Folliculogenesis (repro) Flashcards
1
Q
Germ cells enter the gonad
A
- cells that will become eggs or sperm originate from primordial germ cells (PGC)
- PGCs first identifiable in yolk sac of developing foetus at 3 weeks after contraception
- undergo many cycles of mitosis
- migrate to genital ridge in foetus
- genital ridge becomes the gonad
- further differentiation of the PGC into male/female gametes depend on the sexual differentiation or development of the gonad ie into ovary or testis
2
Q
Changes of PCGs in ovary
A
if PCGs enter the ovary they become oocytes:
- germ cells become oogonia when in ovary
- oogonia are egg-precursors, diploid and divide by mitosis
- once mitosis stops, they enter into meiosis
- they become primary oocytes
- surrounded by layer of germ cells known as primordial follicle
- establishment of the true ‘ovarian reserve’
3
Q
Germ cells to eggs
A
- all eggs that a woman will ever have are made at this stage
- the mitotic divisions are therefore critical
- once the oogonia enter the 1st stage of meiosis, no more division occurs and they become primary oocytes
- primary oocytes are packed into the outer layer of the ovary (the cortex)
- primary oocytes remain in first phase of meiosis until it is ovulated (or dies)
- could be for 52 years
4
Q
primordial follicle
A
- oocytes are arguably the most important cell in body and are in vulnerable 1st meiotic phase for many years
- therefore, each one becomes surrounded by protective layers and cells
- in foetal ovary, surrounding cells condense around the oocyte and differentiate into granulosa cells (GC)
- granulosa cells then secrete an acellular layer called the basal lamina (BL)
- whole structure is called primordial follicle
5
Q
Duplication of chromatids
A
- chromosomes replicate during S phase of cell cycle
- remain attached at the centromere
- each copy known as a chromatid (2 copies are identical) and are sister chromatids
- exact copy of original chromosomes
6
Q
Overview of mitosis
A
- mitosis broadly divides into 4 stages: (interphase), prophase, metaphase, anaphase, telophase
- DNA replication during interphase forms 2 sister chromatids, which are banded together to form a chromosome
- during mitosis, sister chromatids separate and move to opposite ends of the cell
- During cytokinesis, the parent cell divides, forming two daughter cells
- each daughter cell has 2 copies of each chromosome (homologous pairs)
7
Q
Overview of meiosis
A
- the diploid parent cell contains 2 homologous pairs of chromosomes
- DNA replication during interphase forms 2 copies of each chromosome and the copies remain attached
- during meiosis l, homologous chromosomes separate into 2 haploid cells, each containing one member of each homologous pair
- oocyte is arrested in meiosis l
- during meiosis ll, sister chromatids separate and four haploid daughter cells are formed that will develop into gametes
8
Q
Folliculogenesis
A
- defined as growth and development of follicles from earliest ‘resting’ stages as laid down in the foetus through to ovulation
- most of follicles in ovary are not growing - after puberty a cohort of follicles initiate growth each day
- as follicles start to grow, the granulosa cells (GC) multiply and the oocyte secretes another protective acellular layer called zone pellucida (ZP)
- once growth of follicles has started a second layer of cells then differentiate around the basal lamina (BL), the theca which is vascularised
9
Q
follicle growth
A
- factors controlling initiation of growth and the early stages are largely unknown, but granulosa cells multiply and oocyte enlarges (though still in meiotic arrest)
- FSH drives most of folliculogenesis but early growth is independent of FSH ie driven by local factors
- apparent in FSH-deficient patients or those with mutations of FSHr
- also means that even when FSH is suppressed eg on COCP, the follicles will still continue early growth but then die
- as follicle starts to grow it increases rapidly in diameter and granulosa cell layers
- these gape consist of fluid-filled spaces which form an antrum and are filled with follicular fluid
- 2 main phases of follicle growth labelled by absence or presence of antrum
- follicles with an antrum are known as antral or secondary follicles
10
Q
Classification of follicles
A
- Antral or secondary
- Preantral or primary
- Preovulatory or Graafian
11
Q
Antral follicle
A
- Characterised by a cavity or ‘antrum’
- contains fluid formed as exudate of plasma containing secretory products of oocyte and GC
- known as ‘follicular fluid’
- as follicular fluid volume and antrum expands, oocyte is displaced to one side
- AF is visible on ultrasound
- preantral follicles are not visible on ultrasound but are present all the time until menopause
12
Q
Follicle initiation and recruitment
A
- a cohort of early follicles leaving the resting pool and grow continuously
- this is known as follicle initiation
- they will not continue to grow unless they reach the size at which they respond to. changes in FSH that occur in the menstrual cycle
- this is known as follicle recruitment
- human pelvis designed in general to carry a single foetus therefore from the group that are recruited, only one will subsequently be selected for ovulation
13
Q
Theca cells and Granulosa cells
A
- theca cells always have LHR and never FSHr
- LH drives androgen and progesterone production from theca
- granulosa have FSHr
- FSH drives oestrogen production in follicular phase
- LHR acquired from mid-follicular phase onwards (in DF)
- LH will then drive progesterone and oestrogen production as well
14
Q
Many follicles to one
A
- thousands of primordial follicles
- most die through atresia
- a few make in into the menstrual cycle
- even less actually ovulate
- 99.999% die
15
Q
Summary
A
- PGCs differentiate into oogonia, proliferating by mitosis, and enter into meiosis and arrest
- they then form primordial follicles
- there is a bit of basal growth of primordial follicles (PF), but do not progress
- once puberty commences, PF initiate growth as a continuum until all follicles are depleted and menopause has started
- eggs remain arrested in meiosis l until ovulation
- they then enter meiosis ll and arrest again
