AP Biology Reproduction

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In this lecture, we will be covering the topic of reproduction with an emphasis on human reproduction.0002

We are going to start out talking about the two major types of reproduction, which are asexual reproduction and sexual reproduction.0008

Asexual reproduction results in offspring that are genetically identical to the parent.0018

This occurs through mitosis, and via mitosis, daughter cells are produced that carry the same DNA as the parent.0026

Recall that during the survey of animals under the section entitled diversity of life, that group of lectures,0035

I discussed some methods of asexual reproduction as we went through particular types of animals.0043

So, I am going to review some of those now. One type of asexual reproduction is fragmentation.0050

In fragmentation, an entire organism can be regenerated through a fragment of the original animal, so through a piece of the original animal.0058

Fragmentation means regeneration of an entire organism from a piece of the organism.0071

An example is sponges. From a piece of the sponge, can be regenerated an entire sponge.0093

Sea stars can regenerate a portion of the sea star, and there are even a few species that can regenerate an entire sea star just from a piece.0105

A second type of asexual reproduction is fission. In fission, an example, actually, of an organism that undergoes fission is flatworms.0115

And some flatworms, what they do is they can split into two halves, and each half can regrow the missing half.0126

Fragmentation, regeneration from a fragment, whereas, here, if you had a flatworm, and then, it would split in half and regrow the missing half.0148

Another type of asexual reproduction that we discussed was parthenogenesis. In this, the female produces eggs that are not fertilized.0159

But, those eggs, instead, develop into an adult organism, so unfertilized eggs develop into an adult.0173

Examples of organisms that can reproduce this way would be rotifers also honeybees. There are even some reptiles that can reproduce this way.0192

Now, asexual reproduction requires only one parent, whereas, sexual reproduction requires two.0206

And so, there is an inherent efficiency and advantage in asexual reproduction, yet, the dominant mode of reproduction in animals is sexual reproduction.0212

Therefore, there are competitive advantages for the survival of the specie via sexual reproduction, and we are going to talk about that now.0223

And we are also going to just introduce the topic of sexual reproduction and then, go on to talk about the human reproductive system.0234

So, sexual reproduction requires a sperm and an egg, and these two haploid gametes join to produce a diploid zygote.0243

And I just talked about advantages of asexual over sexual reproduction that would0255

make it seem as though asexual reproduction would have won out in the end.0261

And I want to talk about a way in which to view this, which is called the two-fold cost of sexual reproduction.0265

The best way to understand this is to take an example.0279

So, let's say you have a group that is reproducing asexually and another group that is reproducing sexually.0282

And what happens here is we have a female, and she produces, let's say, two offspring.0294

Both of those offspring will also be females, so let's say it is typical in these populations for there to be about two offspring per female.0307

Now, over here in sexual reproduction, this female with a male mate will also produce two offspring.0317

On average, half of the offspring of a female will be male.0333

On average, half of the offspring will be male, half will be female, so here, she has two offspring: one is female, one is male.0339

Now, these two offspring each have two offspring.0347

Now, by the 1, 2, 3, third generation, we are up to four individuals. These are all female.0358

Over here, the male cannot give birth, so we are just counting per female how many individuals are born.0367

And here, we have again this female gives birth to one female, one male. Now, what is happening is that this population is growing more quickly.0376

So, you would expect that asexual reproduction would confer a survival advantage.0387

There is this two-fold cost of sexual reproduction, and yet, this is still the dominant mode of reproduction.0394

What is the advantage to sexual reproduction in terms of the survival of the population?0398

Well, there are a lot of theories on what the advantage is.0404

And one of the major ones, the advantage to sexual reproduction, is the increase in genetic variation in the offspring.0409

And then, that is what accounts for sexual reproduction having one out.0418

What sexual reproduction does do is it increases the genetic variation in the population.0422

Remember to check out the lectures on meiosis, because to understand sexual reproduction, you really need to understand meiosis.0430

And in meiosis, there is going to be crossing over and mixing and matching of different genes and then, the reduction division.0438

And then, the haploid sperm is going to unite with the haploid egg.0447

And by bringing together genes/alleles from two different individuals, you are going to get offspring with a huge variety of traits.0452

So, if conditions change, the environment becomes much warmer or much colder,0458

there is more likely to be some individuals in the population who are well-adapted to that change and could survive.0462

