Embryology-02-First Two Weeks

Front 1

main events of first two weeks

Back 1

transport to implant site, development of bilaminar embryo, amniotic cavity & embryonic disc, implantation

Front 2

2 events of first week following fertilization?

Back 2

embryo transport from ampulla fertilization site to uterine implantation site, and blastula development.

Front 3

blastula cells are what?

Back 3

blastomeres – these are totipotential: each capable of producing complete organism

Front 4

embryo movement down uterine tube why?

Back 4

smooth muscle in the muscularis of the uterine tube, and beat of cilia on epithelial lining of tube.

Front 5

why is there no net growth in size of the embryo during tubal transit

Back 5

keeping embryo within zona pellucida for fertilization, not allowing ectopic implantation by preventing contact between trophoblast and epithelial lining of the tube.

Front 6

what occurs during the 2nd week following fertilization?

Back 6

embryo implants in the lining of the uterus and the amniotic cavity and primitive yolk sac are formed.

Front 7

what is cleavage?

Back 7

rapid mitotic divisions resulting in larger numbers of progressively smaller cells

Front 8

when does the first cleavage division occur?

Back 8

abt. 24 hrs after production of 2nd polar body

Front 9

when do other divisions occur?

Back 9

asynchronous at intervals 12 – 24 hrs.

Front 10

potentiality of blastomeres as development proceeds?

Back 10

becomes progressively restricted.

Front 11

what occurs at the eight cell stage?

Back 11

compaction (between the 3rd & 4th cleavage divisions)

Front 12

what does this mean?

Back 12

blastomeres maximize contact with each other, creating a tightly-packed ball of cells

Front 13

tightly packed ball of cells called what?

Back 13

morula

Front 14

significance of morula stage

Back 14

blastomeres no longer totipotent – can’t remove one without damaging others – too tightly packed.

Front 15

what occurs during subsequent cleavages of the morula?

Back 15

blastomeres become arranged either on the inside or the outside of the morula – this is the first sign of differentiation.

Front 16

outer cells of morula are what, and do what?

Back 16

trophoblast – seal the interior of the morula in.

Front 17

does the trophoblast make a contribution to the embryo

Back 17

NO.

Front 18

trophoblast responsible for what?

Back 18

implantation, and formation of the placenta

Front 19

inner cells of morula called what, forms what?

Back 19

inner cell mass, which forms the embryo.

Front 20

4 days after fertilization, morula is where?

Back 20

has arrived at the uterus at about the 32 cell stage. Fluid filled cavity (blastocyst cavity) appears, displacing cells of inner cell mass to one pole. (embryo is still within the zona pellucida)

Front 21

appears in morula at 4th day?

Back 21

cavity – water is taken on, displacing inner cell mass to one pole

Front 22

what does morula become when cavity has appeared

Back 22

blastocyst (still within zona pellucida)

Front 23

Does blastocyst still have zona pellucida?

Back 23

yes.

Front 24

occurs with trophoblast about 5 days after fertilization?

Back 24

trophoblast produces enzymes - disrupt zona pellucida

Front 25

when the zona pellucida is distrupted?

Back 25

embryo breaks out from zona.

Front 26

name for embryo breaking out from zona

Back 26

hatching

Front 27

after hatching has taken place?

Back 27

trophoblast can make contact with endometrium.

Front 28

endometrium definition?

Back 28

lining of uterus, contains epithelium, connective tissue to receive embryo during implantation, are shed during parturition/menstruation.

Front 29

contact method: trophoblast with endometrium?

Back 29

has tissues that bind with the surface of the uterus.

Front 30

what forms at site of contact of trophoblast & endometrium, and how is it formed?

Back 30

syncytiotrophoblast – trophoblast daughter cells proliferate, but fuse together in multinucleated mass

Front 31

following the formation of the syncytiotrophoblast, what is the trophoblast called?

