The Reproductive system

Meiosis The first concept that we need to understand in reproduction is the

concept of meiosis. Meiosis is important both for genetic diversity and for continuation of life

All our body cells have 23 pairs of chromosomes located in the nucleus. In any pair of chromosomes we obtained one chromosome from our mother and one from our father when we were conceived. But our gametes (ovum in females and spermatozoa in males) contain only 23 chromosomes.

Now our gametes arise from special cells called germ cells and these have 23 pairs of chromosomes (called diploid) so during gamete formation these 23 pairs are reduced to 23 chromosomes (called haploid). This is the process of meiosis.

Exchange of genetic material Meiosis is a very complex process and an understanding is outside

the scope of this course, however, there are a few important things that we should understand and the first one is to do with exchange of genetic material during meiosis

There are two cell divisions during meiosis, one occurring during Meiosis 1 and one occurring during Meiosis II. Just after the cell division in Meiosis 1 in a phase called prophase a most remarkable thing happens. The chromosomes unwind and are replicated. After they are replicated the chromosome material from the father’s chromosome may combine with that of the mother’s chromosome

This means that the chromosomes of the gamete may have differing amounts of both mother’s and father’s genetic material. If this process did not occur then children could be identical copies of either their mother or father. You can see a visualisation of this exchange of genetic material in this slide

The male reproductive system Look carefully at this slide. You should be able

to identify the major structures in the male reproductive system. In particular, make sure that you can identify the scrotum, testes, epididymis, vas deferens, seminal vesicle, prostate, bulbourethral gland and urethra. We will look at these structures now in some detail to come to terms with the male contribution to reproduction

The male reproductive system Look carefully at this slide. Notice how the

spermatic cord contains the vas deferens and nerve and blood supplies to the testes. Where the spermatic cord enters the abdomen it runs through the inguinal ring. This has nothing to do with the reproductive system but it is worth mentioning here that this is a weak spot in the body and the intestine can be pushed through here resulting in an inguinal hernia, as the next two slides demonstrate

The testes Now we return to the male reproductive

system. Look carefully at this slide. You will see that the inside of the testis is composed of a long tube called a seminiferous tubule. This is where spermatozoa are made and it empties into the rete testis. The sperm then pass into the epididymis and thence out through the vas deferens

Production of spermatozoa Each of the two testis is lined with a fibrous coat called

the tunica albugina. You can see it stained blue in this slide. This is a microscopic section magnified about 1000 times

You will be able to see seminiferous tubules surrounded by Leydig cells. These cells are stimulated by Lutenising Hormone to produce testosterone, which diffuses into the seminiferous tubule where it is required for spermatogenesis. There are about 12 sections of seminiferous tubules in this slide cut across the tube. The tube bends around much like a coiled hose and when a section is cut a person often sees many sections of the tube, as we do here.

Spermatogenesis A normal ejaculate can contain 250 million sperm. It

takes about 78 days for the sperm to be produced so about 1000 are produced every second. These are produced in about 600 metres of seminiferous tubule.

The spermatozoa arise from germ cells that line the seminiferous tubules. Special cells called Sertoli cells are activated by Follicle Stimulating Hormone and they secrete several other hormones that initiate meiosis

You can see a cross section of a tubule in these next 2 slides that include Sertoli cells (with a dark nucleus) and lots of developing sperm cells undergoing meiosis.

Rete testis Eventually meiosis in the developing

spermatozoa is completed, a flagalla is formed to give the sperm an ability to move and they empty into the rete testis. Here they stay for a few days whilst maturing further. This slide shows them being discharged into the rete testis. Obviously the image has been magnified several thousand times

Epididymis The sperm then move into the epidiymis.

The epididymis is about 5metres in length and the sperm stay here for about a week. They are further matured by a cocktail of chemicals released by the pseudostratified columnar epithelia that line the epididymis, as you can see in this slide.

Vas deferens When the sperm are released they travel through

the vas deferens. This is a thick tube and you can see in this microscope section that there is a thick layer of smooth muscle surrounding the epithelial lining. The smooth muscle is contracted by sympathetic innervation and a hormone called oxytocin. These contractions force sperm along the tube during ejaculation. Incidentally, the vas deferens is cut in a vasectomy

Seminal vesicles Just before the spermatozoa enter the

prostate the seminal vesicle adds mucus to the ejaculate. This slide shows the seminal vesicle magnified about 500 times. Mucus probably helps protect the sperm in their journey into the female reproductive tract

Prostate and Bulbourethral gland The spermatozoa now pass through the prostate gland.

The cells in this gland provides about a third of the volume during ejaculation and it is composed of fluid that has several roles. The fluid contains substances that assist the sperm in their quest to fertilise an ovum.

The bulbourethral gland empties its content into the semen after the semen passes through the prostate gland. Function of this fluid is not known but is thought to be useful in provide passage of the semen through the male urethra. This slide show a microscope image of the prostate gland

This slide shows an overall longitudinal section of the male reproductive system from the prostate down. Please make sure that you are familiar with the major structures.

