0610 · 16.5Sex hormones in humans

Hormones and puberty

0:00How does an ovary release an egg at roughly the same time that the uterus is preparing to receive an embryo? These organs cannot see what each other is doing, yet their changes are coordinated. The answer is hormones, chemical messages carried in the blood. By the end of this video, you will understand how hormones cause changes at puberty, control the menstrual cycle and help maintain pregnancy. First, separate two structures that are often confused.

0:23The ovaries contain eggs and produce hormones, while the uterus is the organ in which an embryo can develop. The lining of the uterus changes during the menstrual cycle. The central idea is that hormones coordinate changes in the ovaries with changes in this lining, preparing the body for a possible pregnancy. A hormone is a chemical substance produced by a gland and carried by the blood, which alters the activity of specific target organs.

0:46Think of it as a message travelling through the bloodstream rather than a substance travelling directly from one reproductive organ to another. Only suitable target tissues respond to that message. You met the general idea in the video on coordination and response. Here, we will apply it to reproduction. At puberty, the body becomes reproductively mature, and secondary sexual characteristics develop. These are features that distinguish mature males and females, but are not the reproductive organs themselves.

1:12Testosterone, produced by the testes, causes and regulates male secondary sexual characteristics. Examples include facial and body hair, a deeper voice and increased muscle development, so these changes are responses to a hormonal signal rather than separate, unrelated events. Testosterone also supports reproductive maturation, including growth of the penis and testes and the start of sperm production. In females, oestrogen produced by the ovaries causes and regulates secondary sexual characteristics, including breast development and widening of the hips.

1:44Pubic hair also develops during female puberty, and the menstrual cycle begins as the reproductive hormone system becomes active. We will now focus on how that cycle works.

Stages of the menstrual cycle

1:53A menstrual cycle is often shown as lasting about twenty eight days, although its length varies between individuals and between cycles. Day one is the first day of menstruation, not the day an egg is released. In our example, menstruation occurs over approximately the first five days. During menstruation, the uterus lining breaks down, and blood and lining tissue pass out through the vagina. After menstruation, the uterus lining repairs and thickens, while an egg develops inside an ovary.

2:19Around day fourteen in this example, the egg is released from the ovary, a process called ovulation. The uterus lining is then maintained, ready for a possible pregnancy. If pregnancy does not begin, the lining eventually breaks down again, and the next menstrual cycle starts. Why prepare a lining before knowing whether there will be a pregnancy? An embryo needs somewhere suitable to attach and begin developing, so preparation must happen in advance.

2:44The thickened uterus lining contains blood vessels and provides a suitable environment for implantation, which means the embryo becoming embedded in the lining. The cycle therefore coordinates two jobs, preparing an egg and preparing a place where an embryo could develop.

Hormonal control of the cycle

2:58The first two control hormones come from the pituitary gland, located at the base of the brain. Their names are follicle stimulating hormone, or F S H, and luteinising hormone, or L H. Follicle stimulating hormone stimulates an egg to develop in a follicle in the ovary. A follicle is a structure containing a developing egg, so the hormone’s name gives you a useful clue to its target. Follicle stimulating hormone also stimulates oestrogen production by the developing follicle.

3:23This gives us a connected sequence, in which a pituitary hormone stimulates follicle development, and the follicle produces a hormone that acts on the uterus. Oestrogen causes the uterus lining to repair and thicken after menstruation. The developing egg and the developing lining are therefore linked through hormonal messages, rather than changing independently. Oestrogen also helps regulate the signals coming from the pituitary gland. It inhibits follicle stimulating hormone, helping to limit further follicle stimulation.

3:50When oestrogen reaches a high level before ovulation, it stimulates the release of luteinising hormone. You do not need complicated feedback pathways here. The important idea is that a hormone produced by the ovary can influence the gland that originally stimulated it. A sharp rise in luteinising hormone triggers ovulation, releasing the egg from its follicle in the ovary. Notice the distinction between developing an egg and releasing it. Follicle stimulating hormone helps the egg develop, while luteinising hormone triggers its release.

4:19After ovulation, the remaining follicle tissue forms a structure called the corpus luteum, which produces progesterone and also some oestrogen. Progesterone maintains the thickened uterus lining after ovulation. Oestrogen has helped rebuild the lining, and progesterone now helps keep it in place. Progesterone also inhibits follicle stimulating hormone and luteinising hormone, helping prevent another ovulation during this phase. This makes the sequence easier to understand. Prepare an egg, release it, then maintain conditions suitable for a possible embryo.

