The Endocrine System

1 The Endocrine System Chapter 23

CHAPTER SUMMARY

This chapter begins with an introduction to endocrine glands and a comparison of the roles of the nervous and endocrine systems. The distinction between endocrine and exocrine glands is described. The mechanism of action of hormones is explained. The major endocrine characteristics of the hypothalamus and pituitary gland, the thyroid gland, the parathyroid glands, the adrenal glands, the pancreas, the ovaries and testes, the pineal gland, and the thymus are described in detail. The secretions of other endocrine tissues are summarized. The development of the endocrine system is portrayed. The effects of aging on the endocrine system are explained. A glossary of key medical terms associated with the endocrine system is provided. This chapter concludes with a thorough study outline, an excellent self-quiz, critical thinking questions, and answers to questions that accompany chapter figures.

STUDENT OBJECTIVES

1. Distinguish between exocrine glands and endocrine glands. 2. Describe how hormones interact with target-cell receptors. 3. Explain why the hypothalamus is an endocrine gland. 4. Describe the location, histology, hormones, and functions of the anterior and posterior pituitary glands. 5. Describe the location, histology, hormones, and functions of the thyroid gland. 6. Describe the location, histology, hormones, and functions of the parathyroid glands. 7. Describe the location, histology, hormones, and functions of the adrenal glands. 8. Describe the location, histology, hormones, and functions of the pancreas. 9. Describe the location, histology, hormones, and functions of the male and female gonads. 10. Describe the location, histology, hormones, and functions of the pineal gland. 11. List the hormones secreted by cells in tissues and organs other than endocrine glands, and describe their functions. 12. Describe the development of the endocrine system. 13. Describe the effects of aging on the endocrine system.

LECTURE OUTLINE A. Comparison of Nervous and Endocrine Systems (see Table 23.1): (p. 704) 1. The nervous and endocrine systems together coordinate functions of all body systems. 2. The nervous system controls homeostasis via nerve impulses that trigger release of neurotransmitter molecules which act on other neurons, muscle cells, or gland cells. 3. The endocrine system controls homeostasis by secreting hormones, i.e., messenger molecules, into the bloodstream for delivery to virtually all body cells. 4. The nervous and endocrine systems act as a coordinated supersystem called the neuroendocrine system: some neurons stimulate or inhibit the release of hormones and some hormones promote or inhibit the initiation of nerve impulses. 5. The nervous system causes muscles to contract and glands to secrete either more or less of their product; the endocrine system affects virtually all body tissues by altering metabolic activities, regulating growth and development, and influencing reproductive processes. 6. Nerve impulses typically produce their effects within several milliseconds and the effects are relatively brief in duration; hormones may take seconds to hours to produce their effects and these effects are generally longer in duration. 7. Endocrinology is the science concerned with the structure and functions of the endocrine glands and the diagnosis and treatment of disorders of the endocrine system. 2 B. Endocrine Glands Defined (p. 705) 1. The body contains two types of glands: i. exocrine glands (e.g., sudoriferous, sebaceous, and mucous glands) secrete their products into ducts which deliver the secretions into body cavities, into the lumen of an organ, or to the outer surface of the body ii. endocrine glands secrete their products, called hormones, into the surrounding interstitial fluid from which they diffuse into capillaries to be carried away by the blood 2. The endocrine glands (e.g., pituitary, thyroid, and adrenal glands) collectively form the endocrine system; there are several organs of the body (e.g., hypothalamus, thymus, pancreas, stomach, skin, heart, etc.) that contain endocrine tissue but are not endocrine glands exclusively.

C. Hormones (p. 705) 1. Although hormones are delivered by the bloodstream throughout the body, hormones affect only certain target cells. 2. Target cells for a particular hormone are those cells that have the appropriate receptor molecules (proteins or glycoproteins) that can bind to the hormone. 3. The number of receptor molecules in a target cell may increase or decrease in order to increase or decrease, respectively, the sensitivity of that target cell to a particular hormone. 4. The amount of hormone secreted by endocrine cells, usually in short bursts, is regulated to maintain homeostasis and prevent overproduction or underproduction of that hormone.

