6.6 - Hormones, homeostasis and reproduction

Y chromosome

A gene on the Y chromosome causes embryonic gonads to develop as testes and secrete testosterone.

Prostate gland

A gland that produces much of the seminal fluid, including carbohydrates for the sperm

Uterus

A muscular structure where the early embryo implants and develops if a pregnancy occurs

Vas deferens

A muscular tube that carries mature sperm from the epididymis to the urethra during an ejaculation

Vagina

A muscular tube that leads from the external genitals to the cervix; semen is ejaculated here during sexual intercourse

Urethra

After all the glands have added fluids, this is the tube via which the semen leaves the penis

Penis

An organ that becomes erect as a result of blood engorgement in order to facilitate ejaculation

Hormone therapy

As part of the IVF procedure, a woman must have eggs 'harvested' from her ovaries. In order to ensure the proper timing for this, and to maximize the number of available ova, the woman undergoes about a month of hormone therapy. During the rst 2 weeks she injects a drug (or uses a nasal spray of the drug) that suspends her own natural hormones associated with her menstrual cycle. Then for the next 12 days or so she takes hormone injections that include FSH. This ensures that she will produce many Graa an follicles in each ovary and provide many potential ova (oocytes) for harvesting. The production of many more eggs than is typical of a normal menstrual cycle is called superovulation. When the time is right, several eggs (oocytes) are then harvested surgically. To obtain the sperm cells that are needed for fertilization, the man ejaculates into a container. Harvested eggs are mixed with the sperm cells in separate culture dishes. Microscopic observation reveals which ova are fertilized, and whether the early development appears normal and healthy. Between one and three healthy embryos are later introduced into the woman's uterus for implantation. Any healthy embryos from the culturing phase that are not implanted can be frozen and used later if another implantation procedure is needed.

Melatonin

Deep within your brain is a very small gland called the pineal gland. Many animals use their pineal gland to help regulate their daily 24-hour cycle of activity, called the circadian rhythm. The hormone produced and secreted from the pineal gland is called melatonin. The pineal gland produces very little melatonin during the daytime, and is at peak production after dark, with maximum production occurring between 2 a.m. and 4 a.m. The natural circadian rhythm is altered when a person alters his or her period of exposure to light over a short period of time, especially when coupled with a disruption of their normal sleep schedule. This is what is typically called 'jet lag', produced when a person travels through several time zones in a short period of time. Similar disorientation symptoms can be felt by people who work temporary night shifts or have other irregular time patterns of sleep versus being awake. Many people report a decline in the disorienting effects of jet lag by taking melatonin pills until their own circadian rhythm has naturally reset.

Diabetes

Diabetes is a disease characterized by hyperglycaemia (high blood glucose). Type I is typically caused when the β cells of the pancreas do not produce suf cient insulin; type II diabetes is caused by body cell receptors that do not respond properly to insulin. People who have untreated diabetes have sufficient glucose in their blood, but not in their body cells where it is needed. effects: • damage to the retina, leading to blindness • kidney failure • nerve damage • increased risk of cardiovascular disease • poor wound healing (and possibly gangrene, thus making amputation necessary).

Fallopian tubes (oviducts)

Ducts that carry the ovum (or early embryo) to the uterus

Glucose negative feedback mechanism

In the pancreas there are cells known as β (beta) cells that produce the hormone insulin. Insulin is then secreted into the bloodstream and, because all body cells communicate chemically with blood, all cells are exposed to insulin. Insulin's effect on body cells is to open protein channels in their plasma membranes. These channels allow glucose to diffuse into the cell by the process known as facilitated diffusion. There is another important effect attributed to insulin. When blood that is relatively high in glucose enters the liver by the hepatic portal vein, insulin stimulates the hepatocytes to take in the glucose (a monosaccharide) and convert it to glycogen (a polysaccharide). The glycogen is then stored as granules in the cytoplasm of the hepatocytes. The same effect occurs in muscles (see the TEM on the previous page). The two effects of insulin both have the same ultimate result, which is to lower the glucose concentration in the blood or, to put it more simply, to reduce blood glucose. The blood glucose level typically begins to drop below the set point when someone has not eaten for many hours or exercises vigorously for a long time. In either situation, the body needs to use the glycogen made and stored by the liver (and muscle cells). Under these circumstances, α (alpha) cells of the pancreas begin to produce and secrete the hormone glucagon. The glucagon circulates in the bloodstream and stimulates hydrolysis of the granules of glycogen stored in hepatocytes and muscle cells; the hydrolysis produces the monosaccharide glucose. This glucose then enters the bloodstream. The ultimate effect is to increase the glucose concentration in the blood or, to put it more simply, to increase blood glucose (see Figure 6.23). https://i.imgur.com/8eWs7vf.png

