Week 9 - Molecular mechanisms of gametogenesis and fertilisation Flashcards
Define gametogenesis, its biological role
development of haploid sex cell
– mature oocyte and sperm cell in a diploid organism by meiosis
2 types:
Oogenesis
Spermatogenesis
Describe origin of gametes:
- early migration of PGCs,
- epigenetic changes in PGC,
- At about 4 weeks of pregnancy
- Sex cell precursors - PGC (primordial germ cells) arise in the yolk sac
- Alkaline phosphatase – marker of PGC
- – 6. week of pregnancy PGC migrate to genital ridge – bisexual gonads…
- Early migration of PGCs is dependent on the expression of interferon-induced transmembrane proteins 1 and 3 (IFITM1 and IFITM3) in the
surrounding mesoderm
- formation of spermatogonia and oogonia
Explain briefly
Formation of all gonia from promoter germ cells after travel from yolk sac to bisexual gonad, formation depends on presence of Y-chromosome
If Y-chromosome is present → medulla part will develop into embryonic testis
→ PGC will develop into spermatogonia
If Y-chromosome absent → Medulla will NOT develop further but cortical part will → into embryonic ovaries
→ PGC will develop in Oogonia
What determines sex determination and differentation
(short answer)
Dependent on genetic material
X and Y chromosome
OBS not only genes involved in sex determination, many genes in autosomes determine sex
What is SRY
The Male Determining Pathway
SRY - sex-determining region on the Y chromosome
-Main role of SRY consisting in upregulating the expression of SOX9 during a very narrow critical time
window
-In human SRY is expressed in both Sertoli cells and germ cells at fetal and adult stages
What is SOX9?
The Male Determining Pathway:
SOX9 - A Target Of SRY
- SOX9, an autosomal member of the HMG-box protein superfamily
- The master regulator of Sertoli cell differentiation
most likely importante
- SOX9 is expressed at low levels in the bipotential gonads of both sexes under SF1 regulation, but persists only in testicular Sertoli cells after SRY expression has peaked.
Yapping abt general considerations
- Widespread chromatin modifications:
PGCs undergo genome-wide demethylation – reaching a “ground state” in terms of
epigenetic marks
Second wave of DNA demethylation occurs and erases the methylation marks of imprinted genes in the PGC genomes, when they reach the gonads
Remethylation of germ cell genome occurs later during fetal life
What do you see and understand
X chromosome re-activation
- In female PGC (2n)
- 2 X chromosomes active (inactivated x is reactivated)
- Reduction of Xist RNA levels
Do male and female germ cell share certain epigenetic marks?
If so, what are they important for?
Male and female germ cells share certain epigenetic marks, some of which may be very important for gametogenesis, e.g.:
Enzymes, important for synapsis:
▪ Histone methyltransferase, PRDM9 transfers a methyl group to H3K4, an
epigenetic mark generally thought to open up chromatin for transcription
▪ Euchromatic histone-lysine N-methyltransferase 2 protein (EHLM/Ga9), which is important for H3K9me or H3K9me2 repressive marks
Histone acetylation, is also important for proper chromosome segregation withindeveloping gametes
Timing of gametogenesis
males: starts at puberty and continues throughout adult life. Aka spermatogenesis.
females: starts during embryonic life and ends for each particular oocyte after fertilization. Aka oogenesis.
Oogenesis in general
What is it?
How is it achieved?
Additional:
It is sex cell devolopment in females
Achieved mainly by meiosis but also my mitosis.
The 3 periods of oogenesis
- Proliferation period
- Growth period
- Maturation period
When does most of oogenesis occur
During Prenatal period (before the birth)
Proliferation period (Oocytes)
- When does it occur?
- The 2 main steps of this period?
- approx. 3rd month of embryonic development
- Initially: from PGC → oogonia (2n)
- Primary oogonia divides → secondary oogonia (approx. 7 million)
Growth period (Oogenesis)
When does it take place?
What happens?
- Approx. 4. – 6. Month of embryonic
development - Secondary oogonia → primary oocytes
- By the 5th month approx. 7 million
(degeneration begins)
Maturation period (oogenesis)
When does it take place?
What happens?
- Approx. 7. – 8. month of embryonic development – with meiosis I (initiation of meiosis in the fetal ovary is heralded
by the increase in retinoic acid levels)
Leptotene, zygotene, pachytene and diplotene stops
Explan prophase I
Obs ta reda på hur detaljerat man ska kunna det
Prophase I: This is the first phase of meiosis, and it has several steps:
Leptotene: The DNA in the cell starts to organize into thin threads.
Zygotene: The chromosomes (DNA packages) start pairing up like puzzle pieces.
Pachytene: The paired chromosomes exchange bits of DNA, like swapping recipes.
Diplotene: The chromosomes start to pull apart a bit, but they stay connected at certain points.
Stops in Diplotene stage
Oocytes I grow to a large size
What is this picture about?