D. Hypothalamus and Pituitary Gland (p. 707) 1. The hypothalamus is the major integrating link between the nervous and endocrine systems. 2. The hypothalamus receives input from numerous sources and, in addition to many other functions, exerts control over the pituitary gland and is itself a crucial endocrine gland; the hypothalamus and pituitary gland together play important roles in regulation of virtually all aspects of growth, development, metabolism, and homeostasis. 3. The pituitary gland or hypophysis: i. is about 1-1.5 cm in diameter and lies in the sella turcica of the sphenoid bone ii. is attached to the hypothalamus by the infundibulum iii. has two anatomically and functionally separate portions: a. anterior pituitary gland (anterior lobe) accounts for about 75% of the total weight of the gland and contains many glandular epithelial cells that form the glandular part of the pituitary gland; in the adult, it consists of the pars distalis and the pars tuberalis b. posterior pituitary gland (posterior lobe), consisting of the pars nervosa and the infundibulum, contains axons and axon terminals of neurons whose cell bodies are located in the supraoptic and paraventricular nuclei of the hypothalamus - the axon terminals in the posterior pituitary gland are associated with specialized neuroglia called pituicytes iv. has a third region called the pars intermedia that atrophies during fetal development 4. Anterior Pituitary Gland (Anterior Lobe) or Adenohypophysis: i. Secretion of anterior pituitary hormones is regulated by releasing hormones and inhibiting hormones that are delivered from the neurosecretory cells of the hypothalamus by a hypophyseal portal system of blood vessels: a. superior hypophyseal arteries that form the primary plexus at the base of the hypothalamus b. hypophyseal portal veins that pass down outside of the infundibulum to form a secondary plexus in the anterior pituitary; anterior pituitary hormones are carried away by the anterior hypophyseal veins for distribution to target tissues throughout the body ii. Five types of anterior pituitary cells secrete seven major hormones: a. somatotrophs secrete human growth hormone (hGH) or somatotropin which stimulates several tissues to secrete insulinlike growth factors, hormones that stimulate general body growth and regulates aspects of metabolism b. thyrotrophs secrete thyroid-stimulating hormone (TSH) or thyrotropin which regulates activities of the thyroid gland 3 c. gonadotrophs secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH) which affect activities of the ovaries and testes d. lactotrophs secrete prolactin (PRL) which stimulates milk production e. corticotrophs secrete adrenocorticotropic hormone (ACTH) or corticotropin which stimulates the adrenal cortex to secrete glucocorticoids; some corticotrophs also secrete melanocyte-stimulating hormone (MSH) which is thought to affect brain activity and skin pigmentation f. the 5 different types of secretory cells can be classified according to their staining reactions into basophils, acidophils and chromophobes. iii. Some of the anterior pituitary hormones are tropic hormones or tropins, i.e., hormones that influence another endocrine gland. iv. Table 23.2 provides a summary of anterior pituitary gland hormones and their actions. 5. Posterior Pituitary Gland (Posterior Lobe) or Neurohypophysis: i. The posterior pituitary does not synthesize hormones but it does store and release two hormones synthesized in the hypothalamus. ii. The posterior pituitary consists of: a. pituicytes b. axon terminals of hypothalamic neurosecretory cells whose cell bodies are located in the paraventricular and supraoptic nuclei of the hypothalamus - axons of these cells form the hypothalamohypophyseal tract which extends from the hypothalamus to the posterior pituitary - the cell bodies of these cells produce: 1. oxytocin (OT) which enhances uterine contractions during labor and stimulates milk ejection 2. antidiuretic hormone (ADH), also called vasopressin, which decreases urine volume and increases blood pressure - these hormones are delivered in secretory vesicles to and stored in the axon terminals until nerve impulses that are delivered to the axon terminals trigger exocytosis to release the hormones into nearby capillaries iii. Blood is supplied to the posterior gland by inferior hypophyseal arteries that drain into the capillary plexus of the infundibular process; from this plexus, hormones pass into the posterior hypophyseal veins for delivery to target cells in other tissues. iv. Table 23.3 provides a summary of posterior pituitary gland hormones and their actions.