Insulin and glucagon help regulate glucose levels

Insulin and glucagon are hormones that are both produced and secreted by the pancreas. In addition, they are both involved in the regulation of blood glucose levels. Cells rely on glucose for the process of cell respiration. Cells never stop cell respiration and thus are constantly lowering the concentration of glucose in the blood. Many people eat three or more times a day, including foods containing glucose, or carbohydrates that are chemically digested to glucose. This glucose is absorbed into the bloodstream in the capillary beds of the villi of the small intestine, and thus increases the blood glucose level. So one factor that causes our blood glucose levels to uctuate is simply that our blood does not receive constant levels of glucose. The increase and decrease in blood glucose levels goes on 24 hours a day, every day of your life. However, even though blood glucose is expected to uctuate slightly above and below the homeostatic normal level, it must be maintained reasonably close to the body's set point for blood glucose level, and negative feedback mechanisms ensure this. In the intestinal villi, the glucose travels through a multitude of capillaries, small venules, and veins into the hepatic portal vein, which takes the blood to the liver. The glucose concentration in the hepatic portal vein varies depending on the time of your last meal and the glucose content of the food you ate. The hepatic portal vein is the only major blood vessel in the body in which blood levels uctuate to a large degree. All other blood vessels receive blood after it has been processed by liver cells called hepatocytes. Hepatocytes are triggered into action by the two pancreatic hormones, insulin and glucagon. These two pancreatic hormones are antagonistic: they have opposite effects on blood glucose concentration.

Pancreas

Insulin and glucagon are secreted by and cells in the pancreas, respectively, to control blood glucose concentration.

Leptin

Leptin is a hormone that is produced by adipose (fat) tissue in the body. The more fat stored in the body, the more leptin is produced and secreted into the bloodstream. Leptin's target cells are in the hypothalamus of the brainstem. Under ideal circumstances, leptin has the effect of lowering your appetite. Evolutionarily, the logic is simple: if someone has enough fat reserves, that person does not need to eat as much anymore. Unfortunately that simple logic doesn't always hold true, as evidenced by the very large incidence of obesity in modern society today. People who are obese are known to have a greater level of leptin circulating in their bloodstream. Researchers are working on why they appear to have become 'desensitized' to this high level of the appetite-controlling hormone. Some researchers have suggested that the function of leptin is related to increasing appetite when fat reserves are low, but not as an appetite suppressant when fat reserves are high.

Adipose tissue

Leptin is secreted by cells in adipose tissue and acts on the hypothalamus of the brain to inhibit appetite.

Pinean glands

Melatonin is secreted by the pineal gland to control circadian rhythms.

IVF

Natural fertilization typically occurs in one of a female's Fallopian tubes 24-48 hours after ovulation. The resulting zygote begins to divide by mitosis, and takes several more days to travel down the Fallopian tube to the endometrium of the uterus. When the embryo reaches the endometrium, it has already divided mitotically many times and is a ball of about 100 cells. The embryo, called a blastocyst at this stage, will then implant in the highly vascular tissue of the endometrium. Some couples are unable to bear children. There is a wide variety of possible reasons for infertility, including: • males with low sperm counts • males with impotence (failure to achieve or maintain an erection) • females who cannot ovulate normally • females with blocked Fallopian tubes. Reproductive technologies have been developed to help overcome these situations. One of the most common of these new technologies is in vitro fertilization (

Oestrogen and Progesterone

Oestrogen and progesterone cause prenatal development of female reproductive organs and female secondary sexual characteristics during puberty.

Ovaries

Organs that produce and secrete oestrogen. They also produce and release the ovum (in the form of secondary oocytes). The area where ovulation occurs grows into the corpus luteum, which temporarily produces the hormone progesterone

Scrotum

Sacs that hold the testes outside the body cavity so that sperm production and maturation can occur at a temperature cooler than body temperature

Seminal vesicles

Small glands that produce and add seminal fluid to the semen

How does a person become male or female?

So, what happens as a result of the XX or XY combinations? The answer lies in the hormones that are produced by each embryo. Embryos of both sexes are virtually identical until about the eighth week following fertilization. Alleles that interact on both of the X chromosomes of female embryos then result in relatively high oestrogen and progesterone production, resulting in the prenatal development of female reproductive structures. Genes located on the single Y chromosome are responsible for early testes development and relatively high testosterone production, resulting in male reproductive structures during subsequent foetal development. It is interesting to note that the male and female reproductive structures have common origins in the pre-8-week-old embryo. In other words, the same embryonic tissue that becomes the ovaries gives rise to the testes, the same embryonic tissue that gives rise to the clitoris gives rise to portions of the penis, etc. Another way of expressing this is to say that some female and male reproductive structures are homologous.

