we now turn our attention to the reproductive system we're going to focus on the sex organs in this lecture largely focused on how gametes are produced and then in subsequent lectures we'll talk about the control of that gamete production so this mini lecture is largely focused on the question of how the gametes are produced by males and females with particular emphasis on sort of the steps of producing gametes and where that occurs in the anatomy so gametes undergo cell divisions that contrast those of somatic cells we essentially have two types of cells in our body the somatic cells they are all of the cells that you we've referred to thus far in this course and then germ cells which produce the gametes which have half the genetic material of the somatic cells so let's do some basic um review of cell division a somatic cell has a diploid copy of genetic material so in the nucleus of every cell in your body you have a homologous pair of chromosomes and we see one homologous pair shown diagrammatically there if we unfurl all the genetic material and lay out all of that material then we see that there are 23 chromosomes here we see those 23 pairs of genetic material we have the sex chromosome here so this would be a male because it has an xy complement so when cells seek to divide then we the goal is to have the two offspring cells have the same genetic material as the parent cell so first there's a matter of replicating each of the homologous pairs of chromosomes so we see the one pair that we're focused on here has replicated so we have identical copies of both uh chromatids and then there's the matter of cell division where that genetic material peels off to the two daughter cells so we have the same genetic material in the offspring here as in the parent cells and this n or 2n means diploid so there's a total of 46 chromosomes which is 23 pairs now germ cells do something different because the goal is to produce cells that are haploid rather than diploid so here we have um the steps that um that are are undergone in order to produce those haploid cells or actually this is more broad than that because we're going to consider what happens when a sexual intercourse occurs and a zygote is produced which is a cell that is essentially a fertilized egg cell so first there are the steps of gametogenesis and we're going to talk about the details of gametogenesis in a moment but the product of that is a haploid cell that is in females each germ cell and in the ovaries of female all of the germ cells that one is going to have in life are contained within the ovaries so each one of those cells goes through gametogenesis this is actually called oogenesis in the case of females an oocyte is an egg and one egg is produced from the single germ cell and we can see that half of the genetic material essentially dissolves away leaving just one set of chromosomes whereas in males each germ cell produces four sperm each of which is haploid having half the genetic material now if sexual intercourse is success successfully produces a fertilized egg then that fertilization produces this zygote cell which is diploid and a fusion of half genetic material from the sperm and half from the egg so that's the overview but let's consider the steps of gametogenesis because there's a lot we're going to talk a lot about the hormonal control of those stages so in both cases germ cells are produced by mitotic divisions and at this stage they are they're diploid they have the two homologous chromosomes then there's the step of replication this is much like mitotic division but then the subsequent stages are different we have two stages of meiosis so meiosis one we have rather than the two complementary pairs of chromosomes like we saw in mitosis instead these replicated chromosomes divide into one cell and then we have the other set of chromosomes dividing in a separate cell then there's an additional division that separates each of those identical copies of chromosomes this is where we get the four spermatids in the case of the female system instead of four all of th essentially the investment goes into a single cell eggs are much more precious than sperm and you can see that the lion's share of the cytoplasm goes into one of the cells and then the other one which has half of the genetic material of the replication stage is much smaller this is known as a polar body which dissolves away then in meiosis ii half of the replicated genetic material is retained and then we have another polar body which again dissolves away and that the larger cell gives us our egg and then if we have sex successful fertilization then we're left with the diplo zygote there now the germ cells are produced by mitosis so preceding all of this are many many mitotic divisions to produce those germ cells that will ultimately produce the gametes okay so males and females are different in terms of what happens in meiosis one and meiosis ii and as a consequence you've got this four-fold difference in numbers of gametes that are produced from each of the cells another key difference is the timing of these events so as i mentioned women have all of the germ cells uh for their whole lives contained within their ovaries so all potential babies um generated that is all eggs that potentially could be fertilized are generated during embryonic development so when a female is born her ovaries