The second lecture in the reproductive system lectures covers the physiology of the male reproductive system and will only be a short one today. We'll describe the process of spermatogenesis or the production of sperm as well as the hormones which regulate male reproductive functions. So learn objective number one to describe the process of spermatogenesis.
So spermatogenesis is the production of sperm from primordial germ cells. But before we delve into the specifics of spermatogenesis, we need to briefly cover the basics of cell division. So most of the cells of the human body undergo cell division which is the process by which cells reproduce themselves. There are two types of cell division.
We have somatic cell division and reproductive cell division and these two different types have two different end goals. Now a somatic cell is any cell of the body other than a germ cell. A germ cell is a gamete which in males is sperm and in females is oocytes or eggs.
So in somatic cell division, a cell undergoes a type of division called mitosis and it produces two genetically identical cells, each with the same number and kind of chromosomes as the original cell. Somatic cell division will replace dead or injured cells and then adds new ones when we've got tissue growth. Reproductive cell division is the mechanism by which we produce gametes, so our sperm and our eggs, and we need reproductive cell division to form our offspring. So this is a two-step process called meiosis and in this process the number of chromosomes in the nucleus is reduced by half.
When an egg is fertilized, the number of chromosomes is then restored. So we have 23 from the female and 23 from the male. So coming back to spermatogenesis which involves the reproductive cell division and in humans this process takes between 65 and 75 days. It begins with our spermatogonia which contains 46 chromosomes and that's what's being represented by this 2n here.
So referring to the fact that these cells contain a complete set of 46 chromosomes. So our spermatogonia are types of stem cells. When they undergo their cell division some of these spermatogonia remain near this basement membrane here. They stay in an undifferentiated state to serve as a bit of a reservoir or a backup pool of cells for future cell division and therefore subsequent sperm production. The rest of our spermatogonia will undergo developmental changes and differentiate into our primary spermaticides.
Now like spermatogonia our primary spermatocytes have 46 chromosomes as well. Now shortly after our primary spermatocytes form they replicate or double their DNA. and then the process of meiosis begins.
So in the first phase of meiosis the two replications of each chromosome are pulled to each side of the dividing cell and the cell becomes two. The two cells that are formed are called our secondary spermatocytes. Each secondary spermatocytes has 23 chromosomes so just one in here. So each secondary spermatocyte has 23 chromosomes.
However, a chromosome is made up of two strands of DNA. So you might have seen the kind of X shape of a chromosome. Those two strands are identical to one another.
one another and they're called chromatids. In the second phase of meiosis those two chromatids of each chromosome are separated and four cells each containing 23 chromosomes are produced. These are what we call our spermatids.
In the final stage of spermatogenesis, which is called spermiogenesis, is the development of these spermatids into sperm. No cell division occurs in spermiogenesis, but each spermatid becomes a single sperm cell. Now during this process the spherical, so the round shaped spermatids, will transform into the elongated slender sperm that we're familiar with.
An aquasome, which we talked about in the last lecture, remember it sits on the head of the sperm, will form. A flagellum or a tail will form and we have mitochondria multiplying in that middle piece. The sustentacular cells which are these pink cells here that sit in between all of our spermatogenic cells will dispose of the excess cytoplasm so our sperm now look in this typical sperm shape. Finally sperm are then released and enter into the lumen of the seminiferous tubules so this is the hollow of the seminiferous tubules.
Fluids secreted by the sustentacular cells will push sperm along their way towards the ducts of the testes Because remember at this point our sperm are not actually able to swim. Now looking at that process again But just with another image and so we begin with our spermatogonia, which are the germ cell stem cells, so the cells that can differentiate into lots of different things And these cells, which is represented by that 2n here, contain 46 chromosomes. They undergo cell division, or mitosis, and they produce more of the same cell.
The spermatogonia then differentiate into our primary spermatocytes, which, like our spermatogonia, still have those 46 chromosomes. The primary spermatocytes then replicate their DNA or double their DNA and the first phase of meiosis begins resulting in those two secondary spermatocytes. Each of those secondary spermatocytes only has the 23 chromosomes.
