Endocrine glands

Question 1

The endocrine system

Answer 1

Responsible for the synthesis and secretion of chemical messengers, called hormones, into the bloodstream.

Question 2

Structure of endocrine glands

Answer 2

Generally, endocrine glands are composed of islands of secretory epithelial cells with intervening supporting tissue, rich in blood and lymphatic vessels. The secretory cells discharge the hormone into the interstitial spaces, and it is rapidly absorbed into the circulatory system.

Question 3

Endocrine cell characteristics

Answer 3

• prominent nuclei; genes expressed into proteins
• abundant mitochondria; energy for active synthesis
• abundant endoplasmic reticulum and golgi; synthesis and packing of peptide/lipid hormones
• secretory vesicles; store hormones

Question 4

3 classes of hormones

Answer 4

• protein/peptide hormones
• amine hormones
• steroid hormones

Question 5

Storage of peptide/amine hormones

Answer 5

Larger protein (eg. insulin, growth hormone, parathyroid hormone), smaller peptide (eg. anti-diuretic hormone) or amine (thyroxine, adrenaline, noradrenaline) hormones can be stored in highly concentrated secretory granules.

Question 6

Storage of steroid hormones

Answer 6

Steroid hormones (eg. androgens) are stored as the precursor molecule cholesterol.

Question 7

Stucture of the thyroid gland

Answer 7

Composed of spaces called follicles, lined by simple cuboidal epithelial cells. The iodine-containing thyroid hormones T3 and T4 are stored as colloid within the follicles. The connective tissue between follicles is highly vascularised. The walls of follicles also contain parafollicular ‘C’ cells, which are involved in the secretion of calcitonin.

Question 8

Structure of the parathyroid gland

Answer 8

2 cell types are present:

• chief cells; secrete parathyroid hormone (PTH), which is important in the regulation of blood calcium levels
• oxyphil cells

Question 9

Structure of the adrenal glands

Answer 9

Consist of an outer cortex with 3 zones, responsible for the secretion of corticosteroid hormones, and an inner medulla, which secretes catecholamines. The entire gland is surrounded by a fibrous connective tissue capsule.

Question 10

Structure of the cortex

Answer 10

Comprised of 3 zones:

• zona glomerulosa; cells arranged in rounded clusters, secretes mineralocorticoids (eg. aldosterone), controlled by the renin-angiotensin aldosterone system
• zona fasciculata; cells arranged in parallel cords, secrets glucocorticoids (eg. cortisol), controlled by the secretion of adrenocorticotrophic hormone (ACTH) by the anterior pituitary
• zona reticularis; small, closely packed cells arranged in irregular cords, secrete sex hormones (eg. androgens)

Question 11

Structure of the medulla

Answer 11

Neuroendocrine tissue composed of large cells arranged in clumps, with numerous large veins. Secretes catecholamines adrenaline and noradrenaline under the control of the sympathetic division of the autonomic nervous system.

Question 12

Structure of the endocrine pancreas

Answer 12

Composed of isolated clusters of endocrine cells grouped around capillaries called islets of Langerhans, scattered throughout the exocrine glandular tissue. Different cell types within the pancreas all secrete peptide hormones (eg. insulin, glucagon, somatostatin).

Question 13

Types of pancreatic endocrine cell

Answer 13

• beta cells; secrete insulin, make up 60% of cells in the islet, distributed throughout
• alpha cells; secrete glucagon, make up 25% of cells in the islet, located around the periphery
• delta cells; secrete somatostatin
• other cells types can be found in the islets or scattered singly or in small groups between the acini and along the ducts

Question 14

Structure of the pituitary gland

Answer 14

Divided into anterior and posterior lobes, which have different embryological origins, functions and control mechanisms.

Question 15

Structure of the posterior pituitary

Answer 15

Derived from a downgrowth of nervous tissue from the hypothalamus, to which it remains joined by the pituitary stalk. It secretes anti-diuretic hormone (ADH/vasopresin) and oxytocin, which act on non-endocrine tissues.

Question 16

Structure of the anterior pituitary

Answer 16

An epithelial upgrowth derived from Rathke’s pouch. Most secretion is mediated by hormones released from the hypothalamus (eg. TSHRH). Cells within the anterior pituitary are either acetophils, whose hormones act directly, basophils, whose hormones stimulate other glands, or chromophils.

Question 17

5 cell types of the anterior pituitary

Answer 17

• somatotrophs; secrete growth hormone, make up 50% of the bulk of the anterior pituitary
• mammotrophs; secrete prolactin, comprise up to 20% of the anterior pituitary
• corticotrophs; secrete adrenocorticotrophic hormone (ACTH), make up ~20% of the anterior pituitary
• thryotrophs; secrete thyroid stimulating hormone (TSH), make up ~5% of the anterior pituitary
• gonadotrophs; secrete FSH and LH, make up ~5% of the anterior pituitary

Question 18

Structure of the pineal gland

Answer 18

A small, spherical gland which lies in the midline of the brain, connected by a small stalk which contains nerve fibres. Produces melatonin in response to light levels detected by the retina and communicated via the suprachiasmatic nucleus.

Question 19

The diffuse endocrine system

Answer 19

Composed of widely distributed APUD cells, which secrete a variety of peptide hormones. These neuroendocrine cells are particularly prominent in the gastrointestinal and respiratory tracts.