So, this is what may account for the overall advantage that sexual reproduction confers on a population.0467

Now, like I said, we are going to be focusing on human reproduction, so we are going to talk about the joining of a sperm and an egg,0477

one, the gamete from a male, one from a female.0486

But, before we go on, I just want to introduce some terminologies you should be familiar with because some organisms are hermaphrodites.0488

And a hermaphrodite is an individual who has both male and female reproductive organs in one/the same individual.0498

And this actually comes from the word Hermaphrodytus. Hermaphrodytus was the son of the Greek god Hermes, and the Greek goddess Aphrodite,0515

so just some terminologies to be aware of.0526

Now, we are going to go on and, now, focus on human reproduction beginning with an overview of the male reproductive system.0529

For the male reproductive system, the testes are the site of production of sperm, and they are located within the scrotum outside the abdominal cavity.0536

This keeps the testes at a lower temperature than the rest of the body, and that is necessary for the normal production of sperm.0546

So, within the testes are the seminiferous tubules, and outside the seminiferous tubules are Leydig cells.0554

Leydig cells are interstitial cells, and seminiferous tubules are the site of sperm production; and Leydig cells are the site of testosterone production.0564

After sperm is produced in the seminiferous tubules, it enters and travels through a duct called the epididymis.0589

While the sperm is traveling through the epididymis, it gains motility. It gains motility traveling through the epididymis.0601

So, sperm become motile during this phase.0620

During ejaculation, the sperm pass through a muscular duct called the vas deferens.0630

And you may have heard of a procedure called a vasectomy, which renders a male infertile.0639

And the way this works is the vas deferens is cut, so that the sperm will not end up in the semen, so they will not be in the ejaculates.0646

After the sperm travels through the epididymis, then, it goes via the muscular vas deferens and then,0658

exits the body via the urethra, so that is what is shown here.0664

The urethra is both part of the excretory system and the reproductive system in males.0670

There are also some different glands that are involved in the male reproductive system, and right here, there are glands called...0679

There are actually two of them. This is a side view, a side cross-section, but back here, there are actually seminal vesicles.0688

And the seminal vesicles, which are located behind the bladder, are involved in the production of semen.0702

Semen contains mucus. It contains prostaglandins, and it is alkaline.0712

It is alkaline. It has got more of a basic pH.0723

It also contains fructose, which provides energy for the sperm.0727

The prostate gland shown right here secretes a milky-looking fluid that is also part of the semen. It is alkaline as well, and it contains anticoagulants.0739

Finally, below the prostate, but now shown here, are the bulbourethral glands.0759

These are actually located...they would be right here below the prostate, in that region.0769

They secrete a clear fluid, and this is released just prior to ejaculation, and what it does is it is alkaline also.0775

And it helps to neutralize any acidic urine that maybe left in the urethra.0786

This clear fluid, though, can end up containing some sperm that are left in the urethra, so it is a clear fluid that can contain some sperm.0791

OK, so this is a summary of the male reproductive system. Now, we are going to go on to the female reproductive system in humans.0802

In females, the gametes, which are eggs, are produced in the ovaries.0809

And we are going to talk in detail about the production of the eggs, as well as the sperm.0814

And we are going to talk about the menstrual cycle and the ovulation and the release of the egg from the follicles.0826

Within the ovaries are follicles, and eggs are produced in these follicles; so they produce eggs.0834

They also are part of the endocrine system as is the testes. Since the testes secrete testosterone, they are an endocrine organ.0842

And the ovaries are endocrine organs, as well, because they produce the hormones estrogen and progesterone.0850

And you will remember back in the endocrine lecture, I briefly mentioned the ovaries and the testes as part of the endocrine system.0858

Females are born with all of the eggs that they will have in contrast to males who produce sperm throughout their lives.0870

We are going to talk about how these eggs are present in the ovary at birth in females, but they are not fully-developed/mature.0880

What we have here connected to the uterus is a fallopian tube. That is also called an oviduct.0891

And after ovulation, the egg is released here into the abdominal cavity, and the cilia from the fallopian tube help to sweep the ovum into the tube.0903

The fallopian tube is also the site of fertilization.0916

So, sperm enters the female reproductive tract and has to make its way into the fallopian tube where fertilization will take place.0924

