Folliculogenesis Flashcards
Primordial Germ Cell (PGC)
precursor “stem” cells that will become either egg or sperm
Oogonia
precursors to eggs, they are diploid and multiply by mitosis
Primary oocytes
eggs that have entered meiosis and stopped at meiosis I
Secondary oocytes
eggs that have completed meiosis I, entered into meiosis II and stopped
Oogenesis
the process of egg development covering the stages from an immature oogonium to a mature ovulated egg ready for fertilization
Follicle
oocyte-containing structure containing several cell types i.e. granulosa and theca cells
Antrum
fluid-filled space in a follicle
Preantral follicle
follicle without an antrum consisting of various stages depending on number of layers of granulosa cells.
Antral follicle (AF)
follicle with an antrum filled with follicular fluid
Folliculogenesis
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
Sex steroids
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.
Germ cells enter the gonad
•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
If PGCs enter the ovary they become oocytes
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
Germ Cells to Eggs
- 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!
Primary oocytes are packed into the outer layer of the ovary: the cortex
diagram
Primary Oocytes – Primordial Follicle
- 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
If PGCs enter the ovary they become oocytes
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”
Duplication of chromatids
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
Overview of Mitosis
Mitosis broadly divided into 4 stages:
Prophase; Metaphase, Anaphase & Telophase
diagram 1 + 2
Oocyte is Arrested in meiosis I
Folliculogenesis
and Oogenesis
diagrams
Folliculogenesis: the growth of the follicle
• 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
Follicle Growth
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
Follicle Classification
diagrams !
Antral Follicle explained
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