Back 31

cytotrophoblast (to distinguish it from syncytiotrophoblast)

Front 32

what occurs within the cytotrophoblast as the syncytiotrophoblast is forming?

Back 32

cytotrophoblast cells are flattening within the blastocyst cavity, a layer next to the cavity is formed, called the hypoblast.

Front 33

properties of the syncytiotrophoblast?

Back 33

multinucleated, invasive, burrows into uterine lining, drags the embryo into the uterine lining behind it, produces hCG.

Front 34

What is hCG?

Back 34

Human Chorionic Gonadotropin: essential hormone for pregnancy: maintains proliferation of corpus luteum, secretes hormones - keeps uterine lining:receptive state

Front 35

hormone that is basis for pregnancy test?

Back 35

HCG - Human Chorionic Gonadotropin

Front 36

other key role does the syncytiotrophoblast play in maintenance of pregnancy?

Back 36

thought to play a role in keeping maternal immune system from rejecting the embryo as foreign tissue.

Front 37

what is an ectopic pregnancy and why does it occur?

Back 37

implantation in a place other than the body of the uterus – occurs because the trophoblast will implant anywhere convenient – can occur if egg is delayed

Front 38

Approx. what % of pregnancies are ectopic?

Back 38

1 in 100

Front 39

what % of ectopic pregnancies are tubal?

Back 39

more than 90%

Front 40

why are tubal pregnancies a problem?

Back 40

uterine tube unable to enlarge sufficiently to accommodate rapidly growing embryo – will rupture: risks life-threatening hemmorhage

Front 41

what are other factors effecting tubal pregnancy

Back 41

scar tissue or abnormalities (congenital or acquired) in the uterine tube as a result of infection – these delay the passage of the embryo such that zona comes off while embryo still in the tube.

Front 42

what are other locations of ectopic pregnancies?

Back 42

peritoneal or abdominal. (These can come to term, however must be delivered by cesarian.)

Front 43

If a woman presents with vaginal bleeding, what should she be treated as?

Back 43

pregnant until proven not pregnant (with tubal pregnancy and hemmorhage).

Front 44

correct or normal implantation site?

Back 44

upper wall of the uterus.

Front 45

problem with tubal infection?

Back 45

increased risk of tubal pregnancy

Front 46

two possible treatments for tubal pregnancies?

Back 46

salpingotomy: making hole in tube – extracting embryo from tube (problem as some embryo may be left behind, necessitating re-operation) may also create scarring rendering tube useless.salpingectomy – removal of tube.

Front 47

what happens to the inner cell mass during the process of implantation?

Back 47

becomes a two layered structure called the bilaminar embryonic disc – hypoblast is formed facing the blastocyst cavity. cavities forming in the inner cell massà coalesce à form amniotic cavity: floor of this cavity formed by single layer of cells: epiblast.

Front 48

epiblast becomes?

Back 48

embryo

Front 49

what does the hypoblast become?

Back 49

hypoblast cells multiply and spread, grow together to create a sac called the primitive yolk sac (within the cytotrophoblast

Front 50

Does hypoblast contribute to fetus?

Back 50

No. Is replaced by endoderm during gastrulation

Front 51

what is the bilaminar embryonic disc and what is its context within the rest of the embryo?

Back 51

disc= epiblast + hypoblast, sandwiched between two balloons: amniotic cavity + primitive yolk sac.

Front 52

where is the primitive yolk sac?

Back 52

within the cytotrophoblast

Front 53

what % of pregancies are thought to spontaneously abort?

Back 53

50 – 70% (woman wouldn’t know she was pregnant).

Front 54

why do spontaneous abortions go unrecognized by mothers?

Back 54

because they are during the first two weeks (woman simply thinks her period is late)

Front 55

why do embryos spontaneously abort?

Back 55

chromosomal abnormalities, implantation failure, maternal immune response, embryotoxic environmental agents such as heat, X-ray, ionizing radiation etc.