And then, the fertilized egg will travel into the uterus and then, implant there.0940

The uterus is a very muscular organ. However, it can stretch enough to accommodate an 8, 9 or even larger pound baby or sometimes multiple babies.0948

So, the uterus is capable of stretching quite a bit. It is a muscular organ.0960

The lining of the uterus is called the endometrium, and it is the endometrium that is shed each month during the menstrual cycle.0964

Each month, the endometrium builds up. It is very well-vascularized.0978

This lining builds up and with a great blood supply to support a developing fetus if pregnancy occurs. If pregnancy does not occur, this lining is shed.0982

The cervix is located at the bottom of the uterus, and it is the opening into the vagina/birth canal.0994

The other structures that are not shown here that are part of the female reproductive system are the mammary glands,1005

which produce milk to nurse the offspring.1011

Now that we have talked about both the male and female reproductive system,1022

we are going to look at how gametes are produced first in males, then, in females.1026

The production of sperm is called spermatogenesis, and you will recognize this drawing/figure from our discussion on meiosis.1030

Right now, I am not going to go into the details of meiosis. You should already have a good understanding of meiosis.1038

I am going to, instead, focus on the names and the stages of sperm development that go along with each step of meiosis.1043

We start out actually with a diploid spermatogonium, and this is diploid. You can see that there are two chromosomes.1054

We will call this chromosome 1 and chromosome 2. There are two of each, so this is diploid or 2n.1071

Then, mitosis occurs. There will be two daughter cells, so there will actually be another one over here that we are not going to follow.1076

We are just going to follow one, and it is going to produce a primary spermatocyte.1088

Now, starting out with the primary spermatocyte, then, we get meiosis. We get meiosis I.1115

Meiosis I has a reduction division, so we are still at 2n here; but then, during meiosis I, it goes from 2n, the reduction division, to haploid.1132

So, there is just one of each type of chromosome, so now, these cells are n.1141

The primary spermatocyte undergoes meiosis I. The result is two secondary spermatocytes.1146

Each secondary spermatocyte undergoes meiosis II, so M II right here, to result in 1, 2, 3, 4 spermatids also haploid.1158

Now, these will go on to mature into spermatozoas, mature sperm cells, so the spermatids are not fully developed.1174

This right here, these are all spermatids and now, spermatozoa, the mature gamete or just sperm cells.1186

To sum up what has happened, from one primary spermatocyte, the result will be four sperm cells.1199

And this is a continuous process, and we are going to contrast that with oogenesis.1214

By continuous process, I mean that meiosis I and meiosis II follow one after the other without disruption, without stopping, without arrest during this process.1220

It is a continuous process. The other thing is that spermatogenesis occurs from puberty onward in males.1234

So, it occurs throughout the lifetime of a male from the time of maturation onward.1248

These are three features of spermatogenesis that you should note and compare with oogenesis.1256

Oogenesis is the production of egg/an ovum, and unlike spermatogenesis, this process is discontinuous.1262

They are arrested for years and even decades at certain stages.1271

So, we will start out with an oogonium. Mitosis occurs, then, the result would be two primary oocytes.1276

We are just going to follow what happens to one primary oocyte.1287

And note that this cell, the oogonium, is diploid. This is diploid.1294

Then, what happens is the primary oocyte begins meiosis.1300

But then, it is arrested in meiosis I until puberty or until that particular cell/follicle matures for ovulation,1306

so arrested in meiosis until puberty or even later, 10, 20, 30 years later.1322

In a developing female in utero, a fetus, what will happen is within her ovaries, the oogonia will start to mature, or they will undergo mitosis.1330

They will form a primary oocyte, and then, in the ovaries of that female fetus, the primary oocytes will start meiosis I.1346

But, they will be arrested - actually, let's be more specific - in prophase I.1356

Then, the female infant is born, so she has ovaries that have millions of primary oocytes arrested in prophase I.1363

And they will stay like that until puberty, and then, at puberty, each month, she will ovulate; and one or sometimes two of these eggs will mature.1373

So, that means that a particular egg could be arrested as a primary oocyte for 12 years, 20 years, 30 years, even longer.1390

Each month, just one or two eggs - usually one - will mature and then, be released.1400

We have the primary oocyte at puberty. It finishes meiosis I, so here, we have meiosis I.1407

