Female reproductive system
The female reproductive system consists of the ovaries, fallopian tubes, uterus and vagina. It has a number of different roles and produces female gametes and acts as the host for a fertilised embryo. There are a number of different functions that the reproductive system undertakes such as the menstrual cycle and pregnancy.
The primary reproductive organ of the female is the ovaries and these are the site of gamete production. They are oval structures and come as a pair, each situated at the end of a fallopian tube. They are made of connective tissue and have a large number of blood vessels. Within their structures are protrusions known as follicles and within each follicle lies an egg also known as an oocyte. The follicle plays a pivotal role in terms of oocyte development.
During the menstrual cycle, an increase in FSH levels allows the follicles to develop and a single follicle is chosen. In the early stages of development, the follicle is referred to as the primordial follicle and its structure is a single layer of epithelial cells surrounding the oocyte. As it develops, this layer becomes thicker and the cells are known as granulosa cells. Further development leads to the proliferation of these cells and another cell type is produced called theca cells. Both cells play an important role in terms of developing the oocyte. The granulosa cells are the point at which FSH can control the development of a follicle and they are stimulated by FSH to produce chemical messengers for the oocyte. They also release inhibin to control the levels of FSH in turn. They also provide nutrients through the gap junctions in between the cells.
The Fallopian Tubes
From the ovaries extend a tube called the fallopian tube and it is down this that the oocyte will travel once released. Fertilisation usually takes place along this tube and it is approximately ten centimetres long. There are a number of projections down the tube that are known as fimbraie and these can envelop the oocyte. The oocyte travels down the tube in the oviductal fluid and is moved along by the ciliary beating that occurs from the protrusions. Contractions are also used to propel the oocyte down the tube. It takes the oocyte approximately four days to travel from the ovary to the uterus.
The uterus is the centre of the reproductive system and is a cavity in which the fallopian tubes open out in. It is a pear shaped organ that is situated in the pelvic cavity and it can be split into two different regions. The upper region is known as the body and is where the embryo will grow. The lower region is the cervix which is much narrower and is the passageway that leads to the vagina. This canal is often referred to as the birth canal.
The wall of the uterus must be strong in order to house a developing foetus. There are a number of different layers that make up the uterine wall. The outer most layer is called the perimetrium and this is made up of connective tissue. The next layer is the myometrium which is smooth muscle and is responsible for producing contractions. THe final layer is called the endometrium and it is this layer that breaks down during menstruation. The endometrium is made up of two layers called the basal layer which remains intact during menstruation and the functional layer which is shed during menstruation. There are a number of different secretory glands present throughout these layers. The basal layer serves as a base for the functional layer to grow on.
The vagina is the passage out of the uterus and it is approximately eight centimetres long. It is the female organ of copulation for it receives the penis during intercourse. There is a layer of smooth muscle which can contract and glands secrete fluid into the vagina for protection. The fluid is acidic so that it will kill unwanted bacteria at the site ensuring that the female does not get any infections in that area.
The vulva is the external genitalia of the female and it is found around the reproductive tract entrance. It is made up of a number of different structures. The mons pubis is located in the lower pelvis region and is made up of fatty tissue and skin. The labia majoria and the labia minora are the folds of outer and inner skin that protect the vagina. There is also an erectile organ called the clitoris.
Development of the Oocyte
Unlike males, female gametes are limited in number and this number is predetermined before birth. The process of oocyte development is also discontinous as there are a number of meiotic arrests. The female oogonia enter their first division in fetal development. However, they do not complete this division and arrest at the prophase stage. This division is completed during ovulation and another division occurs but this again is arrested, this time at metaphase. The second meiotic division is completed during fertilisation. The process of division is asymmetrical in females for they release polar bodies instead of dividing directly in two. Therefore for every oogonium that divides, there is one mature egg. It is estimated that a female is born with 2,000,000 eggs but only 400-500 are ever ovulated.
The Menstrual Cycle
The female reproductive system is cyclic and therefore it is tightly controlled throughout a period of 28 days. This is known as the menstrual cycle which begins during puberty and ends after menopause. There are a number of different stages that make up the menstrual cycle. The start of the cycle is signalled as the break down of the endometrium on day one.
The first phase is known as the follicular phase or the proliferative phase. During this time, the hypothalamus releases gonadotrophin releasing hormone which acts on the anterior pituitary gland, stimulating it to release Follicle stimulating hormone (FSH) and lutenising hormone (LH). This causes 15 follicles to develop instead of undergo apoptosis. This process is called folliculogenesis and the layers around the follicle proliferate. The theca cells produce androgens which is then used by the granulosa cells to make oestrogen. This level of oestrogen then suppresses the levels of FSH so that a dominant follicle can be chosen. This is usually the follicle with the most FSH receptors as it will continue to develop whilst the others cease. The oestrogen levels increase and this causes a surge in LH and FSH. 18 hours after this surge, the follicle will rupture and the oocyte will be released from its confines. This is approximately day 14 in the cycle. Throughout this time, the endometrium has developed due to the presence of the oestrogen.