4:50A hormone graph is a picture of this sequence, not just a set of curves to memorise. Oestrogen rises as the follicle develops, before ovulation. Luteinising hormone has a sharp peak near ovulation, while progesterone rises afterwards because the corpus luteum has formed. If you understand where each hormone comes from and what it does, the timing of these changes becomes much easier to explain. If fertilisation does not occur, the corpus luteum eventually breaks down.

5:17Progesterone and oestrogen concentrations fall, so the uterus lining is no longer maintained. The lining breaks down, and menstruation begins again. Falling hormone concentrations also remove their inhibition of follicle stimulating hormone, allowing follicle development to begin in the next cycle.

Pregnancy and hormone graphs

5:33Pregnancy changes the outcome because the uterus lining must now be maintained rather than shed. Early in pregnancy, the corpus luteum in the ovary continues producing progesterone and oestrogen. Later, the placenta becomes an important source of both hormones. The placenta is the organ through which exchanges occur between the mother and the developing baby. Its detailed structure belongs in the video on sexual reproduction in humans. During pregnancy, progesterone remains high and maintains the uterus lining, so menstruation does not occur.

6:03Progesterone and oestrogen also inhibit follicle stimulating hormone and luteinising hormone, preventing the usual cycle of further egg development and ovulation. The key connection is that the lining remains because its hormonal support remains. Progesterone does not simply switch off bleeding. It maintains the tissue whose breakdown would cause menstruation. Let us work through a graph question rather than guess from its shape. Suppose progesterone rises after day fourteen, but falls sharply near the end of the cycle.

6:30The question asks you to explain what is likely to happen to the uterus lining. First, identify the hormone’s function. Progesterone maintains the lining, so a falling concentration means that maintenance signal is being removed. Next, connect the hormone change to the tissue response. With less progesterone, the lining is no longer maintained and begins to break down. Finally, name the process that follows, which is menstruation, marking the beginning of another cycle.

6:55We have moved from evidence, to hormone function, to the biological consequence, which is much stronger than simply saying that the graph means there is no pregnancy. Now change one feature of the example, so that progesterone remains high instead of falling. You would predict that the uterus lining remains maintained and menstruation does not occur, as happens during pregnancy. However, a hormone graph by itself is not a complete medical diagnosis.

7:20For this topic, your task is to explain the relationship between progesterone concentration and the condition of the lining.

Common confusions and exam answers

7:27Watch out for three common confusions. Menstruation and ovulation are different processes. Menstruation is the breakdown of the uterus lining, while ovulation is the release of an egg from an ovary. The egg does not come from the uterus, because it develops in an ovarian follicle. Also, about twenty eight days is a model cycle length, not a rule that every person must follow exactly. Another mistake is swapping the hormones that build and maintain the lining.

7:51Oestrogen causes the lining to repair and thicken, while progesterone maintains it after ovulation and during pregnancy. Do not say that all four cycle hormones come from the ovaries, either. Follicle stimulating hormone and luteinising hormone come from the pituitary gland, while oestrogen and progesterone come from ovarian structures and, during pregnancy, the placenta. In an examination, you might be asked to describe changes in the ovary and uterus lining, identify a hormone curve, or explain how hormones control the cycle.

8:21Describe means give the relevant changes or features, while explain means connect them through causes. For example, consider this question. Explain how follicle stimulating hormone, oestrogen and luteinising hormone coordinate events before and during ovulation. Start with the pituitary signal and follow its effects in order. Follicle stimulating hormone stimulates an egg to develop in a follicle in the ovary. Follicle stimulating hormone stimulates oestrogen production by the follicle. Oestrogen causes the uterus lining to repair and thicken.

8:52A high oestrogen concentration stimulates the release of luteinising hormone. Luteinising hormone triggers ovulation, releasing the egg from the ovary. Let us bring the whole system together. At puberty, testosterone and oestrogen cause and regulate the development of secondary sexual characteristics. During the menstrual cycle, pituitary hormones coordinate egg development and release, while ovarian hormones coordinate preparation and maintenance of the uterus lining. If progesterone falls, the lining breaks down and menstruation begins.

9:21During pregnancy, progesterone stays high and the lining remains. Understanding those connections turns a list of hormone names into a coordinated process.