When that is completed, what ends up happening is that there is a secondary oocyte and a polar body.1423

So, notice that this is different than spermatogenesis. There are no polar bodies in spermatogenesis.1433

After meiosis I is complete, the cytokinesis is uneven. It is uneven on most of the cytoplasm, and this is due to uneven cytokinesis.1439

Most of the cytoplasm ends up in the secondary oocyte. The polar body is a dead end, so we are haploid at this point.1452

It may divide again. It may not, but it is just, sort of, a by-product of the process.1461

This is not going to become an egg that can be fertilized and develop into an embryo.1466

So, the primary oocyte, a particular month following puberty will mature and finish meiosis I and become a secondary oocyte.1472

Now, there is a second arrest that occurs. The secondary oocyte begins meiosis II, then, stops until fertilization.1484

It is actually a secondary oocyte that is released during ovulation.1503

And if fertilization occurs, that will trigger the secondary oocyte to finish meiosis II and become a mature ovum, and a second polar body results.1507

So, here, we have meiosis II, and this occurs upon fertilization.1521

Just to sum up, in a female developing embryo/fetus in her ovaries, oogonium undergoes mitosis to form primary oocytes.1544

This primary oocytes begin meiosis I.1554

But, they are arrested in prophase I until that particular egg starts to mature during a month that it is going to be ovulated.1557

So, the primary oocyte is arrested at least until puberty in prophase I.1568

At ovulation, meiosis I is completed to yield a secondary oocyte, which is haploid and one polar body.1574

Upon fertilization, if fertilization occurs, meiosis II will be completed to yield one ovum and another polar body.1584

So, comparing this with spermatogenesis, one, oogenesis is discontinuous.1592

There are phases where the process is arrested for years even, when we are talking about meiosis I, so it is discontinuous.1598

And the second difference is that a primary oocyte only results in one ovum.1614

And that is in contrast to a primary spermatocyte that results in four mature sperm cells.1631

Third: oogenesis does not occur throughout a woman's lifetime. It begins at puberty.1637

Well, much of it continues on at puberty. The first steps were taken before birth, but it stops at menopause.1646

Whereas in males, spermatogenesis occurs throughout the male's life.1657

Alright, now, the next thing we are going to talk about is hormonal control of both the male and female reproductive systems.1665

And we will start out by talking about the female reproductive system specifically the menstrual cycle.1672

Now, the development of the egg, of the ovum each month is coordinated with a cycle that prepares the uterus for the implantation of the embryo.1677

What is really happening are two cycles: the ovarian cycle and the uterine cycle.1688

And when we talk about menstruation, what we are really talking about is the uterine cycle,1695

that the monthly cycle of the endometrial lining building up and being shed.1700

However, often, when there is a discussion of the menstrual cycle, both are included because they are very well-coordinated.1705

One has to occur at the same time as the other, side by side, in parallel, because if they were not,1714

then, an egg would be ovulated when the uterus was not ready to accept the pregnancy, and that would just be a waste.1719

I am going to talk about both the ovarian and the uterine cycle together and then,1726

afterwards, summarize which is which and the differences between the two.1730

Now, the first thing to understand are the hormones that are involved in these processes.1735

There is GnRH, which is produced by the hypothalamus, follicle-stimulating hormone and luteinizing hormone,1740

which are produced by the anterior pituitary and estrogen and progesterone, which are produced by the ovaries.1749

We are going to start and assume a 28-day cycle.1760

Cycle days can vary in different individuals, but we are just going assume a standard textbook 28-day cycle.1765

And things begin up in the hypothalamus, and what the hypothalamus produces is gonadotropin-releasing hormone/GnRH.1771

Remember that these releasing hormones that are produced by the hypothalamus act on the anterior pituitary.1782

And the anterior pituitary, then, is stimulated to release its hormones.1789

So, the anterior pituitary in response to GnRH produces follicle-stimulating hormone/FSH and luteinizing hormone/LH.1795

The major player that we are going to talk about in this first half of the cycle is...1813

So, if we are starting a cycle, day 1 up here, the first half of the cycle about the first 14 days, the major player is follicle-stimulating hormone.1820

And its name tells you what it does. It stimulates the ovaries, and it causes the follicles in the ovaries to mature.1830

As these follicles mature, they produce estradiol, which is a form of estrogen, so I am just going to put estrogen.1846

