Embryology weeks 1-2 and clinical correlates

Question 1

describe the key events in day 1-2 of week 1

Answer 1

Oocyte and sperm fuse membranes and form separate pronuclei.

Oocyte undergoes 2nd meiosis before fusing nuclei together

Ultimately forms the zygote once DNA replication begins.

Question 2

Zona pellucida

Answer 2

Gelatinous membrane of the oocyte that prevents polyspermia and ectopic pregnancies

Question 3

Relationship between embryonic age and gestational age

Answer 3

Embryonic age is gestational age +2

Gestational age is embryonic age -2

Question 4

Corona radiata

Answer 4

Follicular cells that release molecular signals to help sperm get to the egg.

Question 5

Describe the key step in day 3 of week 1

Answer 5

Morula Forms after 4 cleavage stages in a zygote. (12-32 blastomeres)

Question 6

Describe day 4 key steps

Answer 6

Fluid enters degrade zona pellucida of the morula and forms a blastocyst.

• blastocyst forms two cell layers due to fluid fishing into the cell
• trophoblast = outer cell masses
• embryoblast = inner cell masses

Question 7

Describe key steps in days 5-7

Answer 7

Implantation begins in the uterine endometrium when the blastocyst hits the uterine lining.

Trophoblast cells emerge into the uterine endometrium.

Question 8

Describe day 7-8 steps

Answer 8

Trophoblast cells differentiate into two parts

Cytotrophoblast form prenatal immune system

Synctotrophoblasts are tasked to go out into the endometrium, and degrade uterine cells to bring back nutrients to the blastocyst.

Embryoblast forms embryonic disc by separating into two layers

Question 9

Embryoblast two layers are

Answer 9

Epiblast (dorsal)

Hypoblast (ventral)

Generate 1st directional axis

Question 10

Two sacs that form during day 7-8

Answer 10

Amniotic sac
-produces amnioblasts that hug the epiblasts that layer the amniotic cavity.

Yolk sac

Question 11

Key events in Day 9

Answer 11

Amniotic and yolk sacs officially differentiate and the fibrin clot forms to prevent extra bleeding

Question 12

Amnotic sac and yolk sac line what axis

Answer 12

Amnotic lines the dorsal side

Yolk sac lines the ventral side

Question 13

Key events in days 10-12

Answer 13

Extra embryonic mesoderm layer forms between the cytotrophoblast and the amnotic cavity.

• also forms extra-embryonic cavities that are fractured and not fused yet.
• hypoblast cells opposite the connecting stalk form the AVE

Question 14

Key events in day 13

Answer 14

Extraembroynic cavities fuse together and form the chorionic cavity.

The chorionic cavity formation also shrinks the primary umbilical vesicle and eventually generates both a primary and a secondary umbilical vesicles.

Also form the connecting stalk which connects the somatic and splanchnic extraembroynic mesoderm

Question 15

Difference between extra embryonic somatic and extraembryonic splanchnic mesoderm

Answer 15

Somatic = forms blood vessels connecting placenta to the embryo.

Splanchnic = surrounds the umbilical vesicles

Question 16

Spontaneous abortions and abnormal embryos

Answer 16

Can occur due to

• inadequate progesterone and estrogen production
• chromosomal abnormalities after meiosis 2 in the oocyte and fusion of the pronuceli

Question 17

Ectopic pregnancies

Answer 17

Improper implantation of the blastocyst via early emergence from the zona pellucida

• 95% of ectopic pregnancies occur in the uterine (fallopian) tubes.
• always results in embryo death

Question 18

Inhibition of implantation

Answer 18

Large doses of estrogen cause the blastocyst to not implant anywhere.

• reasoning is the endometrium in the uterus does not prepare for implantation, so the blastocyst doesn’t know where to implants

Question 19

IUDs

Answer 19

Prevent implantation via producing an inflammatory reaction and releasing extre progesterone which disrupts uterine endometrium preparation for implantation.

• copper IUDs also disrupt sperm migration

Question 20

Twin transfusion syndrome

Answer 20

Arteriovenous shunts form between twins in which one twin is deprived of nutrients and one is given more than normal nutrients.

• placenta of deprived twin is pale
• placenta of over nourished twin is dark red.

Question 21

Establishing zygosity of twins

Answer 21

Determined via chromosome and molecular testing

• also ultrasound testing

Question 22

Monochoronic-monoamniotic twins

Answer 22

Late division of embroyonic cells cause twins w/ only 1 amnotic and chorionic sacs.

-causes fetal mortality almost 50% of the time.

Question 23

Conjoined twins

Answer 23

Arises with inhibition of embryonic disc division.

Question 24

Dizygotic twinning

Answer 24

Faternal twins that results from two independent oocytes being released during ovulation. Both become fertilized.

-diamniotic and diachorionic twins

Question 25

Monozygotic fertilization

Answer 25

Identical twins that forms via 3 different ways

• morula spontaneously forming two blastocysts during cleavage stages.
• one blastocyst forms, but the inner mass spontaneously cleaves into two separate embryoblasts
• one blastocyst forms but the inner mass partially splits into 2.

Question 26

Monozygotic two blastocyst is charcterized by

Answer 26

Diamniotic, dichorionic

Question 27

Monozygotic single blastocyst with two separate embryoblasts is characterized how?

Answer 27

Diamniotic, monochorionic w/ a single placenta

Question 28

Monozygotic single blastocyst w/ partially split embryoblasts is characterized how?

Answer 28

Monoamnionic, monochorionic w/ common placenta

Question 29

Placenta previa

Answer 29

Implantation at or near the internal os