The Endocrine System

Question 1

Exocrine glands

Answer 1

Release ENZYMES to the external environment through ducts. Include sweat, oil, mucus, and digestive glands.

Question 2

Endocrine glands

Answer 2

Release HORMONES directly into body fluids. The effects of the endocrine system tend to be slower, less direct, and longer lasting than the nervous system. Endocrine hormones do not move directly to their target tissue, but are released into general circulation.

Question 3

Receptors

Answer 3

All hormones act by binding to protein receptors. Hormones NEED a receptor, either on the cell membrane or inside the cell.

Each receptor is highly specific for its hormone. Some hormones have receptors on all cells, others have receptors only on specific tissues.

Question 4

3 types of hormones

Answer 4

1. Peptide
2. Steroid
3. Tyrosine

Question 5

Peptide hormones

Answer 5

Derived from peptides.

Precursor is made in the rough ER and cleaved in the ER lumen, where it becomes a pro hormone. From there, it goes to the Golgi, where it gets cleaved again and sometimes paired with carbs to become it’s final form. The Golgi (post office!) packages the hormones into secretory vesicles and releases them via exocytosis.

Peptide hormones are water soluble: move freely through the blood, but can’t get through the membrane, so they use a membrane receptor.

Question 6

Effector

Answer 6

The target cell of the hormone, the cell that the hormone is meant to affect.

Question 7

Target cell

Answer 7

The cell that the hormone is meant to affect

Question 8

Intracellular second messenger

Answer 8

One affect of a hormone binding to a receptor maybe to activate an intracellular second messenger such as cAMP, cGMP, or calmodulin.

These chemicals are called second messengers because the hormone is the original, or first messenger, to the cell.

The second messenger activates or deactivates enzymes and or ion channels and often creates a cascade of chemical reactions that amplifies the effect of the hormone. The cascade is one way that a small concentration of hormone can have a significant effect.

Review:
The primary messenger binds to membrane-bound receptors which activates a second messenger. The second messenger triggers a cellular response. Peptide hormones typically utilize a secondary messenger system

Question 9

The anterior pituitary hormones

Answer 9

FLAGTOP. FSH, LH, ACTH, HGH, TSH, Prolactin

All water soluble.

Question 10

The posterior pituitary hormones

Answer 10

ADH and oxytocin.

Both water-soluble.

Question 11

The pancreatic hormones

Answer 11

Glucagon and insulin

Both water-soluble.

Question 12

Steroid hormones

Answer 12

Derived from and often chemically similar to cholesterol. Formed in a series of steps mainly taking place in the smooth ER and the mitochondria.

Steroids are lipids and typically require a protein transport molecule, carrier protein, in order to dissolve into the bloodstream.

Being lipid soluble, steroids diffuse through the cell membrane of their effector. Once inside the cell, they combined with a receptor in the cytosol, which transports the steroid into the nucleus.

Steroids act at the transcription level.

The typical effect of a steroid hormone is to increase certain membrane or cellular proteins within the factor.

Question 13

Important steroid hormones for the MCAT

Answer 13

1. Glucocorticoids and mineral corticoids of the adrenal cortex- cortisol and aldosterone
2. The gonadal hormones- estrogen, progesterone, and testosterone.

(Note that estrogen and progesterone are also produced by the placenta.)

Steroid hormones come only from the adrenal cortex, the gonads, or the placenta.

Question 14

Tyrosine derivatives

Answer 14

1. The thyroid hormones: T3 and T4

T3 and T4 are lipid soluble and must be carried in the blood by plasma protein carriers. They are slowly released to their targets and bind to receptors inside the nucleus. Thyroid hormones increase the transcription of large numbers of genes in nearly all cells of the body.

2. The catecholamines formed in the adrenal medulla: epinephrine and norepinephrine

Epinephrine and norepinephrine are water-soluble and dissolve in the blood. They bind to receptors on the target tissue and act mainly through the second messenger cAMP.

