The Ovary
• Primordial germ cell (pgc)- the initial germ cells
o Pgc becomes an oogonia when it reaches a gonad
▪ At this point it is undergoing mitosis
o When mitotic divisions stop, and meiosis starts, it is then called an oocyte
o Once the oocyte is ovulated, until it becomes fertilised, it is an egg
• Initially oogonia, still undergoing mitosis, become surrounded by somatic cells,
forming nests
o Within the nests, there are ‘cysts’ of germ cells that have cytoplasmic
connections to each other, due to incomplete cytokinesis
▪ Incomplete cell cytokinesis- when cells haven’t fully divided
▪ In some species, only one germ cell from the nest will become an egg
o
• If an animal is female the cells will become an ovary
o Neighbouring mesonephros now produces retinoic acid
▪ Retinoic acid induces oogonia to stop undergoing mitosis and start
meiosis
• Germ cells are now oocytes
▪ Retinoic acid is also made in the male mesonephros but is degraded by
Sertoli cell, so it has no effect on prespermatogonia
• The end of mitosis is the beginning of the major difference between male and female
germ cell development
o When mitosis stops in females, new germ cells can’t be produced
▪ Therefore, women are said to have a finite number of oocytes
▪ There has been recent controversy around whether the postnatal ovary
has actually lost the ability to produce more germ cells
• Stem cells from the ovary can be made in vitro
• Oocytes can then be made from the ES stem cells
• Mitosis- DNA in cell duplicates, one cell division makes 2 daughter cells
• Meiosis- DNA in cell duplicates, two cell divisions make 4, non-identical, daughter
cells
, o
• Meiosis in females
o Produces one mature germ cell and two polar bodies
o Oogonium is a 2n (diploid) cell
o Oocyte is an oogonium cell that has undergone mitotic arrest
▪ Chromosomes have replicated in this cell
▪ Sometime after this, birth occurs
o 1st meiotic division- occurs at ovulation
▪ Produces a normal cell and 1st polar body
• 1st polar body is thought to contain junk DNA
o 2nd meiotic division- completed around the time of fertilisation
▪ Creates a second polar body
o
o Polar bodies were thought to be used to get rid of faulty chromosomes
▪ Now known to be untrue as genetic integrity of polar body shown to be
intact
▪ Plays no further role in development
▪ Defective material not put into first polar body
• Follicle formation- happens around the same time as meiotic arrest
o Somatic cells invade the cysts and surround individual oocytes
o Oocytes become surrounded by a single layer of somatic granulosa cells
, ▪ Most of the germ cells don’t manage to make follicles
▪ Around 80% of the germ cells are lost
o The zona pellucida also forms at this time
• Primordial follicles are at a resting stage
o Can stay in this stage for entirety of female’s life
o No new follicles form
o Pool of follicles decreases as some follicles leave and grow
o The menopause happens when so few primordial follicles are left, the ovary
can’t function
▪ Normally reproductive lifespan is the same length as the lifespan of the
animal
▪ Humans have a shorter reproductive lifespan than their actual lifespan
▪
• Growth initiation- the gradual movement of primordial follicles out of the resting
pool, to start to grow and develop
o The oocyte starts to grow as the follicle grows
o Granulosa cells become more rounded instead of flattened
▪ Granulosa cells also undergo mitosis
▪
o At this stage the follicle is no longer a resting follicle
o Growth initiation is thought to be regulated by the PTEN signalling pathway
▪ PTEN signalling keeps primordial follicles from growing
▪ Interfering with the pathway causes all primordial follicles to undergo
initiation
• Causes there to be no reserve pool left
• Follicle growth and development
o Early stages
▪ Called a pre-antral follicle at this stage
▪ Granulosa cells- continue to divide
• Primordial germ cell (pgc)- the initial germ cells
o Pgc becomes an oogonia when it reaches a gonad
▪ At this point it is undergoing mitosis
o When mitotic divisions stop, and meiosis starts, it is then called an oocyte
o Once the oocyte is ovulated, until it becomes fertilised, it is an egg
• Initially oogonia, still undergoing mitosis, become surrounded by somatic cells,
forming nests
o Within the nests, there are ‘cysts’ of germ cells that have cytoplasmic
connections to each other, due to incomplete cytokinesis
▪ Incomplete cell cytokinesis- when cells haven’t fully divided
▪ In some species, only one germ cell from the nest will become an egg
o
• If an animal is female the cells will become an ovary
o Neighbouring mesonephros now produces retinoic acid
▪ Retinoic acid induces oogonia to stop undergoing mitosis and start
meiosis
• Germ cells are now oocytes
▪ Retinoic acid is also made in the male mesonephros but is degraded by
Sertoli cell, so it has no effect on prespermatogonia
• The end of mitosis is the beginning of the major difference between male and female
germ cell development
o When mitosis stops in females, new germ cells can’t be produced
▪ Therefore, women are said to have a finite number of oocytes
▪ There has been recent controversy around whether the postnatal ovary
has actually lost the ability to produce more germ cells
• Stem cells from the ovary can be made in vitro
• Oocytes can then be made from the ES stem cells
• Mitosis- DNA in cell duplicates, one cell division makes 2 daughter cells
• Meiosis- DNA in cell duplicates, two cell divisions make 4, non-identical, daughter
cells
, o
• Meiosis in females
o Produces one mature germ cell and two polar bodies
o Oogonium is a 2n (diploid) cell
o Oocyte is an oogonium cell that has undergone mitotic arrest
▪ Chromosomes have replicated in this cell
▪ Sometime after this, birth occurs
o 1st meiotic division- occurs at ovulation
▪ Produces a normal cell and 1st polar body
• 1st polar body is thought to contain junk DNA
o 2nd meiotic division- completed around the time of fertilisation
▪ Creates a second polar body
o
o Polar bodies were thought to be used to get rid of faulty chromosomes
▪ Now known to be untrue as genetic integrity of polar body shown to be
intact
▪ Plays no further role in development
▪ Defective material not put into first polar body
• Follicle formation- happens around the same time as meiotic arrest
o Somatic cells invade the cysts and surround individual oocytes
o Oocytes become surrounded by a single layer of somatic granulosa cells
, ▪ Most of the germ cells don’t manage to make follicles
▪ Around 80% of the germ cells are lost
o The zona pellucida also forms at this time
• Primordial follicles are at a resting stage
o Can stay in this stage for entirety of female’s life
o No new follicles form
o Pool of follicles decreases as some follicles leave and grow
o The menopause happens when so few primordial follicles are left, the ovary
can’t function
▪ Normally reproductive lifespan is the same length as the lifespan of the
animal
▪ Humans have a shorter reproductive lifespan than their actual lifespan
▪
• Growth initiation- the gradual movement of primordial follicles out of the resting
pool, to start to grow and develop
o The oocyte starts to grow as the follicle grows
o Granulosa cells become more rounded instead of flattened
▪ Granulosa cells also undergo mitosis
▪
o At this stage the follicle is no longer a resting follicle
o Growth initiation is thought to be regulated by the PTEN signalling pathway
▪ PTEN signalling keeps primordial follicles from growing
▪ Interfering with the pathway causes all primordial follicles to undergo
initiation
• Causes there to be no reserve pool left
• Follicle growth and development
o Early stages
▪ Called a pre-antral follicle at this stage
▪ Granulosa cells- continue to divide