Primary oocytes in embryonic ovaries are surrounded by follicular cells and form an embryonic follicle
What happens at postnatal period (after birth)
Oogenesis
- Primary oocytes (700 000) remain in diplotene till the puberty
- The rest of primary oocytes degenerate
The situation in postnatal period – at the puberty
Oogenesis
- 400 000 primary oocytes
- Embryonic (primordial) follicles →
primary follicles - Maturation of primary oocytes
Primary follicle
Oocyte I + follicular cells
- Develop from embryonic follicles
- Structure:
- Oocyte I:
Plastic membrane, nucleus, cytoplasm - Zona pellucida
- One layer of follicular cells
At the puberty
How often does maturation start?
How many are mature?
What devolops?
(Why only few primordial follicles commence folliculogenesis each month?)
Every month 5-15 oocytes I start maturation
- Only one mature
- Primary follicles → secondary (developing) follicles
Why only few primordial follicles commence folliculogenesis each month?
- Not clear
- One possibility – some follicles become progressively more sensitive to the stimulating effects of FSH (Follicle-Stimulating Hormone) as they advance in development or
- selection process is regulated by a complex system of feedback between the pituitary and ovarian hormones and growth factors
Secondary (developing) follicle)
Develops from the primary follicle as follicular cells become large
Structure:
- Oocyte I
- Zona pellucida
- Several layers of follicular cells
- Anthrum enlarges
Mature follicle (Graafian follicle)
Develops from the
secondary follicle
Structure:
- Oocyte I
- Zona pellucida
- Differentiation of follicular
cells:
»Corona radiata
»Cumulus oophorus
- Anthrum is enlarged
more
Maturation period
Understand and describe this image ….
Cytokinesis
- By the help of actin – myosis contractile ring
Cytokinesis is unequal, but complete:
- One large cell – oocyte II (n)
- One small cell – polar body (n)
Regulation of meiosis in oogenesis
1. What controls meiosis?
A molecule called MPF (Maturation Promoting Factor) is the “boss” that tells the egg cell when to start dividing.
MPF is made of two parts: Cdk2 (a helper enzyme) and cyclin B (a protein that switches things on).
2. What does MPF do?
When MPF goes up (↑), it makes things happen:
Chromosomes condense: The DNA in the cell gets packed up tightly.
Nuclear envelope breaks: The “bag” around the nucleus (DNA’s home) dissolves.
Spindle forms: Tiny “ropes” are set up to move the chromosomes around.
This starts meiosis, which is the process of splitting the DNA to make an egg.
3. What happens during the next steps?
When the egg is moving through meiosis, MPF levels go up and down at different times:
MPF goes down (↓): This allows the cell to move to the next stage (from metaphase to anaphase, which are steps in cell division).
4. What’s special about metaphase II?
The egg cell stops in metaphase II (a late stage of meiosis) and waits until fertilization happens.
To keep the egg “on pause,” MPF stays active, and the cell prevents cyclin B (part of MPF) from being destroyed.
TL;DR
MPF is like a switch that starts and controls meiosis.
It helps the egg cell prepare by organizing the DNA and breaking down barriers.
Later, it pauses the egg cell in metaphase II, waiting for sperm to fertilize it.
Regulation of meiosis in oogenesis cont.
This image explains how a protein called Mos kinase helps in controlling the process of egg cell division (oogenesis) by preventing the cell from moving past a specific stage called metaphase II arrest.
What Mos does:
Mos stops a cellular “machine” called the anaphase-promoting complex (APC) from working, which blocks the cell from progressing to the next stage (anaphase).
When Mos is important:
It is made after the first division
(meiosis I) and helps to:
Activate MPF (Maturation Promoting Factor) for the second division (meiosis II).
Maintain MPF activity to keep the cell arrested in metaphase II.
How Mos works:
It uses a chain reaction involving other proteins (MEK, ERK, and Rsk) to maintain this arrest.
In short: Mos acts as a “brake” to pause the cell division until it’s the right time for fertilization.
↑MPF
induces chromosome condensation, nuclear envelope breakdown, and formation of the spindle - so, stimulate beginning of meiosis
Meiosis of oocytes is controlled by
MPF (maturation promoting factor) - complex of Cdk2 and cyclin B
Anaphase promoting complex stimulation
transition from metaphase to anaphase, MPF↓
Unique features that are responsible for metaphase II arrest
maintain MPF activity - avoid proteolysis (degradation) of cyclin B
The Mos protein kinase maintains metaphase II arrest by
inhibiting the anaphase-promoting complex.
The action of Mos is mediated by
MEK, ERK, and Rsk protein kinases.
Mos is specifically synthesized in
oocytes around the time of completion of meiosis I and is then required both:
- increase in MPF activity during meiosis II and
- for the maintenance of MPF activity during metaphase II arrest.
