Answer to the Questions of the Endocrine System and Chemical Messengers
0.14. Describe briefly the endocrine system in Arthropods.
Endocrine system in Arthropods
All groups of arthropods have extensive endocrine systems.
The endocrine system of a crustacean, such as a crayfish, controls functions such as ecdysis (molting), sex determination, and color changes. Fig. 3.9, 3.10.
X-organs areneurosecretory tissues in the crayfish eye stalks. Associated with each X-organ is a sinus gland thataccumulates andreleases the secretions of the X-organ. Other glands, called Y-organs, are at the base of the maxillae.
X-organs and Y-organs control ecdysis as follows.
In the absence of an appropriate stimulus, the X-organ produces molt-inhibiting
hormone (Mill), and the sinus gland
releases it. The target of this hormone is the Y-organ. When
present in high concentrations, the Y-organ is inactive. Under appropriate internal and external stimuli, MIH release is prevented, and the Y-organ releases the hormone ecdysone, which leads to molting.
The sequence of events in insects is similar to that of crustaceans, but if does not involve a molt-inhibiting hormone. The presence of an appropriate stimulus to the central nervous system activates certain neurosecretory cells (pars intercerebralis) in the optic lobes of the brain. These cells secrete the homone ecdysiotropin, which axons transport to the corpora cardiaca (a mass of neurons associated with the brain). The corpora cardiaca produces thoracotrpoic hormone, which is carried to the prothoracic glands, stimulating them to produce and release ecdysone, which induces molting — in particular, the reabsorption of some of the old cuticle and the development of a new cuticle.
It is secreted by the neurosecretory cells in the brain and nerve cords. Bursicon influences certain aspects of epidermal development, such as tanning (i.e., hardening and darkening of the chitinous outer layer). Tanning is completed several hours after each molt. Fi . 3.11.
Endocdne control of molting In a moth typical of Insects haying complete
metamorphosis. Many moths mate in spring or
summer, and egg soon hatch into the first of several larval stages, called testers. Atter the final larval molt, the last andargest larva (caterpillar) spins a cocoon in which it pupates. The pupa ovenvinters and an adult emerges in the springto start a new generation. Juvenile hormone and ecdysone interact to control molting and pupation. Many gens areactivated during metamorphosis, as seen by puffing of chromosomes ( center column). Puffs form in sequence duringsuccessive molts. Changes in cuticle thickness and surface characteristics are shown at right.
Just behind the insect brain are the paired corpora allata. These structures produce juvenile hormone (JH). High concentrations of JH in the blood of an insebt inhibit differentiation. In the absence of an appropriate environmental stimulus, the corpora allata decrease JH production, which causes the insectlarva to differentiate into a pupa. The pupa then forms a cocoon to over winter. In the spring the final surge of ecdysone, in the absence of JH, transforms the pupa into an adult moth
0.15. Differentiate between endocrine and exocrine glands.
Ans. A gland is a secretory organ, and vertebrate glands can be classified as endocrine and exocrine.
Many hormones are secreted by endocrine (Gr-endo. within + krinein, to separate) glands, small, well-vascularized ductless glands composed of groups of cells arranged in cords or plates. Since endocrine glands have no ducts, their only connection with the rest of the body is by the bloodstream; they must
capture their raw materialsfrom the extensive bloodsupply. They receive and Endocrine secrete their finished cells hormonal products into it.
Exocrine (Gr. exo, outside + krinein, to secrete) glands, are provided with ducts for discharging their
secretions onto a free surface.
exocrine glands are sweat Figure 3.12
glands and sebaceous Vertebrate Glands with and without Ducts. (a)An endocrine
gland, such as the thyroid secretes hormones into the glands of skin, salivary e) dracellular fluid. From there the hormones pass into blood glands, mammary glands. vessels and travel throughout the body. (b)An exocrine gland and the various enzyme such as a sudoriferous (sweat) gland, secretes material secreting glands lining the (sweat) into a duct that leads to a body surface. walls of the stomach and intestine F ig 3 lz
Q.16. Write the aspects by which vertebrate hormones differ from each other.