contain all of those replicated eggs and that's the state that um the development freezes at it's the replicated uh state of genetic material so then what occurs starting at puberty is each one of these cells goes through the stages of meiosis 1 and meiosis 2 to produce an egg that is ovulated and so once a month each one of these cells goes through these steps to produce that egg whereas in males when a male is born they're frozen at this much earlier state of mitotic divisions so in in the males you have many mitotic divisions occurring or resuming at puberty and continuing through adulthood that allows for the opportunity of producing many more sperm than female-produced eggs and so that's all because of mitotic divisions and then subsequent to those divisions you have the meiosis one and meiosis ii but as a consequence not just of producing four spermatids for each germ cell but also all of these mitotic divisions you end up having males produce millions of sperm in a lifetime compared to the precious few eggs that are produced once a month by females so we see these differences in meiosis and now we see there's a big difference in terms of the timing of these stages of gametogenesis in a subsequent and subsequent lectures we're going to consider the hormonal control of these differences so there's a lot of similarities but there are key differences and here is the output or the result of those major differences so let's consider where in the anatomy we see these stages of gametogenesis so i've mentioned that the ovaries contain all of those germ cells and females and after ovulation where each one of those eggs is forcibly ejected from the ovary that egg then carries through a tubule known as the fallopian tube which is lined with cilia that propel the egg through it on their way to the uterus the uterus is a muscularized organ that if a zygote is produced or if an egg is fertilized then it houses a baby that develops and during um during labor the uterus by that stage has developed tremendously and offers the muscles that that essentially eject the baby on at birth now the uterus has a lining of cells that build up and break down on a monthly basis that's known as the endometrium so we'll talk a lot about the endometrium later and then on the outside of the uterus we have the vagina or the cervix and the vagina and then uh on as a mirror image there's another ovary and another fallopian tube and on average most of the time the ovaries take turns in which one offers the ovulation so which one's going to produce the egg and so when an egg is generated we see on the right ovary there we've got our egg that's going to be ovulated and so it enters into the fallopian tube and then a few days after that when it's in the fallopian tube is kind of the most ideal position for fertilization so in the case of sexual intercourse and you have a bunch of sperm in the vagina then there's the opportunity for the lucky sperm to make its way into the fallopian tube to fertilize the egg to produce a zygote now the next challenge for producing a baby or an embryo is to take that zymozygote and not lose it and also have a bunch of hormonal machinery set up shop for a pregnancy we'll get into those details later but the first step here is to do what's called implantation where the zygote adheres to the endometrium and eventually a placenta will develop at that site to provide nutrients for the embryo okay but our focus is largely on the production of the eggs gametogenesis and all of those steps that i described the replication meiosis one meiosis ii can be visualized through a histological section of the ovary so here we see a schematic version of that where we have what are called primary and secondary oocytes we're just going to focus our terminology on the stages of division so we have the products of replication in this proximal position in the the ovary and then each one of those replicated cells the oocytes then moves on to eventually becoming ovulated and so we can see if we were to monitor the chromosomes at each of these stages we would see the products of meiosis one and meiosis ii maybe you'd see a polar body produced and each one of these cells is shepherded through its development and physically moved through a bundle of cells called a follicle and the follicle is partially fluid filled and it's surrounded by cells that serve both sensory and integrative functions which we'll talk about later now after ovulation this is called the ovulated oocyte or we'll just refer to it as an egg after ovulation this follicle becomes an important structure that eventually dissolves away it's called the corpus luteum so the corpus luteum produces progesterone and estrogen which are important hormones that we'll discuss later now there's two layers of cells within the follicle that serve as integrating centers in our control and so if we zoom in on the follicle wall we can see those layers now they're called the thecal cells and the granulosa cells just store that information later because we're not going to really talk about what they do until a later lecture but i just wanted to point it out here while we're discussing the anatomical position of everything now in order for a pregnancy to initiate that requires coordination between the ovaries and the uterus the uterus breaks down on a monthly basis so when menstruation uh begins