4. FOLLICULOGENESIS Flashcards
What are the cells that will become eggs or sperm?
- The cells that will develop into eggs (oocyte) or sperm (spermatozoa) are the PRIMORDIAL GERM CELLS
- The PG are first identifiable in the yolk sac at around 3 weeks
- The PGCs undergo may cycles of mitosis, and migrate to the genital ridge which is the precursor for the gonad
- Whether the primordial germ cells become eggs or sperm ,depends on sexual maturation
How do primary oocytes from?
- Primordial germ cells migrate to the ovary
- The germ cells in the ovary are known as oogonia, they are egg precursors & multiply by mitosis
- Once mitosis stops, the oogonia enter the first stage of Meiosis I (Prophase I). The oogonia are arrested in Meiosis I to prevent any further division. At this point, they are known as PRIMARY FOLLICLES
What are primary oocytes packaged into?
- Primary oocytes are packaged into primordial follicles in the outer layer of the ovary known as the cortex
- Females are born with primordial follicles, after puberty these follicles start to grow
How are primary oocytes protected by layers in the primordial follicle?
- Each oocyte is surrounded by cells & layers
- In the foetal ovary, the surrounding cells condense & differentiate into GRANULOSA CELLS
- The granulosa cells then secrete an acellular layer known as the BASAL LAMINA
- The whole structure is known as a primordial follicle
Describe the division of an oogonium to produce daughter cells in meiosis?
- The oogonium will undergo meiosis, but there’s unequal distribution of cytoplasm during cytokinesis so the 4 daughter cells aren’t identical
- Polar bodies are formed along with the ovum/primary oocyte
- The majority of the cytoplasm is in the primary oocyte, as it will be a source of nutrients
- The primary oocyte will be arrested in Meiosis I, until a surge in LH triggers Meiosis I resumption
Define folliculogenesis
- Folliculogenesis refers to the growth & development of follicles from the early resting phase to ovulation
- The early resting phase is primordial follicles. the majority of the follicles are arrested primordial follicles, they don’t begin developing until puberty
- After puberty, a cohort or group of follicles initiate growth each day
Describe the formation of the zona pellucida during follicular growth?
- As the follicle starts to grow, the granulosa cells multiply & become cuboidal instead of flattened
- The primary oocyte within the primordial follicle secretes another acellular layer called the ZONA PELLUCIDA.
- The zona pellucida separates the granulosa layer from the oocyte
Describe the formation of the vascularised theca during follicular growth
- As the granulosa cells continue to proliferate, the theca layer forms
- A layer of cells differentiate around the basal lamina of the follicle forming theca layer
- The theca then becomes vascularised
What is the key driver in folliculogenesis?
- FSH is the key driver of growth & development of follicles
- However, early growth of the follicles is FSH independent. FSH is needed when the antrum starts to form
- FSH is needed for the development beyond the primordial follicle
Describe the formation of an antrum during follicular growth
- As the granulosa cells multiply, gaps begin to from between the granulosa layer
- The gaps within the granulosa cell consist of follicular fluid filled spaces, collectively forming an ANTRUM
What are the two main phases of follicular grwoth?
- PRE-ANTRAL FOLLICULAR GROWTH (absence of antrum)
2. ANTRAL FOLLICULAR GROWTH (presence of antrum)
What is an antral follicle?
- Follicles with an antrum or cavity are known as antral follicles
- The follicular fluid is the exudate & plasma containing secretory products from the oocyte & granulosa cells
- As the follicular fluid volume & antrum expands, the oocyte is displaced to one side
- The cumulus oophorus also forms which is a mound of granulosa cells clustering around the oocyte
What are the 4 stages of follicular development?
- FOLLICLE INITIATION
- BASAL GROWTH
- FOLLICLE RECRUITMENT
- FOLLICLE SELECTION
What happens in follicle initiation?
- In follicle initiation, a cohort of primordial follicles leave the resting phase and grow a little every day after puberty (0.02mm)
What happens during basal growth of follicular development?
- Follicles grow to become pre-antral (FSH independent) & early antral (FSH dependent, as FSH is needed for antral formation)
- (0.2mm)
What happens during follicle recruitment?
- The follicles won’t be able to continue development if they don’t reach the required size needed to respond to changes in FSH during menstrual cycle
- At this stage, the follicles are antral follicles, they must have an antrum and be the right size (2mm)
What happens during follicle selection?
- The human pelvis is only designed to carry a single foetus so only one follicle will be selected from the recruited group of cells known as the dominant follicle
- The other developing follicles will undergo atresia & degenerate
List the order of follicle types from primordial to ovulatory
Primordial follicle -> Primary/Pre-antral -> Early antral -> Antral -> Ovulatory
What are the theca layer of the follicle & what’s the importance of vascularisation?
- Theca interna
- Theca externa
- Theca cells are vascularised meaning they have a rich blood supply. This will allow the follicle to receive anything & anything produced by the follicle can enter the circulation
What is the 2 cell, 2 gonadotrophin theory?
- THECA CELLS - produce androgens & progesterone. Theca cells have LH receptors, the LH binds to this allowing theca to produce androgens
- GRANULOSA CELLS - produce oestrogen. Granulosa cells have FSH receptors.
- Granulosa cells convert androgens produced by theca cells such as androstenedione into oestrogen using aromataose under the influence of FSH
What receptors are present on theca cells?
- Theca cells have LH receptors
What receptors are present on granulosa cells?
- Granulosa cells have FSH receptors, FSH drives oestrogen production in follicular phase
Give one exception where granulosa cells have LH receptors?
- Granulosa cells don’t have LH receptors
- But during dominant follicle selection, LH receptors are accquired in the mid-follicular phase onwards, to counteract the drop in LH
Describe the order of the formation of the different layers & cells of the follicle
- Granulosa cells form (primordial follicle) surrounding primary oocyte
- Basal lamina forms around primordial follicle
- Zona pellucida separates granulosa cells from primary oocyte, surrounding primary oocyte
- Theca cell layer forms around basal lamina
- Cumulus oophorus clusters around oocyte, is a mound of granulsoa cells
- Antrum forms due to gaps in granulosa to produce antral follicle
