Roles of the Female Reproductive System

Oogenesis

 

Oogenesis takes place in the ovaries.  It begins with the formation of oogonia, primordial germ cells that undergo mitosis in the fetus from the second to the fifth month of pregnancy.  These divisions result in the production of almost seven million diploid germ cells.  The majority of these oogonia degenerate throughout life, a process known as atresia.  Only about 40,000 last until puberty.  The germ cells then undergo meiosis, to form haploid cells.  Meiosis involves two divisions.  At the first stage of meiosis, the oogonia are known as primary oocytes.  They remain like this, suspended during the first stage of meiosis, until puberty, although many will also degenerate. 

 

When a woman reaches puberty, a primary oocyte completes the first stage of meiosis and continues developing at each menstrual cycle.  After the completion of phase one of meiosis, two unequal daughter cells form.  One is a large secondary oocyte, the other is smaller and nonfunctioning, called the first polar body.  The secondary oocyte is then again halted at metaphase of meiosis two, and only continues in development if the oocyte becomes fertilised by a spermatozoon. 

 

The developing egg is surrounded by cells.  Together this structure is called a follicle.  At the start of development, the oogonia are unsupported, but after meiosis begins, cells from the ovaries surround them, forming primordial follicles.  The cell layers build up and a primary follicle is formed, that now contains primary oocytes.  The follicular, or granulosa cells maintain the developing oocyte, allowing for the exchange of amino acids and glucose.  

 

A thin membrane forms between the primary oocyte and the follicle cells, called the zona pellucida.  After it thickens a covering derived from the stroma, ovarian connective tissue, surrounds the developing follicle.  Initially this is called the theca folliculi, but eventually splits into two layers, the theca interna containing blood vessels and glands, and the theca externa composed of connective tissue.  Up to this point, there is little influence from hormones.  However, with the onset of puberty the continuing development of the follicles requires follicle stimulating hormone (FSH), released from the anterior pituitary gland.  The FSH binds to the granulosa cells, stimulating oestrogen secretion.  Further development of the follicle is indicated by the formation of a fluid filled antrum, which establishes it as a secondary follicle.

 

Luteinising hormone, also secreted by the anterior pituitary gland, causes secondary follicles to secrete steroid hormones.  The theca interna produces androgens, such as testosterone, which travel to the granulosa cells.  Here the androgens are converted into oestrogen.  At this point one follicle becomes dominant, and the rest of the developing follicles breakdown.  The oestrogen leaves the follicle, causing LH receptors to form on the granulosa cells.   The follicle continues to increase in size, mainly by proliferation of granulosa cells, and puts pressure on the side of the ovary.  The follicle is now a mature or graafian follicle.  Hours before ovulation, meiosis begins again.  The graafian follicle is now ready for ovulation.  The granulosa cells contain lots of receptors for FSH and LH, and a surge of these hormones cause the follicle to be expelled from the ovary.  The follicular and granulosa cells fall away and form the corpus luteum, which secretes progesterone.     

 

The process of oogenesis Image courtesey of wikimedia under the GNU Free Documentation License https://commons.wikimedia.org/wiki/Image:Gray5.svg

Fertilisation

 

After the egg leaves the ovary and dissociates from the follicle it is wafted towards and captured by the fimbriae of one of the uterine tubes.  Once inside, the egg is transported towards the uterus by contractions of the tube.

 

After ejaculation the seminal fluid, which contains 40 to 250 million spermatozoa, enters the upper vagina.  The fluid protects the sperm from the acidic environment of the vagina, as well as buffering it, changing its pH from 4.3 to about 7.2.  The buffering effect lasts long enough for the spermatozoa to reach the cervix using swimming motions aided by its tail.  The swimming motions allow it to move through the thick cervical mucus to the uterine cavity.  Contractions of the uterine walls then propel the sperm towards the uterine tubes.  The sperm then continue to swim up the tube towards the egg.  Peristaltic waves of the tube also move the egg towards the sperm, until they meet, usually around the ampulla.  A sperm is functional within the female reproductive system for about 80 hours.  When the sperm reaches the egg, it has to penetrate its outer layer, called the corona radiata.  This is made up of proteins and carbohydrates in a matrix, and the swimming capabilities of the sperm allow it to pass through.  It then binds to the zona pellucida, initiating the acrosomal reaction, which releases enzymes from the acrosome around the head of the sperm.  These enzymes help to penetrate the zona pellucida, and pass into the perivitelline space. 

 

The spermatozoon now fuses and integrates with the egg plasma membrane.  Shortly after the sperm enters the cytoplasm, the chromatin begins to separate and spread out.  In the egg, the nucleus completes the second mitotic division, and a pronuclear membrane formed mainly from the endoplasmic reticulum encircles the female chromosomal material.  In both the sperm and egg the pronuclei grow, and DNA replicates.  As the egg and sperm haploid pronuclei come closer together, they form two chromatids.  When they eventually come together, their membranes dissolve and the chromosomes mix together quickly arranging around a mitotic spindle in order to undergo a normal mitotic division.  The maternal and paternal genetic material is now intermingled and a fertilised diploid zygote has formed.  This process is also known as conception.     