All tyrosine derivative hormones are formed by enzymes in the cytosol or on the rough ER

Question 15

Endocrine glands and negative feedback

Answer 15

Endocrine glands tend to over-secrete their hormones. Some aspect of their effect on the target tissue will inhibit the secretion. The control point of the feedback is the conduct of the effector, not the concentration of hormone. In other words, the gland lags behind the effector.

So if an MCAT question indicates that the patient has high blood glucose, and asks whether high levels of insulin or high levels of glucagon would be expected, the correct answer is the hormone that is responding to the condition, not creating it, in this case- insulin.

Question 16

Anterior pituitary

Answer 16

Located in the brain beneath the hypothalamus. The hypothalamus controls the release of the interior pituitary hormones. These hormones are carried to the capillary bed of the interior pituitary by small blood vessels. The release of these hormones is, in turn, controlled by various nervous signals throughout the nervous system.

The interior pituitary releases six major hormones and several minor hormones. All of these are peptide hormones.

Question 17

Hypothalamus

Answer 17

Controls the release of the anterior pituitary hormones with releasing and inhibitory hormones of its own.

Question 18

Human growth hormone (hGH)

Answer 18

Peptide which stimulates growth in almost all cells of the body. Stimulates growth by increasing episodes of mitosis, increasing cell size, increasing the rate of protein synthesis, mobilizing fat stores, increasing the use of fatty acids for energy, and decreasing the use of glucose.

This is accomplished by increasing amino acid transport across the cell membrane, increasing translation and transcription, and decreasing the breakdown of protein and amino acids.

Question 19

Adrenocorticotropic hormone (ACTH)

Answer 19

A peptide which stimulates the adrenal cortex to release glucocorticoids via the second messenger system using cAMP. Release of ACTH is stimulated by many types of stress. Glucocorticoids are stress hormones.

Question 20

Thyroid stimulating hormone (TSH)

Answer 20

A peptide which stimulates the thyroid to release T3 and T4 be at the second messenger system using cAMP. TSH increases thyroid cell size, number, and the rate of secretion of T3 and T4. It is important to note that T3 and T4 concentrations have a negative feedback effect on TSH release, both at the anterior pituitary and the hypothalamus.

Question 21

Prolactin

Answer 21

A peptide which promotes lactation by the breasts. The reason that milk is not normally produced before birth is due to the inhibitory effects of the production by progesterone and estrogen.

Although the hypothalamus has a stimulatory effect on the release of all other interior pituitary hormones, it mainly inhibits the release of prolactin. The act of suckling, which stimulates the hypothalamus to stimulate the interior pituitary to release prolactin, inhibits the menstrual cycle. It is not known whether or not this is directly due to prolactin. The milk production effect of prolactin should be distinguished from the milk ejection affective oxytocin.

Question 22

Posterior pituitary

Answer 22

Composed mainly of support tissue for nerve endings extending from the hypothalamus. The hormones oxytocin and ADH are synthesized in the neural cell bodies of the hypothalamus, and transported down axons the posterior pituitary were there released into the blood. Both oxytocin and ADH are small polypeptides.

Question 23

Antidiuretic hormone (ADH)

Answer 23

AKA vasopressin. A small peptide hormone which causes the collecting ducts of the kidney to become permeable to water, reducing the amount of urine and concentrating the urine. Since fluid is reabsorbed, ADH also increases blood pressure. Coffee and beer are ADH blockers that increase volume.

Question 24

Oxytocin

Answer 24

A small peptide hormone that increases uterine contractions during pregnancy, and causes milk to be ejected from the breasts.

Question 25

Adrenal glands

Answer 25

Located on top of the kidneys. Generally separated into the adrenal cortex and the adrenal medulla.

Question 26

Adrenal cortex

Answer 26

The outside portion of the gland. Secrete only steroid hormones.

