2. La nuova biologia.blu Anatomia e fisiologia dei viventi S
David Sadava, David M. Hillis,
H. Craig Heller, May R. Berenbaum
La nuova biologia.blu
Anatomia e fisiologia dei viventi S

3. Animal Hormones

4. What Are Hormones And How Do They Work?
Endocrine cells produce and release chemical signals directly into the extracellular fluid (ECF).
Hormones are endocrine signals that enter the blood and activate target cells far from the site of release.

5. What Are Hormones And How Do They Work?
Paracrines affect target cells near the release site.
Example: histamine, a mediator of inflammation.
Autocrines bind to receptors on the same cells that secrete them.

6. What Are Hormones And How Do They Work?
Three types of hormones:
Peptides or proteins (water-soluble)
Steroid hormones (e.g., estrogen, testosterone) are synthesized from the steroid cholesterol
Amine hormones

7. What Are Hormones And How Do They Work?
Water-soluble hormone receptors are transmembrane glycoprotein complexes with three domains:
binding domain (outside the plasma membrane),
transmembrane domain,
cytoplasmic domain that extends into the cytoplasm.

8. How Do the Nervous and Endocrine Systems Interact?
One hormone can trigger different responses in different types of cells.
Example: epinephrine (amine hormone) and the fight-or-flight response.
The nervous system stimulates endocrine cells in the adrenal glands to secrete epinephrine.

9. What Are the Major Endocrine Glands and Hormones?
Some endocrine cells exist as single cells (e.g., in the digestive tract).
Endocrine glands: aggregations of endocrine cells that secrete hormones into the ECF.
Exocrine glands have ducts that carry their signals to the outside of the body or a body cavity.

10. The Endocrine System of Humans
Figure The Endocrine System of Humans Cells that produce and secrete hormones may be organized into discrete endocrine glands, or they may be embedded in the tissues of other organs, such as the digestive tract or kidneys. The hypothalamus is part of the brain, but it includes cells that secrete neurohormones into the extracellular fluid.

11. What Are the Major Endocrine Glands and Hormones?
The pituitary gland is attached to the hypothalamus of the brain.
The anterior pituitary originates from gut epithelial tissue; contains endocrine cells controlled by neurohormones from the hypothalamus.
The posterior pituitary originates from neural tissue; contains axons from hypothalamic neurons.

12. What Are the Major Endocrine Glands and Hormones?
Hypothalamic neurons secrete two neurohormones into the posterior pituitary:
1. Antidiuretic hormone (ADH) increases the water retained by the kidneys when necessary.
2. Oxytocin stimulates uterine contractions in childbirth and milk flow.

13. What Are the Major Endocrine Glands and Hormones?
The anterior pituitary secretes four tropic hormones that control other endocrine glands:
Thyrotropin (thyroid-stimulating hormone)— TSH
Luteinizing hormone (LH)
Follicle-stimulating hormone (FSH)
Corticotropin (ACTH)

14. What Are the Major Endocrine Glands and Hormones?
The anterior pituitary also secretes growth hormone, prolactin, enkephalins, and endorphins.
Growth hormone (GH) promotes growth by stimulating cells to take up amino acids.
Endorphins and enkephalins are the body’s natural painkillers, acting as neurotransmitters in the brain.

15. What Are the Major Endocrine Glands and Hormones?
Hypothalamic hormones include:
Thyrotropin-releasing hormone (TRH)
Gonadotropin-releasing hormone (GnRH)
Prolactin-releasing and release-inhibiting hormones
Growth hormone-releasing hormone and growth hormone release-inhibiting hormone (somatostatin)
Corticotropin-releasing hormone

16. How Do the Nervous and Endocrine Systems Interact?
Multiple negative feedback loops also control hormone secretion from the anterior pituitary.

17. What Are the Major Endocrine Glands and Hormones?
The thyroid gland secretes two hormones.
Thyroxine, which regulates metabolism, is produced by follicle epithelial cells.
Thyroglobulin is produced and secreted into the lumen of the follicle, where it is iodinated and stored until processed by the epithelial cells to generate T3 and T4.
Triiodothyronine, or T3:
Tetraiodothyronine, or T4:

18. The Thyroid Gland Consists of Many Follicles
Thyrotropin releasing hormone (TRH), produced in the hypothalamus activates the TSH-producing pituitary cells.
Anterior pituitary secretes thyrotropin or thyroid- stimulating hormone (TSH), which activates the follicles to produce thyroxine.
Figure The Thyroid Gland Consists of Many Follicles (A) Cross section through a thyroid gland, showing numerous follicles bounded by epithelial cells. Calcitonin-secreting cells are located in the spaces between the follicles. (B) The epithelial cells of the follicle synthesize thyroglobulin and secrete it into the lumen of the follicle, where it is iodinated and stored until it is processed by the epithelial cells to generate T3 and T4.

19. What Are the Major Endocrine Glands and Hormones?
Regulation of calcium levels in the blood is critical to body function.
There are several mechanisms for controlling blood calcium levels:
Deposition or absorption of bone
Excretion or retention of Ca by kidneys
Absorption of Ca from digestive tract.

