MNSR - 39 Sexual Reproduction: Embryology, syngamy, pre-embryonic cleavage Flashcards
what events occur during fertilisation and how long does that occur
entry of the spermatozoan into the cumulus mass leads to fusion of these 2 membranes lasts 10 to 20 minutes
newly fertilised oocyte is now called a
zygote
zygote immediately faces 2 problems
it must prevent further spermatozoa gaining entry causing polyspermy and,
the (ovum) zygote must complete the 2nd meiotic division
how is the fusion of the egg with the head of the sperm accomplished
seems to be accomplished by microvilli on the egg membrane enveloping the sperm head;
what events occur when 2 membranes of oocyte and head of spermatazoa fuses
once the two membranes fuse there is an immediate cessation of movement by the spermatozoan;
how is polyspermy (fusion of 2 sperms) prevented
cortical reaction
describe cortical reaction
- when fusion occurs calcium ions stored in ER of oocyte are released to ooplasm
- causes hyperpolarising electrical charges across vitelline membrane for hours
- hyperpolarization - increased permeability of K+ ions prevent fusion of other sperm
- Cortical granules fuse with the oocyte membrane releasing contents into perivitelline space/extracellular space
- The granules contain enzymes which act on the zona pellucida preventing entry of further sperm
- The enzymes also seem to effect the vitelline membrane or oöplasma itself as no further spermatozoa will bind to it.
- These events take place rapidly within minutes of fusion. So called ‘slow block to polyspermy’.
syngamy
is the fusion of the male and female gametes.
events following syngamy
metabolic change in egg and burst in protein synthesis
2-3 hours later meiotic metaphase is reactivated and second polar body is excluded
actin containing microfilaments incorporate sperm nucleus at site of fertilization
when the sperm head enters the oöplasm, the nuclear envelope disperses immediately
series of events in syngamy
fusion of speramatazoan head w/ membrane of oocyte in equatorial segment
release of cortical granules
decondensation of sperm nucleus
egg cortex engulfs head of sperm
leading to the incorporation of the sperm head into an intracellular vesicle
what does decondensed sperm chromatin change
associated proteins and DNA replication occurs within 5 to 8 hours (still 1N – haploid; 23 chromosomes each in chromatid pairs)
how is both the male pronucleus and female pronucleus formed
the sperm chromatin begins to swell becoming what is known as the MALE PRONUCLEUS
after oögenesis is completed the remaining chromatin in the ovum becomes diffuse
4 to 7 hours after fusion a FEMALE PRONUCLEUS appears during which DNA replication occurs (still 1N – haploid; 23 chromosomes each in chromatid pairs)
how does fusion of male pronuceli occur
Both pronuclei surrounded by envelopes and contain several nucleoli;
male and female pronuclei line up on the metaphase plate of the ovum, the pronuclear membranes break down and chromosomal union (i.e. syngamy) is complete
the ZYGOTE is now formed and the first mitotic cell division begins (using centrioles from the sperm. The oocyte does not have centrioles)
From early in the two-cell stage the conceptus starts to _____
express its own genes
cell made from cell division following the early 2-cell stage is called
a blastomere - the number of cells increase but size of conceptus is the same because it is enclosed by zona pellucida
when is zona pellucida shed from conceptus
after 2 days of early 2 cell stage
where does conceptus move to and how does it continue to grow
As the conceptus passes towards the uterus it begins to grow by continued cleavage
where does conceptus get nutrients as it reaches to uterus
nutrients are absorbed from the secretions of the Fallopian tube
what event occurs following the 8 and 16 cell stage of conceptus
it undergoes compaction
describe compaction
individual outer blastomeres tightly adhere through gap and tight junctions and lose their individual identity when viewed from the surface
Now called a MORULA, a solid ball of cells
where does the morula move down and what propels it
is propelled down the Fallopian tube by the ciliated epithelium and some 3 to 5 days after ovulation the morula enters the uterus
what is morula called after 32 to 64 cell stage
later at the 32 to 64 cell stage (around day 4) the morula transforms into a BLASTOCYST;
where does blastocyst get its nutrients
formed as secretions from endometrial glands enter the morula through zona pellucida.
describe blastocyst
At 32 cell stage, the fluid collects between the blastomeres and reorganizes them around a large fluid-filled cavity called the blastocyst cavity or blastocoel.
Although it now contains hundreds of cells, it is still about the same size as the original zygote.
where is the blastocyst
The blastocyst floats free in the uterine cavity
why is it important for blastocyst to make effective contact with the maternal endocrine system
the blastocyst must make effective contact with the maternal endocrine system to change the ovarian cyclic pattern to a non-cyclic pregnant pattern
2 distinct cell types of blastocyst
Outer layer of trophoectoderm cells (TROPHOBLAST)
INNER CELL MASS (embryoblast) is asymmetrically placed within the blastocoelic cavity
trophoblast/trophoectoderm cells
these surround an (inner) fluid-filled cavity the blastocoelic cavity
this fluid accumulation is possible because of tight junctional complexes between the trophoectoderm cells
embryoblast
is asymmetrically placed within the blastocoelic cavity
Eventually develops into embryo
The trophoectoderm cells are the first of the ____
EXTRA-EMBRYONIC TISSUES
purpose of trophoectoderm cells/ trophoblast cells
Do not contribute to the embryo/fetus;
Give rise to part of an accessory fetal membrane the TROPHOBLAST of the chorion which is a sac surrounds the fetus and the fetal portion of the placenta (site of exchange of nutrients between mother and fetus
mechanism involved in which the blastocyst hatches from zona pellucida
Mechanism involves digesting a hole in it with an enzyme, and then squeezing through the hole.
This step necessary for implantation (attachment to the wall of the uterus)
what process occurs on 6 to 7 days of the blastocyst occurs
IMPLANTATION-where the blastocyst makes contact with the uterine tissue
how does attachment of blastocyst to the uterine during implantation occur
uterine endometrium epithelium come into contact and the opposing microvilli interlock or fuse with one another; this is called attachment;
the close contact with the trophoblast and uterine ednometrium epithelium induces …
vascularisation and differentiation in the underlying endometrial stromal tissue
in order for successful implantation to occur what must happen
the endometrial wall of the uterus must be primed by the hormones oestrogen and progesterone;
how long does it take for blastocyst to be firmly attached to the uterine wall
7 days after fertilisation
how is the blastocyst nurtured after implantation
After attachment the blastocyst is nurtured at first by uterine secretions but this phase does not last long.
what tissue is affected after implantation of blastocyst
Within an hour of contact it is clear that the stromal tissue of the endometrium has been affected.
how does the invasion of endometrium occur
suggested that enzymes released by the trophoblast cells digest the endometrium;
As the blastocyst sinks into the endometrium the trophoblast continues to erode the decidual tissue;
finished by day 14
ectopic pregnancy
development of embryo (up to week 8) or fetus (after week 8) outside of the uterine cavity.
problems that may arise from ectopic pregnancy
Problems for mother are bleeding, acute abdominal or pelvic pain, rupture of the uterine tube (leading to death of the mother
