The Endocrine System Flashcards
What is a neuroendocrine cell?
Neurons that release their secretions into the bloodstream
Exocrine versus Endocrine Glands
Exocrine glands = ducts, secretions have extracellular effects
Endocrine glands = ductless with a high density of fenestrated capillaries, secretions have intracellular effects
The _____ is suspended from the hypothalamus by the _____ and rests in a depression of the sphenoid bone called the _____.
Pituitary gland/Hypophysis
Infundibulum/Pituitary stalk
Sella turcica
Two Structures of the Pituitary Gland
1) Adenohypophysis
2) Neurohypophysis
The adenohypophysis arises from the _____ that grows upward from the embryonic pharynx while the neurohypophysis arises from a downward growth of the diencephalon called the _____.
Hypophyseal pouch
Neurohypophyseal bud
Two Parts of the Adenohypophysis
1) Anterior lobe (“pars distalis” - these cells create the hormones)
2) Pars tuberalis (a small mass of cells that wraps around the pituitary stalk)
* *The fetus also contains a strip of tissue between the anterior lobe and neurohypophysis called the pars intermedia
Hypophyseal Portal System
While the pituitary gland has no nervous connection to the hypothalamus, it is liked to it by this network of primary capillaries in the hypothalamus. Portal venules travel down the stalk to the secondary capillaries located in the anterior pituitary.
Three Parts of the Neurohypophysis
1) Median eminence (extension of the floor of the brain)
2) Infundibulum
3) Posterior lobe (pars nervosa)
Hypothalamo-Hypophyseal Tract
A bundle of nerve fibers traveling from the hypothalamus to the posterior lobe of the neurohypophysis
Hormones Produced by the Hypothalamus
1) TRH (thyrotropin-releasing hormone)
2) CRH (corticotropin-releasing)
3) GnRH (gonadotropin-releasing)
4) GHRH (growth hormone-releasing
5) PRH
6) PIH (prolactin-inhibiting)
7) Somatostatin (inhibits GH and TSH)
8) Oxytocin
9) Antidiuretic hormone
Oxytocin
- Main hypothalamic nuclei
- Function(s)
- OT comes mainly from neurons in the right and left paraventricular nuclei in the hypothalamus
- Stimulates uterine contractions, ejection of milk from mammary glands, possibly involved in ejaculation of sperm, sexual affection, and mother-infant bonding
Antidiuretic Hormone
- Main hypothalamic nuclei
- Function(s)
- ADH comes mainly from the supraoptic nuclei of the hypothalamus
- Stimulates the kidney to retain water, increasing blood volume and vasoconstriction
- Can also act as a neurotransmitter (arginine vasopressin or AVP)
Definition of Trophic Hormone
The hormones produced by the pars distalis of the Adenohypophysis are called Trophic hormones because they influence other endocrine glands
6 Hormones Produced by the Anterior Lobe of the Adenohypophysis
1) FSH (folliculin)
2) LH (luteotropin)
3) TSH (thyrotropin)
4) ACTH (corticotropin)
5) PRL (luteotropic hormone, LTH)
6) GH (somatotropin, STH)
FSH
- Cell type
- Function(s)
- Follicle-stimulating hormone (aka folliculin)
- Secreted by cells called gonadotropes
- Females = stimulates secretion of female sex hormones and the development of bubblelike follicles that contain the eggs
- Males = sperm production
LH
- Cell type
- Function(s)
- Luteinizing hormone (aka luteotropin)
- Secreted by gonadotropes
- Females = stimulates ovulation (egg release) ~day 14 of cycle, formation of corpus luteum, and secretion of progesterone by the corpus luteum
- Males = stimulates testes to secrete testosterone from interstitial cells
**Sometimes also called ICSH (interstitial cell-stimulating hormone)
TSH
- Cell type
- Functions
- Thyroid-stimulating hormone (aka thyrotropin)
- Secreted by thyrotropes
- Stimulates growth of the thyroid gland and secretion of the thyroid hormones
ACTH
- Cell type
- Functions
- Adrenocorticotropin hormone (aka corticotropin)
- Secreted by corticotropes
- Stimulates the adrenal cortex to release glucocorticoids
PRL
- Cell type
- Function(s)
- Prolactin (aka luteotropic hormone or LTH)
- Secreted by lactotropes (or mammotropes)
- Females = stimulates milk production after birth
- Males = makes testes more susceptible to LH
GH
- Cell type
- Function(s)