Ans. Vertebrates other than birds or mammals have somewhat similar endocrine systems, but differences do exist. Recent research has revealed the following three aspects of endocrinology that relate to species differences among these vertebrates:
1) Hormones (or neuropeptides) with the same function in different species may not be chemically identical.
2) Certain hormones are species — specific with respect to their function, conversely, some hormones produced in one species may be completely functional in another species.
3) A hormone from one species may elicit a different response in the same target cell or tissue of a different species.
0.17. What are the three major neuropeptides secreting regions in jawed fish?
Describe the major functions of hormones produced by these regions.
Ans. In fishes, the brain and spinal cord are the most important producers of
regions secrete neuropeptides. The two in the brain are the pineal gland of the
epithalamus and the preoptic nuclei of the hypothalamus. Fig. 3.13.
- Pineal gland
The pineal gland produces neuropeptides that Chrornaffinaffect pigmentation and apparently inhibit reproductive development, both e which are &annals stimulated by light. One specific hormone thatpinea, the pineal gland produces, melatonin, has gland broad effects on body metabolism by synchronizing activity patterns with light intensity and day length.
- 2.Preoptic nuclei
- The preoptic nuclei produce various other neuropeptides that control different functions in fishes (e.g., growth, sleep, locomotion.)
- Urophysis: The urophysis (Gr. oura, tail + physis, growth) is a discrete structure in the spinal cord of the tail. The urophysis produces neuropeptides that help control water and ion balance, blood pressure, and smooth muscle contractions
- Q.18. What are the functions of prolactin and calcitonin hormone in fishes?
Ans. Prolactin (produced by the pituitary gland) stimulates reproductive migrations in many animals (e.g, the movement of salamanders to water). In some fishes prolactin causes brooding behavior. It also helps control water and salt balances, and is essential for certain saltwater fishes to enter freshwater during spawning runs.
In jawed fishes and primitive tetrapods. several small ultimobranchial glands form ventral to the esophagus. These glands produce the hormone calcitonin that helps regulate the concentration of blood calcium.
Q.19 What role do thyroxine and triiodothyronine play in amphibian m9tamorphosis?
- In amphibians, thyroxine and triiodothyronine apart from regulating overall metabolism( also play on additional role in metamorphosis. Specifically timed changes in the concentrations of three hormones — prolactin, thyroxine, and triiodothyronine — control metamorphosis in the frog.
Low thyroxine and triiodothyronine concentrations and high prolactin concentrations in young tadpoles stimulate larval growth and prevent metamorphosis. As the hypothalamus and pituitary glands develop in the growing tadpole, the hypothalamus releases thyroid — stimulating hormone (TSH) and prolactin — inhibiting hormone. Their release causes the pituitary gland to release thyroid — stimulating hormone and to cease production of prolactin. As a result, the concentrations of thyroxine and triiodothyronine rise, triggering the onset of metamorphosis. Tail resorption and other metamorphic changes follow. Fig. 3.14.
Frog Tadpole Metamorphosis. The thyroid hormone tniodothyronine (T,) and thyroxine (T,) regulate the metamorphosis of an aquatic frog tadpole into a semiterrestnal or terrestrial adult. The anterior pituitary secretes thyroid-stimulating hormone which regulates thyroid gland activity. During the premetamorphosis (tadpole) stage the pituitary and thyroid glands are relatively inactive. This keeps the concentration of thyroid-stimulating hormones T, and T. at low concentrations. The high prolactin concentration in tadpole stimulates larval growth and prevents metamorphosis. During metamorphosis the concentrations of the thyroid hormones markedly increases and protean decreases. These hormonal fluctuations induce rapid differentiation, climaxing in the adult frog.