Topic 6.6 - Hormones, Homeostasis and Reproduction

SRY gene

In males, a protein called testis determining factor (TDF) is responsible for the development of the testis and production of testosterone. Which gene, found on the Y chromosome, codes for the TDF protein? (Answer in one or two words.)

Fimbria

A fringe of tissue adjacent to an ovary that sweep an oocyte into the oviduct

circadian rhythm

A physiological cycle of about 24 hours that is present in all eukaryotic organisms and that persists even in the absence of external cues.

B. In order to facilitate the maturation of eggs prior to collection.

Before her eggs are due to be collected for IVF, a woman is injected around 36 hours before collection. This injection usually contains human chorionic gonadotrophin (hCG). Why is this hormone used prior to the collection of eggs? A. To allow the sperm to fertilise the eggs more easily in vitro. B. In order to facilitate the maturation of eggs prior to collection. C. To stop the secretion of all other menstrual cycle hormones prior to IVF. D. To ensure too many eggs are not released from the ovaries.

Urethra

Conducts sperm / semen from the prostate gland to the outside of the body via the penis (also used to convey urine)

Increase in FSH secretion causes follicle development in the ovary. A follicle contains an oocyte (egg) surrounded by a layer of follicle cells. The follicle cells secrete estrogen. Estrogen causes the endometrium to thicken and become highly vascularized. Estrogen inhibits the secretion of FSH (negative feedback). Estrogen promotes the secretion of LH. LH stimulates the follicle cells to secrete more estrogen. More estrogen promotes a surge in LH (positive feedback). Surge in LH causes ovulation.

Describe how hormones control the menstrual cycle. Part 1 - the events leading to ovulation.

Following ovulation, LH promotes the formation of the the corpus luteum. The corpus luteum secretes progesterone and some estrogen. Progesterone and estrogen maintain the thickened endometrium. Progesterone and estrogen inhibit the secretion of FSH and LH by the pituitary gland (negative feedback). If no embryo implants in the endometrium, the corpus luteum degenerates and no longer secretes progesterone and estrogen. This causes menstruation. FSH secretion is no longer inhibited and the cycle begins again.

Describe how hormones control the menstrual cycle. Part 2 - the events following ovulation.

Leptin is secreted by adipose tissue into the blood. Adipose tissue stores fat in the body. The hypothalamus in the brain is the target for leptin. The hypothalamus is involved in appetite control. An increased amount of adipose tissue (due to increased food consumption) increases the concentration of leptin in the blood. This causes the inhibition of appetite by the hypothalamus. Leptin has been tested as a treatment for obesity in clinical trials. It has shown to be an effective treatment in rare patients who cannot synthesize leptin (due to gene mutation). However, most obese humans have high concentrations of leptin in their blood and are leptin resistant. Clinical trials have had mixed results.

Describe the role of leptin in humans.

Melatonin controls circadian rhythms. Circadian rhythms are in built 24-hour (day-night) cycles of activity. The hypothalamus controls the secretion of melatonin. Melatonin is secreted by the pineal gland (located in the brain). Melatonin levels are high at night, promoting sleep. Melatonin levels are low during the day. If volunteers are placed in continuous dark conditions, the circadian rhythm continues but will extend a little over 24 hours. Light signals that are transmitted to the hypothalamus, allow it to modify the timing of melatonin secretion each day.

Describe the role of melatonin in humans.

Thyroxin is secreted by the thyroid gland, which is located in the neck. Thyroxin contains three or four iodine atoms. Most cells of the body are targets for thyroxin. It regulates metabolism. Thyroxin binding to target cells increases metabolic rate. A high metabolic rate will generate heat. Thyroxin plays a role in regulating body temperature: Low body temperature increases thyroxin release. This increases metabolic activity, which raises body temperature. High body temperature decreases thyroxin release. This reduces metabolic activity, which lowers body temperature. Iodine deficiency in the diet can lead to low levels of thyroxin (leading to an enlarged thyroid gland or goitre). Iodine is commonly added to table salt to avoid this problem.

Describe the role of thyroxin in homeostasis.