- Growth hormone (aka somatotropin)
- Secreted by somatotropes
- Stimulates mitosis and cellular differentiation to promote tissue growth thought the body
- Stimulates the liver and other tissues to produce insulin-like growth factors (IGF-I and -II, aka somatomedins)
Neuroendocrine Reflexes
The release of hormones in response to signals from the nervous system
Ex. Dehydration causes a higher osmolarity of the blood which is detected by hypothalamic neurons called osmoreceptors; ADH is released to promote water conservation
Hormones of the Pars Intermedia
- MSH (melanocyte-stimulating hormone)
- Beta-endorphins (a neuropeptide)
Definition of Involution
Shrinkage of an organ
Hormones of the Pineal Gland
The pineal gland produces serotonin during the day and melatonin at night
- Melatonin has been suspected in a number of mood disorders, such as SAD and PMS
- Reduction of pineal gland results in premature sexual maturation
Phototherapy
Exposes patients to light therapy to reduce the amounts of melatonin
Thymus
- Role(s)
- Hormones
- Involutes following puberty
- Serves roles in the endocrine, lymphatic, and immune systems
- Site of maturation for T cells
- Secretes thymopoietin, thymosins, thymulin (hormones that aid in the production of T cells)
Isthmus
A narrow bridge of tissue connecting the two winglike lobes of the thyroid gland
Thyroid Follicles
The thyroid gland is composed of sacs called thyroid follicles that are filled with a protein-rich colloid and lined by a cuboidal epithelium of follicular cells
- Follicular cells secrete thyroid hormones: thyroxine (T4) and T3
- Surrounding the follicles are nests of C cells or parafollicular cells that secrete calcitonin
Calcitonin
Secreted by the parafollicular cells of the thyroid gland, this hormone responds to rising levels of Ca++ in the blood
- Stimulates osteoblasts activity to deposit calcium
Thyroid Hormone (TH)
Secreted in response to TSH from the pituitary gland
- T3 and T4
- Increases body’s metabolic rate - increases oxygen consumption and had a calorigenic effect, meaning it increases heat production
- Increases heart and respiratory rate, stimulates breakdown of fuels in the body, increases alertness and reflexes, etc
- Also important during embryonic development of the nervous system, so these hormone levels are monitored closely throughout pregnancy
Parathyroid Glands
- Hormone
The parathyroid glands are located on the posterior surface of the thyroid
- Chief cells secrete PTH (parathyroid hormone) in response to hypocalcemia with help from Vitamin D and calcitrol
- Oxyphil cells do not secrete hormones but serve as supportive cells in the parathyroid
- PTH targets:
- Bones (Osteoclast stimulation)
- Kidneys (to retain Ca++)
- Intestine (Ca++ absorption)
Adrenal Medulla
- Role(s)
- Hormones
- Serves in the endocrine system as well as the sympathetic nervous system
- Preganglionic axons of the sympathetic nervous system reach the chromaffin cells of the adrenal medulla which serve as postganglionic neurons; however, they secrete their hormones directly into the bloodstream like an endocrine gland so they are considered neuroendocrine cells
- Chromaffin cells produce epinephrine, norepinephrine and traces of dopamine (catecholamines)
Effects of Catecholamines Released by the Adrenal Medulla
These hormones are released in response to times of stress or fear to prepare the body for physical activity
- Increases cardiac output, increases HR, mobilization of high-energy fuels such as fatty acids, lactate, and glucose, etc
- Glycogenolysis and gluconeogenesis are stimulated in the liver
- Epi has a glucose-sparing effect – insulin is repressed so the cells absorb less glucose and glucose is reserved for the brain
Adrenal Cortex
- Hormones
- Layers
- Produces more than 25 steroid hormones called corticosteroids (or corticoids)
- Three layers, from outer to inner:
- Zona glomerulosa
- Zona fasciculata
- Zona reticularis
Three Major Categories of Corticosteroids and Layer of Adrenal Cortex Secreted From
1) Mineralcorticoids – regulate electrolyte balance
2) Glucocorticoids – regulate glucose metabolism
3) Sex Steroids – regulate various developmental and reproductive functions
