroidal follicles may be found is the head kidney. Thyroid follicles have also been found in brain, eye, esophagus and spleen. These ‘hypertropic’ thyroid follicles are believed to have reached different organs by migration from the pharynx.

The gland is highly vascular and is generally well supplied with blood. The epithelium surrounding the follicle may be thick or thin and the height of cells depends upon its secretory activity. The less active follicles generally show a thin epithelium with a collapsed one in some sections.

Epithelium is composed of two kinds of cells which are cuboidal or columnar in shape with clear cytoplasm. The colloid cells contain droplets of secretory material. The lumen of each follicle is full of colloid which may be basophilic or acidophilic depending upon the secretory activity of the follicle and may show vacuole in it.

Diagrammatic representation of thyroid gland in (A) typical (C) Two follicles of thyroid gland in T.S. CL., Colloid; FC., Follicle cell, TH., Thyroid

Follicle; VA; ventral aorta

Diagrammatic representation of thyroid gland in (A) typical (C) Two follicles of thyroid gland in T.S. CL., Colloid; FC., Follicle cell, TH., Thyroid

Follicle; VA; ventral aorta

The thyroid follicles extract inorganic iodine from the blood and store it in the cells, combine it with tyrosine and later release it in the form of thyroid hormone.

Mono-iodotyrosine, di-iodotyrosine, tri-iodothyronine and thyroxine are thyroid hormones of fish. The secretion of hormone by thyroid gland is under the influence of thyrotropic hormone (TSH).

The release of thyroid hormones from their bond with the glycoprotein thyroglobulin, which is main component of colloid, is also promoted by TSH. MIT and DIT do not normally leave the gland in significant quantities.

The iodothyronines released from the gland are bound to specific plasma proteins and are transported to various tissues in the body where they are metabolized. The secretory epithelium of the thyroid follicles increases in height in the presence of TSH.

In the presence of TSH all aspects of thyroid function are enhanced i.e. increased iodide trapping by the follicle cells, oxidation of iodide to reactive iodine, iodination of tyrosine creating first 3-monoiodotyrosine (MIT) and next 3,5 diiodotyrosine (DIT). T4 is formed by coupling of two DIT molecules with subsequent loss of an alanine side chain. Triidothyronine (T3) results from the coupling of one molecule of MIT with one of DIT.

Metamorphosis and growth

General metabolism

Oxygen consumption

Osmoregulation

Carbohydrate metabolism

Reproduction etc.

Thyroid functions

Secondary Sexual Characters of Fishes

Identification of male and female sexes can be done on the basis of the milt and eggs. The characteristics of sexual dimorphism or sexual difference that enable identification of the sexes are classed as primary and secondary.

Primary sexual characters are those that are concerned actually with the reproductive process. Testes and their ducts in the male and ovaries and their ducts in the female constitute primary sexual characters.

Secondary sexual characters themselves are really of two kinds, these which have no primary relationship with the reproductive act at all and those which are definitely accessory to spawning.

In hagfishes there appears to be title or no known sexual dimorphism.

In lampreys there are several steroid dependent secondary sexual characters. Such as modified anal fin, and cloacal swelling of females, and the urinogenital papilla and modified dorsal fin structures of males. These appear only at the time of maximal development of gonads.

Secondary sexual characters in female elasmobranchs are less striking than those in males. The mature female is larger than the mature male. The female cloaca is larger and more distensible and the female cloacal lining is thicker and more richly supplied with mucous and sensory cells.

The striking secondary sexual characters of the male are the so-called claspers or myxopterygia structures associated with the intromission of sperm. The claspers represent modified margins of the pelvic fins and consist of a pair of scroll like appendages which border the cloaca and which have an intricate jointed cartilaginous skeleton.

Some other characters are modified teeth, stronger jaws, placoid spines on the wings of some skates, which are claw like and retractile.

In the chimaeras the male develops a spiny, stout retractile knob, the frontal clasper on the upper part of the head.

In the bowfin, Amia calva, a dark eye spot develops in the tail region in the young of both males and females but becomes diluted and subdued in the adult females where as in the males it becomes intensified and develops a brightly colored ocellation (eye like ring) around it.

A genital papilla marks the male of Johny darter (Etheostoma nigrum), white bass (Morone chrysops), Clarias & Heteropneustes.

Pearl organs or nuptial tubercles appear on the males of many fishes such as the smelt (Osmerus) and most species of minnows (Cyprinidae) and suckers (Catostomidae). These tubercles are little horny excrescences that become evident just before the spawning season and disapper shortly after.

Fin roughness in male & its length, colour and swelling of genital aperture in female of Indian major carps is other example. In Mystus seenghala a conical genital papilla is present.

An accessory sexual character marks the males of several species in which the anal fin becomes enlarged into an intromittent, copulatory organ known as gonopodium. It occurs in Gambusia affinis, the guppy (Labistes) and in other live bearing topminnows (Poeciliidae).

In some fishes, the caudal fin may show sexual dimorphism. The lower lobe is greatly extended in the males of the swordtail (Xiphophorus helleri) and is somewhat enlarged in the white sucker (Catostomus commersoni).

Humped back and hooked jaws are found in the males of the pink salmon (Oncorhynchus gorbuscha).

Probably the most extreme form of sexual dimorphism occurs in the deap-sea angler fish (Photocorynus spiniceps) where the male is minute and little more than a parasitic sperm factory attached to the much larger female.

Breeding behaviour of Fishes

It includes a diverse range of activities such as sexual nest building and parental behaviors.Agonistic and migratory behavior are also included in it if they have relationship with the reproduction.