There are two types of steroids secreted by the cortex:

1. Mineral corticoids
2. Glucocorticoids

Question 27

Mineral corticoids

Answer 27

One of the steroid hormones secreted by the adrenal cortex. Affect the electrolyte balance in the bloodstream. The major mineralocorticoid is aldosterone.

Question 28

Glucocorticoids

Answer 28

One of the two types of steroids secreted by the adrenal cortex. Increase blood glucose concentration and have an even greater effect on fat and protein metabolism. The major glucocorticoid is cortisol.

Question 29

Aldosterone

Answer 29

A mineral corticoid. Packs in the distal convoluted tubule and the collecting duct to increase sodium and chlorine reabsorption and potassium and hydrogen secretion. Creates a net gain in particles in the plasma, which results and an eventual increase in blood pressure. Has the same effect to a lesser extent on the sweat glands, salivary glands, and intestines.

Question 30

Cortisol

Answer 30

A steroid. A glucocorticoid that increases blood glucose levels by stimulating gluconeogenesis in the liver. Also:

• degrades adipose tissue to fatty acids to be used for cellular energy
• creates a moderate decrease in the use of glucose by the selves
• causes the degradation of non-hepatic proteins, a decrease of non-hepatic amino acids, and the corresponding increase in liver and plasma proteins and amino acids.

A stress hormone. The benefit of excess cortisol under stressful situations is not fully understood. One explanation may include anti-inflammatory properties possessed by cortisol. Also diminishes the capacity of the immune system to fight infection.

Question 31

Gluconeogenesis

Answer 31

The creation of glucose and glycogen, mainly in the liver, from amino acids, glycerol, and/or lactic acid.

Question 32

Catecholamines

Answer 32

The tyrosine derivatives synthesized in the adrenal medulla: epinephrine and norepinephrine, also called adrenaline and noradrenalin.

Because of their “fight or flight” response, the catecholamines are also considered stress hormones.

Question 33

Epinephrine and norepinephrine

Answer 33

Catecholamines that are also called adrenaline and noradrenaline. Effects on the target tissues are similar to their effects in the sympathetic nervous system but they last much longer.

These are vasoconstrictors- they constrict the blood vessels of most internal organs and skin- and are also vasodilators of skeletal muscle, which means they increase blood flow. This is consistent with the “fight or flight” response of these hormones.

Question 34

T3 and T4

Answer 34

Two of the three thyroid hormones, along with calcitonin. Very similar in the fact. Both hormones are lipid soluble tyrosine derivatives that diffuse through the lipid bilayer and act in the nucleus of the cells of their effector. The general effect is to increase the basal metabolic rate. Thyroid hormone secretion is regulated by TSH.

Question 35

Calcitonin

Answer 35

A large peptide hormone released by the thyroid gland. Slightly decreases blood calcium by decreasing osteoclast activity and number. One of the three thyroid hormones, responsible for building bone mass.

Works in opposition to PTH, just like insulin works in opposition to glucagon.

Calcitonin is water-soluble.

Question 36

Basal metabolic rate

Answer 36

Resting metabolic rate

Question 37

Insulin

Answer 37

A peptide hormone released by the beta cells of the pancreas.

Associated with energy abundance in the form of high-energy nutrients in the blood. Released when blood levels of carbohydrates and proteins are high.

Affects carbohydrate, fat, and protein metabolism. In the presence of insulin, carbohydrates are stored as glycogen in the liver and muscles, fat is stored in adipose tissue, and amino acids are taken up by the cells of the body and made into proteins.

The net effect of insulin is to lower blood glucose levels.

Insulin binds to a membrane receptor beginning a cascade of reactions inside the cell. Except for neurons in the brain and a few other cells which are not affected by insulin, the cells of the body become highly permeable to glucose upon the binding of insulin. The insulin receptor itself is not a carrier for glucose.