Now, during the early part of the menstrual cycle, estrogen exerts a negative feedback effect on the hypothalamus.1857

So that as the follicles mature, and a few follicles may start maturing, but eventually, one will dominate.1869

That one will take over. It will finish maturing.1877

It will be released at ovulation. Occasionally, two can be released, and that would result in fraternal - non-identical - twins.1879

But, usually, just one dominant follicle matures and is released.1887

Now, this follicle is producing estrogen, and low levels of estrogen have a negative feedback effect on the hypothalamus.1890

So, negative feedback meaning it is going to prevent the hypothalamus from secreting GnRH, which means that FSH levels will be lowered.1897

And this will prevent more and more follicles from maturing, so there is a negative feedback loop here.1909

Now, this first half of the cycle is called the follicular phase, and really, what I have been talking about is the ovarian cycle.1917

This is really what is going on with the ovaries, so first half of the ovarian cycle.1936

We will talk about the uterus in a minute. This is the follicular phase.1941

Sometimes this first half of the cycle is also known as the estrogenic phase because1945

estrogen is the major player here as far as what the ovaries are producing.1950

Now, towards mid-cycle when the follicles are very close to maturation, the estrogen levels build up pretty high.1954

When that happens, the effect that estrogen has is reversed- the effect that it has in the hypothalamus.1966

The same hormone acting on the same endocrine organ, instead of causing negative feedback, now switches and actually gives positive feedback.1974

And it stimulates the release of GnRH.1984

At the same time, high levels of estrogen secretes also has a positive effect on the pituitary to help stimulate the release of LH.1988

So, there are two types of positive effect here: one on the hypothalamus, one on the anterior pituitary.2000

And the result is a greatly increased level of LH, so this positive feedback causes what is called an LH surge.2006

So, I am going to the space here and show LH surge meaning that there is a very rapid large increase in the level of luteinizing hormone.2023

About a day after that LH surge, ovulation occurs.2041

Now, before we go on to talk about ovulation, I will write that down- ovaries. Follicle ruptures, and that is ovulation.2051

In other words, the egg is released, but I want to stop for a second and talk about what is going on in the uterus.2071

I talked about the effects of estrogen as far as the negative and positive feedback in the hypothalamus, but estrogen has another effect, as well.2079

What it is doing over here is also affecting the uterus.2089

And in the first half of the cycle, what is going to happen is estrogen will cause the proliferation of the endometrium.2096

Therefore, the first half of the uterine cycle is called the proliferative phase.2112

And what this is doing is getting the uterus ready for pregnancy so that everything is coordinated.2128

As a woman is about to ovulate, her uterine lining is built-up and ready for an embryo to be implanted.2135

OK, so, where we left with the ovarian cycle is that there was an LH surge.2145

And about a day after that in the ovary, a follicle ruptures the mature follicle and release an egg.2151

So, this is ovulation. This occurs around day 14.2160

All of this is the first half of the cycle. This is the follicular phase, and then, after ovulation...2168

So, up here, above this line is the follicular phase, and in terms of the uterus, it is the proliferative phase.2177

Now, down here, after ovulation, this is what is known as the luteal phase.2189

What is left behind after the follicle ruptures, differentiates into a corpus luteum.2200

In the ovary, there is the follicle. The follicle ruptures.2209

It releases an egg, and what is left behind is stimulated by luteinizing hormone to form a corpus luteum from the ruptured follicle.2212

The corpus luteum secretes progesterone, so now, we have the ovaries are still secreting estrogen.2231

But, now, we have progesterone levels going up.2237

So, in the second half of the phase, this second half is called the luteal phase. It is sometimes also known as the progesteronic phase.2241

Estrogenic- first half; progesteronic- second half; follicular- first half; luteal phase- second half.2248

Progesterone maintains the uterine lining.2258

And it also stimulates the development of glands in the endometrium and increases the blood supply to the uterus.2263

So, I talked about what estrogen does to the uterus. Now, let's talk about the effect of progesterone on the uterus.2270

Progesterone acts on the uterus to stimulate gland development, and it increases the blood supply to the endometrium.2282

And it just maintains that endometrium that was built up during the proliferative phase.2303

This phase is called the secretory phase, and that name comes from the fact that glands secrete substances.2309

