Oogenesis And Sexual Differentiation Flashcards
Define sexual differentiation.
Sexual differentiation is the series of events whereby sexually indifferent foetuses progressively squire male or female characteristics in the gonads.
Describe the sexually indifferent urogenital system.
The sexually indifferent urogenital system consists of the metanephros (which will differentiate into the kidneys), the mesonephros and the area called the indifferent go as (the gonadal streak).
Associated with the indifferent gonad are the Müllerian duct system and the Wolffian duct system. The urogenital sinus will differentiate into the internal and external genitalia.
When does the very first stage of oogenesis/spermatogenesis begin?
Oogenesis/spermatogenesis really begins with the formation of primordial germ cells. These germ cells arise very early in foetal development (32-37 days). These cells arise at the junction between the yolk sac and the primitive hind gut. They then migrate from this region in the hind gut to the genital region/ridge. They then populate the gonads.
Describe the basics of sexual differentiation.
The basics of sexual differentiation is progression from an undifferentiated state to a differentiated state.
If the individual is XX then the Müllerian duct will progress. Of the individual is XY then the Wolffian duct will progress.
The Wolffian duct will develop into the vas deferens and the epididymis.
The Müllerian duct will develop to form the uppermost part of the uterus, the upper vagina and the oviducts.
The mechanisms for the differentiation process rely on the presence or absence of the SRY gene. The SRY gene is the testis determining factor. It maps to the P arm of the Y chromosome. It is a DNA binding protein that works by an unknown mechanism.
Hypothesis 1 is that SRY upregulates testis-specific genes such as Sox 9 which acts to upregulate anti-Müllerian hormone.
Hypothesis 2 is that SRY antagonises a gene that inhibits testicular development such as DAX-1.
What parts of their respective reproductive systems do the Müllerian and Wolffian ducts develop into?
The Wolffian duct will develop into the vas deferens and the epididymis.
The Müllerian duct will develop to form the uppermost part of the uterus, the upper vagina and the oviducts.
What gene does sexual differentiation rely on? How is the mechanism involving this gene thought to function?
The mechanisms for the differentiation process rely on the presence or absence of the SRY gene. The SRY gene is the testis determining factor. It maps to the P arm of the Y chromosome. It is a DNA binding protein that works by an unknown mechanism.
Hypothesis 1 is that SRY upregulates testis-specific genes such as Sox 9 which acts to upregulate anti-Müllerian hormone.
Hypothesis 2 is that SRY antagonises a gene that inhibits testicular development such as DAX-1.
Describe the determinants of sexual differentiation on the male.
We have a bipotential gonad, of we have the presence of SRY this may induce the production of SOX9 or DAX1. We therefore have the development of the indifferent gonad down the testis route.
The serotoli cells that develop in the testis produce Müllerian inhibiting substance (MIS), also known as anti-Müllerian hormone (AMH).
In addition the differentiating leydig cells which are steroidogenic calls on the testis produce testosterone which is responsible for the development of the internal and external genitalia.
If however the SRY factor is absent because there is no Y chromosome then we have by default the development of the Müllerian ducts and the female phenotype.
In chickens there is both a female and a male determining factor.
What cells in the developing testis producing anti-Müllerian hormone?
The serotoli cells.
What cells produce testosterone?
The Leydig cells.
How similar are the processes of spermatogenesis and oogenesis?
Spermatogenesis and oogenesis are essentially just the same process until you get down to the level of the primary spermatocytes/oocyte where the cells are still diploid.
In the case of spermatogenesis we have a situation where a primary spermatocyte gives rise to 4 spermatozoa, whereas an oocyte through its unequal replication only gives rise to one oocyte.
Is the ovary inactive during childhood?
During childhood the ovary is not inactive. At birth the ovary contains follicles at all stages of development. Unlike in rodents we do not have recruitment of a primordial follicle to a primary follicle as this occurs constantly from the third trimester of foetal development right through to puberty and adulthood.
The oocyte within the primordial follicle is arrested in prophase 1 of meiosis until either the follicle degenerates or it develops after an LH surge.
Briefly describe oogenesis in mice.
During the latter part of gestation we have the organisation of primordial germ cells into germline cysts and then in the period just after birth we have the formation of the primordial follicles and the recruitment of these follicles into the process of follicular genesis.
Formation of oocyte nests is thought to be associated with ovine ross cords of the somatic cells in mice.
Describe foetal ovary development in the bovine.
Essentially the same as human.
Have production of ovigerous cords (germ cell nests) at day 90-180. Have formation of primordial follicles from day 90 - term.
Formation of different stages of follicular genesis, primary, secondary, preantral and antral follicles occur. Antral follicles occur in the last trimester of pregnancy.
When the individual is born the ovary contains a very large number of primordial follicles, but also follicles at different stages.
Antral follicles in al, species occur at a diameter of approximately 250um. The foetal follicles are antral but are usually not so big that you would be able to see them on the surface of the ovary.
Describe how follicle numbers change throughout life.
During mid semester of foetal development you get a big increase in the number of primordial follicles up to around 7 million. After this the numbers absolutely plummet so that at birth there are somewhere between 0.5 and 2 million primordial follicles in the ovary.
There are primary follicles by 15-16 weeks gestation and Graafian follicles present at 23-24 weeks gestation.
At birth humans have about 0.5 million follicles. The number of primordial follicles declines at a pretty steady rate from the time of birth (and there is no change at around the time of puberty) until the age of 35 or 36 when the rate of loss increases quite dramatically. There are only a few hundred follicles at the time of normal menopause in the early 50s.
How is follicle loss controlled?
A large loss of follicles is due to programmed cell death as a result of FIG-alpha and Wnt-4 action.
Multi oocyte follicles is another reason why we lose quite a number of follicles. These are associated with Notch, some of the BMPs and oestrogens.
After the formation of the primary follicles we see a large loss of follicles through atresia.