Question 38

Glucagon

Answer 38

A peptide hormone released by the alpha cells of the pancreas. The effects of glucagon are nearly opposite to those of insulin. Glucagon stimulates glycogenolysis- the breakdown of glycogen- and gluconeogenesis in the liver. It acts via the second messenger system cAMP. At higher concentrations, glucagon breaks down adipose tissue, increasing the fatty acid levels of blood.

The net effect of glucagon is to raise blood glucose levels.

Question 39

Islets of Langerhans

Answer 39

Pancreatic structures which release hormones into the blood. Composed of numerous beta cells, which secrete insulin, and the less numerous alpha cells, which secrete glucagon. In type I diabetes, the islets of Langerhans do not produce insulin.

Question 40

Parathyroid glands

Answer 40

There are four small parathyroid glands attached to the back of the thyroid. The parathyroid glands release parathyroid hormone.

Question 41

Parathyroid hormone (PTH)

Answer 41

A peptide which increases blood calcium. As with other hormones, PTH has multiple factors and its effects may be different for each effector.

It increases osteocyte absorption of calcium and phosphate from the bone, and stimulates proliferation of osteoclasts. PTH increases renal calcium reabsorption and renal phosphate excretion. It increases calcium and phosphate uptake from the got by increasing renal production of the steroid 1, 2, 5-DOHCC, derived from vitamin D.

PTH secretion is regulated by the calcium ion plasma concentration, and the parathyroid glands shrink or grow accordingly.

PTH is water-soluble.

Question 42

Gonads

Answer 42

The male gonads are called the testes.

Question 43

Seminiferous tubules

Answer 43

Production of sperm occurs in this area of the testes.

Question 44

Spermatogonia

Answer 44

Located in the seminiferous tubules. Arise from epithelial tissue to become spermatocytes, spermatids, and then spermatozoa.

Question 45

Leydig cells

Answer 45

Located in the interstitial space between the tubules. Release testosterone when stimulated by LH.

Question 46

Testosterone

Answer 46

Released by Leydig cells when they are stimulated by LH. The primary androgen, stimulates the germ cells to become sperm. Responsible for the development of secondary sex characteristics such as pubic hair, enlargement of the larynx, and growth of the penis and seminal vesicles. Helps to initiate the growth spurt at puberty. Also stimulates closure of the epiphysis of the long bones, ending growth in stature.

Testosterone is lipid soluble.

Question 47

Androgen

Answer 47

Male sex hormone. Testosterone is the primary androgen.

Question 48

Epididymis

Answer 48

Once freed into the tubule lumen, the spermatozoa is carried here to mature.

Question 49

Vas deferens

Answer 49

Upon ejaculation, spermatozoa are propelled through the vas deferens into the urethra and out of the penis.

Question 50

Semen

Answer 50

The complete mixture of spermatozoa and fluid that leaves the penis upon ejaculation. Composed of fluid from the seminal vesicles, the prostate, and the bulbourethral glands, also called Cowper’s glands.

Question 51

Capacitation

Answer 51

Spermatozoa become activated for fertilization in this process, which takes place in the vagina.

Question 52

The female reproductive system- primary follicle

Answer 52

Oogenesis begins in the ovaries of the fetus. All the eggs of the female are arrested as primary oocytes at birth. At puberty, FSH stimulates the growth of cells around the primary oocyte. These cells secrete a viscous substance around the egg called the zona pellucida. The structure at this stage is called the primary follicle.

Question 53

The female reproductive system- secondary follicle

Answer 53

Next, theca cells differentiate from the interstitial tissue and grow around the follicle to form a secondary follicle. Upon stimulation by LH, these cells secrete androgen, which is converted to estradiol by the granulosa cells in the presence of FSH integrated into the blood. The follicle grows and boulders from the ovary. Typically, estradiol inhibits LH secretion by the anterior pituitary.