The first half of the uterine cycle is the proliferative phase. The second half of the uterine cycle is the secretory phase.2320

Now, estrogen and progesterone secrete a negative feedback effect on the hypothalamus.2330

So, at high levels of estrogen just before ovulation, there is that positive feedback from estrogen, but most of the time, it is doing negative feedback.2340

Now, here, we have the corpus luteum, the ovaries producing progesterone, and the ovary is still producing estrogen.2349

So, I am going to put estrogen here, as well, and what these two are doing, the progesterone and the estrogen, is they are suppressing.2358

They are going back up, and they are suppressing the hypothalamus.2367

Again, we are having negative feedback to the hypothalamus. As a result, GnRH levels drop.2376

LH levels drop, and the corpus luteum atrophies. LH maintains the corpus luteum.2382

One of the things that LH did was stimulated the ovaries to ovulate, and the other thing is for the corpus luteum to develop.2396

Without the influence of LH, the corpus luteum atrophies.2402

So, I am just going to put that right here "decreased LH, corpus luteum atrophies".2407

Since the corpus luteum produces progesterone, when the corpus luteum atrophies, progesterone decreases.2418

If progesterone decreases, there is nothing to maintain the endometrium, the endometrial lining. The lining is shed.2425

Menses occur, so the menstrual cycle is a result of the shedding of this built-up2434

lining because the lining will not be maintained if progesterone levels drop.2444

The LH will only maintain the corpus luteum for a couple of weeks.2450

If no pregnancy results, corpus luteum atrophies, progesterone decreases. The endometrial lining is shed, and they cycle begins again with day 1.2454

Now, let's talk about what happens with pregnancy.2465

If pregnancy occurs, the fetus or the embryo produces hCG. hCG is a hormone called human chorionic gonadotropin- hCG.2470

And hCG is an analogue of LH.2492

Therefore, hCG is capable of maintaining the corpus luteum. What it is essentially doing is rescuing the corpus luteum.2499

If a pregnancy does not result, then, within a couple of weeks after ovulation, LH levels would be low. Corpus luteum will involute.2511

It will atrophy, progesterone drops, menses occur.2522

However, if a pregnancy does result, the fetus, the embryo begins producing hCG.2526

And the hCG can act in the same way as LH maintains the corpus luteum.2533

The corpus luteum keeps making progesterone. They uterine lining is preserved, and the embryo can implant.2542

The pregnancy is maintained. Eventually, the corpus luteum does atrophy, but by then, the fetus and the placenta are developed.2551

And the placenta, then, takes over the role of producing progesterone.2560

And actually, early pregnancy tests that are done in the early part of pregnancy, what they are measuring is hCG.2565

Alright, to go ahead and sum up because that was quite a bit of information,2575

there are two cycles occurring together simultaneously, very well-coordinated.2579

One is the ovarian cycle. The first half is known as the follicular phase.2585

It is also sometimes called the estrogenic phase.2599

And what happens during this phase is FSH stimulates the follicles to develop one dominant follicle ends up taking over.2603

There is a positive feedback effect exerted by the estrogen produced by the developing follicle that causes an LH surge, which triggers ovulation.2615

And an egg is released.2628

The second half of the ovarian cycle is the luteal phase. It is also sometimes called the progesteronic phase.2629

During this part of the cycle, under the influence of LH, a corpus luteum develops from the ruptured follicle.2651

The corpus luteum produces progesterone, which maintains the lining of the uterus and stimulates these glands to develop.2662

However, because progesterone and estrogen exert a negative feedback on the hypothalamus,2675

eventually, within a couple weeks, LH levels will drop low enough.2679

So, the corpus luteum will atrophy, will not be producing enough progesterone, so progesterone levels will drop; and that plays into the uterine cycle.2682

In the first half of the uterine cycle, it is the proliferative phase, and under the influence of estrogen,2694

stimulates the endometrium to proliferate, readying the body for potential pregnancy.2709

In the second half, this is the secretory phase, the progesterone being secreted by the corpus luteum maintains2719

the endometrium and stimulates gland development in the endometrium and an increase in the blood supply.2733

If the corpus luteum does atrophy because of the drop in progesterone when LH drops, then, the endometrium will not be maintained.2753

It will be shed, and that is menstruation.2761

Now, if a pregnancy results, the corpus luteum, remember, hCG can maintain the corpus luteum so that progesterone levels will stay high.2764

