ENDOCRINE SYSTEM

ENDOCRINE SYSTEM
Chapter 17

Endocrine Gland is "ductless".

Exocrine Gland has a duct.

Pancreas -- example of an organ which is both an endocrine and exocrine gland. Insulin produced by endocrine gland portion (islets of Langerhans), digestive enzymes produced by exocrine gland portion (pancreatic ducts deliver digestive enzymes to duodenum).

Hormones

Hormones are secreted directly into blood.

Subtypes:

1) Amines -- catecholamines like epinephrine (adrenalin)

2) Polypeptides/Proteins--insulin

3) Steroids--cholesterol derived--like testosterone, estrogen

Target cells have specific receptors for hormones.

Mechanisms of Action: Dependent upon Target Cell

Steroids-- "Mobile-receptor mechanism" Alters transcription.

Thyroxine -- action similar to steroids but it is derived from the amino acid tyrosine.

Amines, Polypeptides, Proteins--bind to receptor on cell membrane ("fixed-membrane-receptor mechanism") which serves to activate a "second messenger." Second messenger is either calcium or cyclic AMP.

Hormone Actions and Interactions

Synergistic--epinephrine & norepinephrine. Hormones act in concert.
Permissive--estrogen & progesterone. Estrogen stimumates initial thickening of endometrium, progesterone further increases thickness. Without estrogen, progesterone effect would not be as effective.
Antagonistic--insulin & glucagon. Insulin decreases blood glucose levels, glucagon increases it.

Major Endocrine Glands

Pituitary (hypophysis): 2 regions, Anterior and Posterior lobes

Histology of Pituitary

Histology of Anterior Pituitary: Glandular

Histology of Posterior Pituitary: Neural

Adenohypophsis: Anterior. Produces 6 major regulatory hormones:

Luteinizing Hormone (LH)
Follicle Stimulating Hormone (FSH)

Cell Types of anterior pituitary.

Melanocyte-stimulating Hormone (MSH) (minor regulatory hormone in humans; controls skin pigment in many vertebrates)

Hypothalamus controls release of hormones of anterior pituitary by producing Releasing Hormones--some inhibit release.

Releasing hormone travels to anterior pituitary gland via the "hypothalamic-hypophyseal portal system"

Neurohypophysis: Posterior lobe. Neural--really an extension of the hypothalamus. Hypothalamus produces hormones--released by neurohypophysis.

2 Hormones produced:

Antidiuretic Hormone (ADH) & Oxytocin (the love hormone?)

Negative Feedback Control of Hormone Release/Production
Levels of Feedback Regulation
Animation

Pituitary Adenoma

Thyroid/Parathyroid

Thyroid: Produces thyroxine and calcitonin.

Thyroxine increases basal metabolism, stimulates growth.

"Thyroxine" (or T4) requires iodide/iodine (iodide is the ion, iodine is I2 -- our need for iodine in the diet is because of its use in T3 and T4). Two forms: T3 and T4 (number relates to the 3 or 4 iodide ions).

T3 & T4 are complexed with thyroglobulin which is stored in colloid within follicle cavity. Histology. Most endocrine glands do not store their hormones. Why would T3 and T4 be stored?

Calcitonin: lowers calcium and phosphate levels in blood (calcium sent to bones or used by cells).
Produced by "parafollicular cells."

Thyroid Gland in situ

Enlarged thyroid (goiter)

Parathyroid: Parathyroid Hormone.
Your Parathyroid (Endocrine Web)

Parathyroid hormone increases level of calcium in blood. (Calcium taken from bone).
Hyperparathyroidism and Osteoporosis

Hyperplasia of Parathyroids

Adrenal Gland: a dual organ

Cortex--produces 3 classes of corticosteroids

Mineralocorticoids
Glucocorticoids
Sex steroids (androgens)

Medulla--produces epinephrine and norepinephrine.

Fight-or-Flight effects

Adrenal glands are affected greatly by stress -- release of cortisol by cortex and epinephrine by medulla.

Adrenal production of androgens (e.g., from congenital adrenal hyperplasia) can cause the masculinization of the genitalia as well as masculinization of a number of behavioral characteristics in females.

Biochemistry of the Adrenal Cortex

Androstenedione

Pancreas: Pancreatic Islets produce:

Insulin: promotes glucose entry into cells from blood. Beta cells.
Glucagon: promotes glucose formation from glycogen. Alpha cells.

Normal Islet

Antibody labelling of Alpha and Beta cells.

Functional Anatomy of Endocrine Pancreas

Insulin Mechanism of Action: Shockwave animation

Testes and Ovaries (gonads): Secrete sex hormones.

Hormones of Reproductive System

Androgens (testosterone). Necessary for maintenance of genitalia and accessory structures, in addition to secondary sexual characteristics.

Estrogens and Progesterone

Ovarian follicles secrete estrogens prior to ovulation. After ovulation the follicle becomes the corpus luteum and secretes progesterone.

Placenta

Human chorionic gonadotropin (hCG) --secreted early by placenta--used to test for pregnancy.

Structure of hCG

Kidneys produce a variety of hormones related to the circulatory system.

Erythropoietin - increases red blood cell numbers.

Renin - increases blood pressure

Pineal Gland: Produces melatonin: role in circadian rhythms (jet lag), sleep cycle.
"Melatonin: The Natural Sleeping Pill"
Does Melotonin decrease as we age?

Heart: produces atriopeptin -- promotes loss of sodium and water in urinary system. Antagonistics to aldosterone.

Digestive system: covered in that section.

Prostaglandins: An "autocrine"--produced in same organ in which it exerts its effect. Serve as regulators in many systems--reproductive, digestive, circulatory, excretory. Example--prostaglandins and uterine contraction: Anaprox (naproxin), a medication for menstrual cramps, inhibits PG synthesis. PG can induce labor, and used in abortions.

Disorders of Endocrine Glands

Pituitary:

Inadequate GH or GH receptor -- dwarfism or midget. Short stature, but body is porportional. (Note: achrondroplasia causes a type of dwarfism in which the limbs are short, but the head and chest are close to "normal." Due to abnormal growth of cartilage).
Too much GH -- gigantism, acromegaly.
Acromegaly: hands

Growth Hormone stimulates liver to produce "insulin-like growth factor type 1" (IGF-1or somatomedin). IGF-1 stimulates growth of bones and muscles -- thus, IGF-1 has direct effects and GH has indirect effects on growth.

Pygmies have differences in GH receptor expression.

Human Growth Hormone: Health Central

Human Growth Hormone: Review, National Acad. Sports Medicine

Under secretion of ADH - diabetes insipidus.

Prolactin & Hyperprolactinemia

Adrenals:

1) Cushing's Syndrome--hypersecretion of corticosteriods. Causes and Symptoms