The next phase is the luteal phase where the endometrium is prepared for implantation. The follicle turns into the corpus luteum which supplies the endometrium of the uterus with a constant supply of progesterone. This ensures that the endometrium is maintained. If the oocyte is fertilised then the developing embryo makes its way down to the uterus and secreted human chorionic gonadotrophin which signals to the uterus that fertilisation has been successful. The hormone maintains the presence of the corpus luteum which continues to supply progesterone so that the endometrium does not break down. Once the placenta has formed at six weeks, it takes over the control of progesterone levels and the corpus luteum becomes th corpus albicans. If fertilisation does not occur then there is no human chorionic gonadotrophin production and after twelve days with no hormone stimulation, the corpus leuteum breaks down to become the corpus albicans and stops producing progesterone. This causes the progesterone levels to drop and the endometrium breaks down so that menstruation can occur. The cycle will then start once more.
Fertilisation is where the sperm and the oocyte meet and the sperm successfully penetrates the oocyte. There are several steps before this can occur and all limit the chances of the sperm fertilising the egg. Firstly, the sperm must swim through the uterus and up the correct fallopian tube to the only egg produced during that cycle. Once it has reach the oocyte, it must then bind to the outer coat. The outer coat is called the corona radiata and the sperm use enzymes called hyaluronidases to penetrate through this outer later. Following this, the sperm then must bind to the zona pellucida. There are a number of different models to explain how the sperm bind to the zona pellucida and the foremost says that there are glycoproteins that are recognised by the sperm and these are what the sperm bind to. Once bound, the sperm must now penetrate the zona pellucida and they can only do so by undergoing the acrosome reaction. During this time, the enzymes contained in the sperm head are released to digest through this layer of the oocyte. Once through, the sperm can bind to the oocyte. There are a number of different candidates put forward for the binding proteins but it is believed that a protein called izumo is responsible for this connection. Fertilisation of the egg then occurs. The embryo must then implant into the endometrium in order to have a clinical pregnancy. This process is long and dangerous with many embryos failing at this stage. Once implanted, the embryo can use the mothers blood supply to gain nutrients and develop.
Female Reproductive Disorders
Sometimes an individual can have reduced fertility or infertility which means that they struggle to concieve naturally. IVF techniques are used to solve the majority of fertility issues but sometimes the only other options are surrogacy or adoption. There are a number of different reasons why a female may be infertile from genetic conditions to sexually transmitted diseases.
Sometimes, there is a problem in the ovulatory cycle. This can mean that the female is not releasing an egg every month. Absent cycles are a factor in infertility and some individuals never start their menstrual cycle whereas others suffer from secondary amenorrhoea where menstruation does not occur anymore. Some females may undergo the cycle but not release an egg. These conditions are usually caused by either stress, obesity, drugs or conditions such as anorexia nervosa and can be treated with placebos as they can be psychological.
Idiopathic ovarian failure is another condition where the ovary does not respond to the hormones secreted. The cause of this condition is unknown. In other cases, the egg may not leave the follicle as it is simply a bad egg. One condition called the abbreviated luteal phase is where the level of progesterone is reduced and the female does not have an effective luteal phase so the environment is more hostile towards an embryo.
Some females suffer from polycystic ovarian syndrome where many follicles develop at one time and they form cysts which damage the the tubes and ovaries.
Female Tract Disorders
There are some conditions which prevent the sperm and the oocyte meeting. These can be due to an infection such as gonorrhoea, chlamydia or tuberculosis. Infections can also occur post pregnancy and prevent women from becoming pregnant once more. Usually, the infections damage the cilia along the fallopian tubes causing scarring and obstruction.
A condition called endometriosis can also cause infertility where the cells that make up the endometrium grow elsewhere in the reproductive tract. They break down in the same manner during menstruation and cause pain and scarring which leads to adhesions.
Sometimes, the environment for the foetus is hostile and the mother cannot carry the baby to full term. One such condition is cervical incompetence where the cervix opens too early and the female suffers a miscarriage. This can be solved by suturing over the cervix and prescribing bed rest until the mother reaches full term.
Implantation defects can occur where the baby implants outside of the uterus, usually in the fallopian tube. As it grows, it puts pressure on the tube which will eventually rupture and cause great danger to the mother. This is known as an ectopic pregnancy and always ends in abortion or miscarriage.
Some females encounter autoimmune defects such as lupus where the immune system attacks the tissue and the foetus. The foetus may also have a different compatibility and the immune system may recognise it as foreign. One example is a Rhesus baby. If the mother is RH- and the father is Rh+ then the foetus will be RH+. Therefore, the mothers immune system will attack the foetus.
Pregnancies can spontaneously abort and some infertility problems are because the mother miscarries each time she is pregnant. Spontaneous abortion usually occurs because of a chromosomal defect which means the foetus can never develop to full term. Some defects can develop to full term but the baby may have a genetic condition or disability. The typical treatment for such abortions is to use pre-implantation genetic diagnosis to select for a health embryo that has the best chance of being carried to full term. IVF is needed in these cases.