MNSR - 39 Sexual Reproduction: Embryology, syngamy, pre-embryonic cleavage

Question 1

what events occur during fertilisation and how long does that occur

Answer 1

entry of the spermatozoan into the cumulus mass leads to fusion of these 2 membranes lasts 10 to 20 minutes

Question 2

newly fertilised oocyte is now called a

Answer 2

zygote

Question 3

zygote immediately faces 2 problems

Answer 3

it must prevent further spermatozoa gaining entry causing polyspermy and,
the (ovum) zygote must complete the 2nd meiotic division

Question 4

how is the fusion of the egg with the head of the sperm accomplished

Answer 4

seems to be accomplished by microvilli on the egg membrane enveloping the sperm head;

Question 5

what events occur when 2 membranes of oocyte and head of spermatazoa fuses

Answer 5

once the two membranes fuse there is an immediate cessation of movement by the spermatozoan;

Question 6

how is polyspermy (fusion of 2 sperms) prevented

Answer 6

cortical reaction

Question 7

describe cortical reaction

Answer 7

1. when fusion occurs calcium ions stored in ER of oocyte are released to ooplasm
2. causes hyperpolarising electrical charges across vitelline membrane for hours
3. hyperpolarization - increased permeability of K+ ions prevent fusion of other sperm
4. Cortical granules fuse with the oocyte membrane releasing contents into perivitelline space/extracellular space
5. The granules contain enzymes which act on the zona pellucida preventing entry of further sperm
6. The enzymes also seem to effect the vitelline membrane or oöplasma itself as no further spermatozoa will bind to it.
7. These events take place rapidly within minutes of fusion. So called ‘slow block to polyspermy’.

Question 8

syngamy

Answer 8

is the fusion of the male and female gametes.

Question 9

events following syngamy

Answer 9

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

Question 10

series of events in syngamy

Answer 10

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

Question 11

what does decondensed sperm chromatin change

Answer 11

associated proteins and DNA replication occurs within 5 to 8 hours (still 1N – haploid; 23 chromosomes each in chromatid pairs)

Question 12

how is both the male pronucleus and female pronucleus formed

Answer 12

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)

Question 13

how does fusion of male pronuceli occur

Answer 13

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)

Question 14

From early in the two-cell stage the conceptus starts to _____

Answer 14

express its own genes

Question 15

cell made from cell division following the early 2-cell stage is called

Answer 15

a blastomere - the number of cells increase but size of conceptus is the same because it is enclosed by zona pellucida

Question 16

when is zona pellucida shed from conceptus

Answer 16

after 2 days of early 2 cell stage

Question 17

where does conceptus move to and how does it continue to grow

Answer 17

As the conceptus passes towards the uterus it begins to grow by continued cleavage

Question 18

where does conceptus get nutrients as it reaches to uterus

Answer 18

nutrients are absorbed from the secretions of the Fallopian tube

Question 19

what event occurs following the 8 and 16 cell stage of conceptus

Answer 19

it undergoes compaction

Question 20

describe compaction

Answer 20

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

Question 21

where does the morula move down and what propels it

Answer 21

is propelled down the Fallopian tube by the ciliated epithelium and some 3 to 5 days after ovulation the morula enters the uterus

Question 22

what is morula called after 32 to 64 cell stage

Answer 22

later at the 32 to 64 cell stage (around day 4) the morula transforms into a BLASTOCYST;

Question 23

where does blastocyst get its nutrients

Answer 23

formed as secretions from endometrial glands enter the morula through zona pellucida.

Question 24

describe blastocyst

Answer 24

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.

Question 25

where is the blastocyst

Answer 25

The blastocyst floats free in the uterine cavity

Question 26

why is it important for blastocyst to make effective contact with the maternal endocrine system

Answer 26

the blastocyst must make effective contact with the maternal endocrine system to change the ovarian cyclic pattern to a non-cyclic pregnant pattern

Question 27

2 distinct cell types of blastocyst

Answer 27

Outer layer of trophoectoderm cells (TROPHOBLAST)

INNER CELL MASS (embryoblast) is asymmetrically placed within the blastocoelic cavity

Question 28

trophoblast/trophoectoderm cells

Answer 28

these surround an (inner) fluid-filled cavity the blastocoelic cavity
this fluid accumulation is possible because of tight junctional complexes between the trophoectoderm cells

Question 29

embryoblast

Answer 29

is asymmetrically placed within the blastocoelic cavity

Eventually develops into embryo

Question 30

The trophoectoderm cells are the first of the ____

Answer 30

EXTRA-EMBRYONIC TISSUES

Question 31

purpose of trophoectoderm cells/ trophoblast cells

Answer 31

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

Question 32

mechanism involved in which the blastocyst hatches from zona pellucida

Answer 32

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)

Question 33

what process occurs on 6 to 7 days of the blastocyst occurs

Answer 33

IMPLANTATION-where the blastocyst makes contact with the uterine tissue

Question 34

how does attachment of blastocyst to the uterine during implantation occur

Answer 34

uterine endometrium epithelium come into contact and the opposing microvilli interlock or fuse with one another; this is called attachment;

Question 35

the close contact with the trophoblast and uterine ednometrium epithelium induces …

Answer 35

vascularisation and differentiation in the underlying endometrial stromal tissue

Question 36

in order for successful implantation to occur what must happen

Answer 36

the endometrial wall of the uterus must be primed by the hormones oestrogen and progesterone;

Question 37

how long does it take for blastocyst to be firmly attached to the uterine wall

Answer 37

7 days after fertilisation

Question 38

how is the blastocyst nurtured after implantation

Answer 38

After attachment the blastocyst is nurtured at first by uterine secretions but this phase does not last long.

Question 39

what tissue is affected after implantation of blastocyst

Answer 39

Within an hour of contact it is clear that the stromal tissue of the endometrium has been affected.

Question 40

how does the invasion of endometrium occur

Answer 40

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

Question 41

ectopic pregnancy

Answer 41

development of embryo (up to week 8) or fetus (after week 8) outside of the uterine cavity.

Question 42

problems that may arise from ectopic pregnancy

Answer 42

Problems for mother are bleeding, acute abdominal or pelvic pain, rupture of the uterine tube (leading to death of the mother