Menstruation

During the menstrual cycle, what occurs in response to a fall in the progesterone level?

LH is produced by pituitary (stimulus) promotes the formation of the corpus luteum. The corpus luteum then produces progesterone (response). The progesterone Inhibits LH secretion

Explain an example of negative feedback in the menstrual cycle

LH is produced by the pituitary gland. LH causes follicle cells to make estrogen. Estrogen promotes production of LH. LH produced increases. This LH production continues to increase until ovulation - then follicle cells become the corpus luteum and make progesterone which inhibits LH production.

Explain an example of positive feedback in the menstrual cycle

There are two hormones, insulin and glucagon, that are responsible for controlling the blood glucose concentrations in the body. Controlling the blood glucose concentrations is an example of a negative feedback loop. After eating, the bodies blood glucose levels are high. In order to control these levels beta cells of the pancreas release the hormone Insulin, which causes a decrease in blood glucose concentration. Additionally, in the liver it stimulates glycogen synthesis, which promotes glucose uptake by the liver and adipose tissue. This then increases the rate of glucose breakdown by increasing cell respiration rates. After exercising, the bodies blood glucose levels are low. In order to control these levels alpha cells of the pancreas release glucagon, which can an increase in blood glucose concentrations. Additionally, in the liver it stimulates glycogen breakdown, which promotes glucose release by the liver and adipose tissue. This then decreases the rate of glucose break down by reducing respiration rates.

Explain the control of blood glucose

This experiment was conducted using mice that were either obese due to a leptin gene mutation or a defective leptin receptor. When the obese mouse with no leptin was parabiotically fused to a healthy mouse: Leptin in the blood of the healthy mouse was transferred to the obese mouse. The obese mouse responded to the leptin and began to lose weight, demonstrating the potential viability of leptin treatment. However, when the obese mouse with a defective leptin receptor was parabiotically fused to a healthy mouse: Leptin was transferred to the healthy mouse (the obese mouse overproduced leptin to compensate for low receptor sensitivity). The obese mouse remained obese as its body could not respond to leptin. The healthy mouse became emaciated due to the abnormally high levels of leptin transferred into its bloodstream

Explain the results of the leptin trials conducted by surgically fusing the blood circulation of obese and healthy mice (parabiosis)?

FSH and LH are produced by the pituitary gland; estrogen and progestin are produced by the ovary; FSH stimulates the ovary to promote development of a follicle; The developing follicles secrete estrogen, which inhibits FSH (negative feedback); Estrogen stimulates growth of endometrium; Estrogen stimulates LH secretion (positive feedback); LH stimulates follicle growth and triggers ovulation;(the secondary oocyte leaves the ovary and) follicle becomes corpus luteum; The corpus luteum secretes estrogen and progesterone; Estrogen and progesterone maintain the endometrium; Estrogen and progesterone inhibit LH and FSH (negative feedback); After (two weeks) the corpus luteum degenerates progesterone and estrogen levels fall; This triggers menstrual bleeding, the loss of endometrium; The pituitary gland secreted FSH and LH, as they are no longer inhibited (and the menstrual cycle continues);

Explain the role of hormones in the regulation of the menstrual cycle

Follicle stimulating hormone (FSH) is secreted from the anterior pituitary and stimulates growth of ovarian follicles. The dominant follicle produces estrogen, which inhibits FSH secretion (negative feedback) to prevent other follicles growing. Estrogen acts on the uterus to stimulate the thickening of the endometrial layer

Explain what happens in the follicular phase in the menstrual cycle

The ruptured follicle develops into a slowly degenerating corpus luteum. The corpus luteum secretes high levels of progesterone, as well as lower levels of oestrogen. Estrogen and progesterone act on the uterus to thicken the endometrial lining (in preparation for pregnancy). Estrogen and progesterone also inhibit secretion of FSH and LH, preventing any follicles from developing

