Answer to the Questions of the Endocrine System and Chemical Messengers

Answer to the Questions of the Endocrine System and Chemical Messengers

Q.20.     Why the thyroid gland is in the neck on the ventral side of the pharynx in all vertebrates?

Ans. From an evolutionary perspective, evidence indicates that the thyroid gland in the earliest vertebrates evolved from a pouch – like structure (the endostyle) that carried food particles in the front end of the digestive tract. This explain that why the thyroid gland is in the neck on the ventral side of the pharynx.

The possible hypothesis that how did this feeding mechanism turn into an . endocrine gland is that; as the developing pouch gradually lost all connection with the pharynx, it became independent of the digestive system both functionally as well as structurally. As a result, a functionally novel structure arose from an ancestral structure with an unrelated function.

Q.20.     What Is the function of melanocyte – stimulating hormone? What Is the function of calcitonln?

Ans. Melanocyte – stimulating hormone (MSH)

In cartilaginous and bony fishes, amphibians and reptiles, MSH is a direct – acting hormone that promotes dispersion of the pigment melanin within the melanocytes, causing darkening of the skin. In birds and mammals, its physiological function remains, unclear. MSH appears unrelated to pigmentation in endotherms, although it will cause darkening of the skin in humans if injected into the circulation. Until recently, many endocrinologists thought MSH was a vestigial hormone in mammals, but interest has been rekindled by studies showing that it enhances memory and growth of the fetus. In addition, MSH has been isolated from specific regions of the hypothalamus, where it has been linked to regulation of ingestive behaviours and metabolism in adult mammals.


Calcitonin is a calcium – regulating hormone, secreted by specialized cells (C cells) in the

thyroid gland of mammals and in the ultimobranchial glands of other vertebrates. Calcium is released in response to elevated PTH ® levels of calcium in the It   rapidly suppresses calcium withdrawal from bone decreases  intestinal absorption of calcium and increases
excretion of calcium by the kidneys. Calcitonin increase in level of Figure 3-15 calcium in the blood,    Regulation of blood calcium in birds and mamimals. just as parathyroid

hormone protects it from a decrease in blood calcium. Calcitonin has been identified in all vertebrate groups, but its importance is uncertain because replacement of calcitonin is not required for maintenance of calcium homeostasis, at least in humans, if the thyroid gland is surgically removed (also removing the C cells). Fig. 3.15.


0.22. Name the endocrine glands in birds, Write a brief outline of

(a) thyroxine (b) testosterone (c) calcitonin, and d (d) bursa of Fabricius In birds.

Ans. The endocrine glands in birds include the ovary, testes, adrenals, pituitary, thyroid, pancreas, parathyroids, pineal, hypothalamus, thymus, ultimobranchial, and bursa of Fabricius. Fig. 3.16.

(a)   Thyroxine

The bird’s thyroid gland produces the hormone thyroxine; it stimulates and maintains metabolic processes, regulates the normal development of feathers and the molt cycle, and plays a role in the onset of migratory behaviorStudies have shown that thyroxine hormone is required for the normal functioning of bone forming cells and for the branching of nerve cells during embryonic development of the brain.

(b)   Testosterone

In male birds, the testes produce the hormone testosterone. Testosterone promote spermatogenesis and also controls the secondary sexual characteristics of the male, such as bright plumage color, comb (when present), and spurs — all of which strongly influence sexual behavior.-


(c)   Calcitonin:

The ultimobranchial glands are small, paired structures in the neck just below the parathyroid   glands. They secrete the hormone calcitonin, which lowers the blood calcium level by inhibiting calcium reabsorption from the bone.



(c)  Bursa of Fabricius:

The bursa of fabricius is a sac that lies just dorsal to the cloaca and empties into it. Although well developed during the bird’s embryological development, it begins to shrink soon after hatching. Its tissues produce secretions that

Figure 3.16a

Steroid hormones from the adrenal cortex and gonads. Cortisol (a glucocorticoid) and aldosterone (a mineralocorticoid), both made in the adrenal cortex, are structurally similar to the sex hormones testosterone (an androgen), estradiol (an estrogen), and progesterone (a progestin). The precursor for the syntheis of all steroid hormones is cholesterol. Most of the androgens (male hormones) that circulate in the blood are made by the testes, and most of the estrognes and progestins (female hormones) are produced by the ovaries; however, small amounts of both types of sex hormones are also made by the adrenal cortex.

are responsible for the maturation of white blood cells (B lymphocytes). which play an important role in immunological reactions.

Q 23: What are the functions of the hormone prolactin in birds and mammals?

