Processes in fertilization
Oocyte
(or
ovum
/
egg
)
activation
is a series of processes that occur in the
oocyte
during
fertilization
.
Sperm entry causes calcium release into the oocyte. In mammals, this is caused by the introduction of
phospholipase C
isoform zeta (PLCζ) from the sperm cytoplasm.
[1]
Activation of the ovum includes the following events:
- Cortical reaction
to block against other sperm cells
- Activation of egg metabolism
- Reactivation of meiosis
- DNA synthesis
Sperm trigger of egg activation
[
edit
]
The sperm may trigger egg activation via the interaction between a sperm protein and an egg surface receptor.
Izumo
is the sperm cell signal, that will trigger the egg receptor
Juno
.
[2]
This receptor is activated by the sperm binding and a possible signaling pathway could be the activation of a tyrosine kinase which then activates
phospholipase C
(PLC). The inositol signaling system has been implicated as the pathway involved with egg activation.
IP
3
and
DAG
are produced from the cleavage of
PIP
2
by phospholipase C. However, another hypothesis is that a soluble 'sperm factor' diffuses from the sperm into the egg cytosol upon sperm-oocyte fusion. The results of this interaction could activate a
signal transduction
pathway that uses
second messengers
. A novel PLC isoform, PLCζ (
PLCZ1
), may be the equivalent of the mammalian sperm factor. A 2002 study demonstrated that mammalian sperm contain PLC zeta which can start the signaling cascade.
[3]
Fast and slow block to polyspermy
[
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]
Polyspermy
is the condition when multiple sperm fuse with a single egg. This results in duplications of genetic material. In sea urchins, the block to polyspermy comes from two mechanisms: the
fast block
and the
slow block
. The fast block is an electrical block to polyspermy. The
resting potential
of an egg is -70mV. After contact with sperm, an influx of sodium ions increases the potential up to +20mV. The slow block is through a biochemical mechanism triggered by a wave of calcium increase. The rise of calcium is both necessary and sufficient to trigger the slow block. In the
cortical reaction
,
cortical granules
directly beneath the plasma membrane are released into the space between the plasma membrane and the
vitelline membrane
(the
perivitelline space
). An increase in calcium triggers this release. The contents of the granules contain
proteases
,
mucopolysaccharides
,
hyalin
, and
peroxidases
. The proteases cleave the bridges connecting the plasma membrane and the vitelline membrane and cleave the bindin to release the sperm. The mucopolysaccharides attract water to raise the vitelline membrane. The hyalin forms a layer adjacent to the plasma membrane and the peroxidases cross-link the protein in the vitelline membrane to harden it and make it impenetrable to sperm. Through these molecules the vitelline membrane is transformed into the fertilization membrane or fertilization envelope. In mice, the zona reaction is the equivalent to the cortical reaction in sea urchins. The terminal sugars from ZP3 are cleaved to release the sperm and prevent new binding.
Reactivation of meiosis
[
edit
]
The
meiotic
cycle of the oocyte was suspended in metaphase of the second meiotic division. Once PLCζ is introduced into the oocyte by the sperm cell, it cleaves phospholipid phosphatidylinositol 4,5-bisphosphate (PIP
2
) into diacyl glycerol (DAG) and inositol 1,4,5-trisphosphate (IP
3
). In most cells, this occurs at the cell membrane however, evidence suggests that the PIP
2
required for oocyte activation is potentially stored in intracellular vesicles dispersed throughout the cytoplasm.
[4]
The IP
3
produced then triggers calcium oscillations which reactivate the meiotic cycle. This results in the production and extrusion of the second
polar body
.
[5]
DNA synthesis
[
edit
]
4 hours after fusion of sperm and ovum, DNA synthesis begins.
[5]
Male and female pronuclei move to the centre of the egg and membranes break down. Male
protamines
are replaced with histones and the male DNA is demethylated. Chromosomes then orientate on the metaphase spindle for mitosis. This combination of the two genomes is called
syngamy
.