And the uterus will be maintained in a state that can support a pregnancy.2777

Alright, so, we talked about most of the hormones involved in a female reproductive system.2785

However, I want to review two more that I mentioned back in the endocrine section, and those are oxytocin and prolactin.2791

Oxytocin, if you will recall, is produced in the hypothalamus but stored in the posterior pituitary.2799

And its function is to stimulate the contraction of the uterus during child birth. In addition, it stimulates the secretion of milk by the mammary glands.2806

So, oxytocin is one, and then, the other hormone you should know is prolactin.2818

Prolactin is actually produced by the anterior pituitary.2824

And it stimulates the development of the mammary glands as well as the production of milk by the mammary glands and lactin, lactate.2828

That helps tell you its function.2837

In addition to the other hormones we talked about, you should keep in mind the roles of these two hormones in the female reproductive system.2840

Now, talking about regulation of the male reproductive system by hormones, there are some similarities especially in the first few steps.2847

So, again, I will start out by talking about the various hormones that are involved, this time, in the male reproductive system.2857

GnRH, which is produced by the hypothalamus, is also a player here. LH and FSH have a role, and in males, the other hormone is testosterone.2863

Again, starting up with the hypothalamus, the hypothalamus secretes gonadotropin-releasing hormone, so same as in females for this step right here.2878

As in females, the GnRH stimulates the anterior pituitary to release follicle-stimulating hormone and luteinizing hormone.2895

Now, here is where things get different in males versus females.2910

What FSH does is it stimulates Sertoli cells. Sertoli cells play a role in the development of sperm.2914

You can look at them as supporting the development of sperm. They help with nutrition and play other roles in the development of sperm.2925

That is FSH. The role of FSH is to stimulate Sertoli cells, which help with the developing sperm.2939

LH stimulates Leydig cells, and if you recall, Leydig cells are interstitial cells in the testes that produce testosterone.2947

So, these produce testosterone. Actually, this is should be...and their role is to produce testosterone.2956

Testosterone has an inhibitory effect on the hypothalamus, so when there is enough testosterone being produced, what it is going to do is go back,2975

suppress the secretion of GnRH by the hypothalamus, which is going to decrease FSH and LH levels and then, decrease testosterone levels.2984

So, we have another negative feedback loop here as we did with estrogen in the early part of the menstrual cycle.2993

Alright, now that we have talked about spermatogenesis, hormonal control of the reproductive systems,3002

oogenesis, we are up to the point of talking about fertilization.3009

Fertilization refers to the union of the sperm and the egg to form a zygote.3013

The sperm is haploid. The egg is haploid, and when those two come together and fuse, they form a diploid zygote.3020

The egg is surrounded by some protective envelopes, and it has an outer transparent envelope called the corona radiata.3028

So, corona radiata is an outer protective envelope.3042

Just beneath the corona radiata...so, the corona radiata, I am going to say, is out here, and then, the next layer is the zona pellucida.3053

There are receptors for sperm on the zona pellucida, and when the sperm binds to the receptors...3069

So, sperm comes along and ends up binding to receptors here, binds to receptors on the zona pellucida.3081

What that triggers is what is called an acrosomal reaction.3094

During the acrosomal reaction, if you look at the head of the sperm, there is this region called the acrosome, and it contains hydrolytic enzymes.3102

So, the sperm that first binds will then trigger this acrosomal reaction, and it is going to start to break down.3113

The sperm will start to break down the zona pellucida. Once it digests this outer membrane, the zona pellucida, it is able to make contact.3127

The head of the sperm can make contact with the plasma membrane of the egg.3136

So, the acrosomal reaction breaks down zona pellucida. Sperm contacts the egg plasma membrane.3140

At that point, a cortical reaction will occur, so that triggers the cortical reaction in the egg.3160

So, we get the sperm-binding receptors on the zona pellucida. That triggers an acrosomal reaction.3172

Enzymes are released.3178

The sperm, then, is able to break through that layer and make contact with the plasma membrane so that the sperm and egg can fuse.3180

Then, this cortical reaction prevents polyspermy. It prevents multiple sperm from fertilizing the egg.3189

What happens is the egg releases the enzymes.3204

So, in the acrosomal reaction, the sperm releases enzymes to break down the zona pellucida.3209

