1. 6.6 Hormones, homeostasis and reproduction

2. Control of blood glucose concentration
Insulin and glucagon are secreted by alpha and beta cells in the pancreas to control blood glucose concentration.
Cells in the pancreas respond to changes in blood glucose levels.
If the glucose concentration deviates substantially from the set point of about 5 mmol L-1, homeostatic mechanisms mediated by the pancreatic hormones insulin and glucagon are initiated.
The pancreas is effectively two glands in one organ.
Most of the pancreas is exocrine glandular tissues that secrete digestive enzymes into ducts leading to the small intestine.
There are small regions of endocrine tissue called islets of Langerhans dotted through the pancreas that secrete hormones directly into the blood stream.

3. Islets of Langerhans
Alpha cells synthesize and secrete glucagon if the blood glucose level falls below the set point.
This hormone stimulates the breakdown of glycogen into glucose in the liver cell and its released into the blood, increasing concentration.
Beta cells synthesize insulin and secrete it when the blood glucose concentration rises above the set point.
This stimulated the uptake of glucose by various tissues, particularly skeletal muscle and liver, in which it also stimulates the conversion of glucose to glycogen.
Insulin therefore reduces blood glucose concentration.
*Like most hormones, insulin is broken down by the cell it acts upon, so its secretion must be ongoing.

4. Diabetes Causes and treatment of type I and type II diabetes.
Diabetes is the condition where a person has consistently elevated blood glucose levels even during prolonged fasting.
Type I: inability to produce sufficient quantities of insulin.
Treated by testing glucose concentrations and regular injections of insulin.
Type II: inability to process or respond to insulin because of deficiency of insulin receptors or glucose transporters on target cells.
Treated by adjusting diet.

5. Thyroxin
Thyroxin is secreted by the thyroid gland to regulate the metabolic rate and help control body temperature.
The hormone thyroxin is secreted by the thyroid gland in the neck.
Thyroxin contains 4 atoms of iodine.
Prolonged deficiency of iodine in the diet therefore prevents the synthesis of thyroxin.
Thyroxin regulates the body’s metabolic rate, so all cells need to respond, but the most metabolically active is liver, muscle and brain.
Higher metabolic rate supports more protein synthesis and growth and it increases the generation of body heat.

6. The importance of thyroxin is revealed by the effects of thyroxin deficiency (hypothyroidism):
Lack of energy
Forgetfulness and depression
Weight gain despite loss of appetite
Feeling cold all the time
Constipation because contractions of muscle in the wall of the gut slow down.
Impaired brain development in children.

7. Leptin
Leptin is secreted by cells in adipose tissue and acts on the hypothalamus of the brain to inhibit appetite.
The concentration of leptin in the blood is controlled by food intake and the amount of adipose tissue in the body.
The target of this hormone is groups of cells in the hypothalamus of the brain that contributes to the control of appetite.
Leptin binds to receptors in the membrane of these cells.
If adipose tissue increases, blood leptin concentration rise, causing long-term appetite inhibition and reduced food intake.

8. Leptin and Obesity
Testing of leptin on patients with clinical obesity and reason for the failure to control the disease.
The discovery that obesity in mice could be caused by a lack of leptin and cured by leptin injections soon led to attempts to treat obesity in humans this way.
73 obese volunteers: different leptin doses or placebo.
Injections caused skin irritation and swelling
Only 47 completed trial
Avg of 7.1 kg lost, but weight was quickly gained back after trial.
Leptin injections affect the development and functioning of reproductive system.

9. Melatonin…a girl’s best friend!!
Melatonin is secreted by the pineal gland to control circadian rhythms.
Humans are adapted to live in a 24-hour cycle and have rhythms in behavior that fit this cycle.
They can continue even if a person is placed in experimentally in light or darkness continuously, because an internal system is used to control the rhythm.
There is very little melatonin during the daytime and peaks around 2am.
Babies do not develop a regular day-night rhythm of melatonin until about three months old.

10. Jet Lag Causes of jet lag and use of melatonin to alleviate it.
Common in people who travel to different time zones (3+).
Difficulty staying awake during day, and sleeping at night.
Fatigue, irritability, headaches and indigestion.
Melatonin can be taken orally at the time when sleep would ideally begin.