[5]
The sperm contributes a
pronucleus
and a
centriole
to the egg. Most other components and organelles are rapidly degraded. Mitochondria are rapidly ubiquinated and destroyed.
Oxidative stress theory
is a hypothesis that it is evolutionarily favourable for mitochondria from the father to be destroyed, as it there is a greater possibility that the mitochondrial DNA has become mutated or damaged. This is because mtDNA is not protected by histones and has poor repair mechanisms. Due to the increased metabolic activity of the sperm compared to the egg, due to its motility, there is greater production of
reactive oxygen species
and therefore greater chance of mutation.
[5]
Furthermore, sperm are exposed to reactive oxygen species from leukocytes in the epididymis during transit.
[5]
Additionally, quality control of spermatozoa is much worse than for the ovum, as many sperm are released whereas only one dominant follicle is released per cycle. This competitive selection helps to ensure the most 'fit' ova are selected for fertilisation.
[5]
Artificial oocyte activation
[
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]
Oocyte activation may be artificially facilitated by calcium
ionophores
, something that is speculated to be useful in case of fertilization failure, such as still occurs in 1?5% of
intracytoplasmic sperm injection
(ICSI) cycles.
[6]
Another of method is by using the drug Roscovitine, this reduces the activity of M-phase promoting factor activity in mice.
[7]
Indications
for artificial oocyte activation include:
References
[
edit
]
- ^
Saleh A, Kashir J, Thanassoulas A, Safieh-Garabedian B, Lai FA, Nomikos M (2020).
"Essential Role of Sperm-Specific PLC-Zeta in Egg Activation and Male Factor Infertility: An Update"
.
Front. Cell Dev. Biol
.
8
(28).
doi
:
10.3389/fcell.2020.00028
.
PMC
7000359
.
- ^
Bianchi E, Doe B, Goulding D, Wright GJ (2014).
"Juno is the egg Izumo receptor and is essential for mammalian fertilization"
.
Nature
.
508
(7497): 483?7.
Bibcode
:
2014Natur.508..483B
.
doi
:
10.1038/nature13203
.
PMC
3998876
.
PMID
24739963
.
- ^
Saunders C, Larman M, Parrington J, Cox L, Royse J, Blayney L, Swann K, Lai F (2002). "PLC zeta: a sperm-specific trigger of Ca(2+) oscillations in eggs and embryo development".
Development
.
129
(15): 3533?44.
PMID
12117804
.
- ^
Yu, Yuansong; Nomikos, Michail; Theodoridou, Maria; Nounesis, George; Lai, F. Anthony; Swann, Karl (2012-01-15).
"PLCζ causes Ca2+ oscillations in mouse eggs by targeting intracellular and not plasma membrane PI(4,5)P2"
.
Molecular Biology of the Cell
.
23
(2): 371?380.
doi
:
10.1091/mbc.E11-08-0687
.
ISSN
1059-1524
.
PMC
3258180
.
PMID
22114355
.
- ^
a
b
c
d
e
f
Johnson, M. (2007).
Essential Reproduction
(6th ed.). Oxford: Blackwell.
ISBN
9781405118668
.
- ^
Kashir, J.; Heindryckx, B.; Jones, C.; De Sutter, P.; Parrington, J.; Coward, K. (2010).
"Oocyte activation, phospholipase C zeta and human infertility"
.
Human Reproduction Update
.
16
(6): 690?703.
doi
:
10.1093/humupd/dmq018
.
PMID
20573804
.
- ^
Iba, T; Yano, Y; Umeno, M; Hinokio, K; Kuwahara, A; Irahara, M; Yamano, S; Yasui, T (2011). "Roscovitine in combination with calcium ionophore induces oocyte activation through reduction of M-phase promoting factor activity in mice".
Zygote
.
20
(4): 321?325.
doi
:
10.1017/S0967199411000591
.
PMID
22008472
.