After fertilization
Oocyte II continues meiosis II
One large cell - mature
oocyte II (egg) and small cell
- polar body (n)
In absence of fertilization
In absence of fertilization
Oocyte II stays in meiosis II metaphase II, Mature follicle shrinks, corpus luteus is formed - secreting hormones, promotes development of the next oocyte, around 450 - 500 mature oocyte II develop during one female’s life
Follicular cells
granulose cells and theca cells
granulosa cells
Produce sex hormones - estrogen and progesterone (depending
on the phase in menstrual cycle), React to different hormones (e.g., LH and FSH), Produce different growth factors
theca cells
Internal - Theca interna - produce androgenes and progesterone
External - Theca externa - connective tissue, important in the ovulation
Spermatogenesis
Starts at puberty and continues through life time, One cycle of spermatogenesis lasts 64 - 72 days, Occurs in testes (semineferous tubules)
Why don’t spermatogonia enter meiosis?
The mesonephros from the indifferent gonad, the lung and adrenal gland, synthesize retinoic acid that acts as a meiosis inducer.
Mouse Sertoli cells express two factors that prevent meiosis onset:
FGF9 and CYP26B1, an enzyme that catabolizes retinoic acid.
NANOS2
another meiosis-preventing protein
In human fetal testis, CYP26B1 does
not seem to be expressed, and the mechanism underlying the inhibition of germ cell entry into meiosis needs to be elucidated
Postnatal period (after the birth) (spermatogenesis)
At the time of birth sex cells are located in the sex cords
Sertoli cells (functions)
Surround developing sperm cells - tight junctions; hemato-testicular barrier, Secrete proteins and fluids, Trophic function, Secrete growth and other factors, needed for development of the sex cells, Synchronize the events of spermatogenesis
Spermatogenesis - periods
- proliferation (spermatogonia)
- growth (primary spermatocytes)
- maturation (secondary spermatocytes and spermatids)
- spermiogenesis (spermatozoa = sperm cells)
Proliferation period (spermatogenesis)
Begins in embryonic life and continues through life time, During embryonic life Sertoli cells produce inhibitor of meiosis - PG D2, PGC → primary (A) spermatogonia (2n), Primary spermatogonia → secondary (B) spermatogonia (2n)
Growth period (spermatogenesis)
Begins in puberty, Secondary spermatogonia grow → primary spermatocytes (2n), Growth period is weak comparing to oogenesis
Maturation period (spermatogenesis)
Two meiosis, Lasts for 22-24 days, Cytoplasmic bridges - spermatogonial syncytium: Synchronise spermatogenesis in one seminiferous tubule, AKAP82 - protein, used for tail formation
Role of syncytium
Akap82 protein - important in tail formation, Akap82 gene is located on X chromosome
Cytokinesis
By help of actin - myosin contractile ring (cleavage furrow), Cytokinesis is equal, but incomplete: Two cells of identical size, Male’s sex cell precursors remain joined by cytoplasmic bridges, which resolve only in the end of spermatogenesis
Spermiogenesis steps
Formation of an acrosome, Nuclear morphogenesis, Formation of tail structures, Rearrangement of organelles, Shedding most of the cytoplasm
Formation of an acrosome
Formed from Golgi complex, Specialised lysosome, contains digestive enzymes (hydrolases) e.g., hyaluronidase, acrosin, neuraminidase, Enzymes allow sperm to penetrate oocyte
Nuclear morphogenesis
Changes in shape (from round to oval), Becomes more dense, Chromatin condensation, Sperm specific histones are produced
Rearrangement of organelles - Formation of tail structures
Elongation of microtubules, Distal centriole elongates, Mitochondria fuse and form spiral around of the tail
Basic structure and function of human sperm cell
Sperm functions:
-To deliver its set of genes to the egg
-To activate the egg
Abnormal sperm and sperm production syndromes
Kartagener syndrome
Spermatogenesis and oogenesis COMPARISON
Explain difference
Write down summary of spermatogenesis and oogenesis
Acrosomal reaction
Acrosome undergoes exocytosis - sperm releases digestive enzymes into the Zona pellucida
Sperm-Egg Recognition & Binding
The sperm migrates through the coat of
the follicle cells (Corona radiata) and binds to a receptor molecule in the Zona pellucida of the egg (secondary oocyte)
Sperm-Egg Recognition & Binding
Receptors of Zona pellucida
Reception of sperm and egg membrane receptors
With the help of acrosomal reaction the sperm reaches the egg, and a membrane protein (ADAM) of the sperm binds to a receptor (ZP2) on the egg membrane
Contact and fusion of sperm and egg membranes
The plasma membranes fuse making it possible for sperm cell to enter the egg, A key signal from the binding - an increase in the level of Ca2+ in the egg cytoplasm → signals the completionof meiosis in the oocyte, is triggered by a Ca2+- dependent activation of the anaphase-promoting complex, Ca2+-induced exocytosis of secretory vesicles (see next slide)
Cortical reaction
Enzymes released from corticular granules harden Zona pellucida,
Block the polyspermy (allow monospermy)
Summary fertilisation
After sperm cell enters the egg …
Basal body of the sperm’s flagellum divides and forms the centrioles of zygote, Male and female pronuclei form synkaryon
Cleveage
Series of mitotic divisions and cytokineses that transform the zygote
Significance of fertilization
Changes in the egg’s cytoplasm are induced, The diploid chromosome number is re-established, Mixing of paternal and maternal chromosomes takes place
From fertilization
till cleveage