Explain what happens in the luteal phase in the menstrual cycle

If fertilisation occurs, the developing embryo will implant in the endometrium and release hormones to sustain the corpus luteum. If fertilisation doesn't occur, the corpus luteum eventually degenerates (forming a corpus albicans after ~ 2 weeks). When the corpus luteum degenerates, estrogen and progesteron levels drop and the endometrium can no longer be maintained. The endometrial layer is sloughed away and eliminated from the body as menstrual blood (i.e. a woman's period). As estrogen and progesterone levels are too now low to inhibit the anterior pituitary, the cycle can now begin again

Explain what happens in the menstruation phase in the menstrual cycle

Midway through the cycle (~ day 12), estrogen stimulates the anterior pituitary to secrete hormones (positive feedback). This positive feedback results in a large surge of luteinizing hormone (LH) and a lesser surge of FSH. LH causes the dominant follicle to rupture and release an egg (secondary oocyte) - this is called ovulation

Explain what happens in the ovulation phase in the menstrual cycle

Islets of Langerhans, alpha and beta cells

From where in the pancreas are insulin and glucagon released? (Cell names are not needed).

All embryos go through the initial stages of growth and development in the same way. By the time an embryo is five weeks old, embryonic gonads have developed that can become either male or female gonads. One gene located on the Y chromosome determines if the embryo will develop male or female gonads. The SRY gene is only found on the Y chromosome. If the embryo has a Y chromosome, SRY gene expression will occur after 7 weeks. The protein coded by the SRY gene will cause the embryonic gonads to develop into testes. A fetus with testes will become male. If there is no expression of the SRY gene by week 13, the embryonic gonads develop into ovaries. A fetus with ovaries will become female. The embryo is by default female; the expression of the SRY gene causes a change in gonad development.

How is sex determined in males and females?

As obese people are constantly producing higher levels of leptin, their body becomes progressively desensitised to the hormone. This means they are more likely to feel hungry, less likely to recognise when they are full and are hence more likely to overeat. Leptin resistance also develops with age, increasing the potential for weight gain later in life (e.g. the 'middle-age spread'). Theoretically leptin hormone would reduce hunger and limit food intake in obese individuals, leading to weight loss.

In obese people the target organ (or target cells) becomes 'resistant' to the leptin hormone. Explain what this word 'resistant' means in this case, and suggest why taking leptin hormone tablets did not help reduce the appetites of obese patients in a medical trial.

adipose cells

Leptin is responsible for suppressing appetite after a meal. From where is leptin secreted?

Vas Deferens

Long tube which conducts sperm from the testes to the prostate gland (which connects to the urethra) during ejaculation

In the 17th century, it was still unknown that gametes fuse to form a zygote. Aristotle proposed that the sperm forms an egg when inside the female, which then forms an embryo. Harvey thought that animals came from eggs and that semen (provided by sexual intercourse) was needed to cause the egg to form an embryo. Harvey dissected a uterus from a female deer that had recently mated; he could not find an embryo. He only saw embryos in deer that had mated several weeks earlier. He was unable to to gain the evidence he needed to describe the process of gamete fusion. He thought his own ideas were not fully correct. The magnifying equipment that existed at that time was not good enough for him to see sperm, eggs, or early stage embryos (these were too small). The discovery of sperm fusing with eggs and embryo development required the invention of the microscope.

Outline William Harvey's investigations into sexual reproduction.

The endocrine system consists of a number of ductless glands that secrete hormones directly into the blood. Examples include the pancreas, adrenal glands, thyroid gland, and pituitary gland. Hormones are chemical messengers (proteins or steroids) transported around the body in the bloodstream. Hormones cause a change in activity of target cells. Homeostasis is the maintenance of the internal environment. Certain factors (such as blood glucose levels and temperature) must remain within certain limits. If changes occur, mechanisms act to bring these factors back to within original limits (this is called negative feedback).

Outline the following: Endocrine system Hormone Homeostasis

In vitro fertilization (IVF) is a procedure for couples with fertility problems to have children. During the first half of the cycle, drugs are given to inhibit FSH and LH secretion. This suspends the menstrual cycle. High doses of FSH are given for about 12 days. This is to stimulate ovaries to develop multiple follicles. This is called superovulation. When the follicles are fully developed, HCG is given to stimulate the follicles to mature. Mature eggs are collected a day and a half later. Eggs and sperm are mixed in a culture dish and placed in an incubator for 2 days. Prior to implantation, the female is given progesterone to thicken the lining of the uterus. One to three embryos are implanted into the uterus. The remaining embryos can be frozen.