Ans. Pro!actin (PRL) is a protein so similar to growth hormone (GH) that it is believed they are encoded in genes that evolved from the same ancestral gene. Unlike tropic hormones, prolactin acts directly on its target tissues rather than through other hormones.


In some birds (e.g., pigeons and doves), the prolactin stimulates the production of “pigeon’s milk” by desguamation (sloughing off cells) in the pigeon’s crop. Prolactin regulates fat metabolism and also stimulates and regulates broodingness and certain other kinds of parental behavior, and along with estrogen, stimulates full development of the brood (incubation) patch. The brood patch helps keep the eggs at a temperature between 33 and 37°C.




In mammals prolactin plays an essential role in many aspects of reproduction. For example, it stimulates reproductive migrations in many mammals, such as elk and caribou. Prolacfin also enhances mammary gland development and milk production in female mammals. (Oxytocin stimulates milk ejection from the mammary glands, but not its production). More recently, prolactin has been shown to be a chemical mediator of the immune system and is important in formation of new blood vessels (angiogenesis).

Q.24. Describe the major hormones in vertebrates.

Ans. Following is a summary of major vertebrate hormones and the tissues of endocrine nature. Fi.. 3.17.

goshi 3

goshi 4

7Q.25. How do the two lobes of pituitary differ from each other?

Ans. The pituitary gland or hypophysis, is a small gland (0.5 g in humans) lying in a well protected position between the roof of the mouth and floor of the brain. It is a two part gland having a double embryological origin. The anterior pituitary (adenohypophysis) is derived embryologically from the roof of the mouth. The posterior pituitary (neurohypophysis) arises from a ventral portion ot the brain, the hypothalamus, and is connected to it by a stalk, the infundibulum. The two lobes (anterior and posterior pituitary) differ in several ways: Fig. 3.18

1. The adenohypophysis is larger than the neurohypophysis:

2.  secretory cells called pituicytes are in the, adenohypophysis, but not in the neurohypophysis: and

3. The neurohypophysis has a great supply of nerve endings


Figure 3.18

Functional Links between the Pituitary Gland and the Hypothalamus. Target areas for each hormone are shown in the corresponding box. The blood vessles that make up the hypothalarnic-hypophyseal portal system provide the functional link between the hypothalamus and the adenohypophysis and the axons of the hypothalamic neuroscretory cells provide the link between the hypothalamus and the neurohypophysis (TSH = thyroidstimulating hormone; PRL = proractin. ACTH = adrenocorticotropic hormone: GH = growth hormone: 8TH = somatotropin; FSH = follicle-stimulating hormone; LH = luteinizing hormone.)

Pituicytes produce and secrete hormones directly from the adenohypophysis, whereas the neurohypophysis obtains its hormones from the neurosecretory cells in the hypothalamus, storing and releasing them when they are needed. These modified hypothalamic nerve cells project their axons down a stalk of nerve cells and blood vessels, called the infundibulum. into the pituitary gland. directly linking the nervous and endocrine systems.

Q.26. What hormones do the posterior pituitary or neurohypophysis secrete?

Ans. Hormones of the Neurohypophysis:

The neurohypophysis does not manufacture any hormones. Instead, the neurosecretory cells of the hypothalamus synthesize and secrete two hormones, oxylocin and vasopressin, which move down nerve axons into the neurohypophysis. where they are stored in the axon terminals until released


Oxytocin has two important specialized reproductive functions in adult female mammals

1)     It stimulates contraction of uterine smooth muscles during parturition (birth of the young). In clinical practice, oxytocin is used to induce delivery dunng a difficult labor and to prevent uterine hemorrhage after birth.

2)     A second action of oxytocin is that of milk ejection by the mammary glands in response to suckling.

Recent work also has established a role for oxytocin in pair – bonding behavior in both sexes in monogamous voles.

Antidiuretic or Vasopressin

Vasopressin, hormone from the second posterior lobe, acts on collecting ducts of the kidney to increase water reabsorption and thus restrict urine flow It is therefore often called antidiuretic hormone (ADH). Vasopressin also increases blood pressure through its generalized constrictor effect on smooth muscles of arterioles. Finally, vasopressin acts centrally to increase thirst, and therefore, drinking behavior.

Both oxytocin and vasopressin or ADH are thought to have evolved from a similar ancestral chemical messenger that helped control water loss and. indirectly, solute concentrations. For example, the neurohypophysis is ilotably larger in animals that live in arid parts of the world, where water conservation is crucial. The structure of the two hormones is similar except for a difference in two of the amino acids.

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