L14 Week 2 & 3 of Development

Question 1

what happens during days 5-6 after fertilisation?

Answer 1

the fluid-filled blastocyst, which is composed of the embryoblast and the trophoblast, hatches which removed the zona pellucide. the zona pellucida restricts the blastocyst from growing.

Question 2

what does the trophoblast differentiate into?

Answer 2

the cytotrophoblast and the syncytiotrophoblast which go on to form support structures

Question 3

what do the Syncytiotrophoblast

cells do?

Answer 3

they bury into the lining of the uterus during implantation

Question 4

what does the embryoblast differentiate into?

Answer 4

the hypoblast (cavity side) and the epiblast (uterus side). these 2 form the bilaminar disk

Question 5

why doesn’t the mother’s immune system attack the blastocyst, which is a foreign body?

Answer 5

It is half a foreign body - half from father and half from mother. It is because of the Syncytiotrophoblasts which bury into the uterine lining but also hide the developing embryo from the mother’s immune system. By secreting factors which hide it from white blood cells. Second of all they are multinucleated so cell membranes in cytotrophoblasts are broken down to form the Syncytiotrophoblasts. That means if the immune system gets to the blastocyst, the white blood cells cannot get through the gaps of the Syncytiotrophoblast

Question 6

how is the bilaminar disk orientated in terms of dorsal/ventral?

Answer 6

The epiblast forms the dorsal surface of the embryo and the hypoblast forms the ventral surface

Question 7

what is the significance of the bilaminar disk?

Answer 7

Establishes the dorsal/ventral axis of the embryo

Question 8

where does the amniotic cavity form and when?

Answer 8

the amniotic cavity forms as a small fluid-filled cavity in the epiblast. day 8

Question 9

what happens during full implantation of the blastocyst and when does this happen?

Answer 9

Syncytitrophoblast have grown rapidly to engulf the whole embryo, cell membranes break down so it is multinucleated. Day 9

Question 10

how is the primary yolk sac formed?

Answer 10

the hypoblast cells migrate to encase the blastocyst cavity (heuser’s membrane). this cavity is now the primary yolk sac. (happens during full implantation)

Question 11

How does developing embryoblast receive nutrients?

Answer 11

Diffusion at first. The syncytiotrophoblast engulf around structures like the uterine gland. By diffusion, nutrients from the mother are received from the uterine gland. Trophoblastic lacunae provide nutrients into the embryo as nutrients empty in to the lacunae “lakes”

Maternal capillary are eventually surrounded by the syncytiotrophoblasts which breakdown the capillary walls.

Question 12

How is uteroplacental circulation established and when?

Answer 12

Syncytiotrophoblast erodes walls of maternal capillaries to establish uteroplacental circulation and the extraembryonic mesoderm will engulf all the developing embryo. Days 10-11

Question 13

what is the extraembryonic mesoderm?

Answer 13

a new layer of connective tissue derived from the yolk sac cells forms between the primary yolk sac and cytotrophoblast = extraembryonic mesoderm

Eventually it develops to cover the amniotic cavity too

Question 14

how does the chorionic cavity form and when?

Answer 14

the cavity forms in the extraembryonic mesoderm on days 12-13

Question 15

how is the secondary (definitive) yolk sac formed and when?

Answer 15

a second wave of hypoblast cells migrate to form the secondary yolk sac. the primary yolk sac is pushed ventrally. remnants of the primary yolk sac will remain.

(blastocyst cvity –> primary yolk sac –> definitive yolk sac)

12-13 days

Question 16

when is the structure like at the end of week 2?

Answer 16

the amniotic cavity and secondary yolk sac are suspended in the chorionic cavity by a connecting stalk. the amniotic cavity is surrounded by epiblast cells and the definitive yolk sac is surrounded by hypoblast cells. the connecting stalk forms the umbilical cord. 14-15 days.

Question 17

A sperm fertilises an oocyte to initially form what?

Answer 17

zygote

Question 18

Migration of which cells form the definitive yolk sac?

Answer 18

Hypoblast

Question 19

Which layer must the sperm penetrate to fertilise the oocyte?

Answer 19

zona pellucida

Question 20

Which layer is composed of multinucleated cells?

Answer 20

Syncytiotrophoblast

Question 21

what does a pregnancy test detect and where is this secreted from?

Answer 21

Syncytiotrophoblast secretes a hormone – human chorionic gonadotrophin (hCG).
Helps maintain endometrium and may play a role in maternal immunotolerance.

Question 22

how often does the embryo implant into an abnormal site?

Answer 22

Occur in 2% of pregnancies and account for 9% of pregnancy related deaths for the mother

Question 23

in which areas can an ectopic pregnancy take place and what are the consequences?

Answer 23

MESENTERY - the fimbrae do not actively sweep the egg into the fallopian tube for fertilisation. It usually would not survive here.

FALLOPIAN TUBE and CERVIX - not specialised for implantation so will not survive. However usually gets quite big to endanger the mother - can rupture the fallopian tube. However vascularisation can occur here, therefore not only rupturing occurring but also haemhorraging

Question 24

what is placenta previa?

Answer 24

the placenta is stuck to the wall of the uterus close to the cervix. it is harder for the baby to be delivered vaginally and may have to be delivered by cesarean. can cause severe bleeding in late pregnancy. If the placenta ruptures, both the baby and the mother are in trouble due to the sharing of blood.

Question 25

what is tubul implantation?

Answer 25

Majority of ectopic pregnancies occuring in the uterine tube (80% in the ampulla). Ruptured tube  shared blood supply causes danger to mother also. Makes further pregnancies more difficult

Question 26

what is abdominal implantation?

Answer 26

the egg implants in the recto-uterine pouch. Usually gets to a certain point of development but then dies where the mother’s body absorbs it

Question 27

what is lithopaedion?

Answer 27

The foetus is too large to be reabsorbed, instead it calcifies (litho = stone; paedion = child)

Can occur when an abdominal pregnancy remains undiagnosed and the foetus dies

This is thought to protect the mother from the necrotic tissue of the foetus

Question 28

what is a hydatidiform mole?

Answer 28

Development of the trophoblast without any embryonic tissue (just ball of supporting structures)

Due to fertilisation of an ‘empty’ egg lacking a nucleus

Suggests that paternal genes favour the formation the trophoblast at the expense of the embryo – parental imprinting

(Syncytiotrophoblast still produces hCG giving a positive pregnancy test even though there is no embryo)

Question 29

SUMMARY - what happens by the end of week 2?

Answer 29

Bilaminar embryo in between amniotic cavity and secondary yolk sac

Fully implanted and established of uteroplacental circulation

Dorsal/Ventral axis established