that would be a horrible time for implantation to occur so we have to coordinate the timing of that and ovulation so you don't want ovulation to occur at the same time that a woman gets her period because that would result in a failed implantation so what we're going to focus on when we talk about the hormonal control of the female reproductive system is what occurs in the ovaries which we see here through it's all of its stages relative to what's going on in the uterus and how those two areas are coordinated with one another now and and this occurs on a monthly or 28-day cycle and in the middle of that cycle we have ovulation okay so that's when the egg is released then fertilization in most cases occurs after that just a few days after ovulation now to parse the timing of all of these events we're going to refer to the development of the follicle which we see on the left is the follicular phase and then when that becomes the corpus luteum then we enter into the luteal phase now while this is occurring we have the coordinated events in the uterus in particular the lining or endometrium of the uterus so we have the uterine cycle where menstruation or a period occurs for about five days at the beginning of the cycle and then we have a building up of the endometrium which is essentially setting up shop for a potential implanted zygote so the female reproductive system controls both gametogenesis and the uterine cycle and this control as we'll see later is super complicated okay it regulates change of both the gametes and the uterus it involves multiple hormones that have negative and positive feedback loops and it's we'll get into those details let's turn our attention to the male reproductive system so we see where the gametes are produced there so we have the penis and the scrotum the scrotum is a muscularized sac that allows for thermoregulation of the testes it turns out that the production of sperm is very temperature sensitive and when cold the muscles bring the testes close to the body and when warm they relax and allow the testes to move further away by the way this is just a little comparative tangent but marine mammals don't have testes they're not very hydrodynamic or i should say they don't have a scrotum they do have testes um and a scrotum wouldn't be very hydrodynamic so instead their testes are internalized and they have this whole um blood cooling system with the capacity for shunting where blood essentially flows from the surface of the body into the interior of the body to regulate the temperature of the testes so it's a trick that humans do not have instead human males have this scrotum where the testes are exterior to the body at any rate sperm travel through a tubule that ultimately is released through the urethra of the penis during sex so they travel through the vas deferens that's the first name for the tubule which the vas deferens can undergo peristalsis much like the esophagus to propel uh sperm the sperm essentially they are capable swimming but they get a little boost through the vas deferens um the seminal fluid gets a major injection of substances from the seminal vesicle and the prostate gland so that includes uh lubricant for sex um there are chemicals for dna protection also the um the female reproductive system the vagina and the uterus are acidic environments so there are buffers um to neutralize the acid in the female reproductive system and as i mentioned oh so for reference here's the urinary bladder so urine travels through the urethra just like the seminal fluid does but the urinary bladder is closed off during sexual arousal so that we don't have a mixture of urine with seminal fluid now most of the seminal fluid is actually not semen itself but rather the fluids that are that are secreted by the seminal vesicle and the prostate gland all right so let's follow the path of a little sperm imagine it's among you know hundreds of thousands of sperm um that make their way uh through that path and then during sex in particular ejaculation then we have the seminal fluid and all the sperm within it emerging from the urethra okay so the testes are the site of sperm production that sperm production occurs in a complex network of tubules so if we take a cross-section through a testis then we can see these tubules they're called the seminiferous tubule and when you make a cross-section through the seminiferous tubule you can see all of those stages of gametogenesis which are called spermatogenesis in the case of males we see that cross section here the most superficial layers are the cells that undergo mitotic divisions and ultimately exhibit replication so if you look at the genetic material then you see that that material is replicated then as you look more towards the interior then you can find the cells for that are produced by meiosis one and ultimately meiosis ii and then the most interior are the sperm the sperm themselves right and there so their question was how are gametes produced by males and females our answer in this lecture is largely anatomical uh we see the male system on the right the female system on the left so we have in the testes the production of sperm and the production of eggs of course in the female system within the ovaries and what we're going to do is essentially revisit this question of gammy production but from a control perspective in the subsequent lectures