E. Thyroid Gland (p. 711) 1. The butterfly-shaped thyroid gland is located just below the larynx. 2. It has right and left lateral lobes, located on either side of the trachea, that are connected by an isthmus that lies in front of the upper end of the trachea; a small pyramidal-shaped lobe sometimes extends upward from the isthmus. 3. It contains thyroid follicles; the wall of each follicle consists of two types of cells: i. numerous follicular cells which secrete: a. thyroxine or tetraiodothyronine or T4 b. triiodothyronine or T3 - these two hormones are collectively called the thyroid hormones - these hormones regulate the rate of metabolism, growth, and development ii. less numerous parafollicular cells or C cells which secrete calcitonin - calcitonin influences calcium homeostasis by decreasing blood calcium concentration 4. The thyroid gland receives its blood supply from the superior and inferior thyroid arteries; blood is drained by the superior, middle, and inferior thyroid veins. 5. The thyroid gland is innervated by postganglionic fibers from the superior and middle cervical sympathetic ganglia; the preganglionic fibers of these ganglia emerge from the second through seventh thoracic segments of the spinal cord. 6. Table 23.4 provides a summary of thyroid gland hormones and their actions. 4 F. Parathyroid Glands (p. 713) 1. There is usually one superior and one inferior parathyroid gland attached to the posterior surface of each lateral lobe of the thyroid gland, i.e., usually 4 in total. 2. The parathyroid glands contain two types of cells: i. numerous principal cells which secrete parathyroid hormone (PTH) or parathormone - this hormone increases blood calcium and magnesium levels, decreases blood phosphate levels, and promotes the formation of calcitriol, which is the active form of vitamin D ii. less numerous oxyphil cells whose function is currently unknown 3. The parathyroid glands receive their blood supply from the superior and inferior thyroid arteries; blood is drained by the superior, middle, and inferior thyroid veins. 4. The parathyroid glands are innervated by the thyroid branches of cervical sympathetic ganglia. 5. Table 23.5 provides a summary of parathyroid hormone actions.

G. Adrenal Glands (p. 715) 1. The two adrenal (suprarenal) glands lie superior to the two kidneys. 2. Each gland is differentiated into two regions: i. large, peripherally located adrenal cortex which is subdivided into three zones: a. outer zona glomerulosa that secretes mineralocorticoids which affect mineral (especially sodium and potassium) levels in the blood b. middle zona fasciculata that secretes glucocorticoids which affect glucose metabolism c. inner zona reticlaris that secretes minute amounts of weak androgens ii. small, centrally located adrenal medulla which consists of chromaffin cells; these cells: a. are innervated by sympathetic preganglionic neurons b. are specialized sympathetic postganglionic neurons that secrete about 80% epinephrine or adrenaline and about 20% norepinephrine (NE) or noradrenaline - both hormones have sympathomimetic effects 3. The adrenal glands receive their blood supply from several superior, the middle, and the inferior suprarenal arteries; blood is drained by the right and left suprarenal veins. 4. The adrenal medullae are innervated by preganglionic fibers from the thoracic splanchnic nerves. 5. Table 23.6 provides a summary of adrenal gland hormones and their actions.

H. Pancreas (p. 717) 1. The pancreas is both an endocrine gland and an exocrine gland. 2. It is a flattened organ located posterior and slightly inferior to the stomach. 3. It consists of three regions: head, body, and tail. 4. Scattered among the exocrine portions (acini) of the pancreas are tiny clusters of endocrine tissue called pancreatic islets or islets of Langerhans. 5. The pancreatic islets contain four types of cells: i. alpha (or A) cells which secrete glucagon - glucagon increases blood glucose concentration ii. beta (or B) cells which secrete insulin - insulin decreases blood glucose concentration iii. delta (or D) cells which secrete somatostatin - somatostatin inhibits the secretion of glucagon and insulin iv. F cells which secrete pancreatic polypeptide - this hormone inhibits secretion of somatostatin and pancreatic digestive enzymes 6. The pancreas receives its blood supply from the superior and inferior pancreaticoduodenal arteries and from the splenic and superior mesenteric arteries; blood drains into the splenic vein and the superior mesenteric vein. 7. The pancreas is innervated by autonomic nerves derived from the celiac and superior mesenteric plexuses; included are preganglionic vagal, postganglionic sympathetic, and afferent fibers (but note that pancreatic secretion is controlled primarily by the intestinal hormones secretin and cholecystokinin). 8. Table 23.7 provides a summary of pancreatic hormones and their actions. 5 I. Ovaries and Testes (p. 719) 1. The ovaries are paired oval structures located in the pelvic cavity; they secrete several hormones: i. estrogens and progesterone, the female sex hormones, which are responsible for the development and maintenance of female sexual characteristics ii. inhibin which inhibits secretion of FSH iii. relaxin which relaxes the pubic symphysis and helps dilate the cervix just before the birth of a baby 2. The testes are paired oval structures located in the scrotum; they secrete: i. testosterone, the male sex hormone, which regulates production of sperm and stimulates growth and development of male sexual characteristics ii. inhibin which inhibits secretion of FSH 3. Table 23.8 provides a summary of hormones secreted by the ovaries and testes and their principal actions.