Zona glomerulosa = Mineralcorticoids
Zona fasciculata = Glucocorticoids and Androgens
Zona reticularis = Glucocorticoids and Androgens
Aldosterone
- Origin
- Function(s)
- Zona glomerulosa
- Mineralcorticoid
- Stimulates kidneys to retain sodium and excrete potassium
- Maintains blood pressure and volume
Cortisol
- Origin
- Function(s)
- Glucocorticoid
- Secreted by both the Zona fasciculata and Zona reticularis
- Release is stimulated by ACTH in the pituitary
- Helps body adjust to stress by stimulating release/catabolism of glucose, proteins, FAs, etc
- Also has an anti-flammatory effect
Androgens
- Origin
- Function(s)
- Major sex steroids produced by the Zona fasciculata and Zona reticularis
- The major androgen is DHEA (dehydroepiandrosterone) that is converted by other tissues testosterone and dihydrotestosterone
- Important in male development
Estradiol
- Origin
- Function(s)
- While the adrenal cortex produces mainly androgens, it also produces estrogen (the main being estradiol)
- Small amount compared to that produced by the ovaries through menopause
Pancreas
- Primarily an exocrine digestive gland (acinus cells)
- Pancreatic islets of Langerhans are endocrine cell clusters
- ~20% alpha cells, 70% beta cells, 5% delta cells, and small numbers of PP and G cells
Glycemia
Blood glucose concentration, regulated mostly by hormones secreted by the pancreas
Alpha Cells
- Cells of the pancreatic islets of Langerhans
- Secrete glucagon in response to low blood glucose levels
- Stimulates gluconeogenesis and glycogenolysis, release of FAs from adipose tissue, amino acid absorption
Beta Cells
- Cells of the pancreatic islets of Langerhans
- Secrete insulin in response to high blood glucose levels
- Target primarily liver, skeletal muscle, and adipose tissue
- Stimulates absorption of glucose, FAs and amino acids, synthesis of glycogen, fats, proteins
Delta Cells
- Cells of the pancreatic islets of Langerhans
- Secrete somatostatin (growth hormone inhibiting hormone) along with release of insulin by beta cells
- Inhibits some digestive enzyme secretion and nutrient absorption
PP Cells (F cells)
- Cells of the pancreatic islets of Langerhans
- Secrete pancreatic polypeptide
- Inhibits gallbladder and intestinal contractions, secretion of stomach acid and pancreatic digestive enzymes
G Cells
- Cells of the pancreatic islets of Langerhans
- Secrete gastrin (just like the small intestine and stomach) to stimulate stomach acid secretion, motility, and emptying
Hyperglycemic versus Hypoglycemic Hormone
Hyperglycemic - any hormone that raises blood glucose concentration
(Ex glucagon, growth hormone, epinephrine, norepinephrine, cortisol, corticosterone)
Hypoglycemic - any hormone that lowers blood glucose concentration
(Ex insulin)
Heart, Liver, Stomach, Small Intestine, Placenta, Gonads
- Acting as endocrine gland
Heart = atrial natriuretic peptide (ANP) – increases Na+ and urine output; BP decreases
Liver = angiotensinogen produced by the liver is converted to angiotensin I by the enzyme renin and then to angiotensin II by enzyme ACE
= erythropoietin is also secreted by the liver to produce RBCs (though most is from the kidneys)
Stomach and Small Intestines = enteroendocrine cells that secrete enteric hormone
Placenta = estrogen, progesterone
Gonads = estradiol, progesterone, inhibin, testosterone
The Gonads
- Both endocrine and exocrine
- “Cytogenic glands” = exocrine products are whole cells (eggs and sperm)
3 Chemical Groups of Hormones
1) Steroid Hormones - derived from cholesterol
- Sex steroids
- Corticosteroids
2) Monoamines (biogenic amines)
- Catecholamines (NE, E, DA)
- Thyroid hormones
3) Peptides
- ADH and OT
- Releasing and inhibiting hormones of the hypothalamus
- Most ant. Pituitary hormones
Synthesis of Thyroid Hormones (Monoamines)
- Follicular cells absorb I- where it is oxidized to neutral I
- I is attached to tyrosine on a protein called thyroglobulin to form monoiodotyrosine (MIT)
- Another I attached to form diiodotyrosine (DIT)
- DIT + DIT = tetraiodothyronine or thyroxine (T4)
- DIT + MIT = triiodothyronine
Hormone Transport (hydrophilic and hydrophobic hormones)