Question 54

Estradiol

Answer 54

The type of estrogen. A steroid hormone that prepares the uterine wall for pregnancy. Typically inhibits LH secretion by the interior pituitary. Just before ovulation, estradiol rises rapidly, actually causing a dramatic increase in LH secretion. This increases called the luteal surge.

Question 55

Ovulation

Answer 55

The bursting of the follicle.

Question 56

Luteal surge

Answer 56

Results from a positive feedback loop of rising estrogen levels which increased LH levels, which increase estrogen. The luteal search causes the follicle to burst, releasing the egg, now a secondary oocyte, into the body cavity.

Question 57

Fallopian tube

Answer 57

Also called uterine tube or oviduct. After the luteal search, the egg is swept into the fallopian tube by the fimbriae. The remaining portion of the follicle is left behind to become the corpus luteum. Once in the fallopian tube, the egg is swept toward the uterus by cilia.

Fertilization normally takes place in the fallopian tubes.

Question 58

Corpus luteum

Answer 58

The remaining portion of the follicle is left behind to become this structure. It secretes estradiol and progesterone throughout pregnancy, or in the case of no pregnancy, for about two weeks until it degrades into the corpus albicans.

Question 59

Menstrual cycle

Answer 59

The cycle of primary and secondary follicles and ovulation repeats itself approximately every 28 days until puberty unless pregnancy occurs. With each menstrual cycle, several primordial oocytes may begin the process, but normally only one completes the development ovulation.

The cycle is divided into three phases:
1. The follicular phase, which begins with the development of the follicle and ends at ovulation,

2. The luteal phase, which begins with ovulation and ends with the degeneration of the corpus luteum into the corpus albicans,
3. Flow, which is the shedding of the uterine lining, lasting approximately five days.

Question 60

Ovum

Answer 60

After the second meiotic division, the oocyte becomes an ovum and releases a second polar body.

Question 61

Zygote

Answer 61

Fertilization occurs when the nuclei of the ovum and sperm fuse to form the zygote

Question 62

Cleavage

Answer 62

Begins while the zygote is still in the fallopian tube.

Cleavage can be meroblastic or holoblastic.

Question 63

Morula

Answer 63

When the zygote is comprised of eight or more cells, it is called the morula. The first eight cells formed by cleavage or equivalent in size and shape and are said to be totipotent, meaning they have the potential to express any of their genes. Any one of these eight cells at this stage could produce a complete individual.

Question 64

Blastocyst

Answer 64

The cells of the morula continue to divide for four days forming a hollow ball filled with fluid. It is the blastocyst that lodges in the uterus in a process called implantation on about the fifth to seventh day after ovulation. The blastocyst is made up of embryonic stem cells that each have the ability to develop into most of the type of cells and the human body.

Question 65

Implantation

Answer 65

When the blastocyst lodges in the uterus on the fifth to seventh day after ovulation. Upon implantation, the female is said to be pregnant.

Upon implantation, the egg also begins to creating a peptide hormone called hCG, which prevents the degeneration of the corpus luteum, and maintains its secretion of estrogen and progesterone.

Question 66

Human chorionic gonadotropin (hGC)

Answer 66

Upon implantation, the cell begins treating a peptide hormone called HCG. This prevents the degeneration of the corpus luteum, and maintains its secretion of estrogen and progesterone. HCG in the blood and urine of the mother is the first outward sign of pregnancy. It is also what pregnancy tests are looking for.

Question 67

Placenta

Answer 67

The placenta is formed from the tissue of the egg and the mother, and takes over the job of hormone secretion. The placenta reaches full development by the end of the first trimester, and begins secreting its own estrogen and progesterone while lowering it secretion of hCG.

Question 68

Determination

Answer 68

CELL FATE.

As the embryo develops past the eight cell stage, the cells become different from each other due to cell-cell interactions. The process where a cell becomes committed to a specialized developmental path is called determination. Cells become determined to give rise to particular tissue early on.