Testosterone

Testosterone causes prenatal development of male genitalia and both sperm production and development of male secondary sexual characteristics during puberty.

Epididymis

The area where sperm are received, become mature, and are capable of swimming motion via movement of their flagella

Thyroxin

The gland that produces and secretes thyroxin is a 'butter y'-shaped gland located in your neck called the thyroid gland. Thyroxin is created from an amino acid and iodine, and exists in two forms, one called T4 and the other called T3. The numbers indicate the number of iodine atoms within the structure. Both T3 and T4 enter the target cells (almost all cells in the body), where the T4 form is typically converted to the T3 form. The T3 form enters the nucleus of the cell and acts as a transcription regulator, leading to an increase in messenger (m)RNA and thus a resultant increase in proteins. Ultimately these proteins lead to an increase in the metabolism of the cell. Thus a cell under the in uence of thyroxin will have a greater need for oxygen and other indicators of an increased metabolic rate. Someone who secretes too much thyroxin is said to have hyperthyroidism, and someone who secretes too little is said to have hypothryroidism. Both conditions can have serious symptoms. In addition to increasing the metabolic rate, thyroxin helps to regulate internal body temperature. An increase in metabolic rate produces more heat from the increased chemical reactions that are occurring. Therefore an increase in thyroxin will lead to an increase in body temperature, and vice versa.

Endometrium

The highly vascular inner lining of the uterus

Homeostasis

The human body typically stays within certain limits for many physiological variables. This is referred to as homeostasis. Here are some representative physiological variables: • blood pH • body temperature • blood carbon dioxide concentration • water balance within tissues. • blood glucose concentration

Cervix

The lower portion of the uterus, which has an opening to the vagina that allows the sperm to enter for fertilization and provides a pathway for childbirth

Testis

The male gonads: the sperm are produced here in small tubes called seminiferous tubules

Negative and positive feedback

The menstrual cycle is controlled by negative and positive feedback mechanisms involving ovarian and pituitary hormones.

Endocrine system

The nervous and endocrine systems work cooperatively in order to ensure homeostasis. Many of the homeostatic mechanisms initiated by your nervous system are under the control of your autonomic nervous system. The endocrine system consists of numerous glands that produce a wide variety of hormones. Each hormone is transported by the bloodstream from the gland where it is produced to the specific cell types in the body that are infuenced by that particular hormone. https://i.imgur.com/DyIgJry.png

Negative feedback mechanism

The physiological processes that bring a value back towards to a set point are called negative feedback mechanisms. Think of negative feedback control as working like a thermostat. The thermostat triggers one set of actions that is required when a value rises above its set point, and another set of actions when a value falls below its set point. Thus negative feedback functions to keep a value within the narrow range that is considered normal for homeostasis.

Secondary sex characterises of females

The secondary sex characteristics of females that arise as a result of increased oestrogen and progesterone production at puberty are: • enlargement of breasts • growth of pubic and underarm hair • widening of hips.

Secondary sex characterises of males

The secondary sex characteristics of males that arise as a result of increased testosterone production at puberty are: • growth of facial, underarm, chest, and pubic hair • enlargement of the larynx and associated deepening of the voice • increased muscle mass • enlargement of the penis.

Thyroid gland

Thyroxin is secreted by the thyroid gland to regulate the metabolic rate and help control body temperature.

Type I diabetes

Type I diabetes is an autoimmune disease. The body's own immune system attacks and destroys the β cells of the pancreas so that little or no insulin is produced by individuals with type I diabetes. Less than 10% of diabetics have this type of the disease. Type I diabetes most often develops in children or young adults, but can develop in people of any age.

Type II diabetes

Type II diabetes is the result of body cells no longer responding to insulin as they once did. This is known as insulin resistance. Initially, the pancreas continues to produce a normal amount of insulin, but this level may decrease after a period of time. Type II diabetes is the most common form of diabetes; approximately 90% of diabetics have this type. Type II diabetes is often associated with genetic history, obesity, lack of exercise and advanced age, and is more common in certain ethnic groups.

role of sex hormones during puberty

When females and males reach puberty, the same hormones that rst determined their physical sex are produced and secreted in higher amounts. The increased production of hormones at this time results in the secondary sex characteristics (the attributes that are characteristic of a sex that only appear at puberty).