In the cortical reaction, the egg releases the enzymes to prevent the entry of additional sperm.3213

So, contact between the sperm and the plasma membrane of the egg triggers the egg to release enzymes that block the entry of other sperm.3226

And it prevents polyspermy.3237

This is a type of what we call a slow block to polyspermy.3245

There is a second type of block to polyspermy called a fast block to polyspermy, but it does not occur in mammals.3255

It does occur in sea urchins and other organisms that have been used, that have been well-studied as far as fertilization and embryology.3263

So, fast block to polyspermy has a different mechanism than this one, and an animal can even have both mechanisms.3272

What this is, is depolarization of the cell membrane of the egg.3281

When the sperm comes into contact with the cell membrane, in some animals,3298

that will cause a depolarization of the cell membrane of the egg that prevents fertilization by a second sperm.3305

And then, there is another mechanism that is the release of enzymes by the egg to prevent entry of additional sperm.3316

And this is known as the slow block to polyspermy. Mammals only have the slow block.3326

Alright, what we are going to do next is some review on reproduction.3332

Example one: list three differences between spermatogenesis and oogenesis.3338

Well, spermatogenesis is continuous meaning that meiosis I to meiosis II to the formation of sperm occurs without disruption.3343

By contrast, oogenesis, there is an arrest in prophase I, and there is a second arrest in meiosis II; so oogenesis is discontinuous.3364

The second difference is that a primary spermatocyte produces four sperm,3388

whereas a primary oocyte will only produce one egg, plus there will be a couple of polar bodies.3406

A third difference is that spermatogenesis occurs throughout the lifetime of the male. Oogenesis seizes at menopause in females.3420

So, these are three differences between spermatogenesis and oogenesis.3449

Example two: match the following terms to their descriptions:3455

One, epididymis: develops from the ruptured follicle and produces progesterone- well, that is incorrect.3459

The site where sperm gain motility- that is correct.3468

Just to double check though, produce testosterone- that is not true.3472

Structure in which fertilization takes place- not true.3476

So, no. 1, the epididymis is the site where the sperm gain motility.3479

Two, fallopian tubes also known as oviducts: develop from the ruptured follicle and produce progesterone- that is not accurate.3484

Produce testosterone- not accurate. Structure in which fertilization takes place- that is correct, D.3494

Corpus luteum: now, the corpus luteum is what develops from the ruptured follicle.3502

And it does produce progesterone that maintains the lining of the uterus, A.3508

Finally, Leydig cells do produce testosterone, so B, D, A and C.3514

Example three: what role does LH - so, that is luteinizing hormone - play in maintaining pregnancy?3526

Well, recall that what LH does is it maintains the corpus luteum.3533

And that indirectly maintains pregnancy because the corpus luteum produces progesterone. Progesterone maintains the endometrium.3541

Without progesterone influence, the endometrium would shed. The pregnancy would be lost along with that.3557

Therefore, LH is responsible indirectly for maintaining the pregnancy.3563

During what part of the ovarian cycle does estrogen exert negative feedback?3571

Well, recall that during the early part of the cycle, estrogen has a negative feedback effect on the hypothalamus.3576

And this when there are relatively low levels of estrogen.3593

Now, during what part of the ovarian cycle does estrogen exert a positive feedback?3598

So, at mid-cycle, late in the first half, high levels of estrogen have the effect of positive feedback on the hypothalamus.3603

And remember that the result there is going to be an LH surge.3621

Estrogen goes from early part of the cycle, low levels of estrogen, negative feedback, and then, at mid-cycle, it switches.3631

There are high levels of estrogen, and they exert a positive feedback effect.3638

Example four: what hormone prevents atrophy of the corpus luteum when LH levels decrease?3645

LH levels decrease at mid-cycle, and if there is no pregnancy, then, the corpus luteum will atrophy. Progesterone will decrease.3651

The endometrium will shed, and that is menstruation. The cycle will begin again.3662

Now, if there is a pregnancy, the embryo will produce hCG, and hCG is what prevents the corpus luteum from atrophy.3667

And remember, this is only if there is a pregnancy that results.3681

What is the name of the hormone that stimulates uterine contractions? That hormone is oxytocin, OK?3686

Alright, that concludes this lesson on reproduction.3695

Thank you for visiting Educator.com.3699