20. What Are the Major Endocrine Glands and Hormones?
These mechanisms are controlled by three hormones:
1. Calcitonin is released by the thyroid gland and lowers blood Ca by regulating bone turnover.
2. Parathyroid hormone (PTH) is secreted by the parathyroid glands.
When Ca levels are low, PTH is released, which stimulates bone turnover and increases blood Ca levels.
3. Calcitriol (synthesized from vitamin D)
Calcitriol promotes absorption of Ca from the gut.

21. Hormonal Regulation of Calcium
Figure Hormonal Regulation of Calcium Calcitonin, parathyroid hormone (PTH), and calcitriol (the active form of calciferol, or vitamin D) regulate Ca2+ levels in the blood.

22. What Are the Major Endocrine Glands and Hormones?
Islets of Langerhans— endocrine cells in the pancreas that produce three hormones:
Beta (β) cells produce insulin
Alpha (α) cells produce glucagon—stimulates liver to convert glycogen to glucose when blood glucose levels fall below normal
Delta (δ) cells produce somatostatin—inhibits release of both insulin and glucagon

23. What Are the Major Endocrine Glands and Hormones?
The adrenal gland is a gland within a gland.
The adrenal medulla produces epinephrine (adrenaline) and norepinephrine in response to stressful situations.

24. What Are the Major Endocrine Glands and Hormones?
Epinephrine and norepinephrine bind to adrenergic receptors on target cells.
α-adrenergic receptors respond more to norepinephrine.
β-adrenergic receptors respond to both.

25. What Are the Major Endocrine Glands and Hormones?
The outer adrenal cortex produces corticosteroids from cholesterol:
Glucocorticoids influence blood glucose concentration
Mineralocorticoids influence salt and water balance
Sex steroids are important in sexual development, behavior and anabolism.
Aldosterone, the main mineralocorticoid, stimulates kidneys to conserve sodium and excrete potassium.

26. What Are the Major Endocrine Glands and Hormones?
Cortisol (a glucocorticoid) mediates metabolic stress responses.
Cortisol and similar drugs are used to reduce inflammation and allergic responses.
The anterior pituitary controls cortisol release by corticotropin (adrenocorticotropic hormone, or ACTH).
ACTH release is controlled in turn by corticotropin- releasing hormone from the hypothalamus.

27. What Are the Major Endocrine Glands and Hormones?
Gonads produce sex steroids.
Androgens—male, testosterone
Estrogens and progesterone—female, estradiol
In development, sex hormones determine whether a fetus will become male or female.
Sex hormones exert their effects by the seventh week of human development.

28. What Are the Major Endocrine Glands and Hormones?
Androgens, produced in the presence of a Y chromosome, are required for male reproductive structures to develop.
Without androgen, female reproductive structures develop.

29. What Are the Major Endocrine Glands and Hormones?
At puberty, production of sex hormones increases.
Gonad activity is controlled by tropic hormones called gonadotropins from the anterior pituitary:
Luteinizing hormone (LH)
Follicle-stimulating hormone (FSH)
Gonadotropins are controlled by hypothalamic gonadotropin-releasing hormone (GnRH)—its release increases at puberty.

30. What Are the Major Endocrine Glands and Hormones?
The pineal gland produces the amine hormone melatonin from tryptophan.
Melatonin is released in the dark and indicates length of the night—light inhibits its release.
Involved in photoperiodicity—seasonal changes in day length trigger physiological changes.

31. How Have Hormones Evolved?
Chemical signaling was critical for the evolution of multicellular organisms.
Signaling molecules are highly conserved, but their functions differ.
As organisms diversified, the same hormone–receptor systems diversified to serve different functions.
Example: the hormone prolactin.

32. Prolactin’s Structure Is Conserved, but Its Functions Have Evolved
Figure Prolactin’s Structure Is Conserved, but Its Functions Have Evolved The hormone prolactin is found in all vertebrate groups and has a long evolutionary history. Its probable function in early vertebrates was in regulating the body’s salt and water balance (osmoregulation). It maintains this function in some species, and has evolved in others to control a number of physiological processes, most of which are associated with reproduction.

33. How Have Hormones Evolved?
In arthropods, hormones control molting and metamorphosis.
Three hormones regulate molting and maturation:
PTTH (prothoracicotropic hormone) from cells in the brain stimulates the prothoracic gland to secrete ecdysone.
Ecdysone diffuses to target tissues and stimulates molting.

34. How Have Hormones Evolved?
Juvenile hormone is also released from the brain, which prevents maturation to adult form.
Control of development by juvenile hormone is important in insects with complete metamorphosis.

35. What Are the Major Diseases?
Diabetes mellitus is caused by lack of the protein hormone insulin (type I) or by a lack of insulin responsiveness in target cells (type II).
Glucose transporters in cell membranes are controlled by insulin. When insulin is absent, transporters are returned to the cytoplasm, and glucose uptake is inhibited.
Today, insulin replacement therapy using manufactured insulin allows people with type I diabetes to lead almost normal lives.

36. What Are the Major Diseases?
A goiter is an enlarged thyroid gland, associated with either hyperthyroidism (excess thyroxine) or hypothyroidism (thyroxine deficiency).
The most common cause of hypothyroid goiter is iodine deficiency—follicle cells can’t produce thyroxine.
Without thyroxine in the blood, TSH levels remain high, and the thyroid continues to produce thyroglobulin.

37. Inc. All rights reserved
Adapted from
Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014
Inc. All rights reserved