11. Sex determination in males
A gene on the Y chromosome causes embryonic gonads to develop as testes and secrete testosterone.
Initially the development of the embryo is the same in all embryos and embryonic gonads develop that could either become ovaries or testes.
The developmental pathway of the embryonic gonads and the whole baby depends on the presence or absence of one gene.
If the gene SRY is present, the embryonic gonads develop into testes.
This gene is located on the Y chromosome.
SRY codes for a DNA-binding protein called TDF (testis determining factor).
TDF stimulates the expression of other genes that cause testis development.

12. Testosterone
Testosterone causes prenatal development of male genitalia and both sperm production and development of male secondary sexual characteristics during puberty.
The testes develop from the embryonic gonads in about the eighth week of pregnancy, at the time when the embryo is becoming a fetus and is about 30mm long.
Testosterone secretion causes male genitalia to develop.
During puberty, the secretion of testosterone increases.
This stimulates sperm production in the testes, which is the primary sexual characteristic of males.
Testosterone also cause the development of secondary sexual characteristics during puberty such as growth of pubic hair and deepening of the voice due to the growth of the larynx.

14. Structure
Function
Testis
Produce sperm and testosterone
Scrotum
Hold testes at lower than core body temperature
Epididymis
Store sperm until ejaculation
Sperm duct (vas deferens)
Transfer sperm during ejaculation
Seminal vesicle and prostate gland
Secrete fluid containing alkali, proteins and fructose that is added to sperm to make semen
Urethra
Transfer semen during ejaculation and urine during urination
Penis
Penetrate the vagina for ejaculation of semen near the cervix

15. Sex determination in females
Estrogen and progesterone cause prenatal development of female reproductive organs and female secondary sexual characteristics during puberty.
If there is no Y chromosome present, the gonads develop into ovaries.
Estrogen and progesterone are secreted instead of testosterone.
During puberty the secretions of estrogen and progesterone increases, causing the development of secondary sexual characteristics.
Enlargement of breast and growth of pubic and underarm hair.

17. Structure
Function
Ovary
Produce eggs, estrogen and progesterone
Oviduct
Collect eggs at ovulation, provide a site for fertilization then move the embryo to the uterus
Uterus
Provide for the needs of the embryo and then fetus during development (embryo implants here).
Cervix
Protect the fetus during pregnancy and then dilate to provide a birth canal
Vagina
Stimulate penis to cause ejaculation and provide a birth canal
Vulva
Protect internal parts of the female reproductive system

18. Menstrual Cycle
The menstrual cycle is controlled by negative and positive feedback mechanisms involving ovarian and pituitary hormones.
Menstrual cycle occurs from puberty to menopause; except during pregnancy.
The first half of the menstrual cycle is called the follicular phase because a group of follicles is developing in the ovary.
In each follicle an egg is stimulated to grow.
At the same time the lining of the uterus (endometrium) is repaired and starts to thicken.
The most developed follicle breaks open, releasing its egg into the oviduct.
The other follicles degenerate.

19. The second half is called the luteal phase because the wall of the follicle that released the eff becomes a body called the corpus luteum.
Continued development of the endometrium prepares it for the implantation of an embryo.
If fertilization does not occur the corpus luteum in the ovary breaks down.
The thickening of the endometrium in the uterus also breaks down and is shed during menstruation.

20. The four hormones all help to control the menstrual cycle by both positive and negative feedback.
FSH and LH are protein hormones produced by the pituitary gland that bind to FSH and LH receptors in the membranes of follicle cells.
Estrogen and progesterone are ovarian hormones, produced by the walls of the follicle and corpus luteum.
They are absorbed by many cells in the female body, where they influence gene expression and therefore development.

21. In vitro fertilization (IVF)
The use in IVF of drugs to suspend the normal secretion of hormones, followed by the use of artificial doses of hormones to induce superovulation and establish a pregnancy.

22. William Harvey
William Harvey’s investigation of sexual reproduction in deer.
“Seed and soil” theory of Aristotle states that the male produces a seed which forms an egg when mixed with menstrual blood. The egg then develops into a fetus inside the mother.
Harvey studied animal reproduction. He dissected female deer after mating to see changes in their reproductive organs.
Harvey came to understand that menstrual blood did not contribute to the formation of a fetus.