Lecture 17: Oogenesis

Question 1

Folliculo-genesis

Answer 1

formation and development of ovarian follicles, which are endocrine structures that participate in the coordination of reproductive cycles and support the development of the female germ cell- the oocyte

Question 2

Oogenesis

Answer 2

oogenesis is a protracted process that encompasses the differentiation and release of a mature oocyte for fertilization and successful propagation of the species

oogenesis begins during embryonic development of the female and continues throughout her reproductive lifetime

Question 3

Oogenesis- Mitotic Phase

Answer 3

characterized by mitosis of primordial germ cells and oogonia

1. mitotic active PCGs colonize the bipotential gonads of the developing fetus
2. bipotential gonads differentiate into ovaries
3. PGCs differentiate into oogonia
4. oogonia continue to divide by mitosis unit formation of primordial follicles
5. mitotic divisions determine the size of the ovarian reserve of the female

Question 4

Oogenesis- Meiotic phase with nuclear arrest

Answer 4

characterized by the onset of meiosis and formation of primary oocytes
1. formation of primordial follicles: the first layer of granulosa cells creates a new biochemical environment in the follicle and oogonia become primary oocytes

2. primary oocytes enter meiosis I
3. meiosis I is arrested at prophase I (diplotene stage)
4. formation of germinal vesicle (GV= large nucleus with arrested DNA highly packed )
   a. cell division remains arrested for a long period of time: meiosis is resumed only shortly before ovulation
   b. the purpose of the nuclear arrest is to inactivate DNA and protect it from insults during its lifetime, which could compromise developmental capacity

Question 5

Oogenesis- Cytoplasmic Development

Answer 5

characterized by substantial growth of the cytoplasm and formation of special cellular structures
1. activation of primordial follicles

2. oocytes enclosed in primary and secondary follicles undergo substantial growth of cytoplasm volume
   a. promoted by granulosa cells
   b. accumulation of molecules that will be critical for early embryogenesis
3. formation of zona pellucida
   a. synthesized by the oocyte
   b. covers and protects the entire oocyte
   c. ZP proteins- important for fertilization
4. formation of gap junctions between the oocyte and granulosa cells in close proximity
   a. cytoplasmic communication that crosses the zona pellucida
   b. critical for communication between the two cell types

Question 6

oogenesis- Meiotic Resumption

Answer 6

characterized by the resumption of meiosis I after LH surge
1. estradiol secreted by the preovulatory follicle has positive feedback in the surge center of the hypothalamus and induces a surge of GnRH

2. the surge of GnRH reaches the pituitary and induces a surge of LH
3. LH reaches the preovulatory follicle in the ovary(ies) and cause changes
4. Deterioration of gap junction s
   a. reduction of oocyte cAMP –> deactivation of kinases –> activation of MPF (maturation promoting factor)
5. Meiosis I is completed
   a. asymmetric cytoplasmic division
   b. extrusion of 1st polar body
6. onset of meiosis II- arrested at the metaphase II stage

Question 7

Crossing over in meiosis I

Answer 7

segments of one chromosome crossing over to a homologous chromosome, resulting in a random assortment of DNA segments between chromatids and ensuring genetic diversity in the daughter

at the end of meiosis, each oocyte is genetically unique

oocytes are ovulated shortly after crossing over, thus, control of the immune system in the ovary is not as critical as the one observed in the testes