Second Week of Development

Question 1

When does the blastocyst implant

Answer 1

6-10 days after fertilization

Question 2

What does the “Week of twos” refer to

Answer 2

• Trophoblast differentiates into the syncitiotrophoblast and cytotrotrophoblast
• Embryoblast differentiates into the epiblast and hypoblast
• The bilaminar disc forms

Question 3

Epiblast

Answer 3

derived from the embryoblast, associates with the amniotic cavity. Amnions are derived from the epiblast

Question 4

Hypoblast

Answer 4

Derived from the embryoblast, adjacent to exocoelomic cavity

Question 5

Amniotic Cavity Formation

Answer 5

As implantation of the blastocyst progresses, a small space appears in the embryonic cavity

Question 6

Embryonic disc formation

Answer 6

Morphologic changed occur in the embryoblast that result in a flat, almost circular, bilaminar plate of cells

Question 7

Amniotic cavity

Answer 7

new cavity that appears between the nner cell mass and the cytotrophoblast

Question 8

What does the umbilical vesicle form from

Answer 8

blastocyst cavity

Question 9

what is the end of the second week characterized by

Answer 9

appearance of the primary chorionic villi

Question 10

What does the extraembryonic coelom split into

Answer 10

extraembryonic somatic mesoderm and extraembryonic splanchnic mesoderm

Question 11

Extraembryonic somatic mesoderm

Answer 11

Derived from the extraembryonic coelom, lines the trophoblast and covers the amnion

Question 12

Extraembryonic Splanchnic Mesoderm

Answer 12

Derived from the extraembryonic coelom, surrounds the umbilical vesicle (Yolk sac)

Question 13

Chorion

Answer 13

Formed from the extraembryonic somatic mesoderm and the 2 layers of the trophoblast. Forms the wall of the chorionic sac

Question 14

Chorionic sac

Answer 14

Contains the embryo and its amniotic sac and umbilical vesicle, which are surrounded by the connecting stalk

Question 15

Chorionic Cavity

Answer 15

Extraembryonic cavity once the chorion has formed

Question 16

Extraembryonic Mesoderm

Answer 16

Develops between the trophoblast and the amnion and umbilical vesicle. Spaces appear within the mesoderm which forms the extraembryonic coelom which splits in two

Question 17

Prechordal plate

Answer 17

Localized thickening of the hypoblast, indicates the future cranial region of the embryo and the future site of the mouth

Question 18

hCG

Answer 18

Human chorionic gonadotropn, secreted by the blastcyst to stimulate the corpus luteum to produce estrogen and progesterone

Question 19

When does the placenta take over hormone production

Answer 19

Week 20

Question 20

Implantation of Blastocyst

Answer 20

Occurs in the endometrium of the uterus, can be detected by ultrasonography and highly sensitive radioimmunoassays of hCG as early as the second week

Question 21

Placenta previa

Answer 21

When implantation occurs in the lower segment of the uterus

Question 22

Ectopic pregnancies

Answer 22

When implantaton doesn’t occur in the uterus

Question 23

Summary of implantation

Answer 23

• Day 5: Zona pellucida disappears due to blastocyst enlargement and enzymatic lytic degradation
• Day 6: Blastocyst adheres to the endometrial epithelium
• Day 7: Trophoblast differentiates
• Day 8: Syncitiotrophoblast erodes endometrial tissues, blastocyst embeds in the epithelium
• Day 9: Blood-filled lacunae appear in the syncitiotrophoblast
• Day 10: Blastocyst sinks below epithelium, defect is filled by a closing plug
• Days 10/11: Lacunar networks form by fusion of adjacent lacunae
• Days 11/12: Syncitiotrophoblast erodes blood vessels, allowing maternal blood to seep in/out of lacunar networks, establishing uteroplacental circulation
• Days 12/13: Defect in epithelium is repaired
• Days 13/14: Primary chorionic villi develop

Question 24

Uteroplacental Circulation

Answer 24

Established by the erosion of blood vessels by the syncitiotrophoblast, allowing maternal blood to seep in/put of lacunar networks

Question 25

Summary of the second week

Answer 25

• Rapid proliferation and differentiation of the trophoblast
• Primary umbilical vesical forms, extraembryonic mesoderm develops, extraembryonic coelom forms from spaces that develop in the mesoderm, coelom becomes the chorionic cavity
• Primary umbilical vesicle disappears are the secondary vesicle appears
• Amniotic cavity appears as a space between the cytotrophoblast and the embryoblast
• Embryoblast differentiates into the bilaminar embryonic disc: epiblast and hypoblast
• Prochordal plate develops