Folliculogenesis

Question 1

Primordial Germ Cell (PGC)

Answer 1

precursor “stem” cells that will become either egg or sperm

Question 2

Oogonia

Answer 2

precursors to eggs, they are diploid and multiply by mitosis

Question 3

Primary oocytes

Answer 3

eggs that have entered meiosis and stopped at meiosis I

Question 4

Secondary oocytes

Answer 4

eggs that have completed meiosis I, entered into meiosis II and stopped

Question 5

Oogenesis

Answer 5

the process of egg development covering the stages from an immature oogonium to a mature ovulated egg ready for fertilization

Question 6

Follicle

Answer 6

oocyte-containing structure containing several cell types i.e. granulosa and theca cells

Question 7

Antrum

Answer 7

fluid-filled space in a follicle

Question 8

Preantral follicle

Answer 8

follicle without an antrum consisting of various stages depending on number of layers of granulosa cells.

Question 9

Antral follicle (AF)

Answer 9

follicle with an antrum filled with follicular fluid

Question 10

Folliculogenesis

Answer 10

the process of follicle development covering the stages of growth from a resting primordial follicle to antral follicles and selection of the dominant follicle destined for ovulation

Question 11

Sex steroids

Answer 11

large group of molecules derived from common sterol precursor: cholesterol. There are 4 main families of steroids – the progestogens; androgens; oestrogens (American spelling estrogen) and corticosteroids. Only the first three are defined as sex steroids. Within each family there are several members.

Question 12

Germ cells enter the gonad

Answer 12

•Cells that will become eggs or sperm originate from primordial germ cells (PGC)
• PGCs first identifiable in the yolk sac of the developing foetus at 3 weeks after conception
– Undergo many cycles of mitosis
– They migrate to the genital ridge in the foetus
– The genital ridge becomes the gonad
– Further differentiation of the PGC into male/female gametes depend on the sexual differentiation/development of the gonad i.e. into ovary or testis

Question 13

If PGCs enter the ovary they become oocytes

Answer 13

diagram
• Germ cells become oogonia when in the ovary
• Oogonia are egg-precursors, diploid and multiply by mitosis
• Once mitosis stops and they enter into meiosis, known as primary oocytes

Question 14

Germ Cells to Eggs

Answer 14

• All the 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
• The primary oocytes remains in the first phase of meiosis until it is ovulated (or dies)….maybe for 52 years!

Question 15

Primary oocytes are packed into the outer layer of the ovary: the cortex

Answer 15

diagram

Question 16

Primary Oocytes – Primordial Follicle

Answer 16

• Oocyte arguably the most important cell in the body and will in the vulnerable 1st meiotic phase for many years
• Therefore each one becomes surrounded by protective layers and cells
• In the foetal ovary, the surrounding cells condense around the oocyte and differentiate into the granulosa cells (GC)
• The granulosa cells then secrete an acellular layer called the basal lamina (BL)
• The whole structure is called the primordial follicle

Question 17

If PGCs enter the ovary they become oocytes

Answer 17

diagram

• Germ cells become oogonia when in the ovary
• Oogonia are egg-precursors, diploid and multiply by mitosis
• Once mitosis stops and they enter into mieosis, known as primary oocytes
• Surrounded by layer of GC – now known as primordial follicle
• Establishment of the true “ovarian reserve”

Question 18

Duplication of chromatids

Answer 18

diagram
Duplication of chromatids
• Chromosomes replicate during S-phase of cell cycle
• Remain attached at the centromere
• Each copy known as a chromatid → the 2 copies are identical to each other → “sister” chromatids
• Exact copy of original chromosomes = Duplication of chromatids

Question 19

Overview of Mitosis

Answer 19

Mitosis broadly divided into 4 stages:
Prophase; Metaphase, Anaphase & Telophase

diagram 1 + 2
Oocyte is Arrested in meiosis I

Question 20

Folliculogenesis

and Oogenesis

Answer 20

diagrams

Question 21

Folliculogenesis: the growth of the follicle

Answer 21

• Defined as the growth and development of follicles from the
earliest “resting” stages as laid down in the foetus, through to
ovulation
• Most of the follicles in the ovary are not growing – after puberty a
cohort of follicles initiate growth each day
• As the follicles start to grow, the granulosa cells (GC) multiply and
the oocyte secretes another protective acellular layer called zona
pellucida (ZP)
• Once growth of the
follicles has started a
second layer of cells
then differentiate
around the basal
lamina (BL): the
theca, which is
vascularised

diagram

Question 22

Follicle Growth

Answer 22

factors controlling initiation of growth and the early stages are
largely unknown, but granulosa cells multiply & oocyte enlarges
(though still in meiotic arrest)
• FSH drives most of folliculogenesis but early growth is independent of
FSH i.e. 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
cells divisions increase, but gaps begin to form in the granulosa cell
layers
• These gaps consist of fluid-filled spaces which form an antrum & 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

Question 23

Follicle Classification

Answer 23

diagrams !

Question 24

Antral Follicle explained

Answer 24

Characterised by a cavity or “antrum”
•Contains fluid formed as exudate of plasma containing
secretory products of oocyte & GC
•Known as “follicular fluid”
•As follicular fluid volume and antrum expands, oocyte is
displace to one side

diagrams

Question 25

AF visible on ultrasound

Answer 25

ultrasounds
Dominant healthy follicle with small AFs in top
left-hand corner

Atretic follicle with thin walls and a few
SAFs in right hand corner

Pre-antral follicles are NOT visible on u/s but are
present all the time until menopause

Question 26

1 follicle initiation

Answer 26

A cohort of early follicles
leaving the resting pool and
grow continuously

Question 27

2 follicle recruitment

Answer 27

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

Question 28

3 one follicle

Answer 28

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

Question 29

Folliculogenesis (stage of follicle growth)

Answer 29

diagrams

Question 30

The Antral

Ovarian Follicle

Answer 30

diagrams

Question 31

Dominant Follicle

Answer 31

picture

Question 32

Theca Vasculature

Answer 32

picture

Question 33

The ovarian follicle produces steroids

Answer 33

diagrams

Question 34

Control of folliculogenesis

Answer 34

diagram

Question 35

Many follicles to one

Answer 35

Thousands of primordial follicles…most die through atresia, a few make it into
the menstrual cycle & even less ovulate

diagram