Outline the hormone therapy given for IVF.

Prenatal development of female reproductive structures. If the fetus has ovaries, then no testosterone is produced. In the absence of testosterone and the presence of estrogen and progesterone from the mother, regions of the fetus develop into female reproductive structures. Estrogen and progesterone are steroid hormones. Development of secondary sexual characteristics. Secretion of estrogen and progesterone by the ovaries increases at puberty. This causes: Enlargement of breasts. Pubic and underarm hair growth. Widening of the hips.

Outline the roles of estrogen and progesterone in females.

Prenatal development of male reproductive structures. Once testes are developed by a fetus, they start to secrete testosterone. Testosterone is a steroid hormone. Testosterone causes some regions of the embryo to develop into male reproductive structures. Development of secondary sexual characteristics. Secretion of testosterone by the testes increases at puberty. This causes: Sperm production. Growth of the penis and testes. Pubic, body, and facial hair growth. Increased muscle mass.

Outline the roles of testosterone in males.

LH

Ovulation occurs after the peak in which hormone of the menstrual cycle?

Vagina

Passage leading to the uterus by which the penis can enter (uterus protected by a muscular opening called the cervix)

Prostate Gland

Secretes an alkaline fluid to neutralise vaginal acids (necessary to maintain sperm viability)

Seminal Vesicle

Secretes fluid containing fructose (to nourish sperm), mucus (to protect sperm) and prostaglandin (triggers uterine contractions)

Epididymis

Site where sperm matures and develops the ability to be motile (i.e. 'swim') - mature sperm is stored here until ejaculation

The testis (plural: testes) is responsible for the production of sperm and testosterone (male sex hormone)

Testis

Endometrium

The mucous membrane lining of the uterus, it thickens in preparation for implantation or is otherwise lost (via menstruation)

Uterus

The organ where a fertilised egg will implant and develop (becoming an embryo)

Oviduct

Transports the oocyte to the uterus - it is also typically where fertilisation occurs

Jetlag is caused by travelling through multiple time zones in a short period of time. Travelling eastwards is a particular problem. The circadian rhythm (set at the origin of travel) no longer matches the day-night times in the new location. This can result in the person feeling sleepy in the day or being awake during the night. It takes a few days for the circadian rhythm to adjust to local conditions, by light signals transmitted to the hypothalamus modifying melatonin secretion. Some travellers take melatonin to match the night time in their destination. Melatonin can help reduce jetlag.

What are the causes of jet lag and how can it be prevented?

Type I diabetes is caused when beta cells are destroyed by the immune system. The disease can start during childhood. The body produces little or no insulin. The means there is a loss of control of blood glucose levels. To treat it, patients have to inject insulin. Blood glucose concentrations need to be regularly monitored. The timing of injections must correspond with high blood glucose concentrations. Injections are given before meal times. Type II diabetes is associated with various risk factors, such as obesity, lack of exercise, age, and genetic factors. It normally starts in adults. The body has a insulin insensitivity (due to reduced number of insulin receptors). Loss of control of blood glucose levels. To treat it, patients must try to changes their lifestyle by reducing carbohydrates in the diet, eating little and often, regularly exercising. Insulin injections can be given to some patients (as insulin production can reduce over time).

What are the causes of type I and type II diabetes and how are they treated?

B. Peak levels stimulate follicle development and estrogen secretion of the follicle.

What are the effect(s) of high levels of FSH in the menstrual cycle? A. Peak levels cause ovulation. B. Peak levels stimulate follicle development and estrogen secretion of the follicle. C. Peak levels stimulate follicle development and progesterone secretion of the follicle. D. Peak levels cause repair of the lining of the uterus.

follicular phase, ovulation, luteal phase and menstruation

What are the four stages in the menstrual cycle

Melatonin is the hormone responsible for synchronising circadian rhythms and regulates the body's sleep schedule. Melatonin secretion is suppressed by bright light (principally blue wavelengths) and hence levels increase during the night

What effect does this evening melatonin level have on the body?