In the prolific open-water spawners, all the eggs do not mature at one time. Just enough mature in each batch. It is in order to avoid undue distention of female with the enlarged sex products.

Many fishes have a very short breeding season, males and females are in fully ripe condition at the same instant and both then are capable of exhibiting their complex breeding reactions. The secondary sexual characterstics develop simultaneously.

A few fishes show marvelously timed relationships for breeding. The best known of these is the grunion (Leuresthes tenuis) of the California coast. This fish spawns just after the turn of high tide at certain times of the year. It spawns out of water where they are carried by high tide.

It deposits eggs and sperm in pockets in the wet sand in such a position that eggs are not likely to be washed out by waves until two weeks or a month.

At the time of next high tide waves come on the sand again and stir up the sand where nests are placed, the young hatch almost instantly and go with the tide before it recede. In North America some fishes spawn at almost any time.

Most fish species have definite seasons for spawning as a part of their timed reproductive relationships.

1. Warm water fishes are summer spawners and coldwater fishes, fall and winter spawners.

2. Species tolerating intermediate temperatures are spring spawners.

3. Some tropical species spawn the year around.

Timing of reproduction also makes it possible for more than one species to use the same breeding grounds in a calendar year.

Thus in American streams spring spawning suckers (catostomus) may use the same nest (redd) sites as fall spawning brook trout (Salvelinus fontinalis).

For external fertilization, proximity of two individuals of the opposite sex for spawning is the most common means employed.

Actual pairing and some form of holding (amplexus)is sometimes used as a special development of proximity.

In pairing some fishes come side by side in actual contact and simultaneously emit eggs and sperms and in other instances the male twists his body around that of the female in a semicircle or even in a corkscrew spiral for a fish with much elongated body such as lamprey.

The parasitic dwarf male of deep-sea anglerfishes (Photocorynus) adheres to the skin of the female by its mouth. Here the male has solely a reproductive function. Concentration of eggs in masses is a feature

that ensures external fertilization.

Eggs are roped together with water jelly in perches. Eggs are massed underneath stones in Johny darter (Etheostoma nigrum) and fathead minnows (Pimephales).

The marine scorpion fishes (Scorpaena) make eggs into two-lobed baloon. Mass aggregations for spawning are also a means employed for ensuring external fertilization. In Bitterlings (Rhodeus) eggs are deposited in the another animal mussel.

For internal fertilization several devices have evolved in fishes. Most common among them is the placement of sperm by the male into the reproductive tract of the female in the process of intromission for which important adaptations include: special modifications of the pelvic fin (Elasmobranchi), anal fin (Poecilidae), and development of particular genital organs in the region of the genital pore (blind cavefishes, Amblyopsidae).

These structures bring the sperms into the oviduct of the female.

In Horaichthys (India) spermatozoa are packaged by male into a tight mass similar to speramtophore.

The sperm ball carrying an arrow tipped stalk is impaled on the skin of the female near the opening of the oviduct in a position ready to ensure fertilization as one egg passes at a time.

The basking shark, Cetorhinus maximus also has spermatophores.

Stimulus provided by one sex to the other to release the sex product at right time also a mean for ensuring successful fertilization. This is brought about by definite courtship behaviour patterns. In which round emission and pheromones play a role.

The male swim circular around the female or prod her, bunt her, rub, fan her to signify that now is the time.

Fish with best defined strongest courtships appear to have produced small numbers of eggs than those in which is little or none.

In fishes in which the males are very active courters they usually assume the task of caring for the eggs.

Various means for affording care to fertilized eggs and young by one or both sexes. (1) One mean is the get the eggs into the right place the place for which their developmental machinery is suited.

Anadromous sea lamprey (Petromyzon marinus), sea-run sturgeons (Acipenser) and salmons (Salmo, Oncorhynchus) ascend freshwater and freshwater eels (Anguilla) descends to sea.

Once on the spawning grounds and selection of spawning site completed careful nest building commences.

Best territory defending example is Siamese fighting fish, Betta splendens, which defends its territory to the death.

Of west building fishes there are broadly of two kinds. (1) Make a nest and then desert it (2) Build one and stay with it to guard the site, egg, and / or young.

Fishes, that build excavated nets then protectively fill them with stones interspersed with eggs, are the creek chub (Semotilus), the trouts (Salmo), the salmons (Oncorhynchus).

Fish excavate a nest and then defend it are bleentose and fathead minnows (Pimephales) and the sunfishes and blackbasses (Centrarchidae).

In all the male is the aggressive partner.

Bullheads (Ictalurus) dig a nest and stay on guard.

The Betta blows a surface bubble nest and defends.

Sticklebacks (Culaea and Gasterosteus) defend theirs made of twigs, leaves and detritus on a substrate.

Internal incubation or gestation Protection is accorded to the young by males as in

seahorses (Hippocampus) and pipefishes (Syngnathus) in which eggs are placed into brood pouch of the male.

In a Brazilian catfish male develop enlarge lower lip to form a pouch on which incubation of egg takes place.

The marine catfishes (Ariidae), gafftopsail catfish (Bagre marinus), sea catfish (Goleichthys felis) employ mouth as incubator.

True internal incubation-sharks-non-placental or ovoviviparous, some true viviparous (genera Triakis, Carcharhinus and Mustelus, Rays, skates ovoviviparous & oviparous skates (Rajidae)

Live bearing among bony fishes highly developed in topminnows (Cyprinodontiformes), in guppy (Lebistes) and mosquito fish, Poecillidae, Goodeidae, Anablepidae, Jenynsiidae. The living coelacanth Latimaria is also ovoviviparous.