The Female Reproductive system Study these next two slides carefully to

make sure that you are familiar with the structure of the female reproductive system. In particular you should be able to identify the ovary, infundibulum, fallopian tube, uterus and cervix

The ovary This slide is a schematic representation of the different

structures in the ovary. The ovary has a poor blood supply but is very rich in follicles at different stages of development. To understand ovum development we need to come to terms with the different stages of follicle development

But first we must note that meiosis stops at the first prophase stage in the ovary before the girl is born and it only resumes an hour before ovulation in the ovum that is to be released.

Note carefully that this picture shows us primordial follicles, primary follicles, secondary follicles and a Graffian follicle. We will look at these in due course and cover the functions of the corpus luteum

Primordial follicle Primordial follicles develop in the foetal ovary.

About 10 million may form but most degenerate before birth leaving a million or so in the ovary when the girl is born. By the time the female reaches puberty she may have about 400,000 left. Only about 400 will be ovulated in her reproductive life. This slide shows a central primary oocyte arrested at the first meiotic division. You can see the nucleus of the ovum as the dark material in the lighter coloured ovum. The rest is the follicle

Follicle development After birth the human ovary contains 2 types of follicles,

those growing and those not growing The ones not growing form a stockpile for those which

grow (about 15/day for a 20 year old and one per day for a 40 year old)

These are primary follicles, which have developed slowly from primordial follicles. You can see the ovum in the large follicle near the centre as a milky coloured substance surrounded by dark cells. In the middle is the clearer nucleus with black chromosome material

Secondary follicle development As the follicle develops the cells in the follicle

itself divide and initial development of the zone on the periphery of the follicle takes place. This is called the zona pellucida. This is the region where eventually a sperm will attach to the ovum. The zona pellucida is the darkish area on the outside of the developing ovum.

It is estimated that it takes 270 days to develop from primordial follicle to this secondary pre-antral follicle

Antral follicle The secondary follicle now develops into an

antral follicle. Development of follicular fluid marks the stage of the antral follicle. Follicle Stimulating Hormone (FSH) stimulates the granulosa cells that surround the ovum to secrete this fluid. Development of the follicle further is now critically dependant on FSH and lutenising hormone (LH). Lack of FSH causes the follicle to degenerate

Pre-ovulatory follicle Groups of antral follicles are either

recruited to continue growth or undergo degradation. The largest one is selected and becomes dominant. The space within the follicle is filled with follicular fluid, which assists maturing of the follicle

Ovulation A few hours before ovulation the oocyte

resume meiosis 1 producing a secondary oocyte and a tiny polar body

Secondary oocyte goes into metaphase II but stays there until ooycte is fertilised. You can see quite clearly the chromosome in the nucleus lining up in meiosis

The anterior pituitary secretes FSH and LH and the levels of these continue to rise. FSH acts on cells in the antral follicle, which secrete oestrogen. Eventually the level of oestrogen is high enough to stimulate a burst of lutenising hormone from the pituitary. This stimulates meiosis of the ovum to the middle of the second meiotic division where it stops. The follicle wall bulges out and ruptures releasing the ovum into the infundibulum

Late luteal phase ovary Where the follicle was in the ovary is now

transformed into a gland called the corpus luteum, which secretes large amounts of progesterone and some oestrogen. This stops FSH and LG being produced by the pituitary. As LH blood levels drop the corpus luteum begins to degenerate. You can clearly see the corpus luteum in this slide.

Corpus luteum The CL is pivotal to the control of the menstrual

cycle as it secretes androgens, growth factors (inhibin A), progesterone and oestrogen. These inhibit FSH. You can see the granulosa luteal cells (yellow colour) in this slide. This is the degenerating corpus luteum. Eventually the corpus luteum will degrade and become the corpus albicans. In due course that section of the ovary will return to normal

Fallopian tube This slide shows the fallopian tube at the

ampullary end. The epithelial cells secrete fluid to maintain the oocyte (ovum). The cells are ciliated to encourage ovum transport and this is amplified by contractions of smooth muscle, which help push the ovum along the tube

The uterus The fallopian tubes empty into the uterus. The outside

layer of the uterus is called the myometrium and it is composed of smooth muscle, collagen and elastin

The inside layer is called the endometrium and is composed of stroma, glands and epithelium divided into the basal layer and the functional layer. See if you can identify the different layers. If a fertilised ovum is present it will lodge on the endometrium and become attached through growth of the cell mass. The endometrium has been prepared for ovum implantation by progesterone secreted by the corpus luteum

The cervix The neck of the uterus is called the cervix.

It is a fibrous structure with two different epithelial layers. The exocervix contains squamous epithelial cells and endocervix contains columnar epithelial cells. Some of the columnar epithelial cells are replaced by squamous cells after puberty. You can see a schematic representation of the cervix in this slide

Vagina The epithelial surface is composed of non-

keratinised stratified squamous epithelium. There is an extensive vascular network in the sub-mucosal layer. If you look carefully you will be able to see abundant lactobacilli. These utilise glycogen (from cervix and squamous epithelia) producing lactate acid and maintaining a low pH to help kill any invading bacteria and fungi.

Pregnancy If a sperm does penetrate the zona pellucida of the ovum

it adds its 23 chromosome to the ovum’s 23 chromosomes forming the first cell. This fertilised cell then completes meiosis and fertilisation is complete. The fertilised egg then travels to the uterus where it is implanted in the endometrium. Study this slide carefully to review the miracle of conception. The following 4 slides give representations of the first 6 days after conception