The natural melatonin levels in the body in the evening increasing int the evening up to midnight.

What happens to the natural melatonin levels in the body in the evening up to midnight?

Insulin is released from beta (β) cells of the pancreas and cause a decrease in blood glucose concentration. This may involve stimulating glycogen synthesis in the liver (glycogenesis), promoting glucose uptake by the liver and adipose tissue, or increasing the rate of glucose breakdown (by increasing cell respiration rates)

What happens when blood glucose levels are high (e.g. after feeding)?

Glucagon is released from alpha (α) cells of the pancreas and cause an increase in blood glucose concentration. This may involve stimulating glycogen breakdown in the liver (glycogenolysis), promoting glucose release by the liver and adipose tissue, or decreasing the rate of glucose breakdown (by reducing cell respiration rates)

What happens when blood glucose levels are low (e.g. after exercise)?

surges cause ovulation, results in the formation of a corpus lutuem maturation of follicle cells

What is the effect of FSH in the menstrual cycle

Promotes the release of an egg during ovulationand the formation of the corpus luteum.

What is the effect of LH in the menstrual cycle

thicken uterus lining, inhibits LH + FSH for most of the cycle, stimulates FSH + LH release pre-ovulation

What is the effect of estrogen in the menstrual cycle

maintains the thickness of uterus lining, inhibits LH + FSH

What is the effect of progesterone in the menstrual cycle

C. To increase the feeling of satiety after a meal.

What is the main role of leptin? A. To control the circadian rhythm. B. To increase metabolic activity in the tissues of the body. C. To increase the feeling of satiety after a meal. D. To break down glycogen into glucose in order to increase blood sugar levels.

The menstrual cycle starts in females at puberty and continues until menopause. In the first half of the 28 day cycle, a follicle develops in the ovary and the lining of the uterus (the endometrium) is prepared for possible implantation. Ovulation (release of an oocyte) occurs mid-cycle. If no implantation occurs, the endometrium breaks down and menstruation occurs. The cycle is controlled by four hormones. Two are secreted from the pituitary gland and two are secreted by the ovaries. Pituitary hormones: FSH and LH. Ovarian hormones: Estrogen and progesterone. The cycle involves both negative and positive feedback. Negative feedback mechanisms act against a change. Positive feedback mechanisms will reinforce a change.

What is the menstrual cycle?

D. To break down glycogen into glucose in order to increase blood glucose levels.

What is the role of glucagon? A. To produce glycogen from glucose in order to decrease blood sugar levels. B. To break down glycogen into glucose in order to decrease blood glucose levels. C. To produce glycogen from glucose in order to increase blood sugar levels. D. To break down glycogen into glucose in order to increase blood glucose levels.

pineal gland

Where is melatonin secreted from? (Answer with two words.)

Ovary

Where oocytes mature prior to release (ovulation) - it also responsible for estrogen and progesterone secretion

Melatonin

Which hormone can be administered in order to help overcome jet lag?

D. I, II and III

Which of the following are roles of the corpus luteum in the menstrual cycle? I. Secreting oestrogen II. Secreting progesterone III. Inhibiting the secretion of FSH and LH A. III only B. I and III only C. II and III only D. I, II and III

C. The stimulus causes a response in a target organ that reduces the stimulus.

Which of the following best describes the idea of negative feedback in hormonal control? A. The hormone has a negative effect on the target organ. B. The hormone produces a response in the target organ which is negative. C. The stimulus causes a response in a target organ that reduces the stimulus. D. The stimulus produces a response in the target organ which tells the gland to stop producing hormone.

A. Glucagon is released from the α cells of the pancreatic islets.

Which response takes place when blood glucose levels are low? A. Glucagon is released from the α cells of the pancreatic islets. B. Glucagon is released from the β cells of the pancreatic islets C. Insulin is released from the α cells of the pancreatic islets. D. Insulin is released from the β cells of the pancreatic islets.