Human reproduction

basically a male gamete (sperm) fertilizing a female gamete (egg or ovum). This cellular union ensures that half of the genetic makeup of the resulting zygote is derived from each parent. Thus, like all forms of sexual reproduction, reproduction in humans serves the bigger purpose of ensuring genetic variation in the species. In both sexes, hormones play a key role in both the development of sexual dimorphism (different body forms of males and females) and the regulation of sexual physiology.

The effects of FSH and LH on the ovaries

he hormones FSH and LH have several effects on the ovaries. One of these effects is to increase the production and secretion of another reproductive hormone by the follicle cells of the ovary. This hormone is oestrogen. Like all hormones, oestrogen enters the bloodstream. Its target tissue is the endometrium of the uterus. One effect of oestrogen is an increase in the density of blood vessels of the endometrium, that is, as stated earlier, the endometrium becomes highly vascular. Another effect of oestrogen is to stimulate the pituitary gland to release more FSH and LH. This is the positive feedback loop of the menstrual cycle, speci cally these two sets of hormones increasing becau of the increase of the other(s). Another effect of FSH and LH is the production of structures within the ovaries known as Graa an follicles. Within the ovaries are cells known as follicle cells, and the true reproductive cells that are at a stage of development called oocytes. Under the chemical stimulation of FSH and LH, the somewhat randomly arranged follicle cells and oocytes take on a cellular arrangement known as a Graa an follicle. A spike in the level of FSH and LH leads to ovulation (the release of the oocyte from the Graa an follicle). The oocyte is accompanied by the inner ring of follicle cells of the Graa an follicle. This entire structure is known as a follicle, and typically enters the Fallopian tube soon after ovulation. The outer ring of follicle cells remains within the ovary. These follicle cells begin to produce and secrete another hormone, progesterone. The cells of this outer ring begin to divide and ll in the 'wound' area left by ovulation, and this forms a glandular structure known as the corpus luteum. The corpus luteum will be hormonally active (producing progesterone) for only 10-12 days after ovulation. Progesterone is a hormone that maintains the thickened, highly vascular endometrium. As long as progesterone continues to be produced, the endometrium will not break down and an embryo will still be able to implant. In addition, the high levels of both oestrogen and progesterone at the same time provide a negative feedback signal to the hypothalamus. The hypothalamus does not produce GnRH when the oestrogen and progesterone levels are high, so FSH and LH remain at levels that are not conducive to the production of another Graa an follicle during this time. Assuming there is no pregnancy, the corpus luteum begins to break down after 10-12 days, and this leads to a decline in both progesterone and oestrogen levels. As both of these hormone levels fall, the highly vascular endometrium can no longer be maintained. The capillaries and small blood vessels begin to rupture and menstruation begins. The drop in progesterone and oestrogen also signals the hypothalamus to begin secreting GnRH, and thus another menstrual cycle begins. Because the menstrual cycle is a cycle, there is no true beginning or ending point. The rst day of menstruation is designated as the rst day of the menstrual cycle simply because this is an event that can be easily discerned (see Figure 6.28).

Hormones and linings in menstrual cycle

https://i.imgur.com/6H332vZ.png

Flow chart of hormones from hypothalamus and pituatiar gland

https://i.imgur.com/FgpRyAA.png

Draw male reproductive anatomy and functions

https://i.imgur.com/J0WUpCa.png https://i.imgur.com/6GmwY1X.png

Diagram of the cycle for eggs

https://i.imgur.com/i7eEEEe.png

Female reproductive drawing and functions

https://i.imgur.com/x6P9Mu3.png https://i.imgur.com/bELxQM9.png

Menstrual cycle

tarting at puberty, human females begin a hormonal cycle known as the menstrual cycle. Each cycle lasts, on average, 28 days. The purpose of the menstrual cycle is to time the release of an egg or ovum (ovulation) for possible fertilization and later implantation into the inner lining of the uterus. This implantation must occur when the uterine inner lining (the endometrium) is rich with blood vessels (i.e. highly vascular). The highly vascular endometrium is not maintained if there is no implantation. The breakdown of the blood vessels of the endometrium leads to the menstrual bleeding (menstruation) of a typical cycle. This menstruation is a sign that no pregnancy has occurred.

Hormones from the hypothalamus and pituitary gland

ypothalamus is the regulatory centre of the menstrual cycle. The hypothalamus produces a hormone known as gonadotropin-releasing hormone (GnRH). The target tissue of GnRH is the nearby pituitary gland, and it results in the anterior pituitary producing and secreting two hormones into the bloodstream. These two hormones are follicle-stimulating hormone (FSH) and luteinizing hormone (LH). The target tissues for these two hormones are the ovaries.

Testosterone in male

• determines the development of male genitalia during embryonic development • ensures the development of secondary sex characteristics during puberty • ensures sperm production as well as maintains sex drive following puberty.