Development of the Female Reproductive System

 

Before Birth

As soon as the female egg released from the ovary is fertilised by a spermatozoon, development of the embryo begins.  The first thing that happens is cleavage, the mitotic division of cells.  The embryo initially has two cells, one from the ovum and one from the spermatozoon.  Separately these cells are known as blastomeres.  At first division is relatively slow, one division occurring each day for the first 3 to 4 days.  When there are about 16 cells, the embryo becomes a morula.  The cells continue to divide, and the outer blastomeres merge into each other.  In the centre fluid accumulates.  The whole embryo is now known as a blastocyst.  The blastocyst is composed of the outer layer of cells, which is now known as the trophoblast, and cells on the inside, which are the inner cell mass.  This occurs about 5 days after fertilisation, and is happening as the embryo moves along the uterine tube towards the uterus.

 

The development of these two distinct cell types indicates that much of the main organisation has occurred in the embryo.  The trophoblast cells will give rise to the outer layers of the placenta, and other extraembryonic structures, while the inner cell mass cells form the body of the embryo, as well as some extraembryonic features.  While the embryo implants in to the endometrium the inner cell mass rearranges and two layers form, the epiblast at the top and the hypoblast below it.  The epiblast will give rise to the embryo itself, while the hypoblast is known as extraembryonic endoderm and eventually forms the endodermal layer of the yolk sac.  A layer of extraembryonic ectoderm, the amnion, surrounds the entire embryo in a fluid-filled cavity called the amniotic cavity.  The fluid is called amniotic fluid, and this serves the purpose of allowing the fetus to move, as well as providing protection from internal and external harm.  Continued development leads to gastrulation, the formation of the three embryonic germ layers that eventually give rise to all the structures of the body.  These are derived from the epiblast, and are called the endoderm, mesoderm and ectoderm.  The female reproductive system is formed from the mesoderm.                   

 

The trophoblast gives rise to the placenta which begins to develop from implantation of the embryo.  Part of it is derived from the trophoblast, and part of it is derived from the mother.  It is highly vascularised and allows the transfer and exchange of nutrients, oxygen and blood from mother to fetus, and removal of carbon dioxide and urine from the fetus to the mother.  The fetus is basically parasitic while in the womb. 

 

The embryo and some of it's derivatives Image courtesy of Wikimedia under the creative commons license https://upload.wikimedia.org/wikipedia/commons/d/d4/Cell_differentiation.gif

At the start of development male and female embryos are the same and it is only later that differences occur.   However the gender of an embryo is determined at fertilisation by the fusion of the male and female chromosomal material.  Females have two X chromosomes, while males have one X and one Y.  The sex of a fetus is determined by the presence of a Y chromosome.  If the sperm provides an X chromosome the fetus will be female and develop ovaries, if it provides a Y chromosome the fetus will be male and have testis.  In females, when two X chromosomes are present, one of them is non-functional and breaks down to form sex chromatin, called a Barr body.  There is no Barr body present in males, as the single X chromosome does not condense.  

 

Both the male and female gonads develop from the urogenital ridge.  The development of testis at around the seventh week after fertilisation is due to the presence of the SRY gene on the Y chromosome.  Due to the absence of a Y chromosome, and therefore the SRY gene, female embryos do not develop testis, but instead develop ovaries at around the eleventh week post fertilisation.  Previous to differentiation, the primordial gonads have two duct systems, Wolffian ducts and Müllerian ducts.  Developing testis induce the formation of Leydig cells, which secret testosterone.  Testosterone stimulates the development of male reproductive organs by promoting the differentiation of Wolffian ducts.  SRY also promotes the testis to secrete Müllerian-inhibiting substance (MIS), which causes the disintegration of the Müllerian duct system. 

 

 

Due to the absence of the SRY gene female embryos do not secrete testosterone or MIS and do not develop testis.  Instead the Müllerian ducts differentiate into a uterus and uterine tubes.  The absence of testosterone cause the Wolffian ducts to regress, and promotes the formation of female external genital organs. 

 

At first the development of the genital organs are the same in males and females.  Three pronounced swellings form from ectodermal cells.  One swelling becomes the genital tubercle, and a swelling on either side becomes the genital fold.  The genital tubercle extends to become the glans, and entodermal cells from the urogenital sinus proliferate to become the root of the phallus.  Two more swellings appear lateral to the genital folds, called genital swellings.  At this point the development of the genital organs begins to differ, although the female changes are not as drastic as the male.  In females bending of the phallus forms the clitoris, and the genital folds become the labia minora.  In males the genital folds fuse to become the urethra.  The genital swellings get bigger and eventually form the labia majora.   

 

Internal structure formation

 

The uterus forms from part of the paramesonephric ducts that fuse together.  Where they join it becomes a rounded dome that eventually forms the fundus of the uterus.  Continuing fusion of the ducts leads to the formation of a hollow cavity that becomes the body of the uterus and the cervix.  The vagina develops from the wall of the urogenital sinus.  The fusion of the paramesonephric ducts touch the posterior wall of the urogenital sinus and two outgrowths form called the sinovaginal bulbs.  These proliferate to form the vaginal plate, which thickens and stretches around the fused ends of the ducts, eventually forming the vaginal fornices.     

 

 

Puberty

 

The development of the female reproductive system slows from birth until the girl reaches puberty.  Puberty starts in a girl between 10 and 16 years old, and is the process by which physical and hormonal changes ready the child’s body for reproduction.  Up until puberty, the only major physical differences between males and females are the external genital organs.  When puberty occurs several changes take place that completely alter male and female appearances.  These are called secondary sexual characteristics.  The main reason for these changes is the change in specific hormone release.  The most important hormone involved is gonadotrophin releasing hormone (GnRH).  At around the age of puberty the concentration of GnRH released from the hypothalamus increases. 

 

This increase triggers the release of the hormones FSH and LH from the anterior pituitary gland, which act on the ovaries.  These secrete the female sex hormone oestrogen which stimulates the maturation of the uterus and promote the development of secondary sexual characteristics, such as breasts.  It also promotes thickening of the endometrial lining and widening of the cervix ready for child birth.  When the levels of oestrogen reach a specific level, the girl begins to menstruate.  From this point onwards the ovaries are stimulated to release a follicle containing an egg each month up until they undergo the menopause at about 50 to 55 years old.  Menstruation also stops if a woman is pregnant, and doesn’t return for a considerable period after the woman gives birth.    

 

 

Menopause

 

The menopause is the stopping of menstruation in women.  It is a result of failure of the ovaries to respond to GnRH due to the fact that most of the eggs and follicles in them have degenerated.  This results in a decrease in oestrogen being released form the ovaries.  A woman is no longer able to reproduce after a year of having no menstruation.  Oestrogen has many functions within the body.  Therefore a decrease in the levels of the hormone leads to the menopause having many different symptoms.  These include hot flushes, vaginal dryness due to the thinning of the vaginal lining, lowered libido and emotional changes.  The onset of the menopause also leads to other health issues.  The likelihood of developing osteoporosis increases, as well as heart disease and uterine, endometrial and breast cancer.     

 

The development of the external female genitalia Image courtesy of Wikimedia after the copyright expired https://upload.wikimedia.org/wikipedia/commons/c/c7/Femaledevelopment.png