In that second phase of meiosis the two chromatids of each chromosome are pulled apart and four spermatids are produced each with 23 chromosomes. Each spermatid then undergoes spermiogenesis, so lots of new terms here, where they develop the head, the middle piece and the tail that we're familiar with. Each sperm has a set of 23 chromosomes. Now that process in text, and I'm not going to go over that with you again here, but you can just use this slide, I guess, to refer back to. if you're unsure about working through this process on your own with those images.
Moving on to the second and the final learning objective for the physiology of the male reproductive system which is to describe the hormones which regulate male reproductive functions. Now although it's not exactly clear what initiates these changes, at puberty cells in the hypothalamus increase their secretion of our gonadotropin-releasing hormone. This hormone then in turn stimulates the anterior pituitary gland to secrete luteinizing hormone and follicle-stimulating hormone. Our luteinizing hormone stimulates our interstitial cells, which remember are located between the seminiferous tubules in the testes.
Their job is to secrete testosterone. Testosterone easily diffuses out of the interstitial cells into the interstitial fluid and then into the blood and via negative feedback testosterone suppresses the secretion of luteinizing hormone as well as a secretion of gonadotropin releasing hormone from the hypothalamus. Follicle stimulating hormone acts indirectly to stimulate spermatogenesis, so that process we just talked about.
Our follicle-stimulating hormone and testosterone act synergistically or together on the sustentacular cells to stimulate the secretion of our androgen binding protein. This goes into the lumen of the seminiferous tubules and into the interstitial fluid around our spermatogenic cells. Now the function of our androgen binding protein is that it binds to testosterone.
It keeps it within the blood, so it keeps that concentration of testosterone high. This is important because testosterone stimulates those final steps of spermatogenesis in the seminiferous tubules, so important to have high levels of testosterone. Once the degree of spermatogenesis required for male reproduction has been achieved, the sustentacular cells will then release another hormone called inhibin. This is a protein named for its role in inhibiting the secretion of follicle stimulating hormone.
If this somatogenesis process is proceeding too slowly, then we simply secrete less inhibin, which promotes more release of follicle stimulating hormone and then a faster rate of somatogenesis. Now outside of the effects on spermatogenesis, testosterone acts on target tissues in several ways. Before birth, testosterone stimulates the development of the male reproductive system ducts and the descent of the testes. So the testes are formed up in the pelvic cavity and then they descend into the scrotum.
At puberty, testosterone brings about the development and the enlargement of our male sex organs. as well as the development of masculine secondary sexual characteristics. A secondary sexual characteristics are traits which distinguish males from females physically, but they do not have a direct role in reproduction.
So these include things like the muscular and skeletal growth that results after puberty, and we have the wide shoulders and the narrow hips in males, the growth of facial hair and chest hair, thickening of the skin, increased secretion from oil glands, and the enlargement of the larynx or the Adam's apple, which consequently deepens the voice. In adulthood, or really any time after puberty, testosterone also contributes to male sexual behavior and things like sex drive and libido. Just a bit of a summary table to put all of that information in one place for you. So we have the six hormones that are important in regulating male reproductive functions.
We have their site of production and secretion and then their function. So you might like to do a table like this on your own. But we have our gonadotropin releasing hormone which is produced up in the hypothalamus. This stimulates the secretion of luteinizing hormone and follicle stimulating hormone. We have luteinizing hormone which as we now know comes from the anterior pituitary gland.
It stimulates the secretion of testosterone from those interstitial cells in between the seminiferous tubules. We have follicle stimulating hormone which is also from our anterior pituitary. This stimulates the secretion of our androgen binding protein from those sustentacular cells. We have testosterone which is our main male reproductive hormone. It's produced in our testes and in particular in those interstitial cells.
Does quite a wide range of things including developing the sex organs, developing the secondary sexual characteristics as well as being essential for spermatogenesis to occur. We have androgen binding protein which is also from the testes but particularly from the sustentacular cells. As we mentioned this binds to the testosterone protein so that it remains in the blood and remains in high concentration primarily to facilitate this spermatogenesis.
And then lastly we have inhibin released also from the sustentacular cells in the testes and this inhibits the secretion of follicle stimulating hormone. when we've had a sufficient production of our sperm. And so that is all for the male reproductive system.
In the next and final series of lectures, which is exciting, we will cover female anatomy and female reproductive physiology.