Oogenesis Flashcards
Oogenesis
Formation of female gamete
Starts from undifferentiated primordial germ cells
Starts during early embryonic life and completed in adulthood
Spermatogenesis
Formation of male gamete
Miosis
Primordial gem cell- diploid
Diploid, 2n have two copies (homologs) of each chromosome, one from mother and one from father
Prior to fertilization the number of chromosomes needs to be reduced because haploid gametes must be produced
Mitosis
Cell division, diploid to diploid cell because replication occurs before division, still have 2 chromosomes and 2 chromatids- identical cells produced
Miosis cell division
Diploid cell to haploid cell
Number of chromosomes reduced- resulting cells only have 1 copy of one homolog each- DNA synthesis after mitosis and then division where the two chromosomes are separated results in haploid cells.
Later becomes 1 chromosome per chromatid
1st meiotic division
Separates homologous chromosomes- each daughter cell has only one copy of each homolog
2nd meiotic division
Separates the two sister chromatids from each other
1St meiotic division
Synthesis phase and prophase
Synthesis phase
Results in two chromatids chromosomes
Prophase
Takes a long time
Subdivided into 5 stages- leptotene, zygotene, pachytene, diplotene, diakinesis
Leptotene
Chromosomes appear as
Thin as threads
Zygotene
Condensation of the thinly stretched out chromatin
Homologous chromosomes pair- synapses directed by synaptonemal complex
Pachytene
Condensation proceeds
Formation of tetrads
Crossing over- genetic recombination
Diplotene
Synaptonemal complex breaks down
Homologs start to separate
Diakinesis
Centromers move away from each other
Nuclear membrane breaks down
Metaphase I
Homologs align equidistant to each (metaphase plate)
Anaphase I
Homologs are separated from each other
Telophase I
Two daughter cells formed
Each daughter cell contains one copy of the homologous chromosome
2nd meiotic division
Separates the two sister chromatids from each other
1n, 1c
Meiosis
Two divisions
Primordial germ cells
Stem cell of oogenesis
Migrate from yolk sac to the genital ridge (chemotaxis)
PGC- oogonia
Oogonia
Interconnected by cytoplasmic bridges
Lose motility
Multiply by mitosis
Oogenesis process
Oogonium (in gonadal ridge ) my multiplication by mitosis to an oogonia man which is diploid as well 2n, 2c
Then initiation of meiosis to primary oocyte which is the primordial follicle (2n 4c)
Primary oocytes
Initiation by meiosis (independent of stimulus)
Intercellular bridges disappear
During embryonic life (in ,cattle ~140 days gestation)
1st meiotic arrest(block) of meiosis
Primary oocyte enters meiosis but does not finish the first meiotic division immediately
Meiosis is arrested iN the late diplotene stage (dictyate)
Arrest occurs prenatally and is kept through the adulthood until activation of th oocyte in response to ovulatory gonadotropic stimulus (sexual maturity)
Oocyte grows
Zone pellucida formed
Follicular development proceeds
Anisogamy
Difference in size of male and female gametes
Oogenesis stages
1st meiotic arrest- oocyte growth and follicular development
Does not divide and polar body and 2ndary oocyte not produced until out of arrest and go through first meiotic division
Resumption of meiosis and 2nd meiotic arrest (block)
Resumption of meiosis occurs in response to the ovulatory gonadotropic stimulus
First meiotic division ends with extrusion of the first polar body
2nd meiotic division initiated- also arrested at metaphase 2
Second arrest terminated upon fertilization and ends with extrusion of the 2nd polar body
Oogenesis
Secondary oocyte ovulation and fertilization
LH primary oocyte
In secondary have zone pellucida and end up with two polar bodies one from each oocyte (primary and secondary) and ends with ovum
Meiotic events
Before birth oogonium
Then primary oocyte via meiosis initiation and then arrest while develop and grow
Before puberty ovary inactive
After puberty primary oocyte still arrested in prophase (2n 4c)
Then LH surge and meiosis 1 resumed
Once meiosis 1 completed- first polar body and secondary oocyte arrested in metaphase 2 (1n, 2c)
Meiosis 2 completed only if sperm penetration occurs- ovum and second polar body
Oogenesis new idea
Females like males have the capacity to renew their germ cell pool during adult life
Old dogma- cease production of oocytes shortly after birth
Folliculogenesis
Interwoven with oogenesis
Formation of an antral follicle starting from a primordial follicle
Follicle- oocyte surrounded by somatic cells
Different stages of development- primordial, primary, secondary, and tertiary follicle
Primary and secondary follicles develop independent of gonadotropins
Development beyond secondary follicle relies heavily on gonadotropins
Hormonal regulation
GnRH
FSH and LH
FSH- stimulates follicular growth
LH- maturation of oocyte, triggers ovulation
Anterior pituitary
LH and FSH
FSH receptors
Exclusively located on granulosa cells
LH receptors
Early stages- only on thecal cells
Later stages- also on granulosa cells
Hormonal regulation
Two cell two gonadotropin hypothesis
LH on theca cell- androgens from cholesterol
Androgens diffuse to granulosa cells
Granulosa cells androgens via aromatase converts estrogen
Negative feedback
Estrogen from ovary has negative feedback on ant. Pituitary and hypothalamus
+ feedback on hypothalamus as well
Ovulation
Estrogen feedback
Negative feedback on GnRH, FSH, LH at low levels
Positive once reaches threshold- burst of GnRH release, surge of LH, follicular maturation and ovulation
Ovulation LH surge
Surge of LH necessary for ovulation
LH pulse is short lived in most species, except mares
LH surge post estrus, on last day 0-21?
Follicular maturation
LH surge
Resumption of meiosis- meiotic inhibitors removed at LH surge
Meiosis resumes
1st meiotic division occurs- 1st polar body extruded
Meiotic arrest occurs (metaphase II) and remains until fertilization or atresia
Preovulatory luteinization
Following LH surge, theca cells begin to produce low levels of progesterone
Stimulation of collagenase
Degradation of connective tissue and follicle weakens
Ovulation
Results from weakening of follicular wall
Not increased follicular pressure/bursting
Corpus luteum formation
Collapsed follicle fills with blood- corpus hemorrhagicum
Rapid remodeling of follicle- theca cells- small literal cells
Granulosa cells- large luteal cells
Progesterone production slowly increases over first few days
Corpus luteum roles
Secretion of progesterone
Progesterone required for maintenance of pregnancy
Corpus luteum-luteolysis
Non pregnant females must regress their corpus luteum in order to initiate another cycle
Prostaglandin F2a produced by the endometrium of the uterus, causes the CL to regress in large animals
Destruction of the corpus luteum results in formation of the nonfunctional corpus albicans
CA does not produce progesterone
Luteolysis
PGF2a
Ruminants- unilateral pathway (counter current diffusion mechanism)
Mares- systemic uterine ovarian pathway
Ewe=local exchange via ovarian artery
Cow
Countercurrent diffusion mechanism for PGF2a
