Neuroendocrine of Crustacea

7/29/2019 Neuroendocrine of Crustacea

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Neuroendocrine system of Crustaceans

and Reproduction

Endocrine system of decapoda

The crustacean endocrine system, like that of

other higher invertebrates, has neurosecretory

cells and some classical endocrine glands.

The following are the principal endocrine areas.

The eye stalk contains a number of, so called, X-organs including medulla externa (me), sensory

pore, medulla interna (mi), and medulla

terminalis (mt)


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and the neurohemal organ, the sinus gland (sg).

Axons from neurosecretory cells in the brain

and other parts of the nervous system pass tothe sinus gland via a neurosecretory tract.

The postcommisural organs receive axons from

4 neurosecretory cells in each side of thecircumesophageal connectives: only one is

shown on the left side.


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The pericardeal organs are located over

openings of branchiocardiac veins into the

aorta: Numerous nerves run from the nervoussystem to the organs. The dorsal nerve of the

heart (n. dors) and nerves to muscles (n. mot)

are also shown.

The androgenic gland often is a vermiform mass

of secretory cells attached to the distal portion

of vas deferens. The ovaries secrete female sex

steroids in many species.


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The generalized neurosecretory system

(except the eyestalk) consist of cerebral

neurosecretory cells (ns 1-5) and thoracicganglia neurosecretory cells.

Axons of neurosecretory cells in all parts of

the brain pass to the sinus gland, but onlyaxons from ganglionic neurosecretory cells

pass out through the pedal nerves.

The Y-organ is an endocrine gland withoutinnervation; it is an ovoid disk of

hypertrophied epidermis, which secretes

molting hormone.


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Molting cycle Molting, that is the periodic shedding of the

exoskeleton, is under endocrine regulation incrustaceans as well as in other arthropods.

The event of exoskeleton shedding is termed,

ecdysis, and names of the other phases in themolt cycle relate to ecdysis.

Thus, the molt period starts with the proecdysis,

which leads into the ecdysis.

The period immediately following ecdysis is

called metecdysis, and periods between molts

are called anecdysis.


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During proecdysis, the epidermis starts to

separate from the old cuticule (exoskeleton)

and the epidermal cells enlarge and begin tosecrete the new exoskeleton.

Minerals and macromolecules are reabsorbed

from the old exoskeleton and temporarilystored elsewhere for later incorporation into

the new exoskeleton.

Amino acids are stored in the hemolymph,while Ca is deposited at various locations

depending on species.


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The muscles of the major limb segments atrophy

during proecdysis so that the limbs can be pulled

out through the narrow basal segment at

ecdysis. Regeneration of lost limbs also begins

during proecdysis.

At ecdysis, the old exoskeleton cracks open,

typically in the rear end, and the animal backs

out of the old shell.

When newly emerged, the new exoskeleton ispale and soft.


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In many species the molted animal eats the old

exoskeleton to regain some of the minerals and

other nutrients.

During metecdysis, Ca and other components

are laid down in the new exoskeleton and the

limb muscles grow.


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Ecdysone in lobster

Molting in crustaceans is induced by the steroid,20-hydroxyecdysone, which is produced and

released as a precursor from the Y-gland,the

concentration of 20-hydroxyecdysone in

hemolymph of a lobster during the molt cycle.

The molting event, or ecdysis, is indicated by an

M. The peak in 20-hydroxyecdysone occurs just

prior to the molt.


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There is however more to the story, because

20-OH ecdysone is not the only hormone

controlling molting.

Already in 1905 there was a paper reporting

that bilateral ablation of the eyestalks of crabs

accelerated the molting cycle.

an eyestalk ablation experiment with lobster.

Again, 20-hydroxyecdysone concentrations in

hemolymph were measured during the moltingcycle.


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One of the groups, ablation substantially

reduced the lag before the 20-hydroxyecdysone peak and thereby also the

time to molt.

The net effect of eyestalk ablation is prettydramatic. This picture shows two lobster

siblings. The larger animal was bilaterally

eyestalk ablated at the 2nd larval stage andthen kept under identical conditions during 5

months.


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The explanation for this dramatic effects is that

ecdysone release from the Y-gland is under

negative control of a peptide hormone, molt-inhibiting hormone (MIH), which is released

from the sinus gland of the eyestalk.

MIH is a protein with a molecular weight of7000-9000, depending on species.

MIH release, in turn, is stimulated by a

neurotransmitter, 5-HT. Exactly how theperiodicity of molting is regulated seems to be a

matter of current research.


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Reproduction

Already in 1943 it was demonstrated that the

sinus gland is involved in reproductive

regulation as well.

In a classic experiment, Panouse showed that

eyestalk ablation from prawn females leads to

rapid increase in ovarian size and to

precocious egg deposition.


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It was further demonstrated that an ovarian-

inhibiting factor was released by the sinus

gland.

The factor is now known to be a peptide

hormone with a MW of 7,000 8,000, and it is

generally called, gonad-inhibiting hormone

(GIH).

The sinus gland is the GIH-releasing organ, but

the source of this neurohormone is the medullaterminalis (mt) X-organ.


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In females, the GIH level in the plasma varies

during the course of the annual reproduction

cycle and ovary development is stimulated when

the GIH concentration of the hemolymph is low.

In many species, sexual maturation and molt

cycle is synchronous. In such species, follicle

growth and oocyte development appear to be

related not only to the absence of GIH but also

to the low levels of 20-OH ecdysone present atthe beginning of each molt cycle.


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Vitellogenesis

In all oviparous species that have beeninvestigated, the yolk of the egg is not produced

in the follicle itself but elsewhere in the body

and is transported to the oocyte.

The yolk protein is called vitellogenin (VTG) in

arthropods and well as in chordates. The process

of vitellogenin production and its subsequent

transport to and uptake by oocytes is collectively

called vitellogenesis.


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In crustaceans, VTG synthesis occurs in an organ,

called the fat body, and possibly at other sites as

well. VTG is transported in the hemolymph to the

ovaries where the protein is incorporated into

the oocytes. The follicle cells that surround theoocyte have an extensive tubular network

through which the VTG molecules can pass.

Uptake of VTG in the oocyte occurs by means ofreceptor mediated endocytosis.


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There seems to be two targets for GIH: (1)

Inhibition of VTG synthesis, and (2) binding of

VTG to the membrane receptors on the oocyte. When the level of GIH in the hemolymph goes

down the vitellogenesis can proceed, which

leads to maturation of the follicle.


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Endocrine control of Reproduction

In insects sexual differentiation is by geneticmechanism.

But in Crustacea both sexual differentiation and

gonadal activity are influenced byhormones situation

resembling as in vertebrates.

Most of Malacostraca are unisexual only few groups

show protandric hermaphroditism.

In these Crustacea sex is determined genetically butmorphological and functional expression of sex is

very much influenced by hormones.


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At the time of hatching/ at the end of larval period-

indistinguishable with reference to sexes.

During successive molt sexual differentiationcommences but only completes after gonadal maturity.

Sexual dimorphismPenis in male and oostegites in

female.

In undifferentiated young crustacean primordial of

vasdeference are present in both the sexes.

Attached near each vasdeference is androgenic gland.

In genetic female androgenic gland fail to develop butin the male the glands enlarge to form a solid strand of

cells.


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Androgenic gland secrets male hormone.

Young male ----------Remove Androgenic gland------ instead of spermatocytes-----oocyte

development commences


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Neuroendocrine of Crustacea