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PHYS - REPRO > 4. FOLLICULOGENESIS > Flashcards

4. FOLLICULOGENESIS Flashcards

1
Q

What are the cells that will become eggs or sperm?

A
  • 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
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2
Q

How do primary oocytes from?

A
  1. Primordial germ cells migrate to the ovary
  2. The germ cells in the ovary are known as oogonia, they are egg precursors & multiply by mitosis
  3. 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
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3
Q

What are primary oocytes packaged into?

A
  • 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
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4
Q

How are primary oocytes protected by layers in the primordial follicle?

A
  1. Each oocyte is surrounded by cells & layers
  2. In the foetal ovary, the surrounding cells condense & differentiate into GRANULOSA CELLS
  3. The granulosa cells then secrete an acellular layer known as the BASAL LAMINA
    - The whole structure is known as a primordial follicle
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5
Q

Describe the division of an oogonium to produce daughter cells in meiosis?

A
  • 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
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6
Q

Define folliculogenesis

A
  • 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
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7
Q

Describe the formation of the zona pellucida during follicular growth?

A
  • 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
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8
Q

Describe the formation of the vascularised theca during follicular growth

A
  • 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
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9
Q

What is the key driver in folliculogenesis?

A
  • 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
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10
Q

Describe the formation of an antrum during follicular growth

A
  • 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
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11
Q

What are the two main phases of follicular grwoth?

A
  1. PRE-ANTRAL FOLLICULAR GROWTH (absence of antrum)

2. ANTRAL FOLLICULAR GROWTH (presence of antrum)

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12
Q

What is an antral follicle?

A
  • 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
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13
Q

What are the 4 stages of follicular development?

A
  1. FOLLICLE INITIATION
  2. BASAL GROWTH
  3. FOLLICLE RECRUITMENT
  4. FOLLICLE SELECTION
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14
Q

What happens in follicle initiation?

A
  • In follicle initiation, a cohort of primordial follicles leave the resting phase and grow a little every day after puberty (0.02mm)
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15
Q

What happens during basal growth of follicular development?

A
  • Follicles grow to become pre-antral (FSH independent) & early antral (FSH dependent, as FSH is needed for antral formation)
  • (0.2mm)
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16
Q

What happens during follicle recruitment?

A
  • 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)
17
Q

What happens during follicle selection?

A
  • 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
18
Q

List the order of follicle types from primordial to ovulatory

A

Primordial follicle -> Primary/Pre-antral -> Early antral -> Antral -> Ovulatory

19
Q

What are the theca layer of the follicle & what’s the importance of vascularisation?

A
  1. Theca interna
  2. 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
20
Q

What is the 2 cell, 2 gonadotrophin theory?

A
  1. THECA CELLS - produce androgens & progesterone. Theca cells have LH receptors, the LH binds to this allowing theca to produce androgens
  2. 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
21
Q

What receptors are present on theca cells?

A
  • Theca cells have LH receptors
22
Q

What receptors are present on granulosa cells?

A
  • Granulosa cells have FSH receptors, FSH drives oestrogen production in follicular phase
23
Q

Give one exception where granulosa cells have LH receptors?

A
  • 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
24
Q

Describe the order of the formation of the different layers & cells of the follicle

A
  1. Granulosa cells form (primordial follicle) surrounding primary oocyte
  2. Basal lamina forms around primordial follicle
  3. Zona pellucida separates granulosa cells from primary oocyte, surrounding primary oocyte
  4. Theca cell layer forms around basal lamina
  5. Cumulus oophorus clusters around oocyte, is a mound of granulsoa cells
  6. Antrum forms due to gaps in granulosa to produce antral follicle

PHYS - REPRO (10 decks)

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