Question 69

Differentiation

Answer 69

SPECIALIZATION INTO A SPECIFIC TISSUE.

The specialization that occurs at the end of the development forming a specialized tissue cell is called differentiation. Although the fate of a cell is typically determined early on, that same cell usually doesn’t differentiate into a specialized tissue cell until much later, at the end of the developmental process. Recent research a shown that the fate of even a fully differentiated cell can be altered given the proper conditions.

Question 70

Differentiation of the embryo from zygote to gastrula

Answer 70

See EK 136

Question 71

Gastrulation

Answer 71

The formation of the gastrula occurs in the second week after fertilization and a process called gastrulation. Cells begin to slowly move about the embryo for the first time. In mammals, the primitive streak is formed, which is analogous to the blastopore in aquatic vertebrates. Cells destined to become mesoderm migrate into the primitive streak.

During gastrulation, the three primary germ layers are formed.

Question 72

Ectoderm

Answer 72

Develop into the outer coverings of the body, such as the outer layers of skin, nails, into the enamel, and into the cells of the nervous system and sense organs.

Question 73

Mesoderm

Answer 73

The stuff that lies between the inner and outer covering of the body, including the muscle, bone, and the rest.

Question 74

Endoderm

Answer 74

Develops into the lining of the digestive tract, and into much of the liver and pancreas.

Question 75

Neurulation

Answer 75

In the third week, the gastrula develops into a neurula in a process called neurulation.

The notochord, made from mesoderm, induces the overlying ectoderm to thicken and form the neural plate. The notochord eventually degenerates, while a neural tube forms from the neural plate to become the spinal cord, brain, and most of the nervous system.

Question 76

Induction

Answer 76

When the notochord induces the overlying ectoderm to thicken and form the neural plate. Induction occurs when one cell type affects the direction of differentiation of another cell type.

Question 77

Apoptosis

Answer 77

Programmed cell death. Essential for development of the nervous system, operation of the immune system, and destruction of tissue between fingers and toes to create normal hands and feet in humans. Damaged cells may undergo apoptosis as well. Failure to apoptosize may result in cancer.

Question 78

Three effects of the endocrine system

Answer 78

The effects of the endocrine system are:

• to alter metabolic activities,
• to regulate growth and development
• to guide reproduction

Question 79

What’s interesting about the pancreas?

Answer 79

Note that the pancreas acts both as an exocrine gland– releasing digestive enzymes through the pancreatic duct– and as an endocrine gland– releasing insulin and glucagon directly into the blood.

Question 80

How do the endocrine and nervous systems work together?

Answer 80

The endocrine system works in conjunction with the nervous system. Many endocrine glands are stimulated by neurons to secrete their hormones.

Question 81

One way to regulate hormones

Answer 81

One method of hormone regulation: increase or decrease receptors when you have high or low hormone concentrations.

Question 82

Once bound by a hormone, a membrane receptor can do one of several things:

Answer 82

1. It may act as an ion channel, increasing membrane permeability to a specific ion.
2. It may activate or deactivate other intrinsic membrane proteins also acting as ion channels.
3. It may activate intracellular second messenger.

Question 83

What regulates apoptosis?

Answer 83

Protein activity in humans, as opposed to regulation of the transcription or translational level. The proteins involved in this process are present but inactive in a normal healthy cell. In mammals, mitochondria plan important role in apoptosis.

Question 84

Placental hormones

Answer 84

The placental hormones are hCG, estrogen, and progesterone.

HCG is water-soluble. Estrogen and progesterone are both lipid soluble.

Question 85

Where does fertilization take place?

Answer 85

Fertilization normally takes place in the fallopian tubes.

Question 86

Sertoli cells

Answer 86

Sertoli cells stimulated by FSH surround and nurture the spermatocyte and spermatids. They also secrete inhibin, a peptide hormone which acts on the pituitary gland to inhibit FSH secretion.