- Monoamines and peptides are hydrophilic hormones so they are able to transport freely in the blood (unbound hormones)
- Steroid hormones are hydrophobic so they require transport proteins such as albumin and globulins (bound hormones)
- Transport proteins also enhance the half-life of the hormone – bound hormones can circulate for hours to weeks whereas unbound hormones can be removed from blood in a few minutes
Hormone Receptors
Hormones stimulate only those cells that have specific receptors for them
Receptors are protein or glycoprotein molecules located on the plasma membrane, cytoplasm, or in the nucleus
**Receptor defects are often the cause of endocrine disease
Up-regulation versus Down-regulation
Target cells can adjust their sensitivity to a hormone by changing the number of receptors for it
- can up-regulate or down-regulate
- can occur naturally or by drugs
3 Kinds of Hormone Interactions
1) Synergistic – two or more hormones work together to produce a greater effect (ex FSH and testosterone to produce sperm)
2) Permissive – where one hormone enhances a target organ’s response to a secondary hormone secreted later (ex estrogen stimulates up-regulation of progesterone receptors in the uterus)
3) Antagonistic – one hormone opposes the effects of the others (ex insulin and glucagon)
Stage of Resistance
If a stage of stress is not resolved within a few hours, the body enters a stage of resistance to provide alternative fuels for metabolism
CRH –> ACTH –> Cortisol, others
- Promotes breakdown of fats and proteins to use in gluconeogenesis in the liver
- Glucose-sparing effect
- Long-term elevation of cortisol sacrifices one’s immune response by inhibiting the synthesis of antibodies and lymphocytes
Stage of Exhaustion
- Fat stores depleted, body relies on protein breakdown
- Results in decline and death
Diabetes Insipidus
- Result if hyposecretion of ADH
- Kidneys cannot conserve water, resulting in polyuria without glucose and polydipsia
Common Pituitary Disorders
- Acromegaly
- Gigantism
- Dwarfism
- Acromegaly results from hyper secretion of the growth hormone in adults and is the thickening of bones and soft tissues, especially noticeable with the hands, feet and face
- In childhood/adolescence, hyper secretion of GH results in gigantism while hyposecretion results in pituitary dwarfism
Common Thyroid Disorders
- Congenital hypothyroidism
- Myxedema
- Endemic goiter
- Congenital hypothyroidism is thyroid hyposecretion present from birth
- Myxedema is prolonged adult hypothyroidism, characterized by slow metabolism, dry skin/hair, sleepiness, weight gain, etc
- Endemic goiter typically occurs in localities lacking iodine in the standard diet - thyroid hormones cannot be produced without iodine, so the pituitary gland overstimulates the thyroid resulting in hypertrophy of the thyroid gland
Common Parathyroid Disorders
- Hypoparathyroidism
- Hyperparathyroidism
- If the parathyroids are accidentally removed or degraded, Hypoparathyroidism causes a rapid decline in Ca++ levels in the blood and can be fatal within 3-4 days
- Hyperparathyroidism results in soft, deformed and fragile bones as well as the formation of kidney stones
Adrenal Disorders
- Cushing Syndrome
- Adrenogenital Syndrome (AGS)
- Cushing syndrome is due to excess cortisol secretion which can be caused by excess secretion of ACTH by the pituitary gland or can be independent of ACTH - symptoms include muscle and bone weakness due to disruption in carbohydrate and protein degradation
- AGS is the hypersecretion of adrenal androgen; in infants it can result in premature puberty and masculinized features in females
- AGS often accompanies Cushing syndrome
Diabetes Mellitus
- 3 “polys”
- Two forms
Diabetes Mellitus results from hyposecretion or inaction of insulin and exhibits polyuria (excessive urine) polydipsia (excessive thirst), and polyphagia (hunger)
- type one diabetes has a genetic component and is the result of low levels of insulin so it is typically treated by providing doses of insulin
- type two diabetes also has a genetic component and is the result of insulin resistance the body - this form of diabetes is often managed through diet and exercise