 

 

 

Implantation

 

Immediately after fertilisation, the embryo begins to undergo cleavage.  Usually one division occurs per day in the first three or four days, and it is often asynchronous.  The embryo moves down the uterine tube towards the uterus, eventually reaching it 6-7 days after fertilisation.  At this point the embryo loses the zona pellucida surrounding it, and begins to attach to the posterior endometrial lining.  It is eventually completely engulfed into the endometrial stroma, about 10-12 days after fertilisation.  It is here that the embryo remains and begins to properly develop.

 

 

Pregnancy and Birth

 

Pregnancy involves the carrying of one or more fetuses in the uterus.  It begins when the fertilised egg implants in to the endometrial lining.  The total length of a normal pregnancy is usually about 40 weeks, or nine months.  From the point of fertilisation till about eight weeks of pregnancy, the offspring is called an embryo, after which it is a developing fetus.  Pregnancy is divided in to three trimesters, relating to the different stages of development.  The trimesters last about three months each.  In the first trimester the placenta forms around the embryo, and much of the major organ development occurs.  Miscarriage is most common in this trimester.  The second trimester starts around month 4.  The woman begins to put on weight, and morning sickness subsides.  The fetus can be felt to move, and the placenta functions completely.  The reproductive organs of the fetus are developed at this point, so the sex can be determined.  In the third trimester, the woman continues to gain weight and the fetus grows quickly.  It moves in the womb, to a downward position, ready for birth.  At this stage the fetus is often moving, and it can be very uncomfortable for the woman, causing bladder weakness and back ache.  It is possible for a baby born early in the third trimester to survive.  When the fetus is fully developed, at around 40 weeks after conception, the woman goes in to labour. 

 

Child birth has three main stages, the dilation of the cervix, the delivery of the child, and the delivery of the placenta.  The first stage, the latent phase begins a few days before birth.  Small contractions begin and the cervix stretches.  By the end of the latent phase the cervix should be about 3cm dilated.  At this point contractions start properly, and the cervix continuously dilates.  At each contraction muscles around the walls of the uterus pull the cervix up over the fetus’ head, gradually forcing the baby out.  A woman is fully dilated when the cervix reaches about 10cm, or the size of the baby’s head.

 

The next stage is the actual birth of the baby.  At the point of full dilation, the widest part of the head has already passed through the pelvic brim, and, normally, through the narrowest part of the pelvis.  The fetus just has to move down the birth canal, below the pubic arch.  The mother aids this process by pushing or ‘bearing down’.  The fetus crowns, and the rest of the baby is delivered.  After the birth, usually up to 30 minutes, the placenta has to be delivered.  This occurs due to continuous contractions of the uterus, as well as pulling on the umbilical cord that comes out attached to the baby.  The length of labour differs from person to person, and usually takes longer if it is the woman’s first child.  

 

The Menstrual Cycle         

 

Puberty usually starts when a girl is between 12 and 16 years old.  One of the things that happens during puberty is the start of the menstrual cycle, lasting about 28 days where one egg is released per cycle from the ovaries ready to be fertilised.  The menstrual cycle is under direct control from different hormones and is necessary for reproduction.  It can be split in to two stages, follicular and luteal.  Each cycle starts with overt menstruation, the breakdown of the endometrial lining leading to bleeding from the vagina.  This occurs from days 1 to about 5.  The follicular stage is also known as the proliferative phase, where hormones cause the proliferation of the endometrial lining.  This lasts until about day 14, when the egg is released form the ovary.  After this the cycle goes into the luteal phase, starting after ovulation and lasting until the degeneration of the corpus luteum at around day 28.

 

Hormonal control

 

At the beginning of the cycle follicle stimulating hormone (FSH) and luteinising hormone (LH) secretion increases from the pituitary gland due to low levels of oestrogen, progesterone and inhibin.  The FSH triggers the development of several antral follicles, which enlarge and start secreting oestrogen, causing the plasma oestrogen concentration to increase.  Oestrogen also acts on the follicles themselves, causing the granulosa cells to proliferate.  LH acts on newly synthesized LH receptors on theca cells of the developing follicle, causing them to produce androgens which are converted into oestrogen by the enzyme aromatase.  At this point one follicle becomes dominant, due to the fact that levels of FSH decrease causing the other developing follicles to degenerate.  The dominant follicle does not degenerate because it has developed a higher sensitivity to FSH due to a larger number of FSH receptors on the granulosa cells.  The granulosa cells also become sensitive to LH.

The levels of FSH decrease due to the continuing rise of plasma oestrogen and the secretion of inhibin from the granulosa cells of the follicle.  The high oestrogen levels exert an opposite effect on LH, causing a positive feedback mechanism to increase LH levels just before ovulation.  The egg finishes its first meiotic division, and the matured follicle begins to secrete less oestrogen and some progesterone.   The egg is ovulated and the remaining follicular cells that are expelled along with the ovum degenerate into the corpus luteum.  This secretes large amounts of progesterone, and is maintained by low levels of LH for about 14 days.  If the egg of the woman becomes fertilised the corpus luteum is maintained to secrete progesterone to build up the endometrium ready for implantation.  If the egg is not fertilised, the corpus luteum breaks down to form the corpus albicans, progesterone levels decrease, and the endometrial lining is lost.  With the break down of the corpus luteum oestrogen levels also decrease, so FSH and LH are no longer inhibited.  Levels of these rise and the cycle starts over again.            

 

Stages of the menstrual cycle Image courtesy of wikimedia under the creative commons cc-by-sa 2.5 license https://upload.wikimedia.org/wikipedia/commons/f/f0/MenstrualCycle.png