J. Pineal Gland (p. 720) 1. The pineal gland is attached to the roof of the brain’s third ventricle. 2. It is part of the epithalamus and is covered by a capsule formed by the pia mater. 3. It consists of masses of neuroglia and secretory cells called pinealocytes; it is innervated by sympathetic postganglionic fibers from the superior cervical ganglia. 4. The pineal gland secrets melatonin which is believed to promote sleepiness and play a role in setting of the body’s biological clock. 5. The pineal gland receives its blood supply from the posterior cerebral artery; blood drains into the great cerebral vein.

K. Thymus (p. 720) 1. The thymus plays a major role in immunity and was described in Chapter 16; review the details of structure and function of the thymus as previously described in Chapter 16. 2. Hormones secreted by the thymus gland include: i. thymosin ii. thymic humoral factor (THF) iii. thymic factor (TF) iv. thymopoietin 3. These hormones promote the proliferation and maturation of a group of lymphocytes called T cells.

L. Other Endocrine Tissues (p. 721) 1. In addition to the endocrine glands described above, there are other organs that contain endocrine cells which secrete hormones; these organs include (see Table 23.9): i. gastrointestinal tract which secretes: a. gastrin b. glucose-dependent insulinotropic peptide (GIP) c. secretin d. cholecystokinin (CCK) ii. placenta which secretes: a. human chorionic gonadotropin (hCG) b. estrogens c. progesterone d. human chorionic somatomammotropin (hCS) iii. kidneys which secrete renin, erythropoietin (EPO) and calcitriol iv. heart which secretes atrial natriuretic peptide (ANP) v. adipose tissue which secretes leptin

M. Development of the Endocrine System (p. 721) 1. The pituitary gland (hypophysis) develops from two different regions of the ectoderm: i. the posterior pituitary gland (neurohypophysis) develops from an outgrowth of ectoderm called the neurohypophyseal bud, located on the floor of the hypothalamus ii. the infundibulum is also an outgrowth of the neurohypophyseal bud and connects the posterior pituitary gland to the hypothalamus 6 iii. the anterior pituitary gland (adenohypophysis) develops from an outgrowth of ectoderm called the hypophyseal (Rathke’s) pouch, located in the roof of the mouth 2. The thyroid gland develops as a midventral outgrowth of endoderm called the thyroid diverticulum, from the floor of the pharynx. 3. The parathyroid glands develop from endoderm as outgrowths from the third and fourth pharyngeal pouches. 4. The adrenal cortex develops from intermediate mesoderm from the same region that produces the gonads. 5. The adrenal medulla develops from ectoderm, namely the neural crest. 6. The pancreas develops from two outgrowths of endoderm from the part of the foregut that later becomes the duodenum; these two outgrowths eventually fuse to form the pancreas. 7. The development of the ovaries and testes is described in Chapter 27. 8. The pineal gland develops as an outgrowth between the thalamus and colliculi from ectoderm associated with the diencephalon,. 9. The thymus develops from endoderm of the third pharyngeal pouches.

N. Aging and the Endocrine System (p. 723) 1. Endocrine glands generally atrophy with aging. 2. However, the secretion of only some hormones is decreased, e.g., hGH, thyroid hormones, cortisol, aldosterone, insulin, and sex hormones.

O. Key Medical Terms Associated with the Endocrine System (p. 726) 1. Students should familiarize themselves with the glossary of key medical terms.