14 - Zygote to Embryo 2

Question 1

What are the major events of the second week of development (7-12)?

Answer 1

Completion of implantation, amnion and umbilical vesicle formation, and extraembryonic mesoderm formation.

Question 2

What is the big picture of implantation? What are the key players?

Answer 2

Occurs between days 5-12, bridging the gap between weeks 1 and 2.

Relies on cooperation between embryo and uterus.

Initiation of implantation influenced by estrogen and progesterone.

Question 3

What is the first phase of implantation?

Answer 3

Receptor mediated attachment of blastocyst to endometrium via selectins on surface of trophoblast on the side of the ICM (embryonic pole).

Question 4

How does the uterus react to attachment of the blastocyst?

Answer 4

Decidual reaction triggered: cells beneath epithelium enlarge and secrete carbs and lipid fluid, creating a layer called the compact layer in the endometrium of the uterus.

Creates a cellular bed for blastocyst to implant into.

Question 5

What is the second phase of implantation?

Answer 5

Penetration and embedment into compact layer.

Attachment stimulates trophoblast cells to divide to form large mass of cytoplasm called syncytiotrophoblast with many nuclei but no cell borders.

Outer surface has microvilli for absorption of nutrients.

Question 6

What happens are the blastocyst embeds further into the endometrium’s compact layer? Describe the name of the previous structure.

Answer 6

More syncytiotrophoblast is formed around the trophoblast.

Since the original trophoblast was cellular, it is referred to as the cytotrophoblast.

Question 7

What happens within the syncytiotrophoblast?

Answer 7

The spaces form a lacunar network that maternal vessels and glands can empty into so that the embryo can absorb them via the microvilli.

Question 8

At what point is the embryo fully embedded in the endometrium?

Answer 8

Day 10-12

Question 9

How are pregnancy tests done? Why is this the case?

Answer 9

The syncytiotrophoblast surrounding the embryo secretes human chorionic gonadotropin, which is detected to confirm pregnancy.

Question 10

Where is implantation of the blastocyst supposed to occur? Where can it occur that it’s not supposed to?

Answer 10

Supposed to be in the middle region of the uterus.

Extrauterine (ectopic):

• near the cervix
• abdominal or pelvic cavity
• ovary
• outer 1/2 of oviduct (most common)
• vagina

Question 11

What can happen if implantation occurs near the cervix?

Answer 11

The mucosa in the cervix is insufficient for implantation, so detachment can occur too early.

Called placenta previa.

Question 12

What is spontaneous abortion(aka miscarriage)? How common is this and what is the reasoning?

Answer 12

Inadequate implantation.

50% of all pregnancies end in spontaneous abortion.

It’s a natural process for ending abnormal development, especially in the first 3 weeks.

Question 13

What occurs after the embryo is embedded in the endometrium (day7-8/9)?

Answer 13

An extraembryonic membrane called the amnion, is formed from the epiblast.

A cavity forms within the amnion membrane, creating a space for amniotic fluids.

The amnion surrounds the embryo by week 8.

Question 14

What is the function of the amnion and amnionic fluids?

Answer 14

Amnion is initial source of amniotic fluid.

Fluid:

• protects embryo and allows symmetrical growth
• Proper lung growth
• fluid homeostasis
• embryo movement
• helps control embryo body temp

Question 15

What occurs after the formation of the amnion membrane and cavity on days 8-13?

Answer 15

Hypoblast cells at edge of the embryonic disc flatten and migrate outwards to form the extaembryonic exoceolomic membrane.

This membrane bounds a space called the primary umbilical vesicle or yolk sac.

Question 16

What happens on day 13 after the primary yolk sac has formed? ?

Answer 16

Second group of flattened hypoblast cells migrate away from edges of embryonic disc to form a smaller secondary yolk sac.

Question 17

What happens to the remained of the primary yolk sac after the secondary one is formed?

Answer 17

It atrophies but may persist as a small vesicle at the aembryonic pole of the blastocyst.

Question 18

What is the function of the UV (yolk sac)? What occurs here?

Answer 18

Metabolizes and transports nutrients from the trophoblast to the embryo.

Site of initial vascular formation.

Primordial germ cells segregate within the UV wall and helps control epiblast migration.

Question 19

How does the extraembryonic mesoderm form on days 9-12?

Answer 19

The current opinion is that it’s formed by the delamination of the extraceolomic membrane that bounds the yolk sac.

Question 20

Describe the characteristics of extraembryonic mesoderm?

Answer 20

It’s a mesenchyme:

• non-polar, free cell derivative of embryonic epithelium
• cells often motile and irregular in shape
• no cell junctions or layers

Question 21

At the end of the second week of embryonic development, what are the two tissue types present? What is unique about this?

Answer 21

Epithelium that forms layers and mesenchyme (extraembryonic mesoderm) that are free cells.

During development one form can change into the other and vice versa.

Question 22

Describe the embryo at the end of week 2?

Answer 22

1. Extraembryonic mesoderm coats the inside of the trophoblast and the outside of the amnion and yolk sac.
2. Split in the EEM forms the chorionic cavity.
3. Primary chorionic villi form
4. Extraembryonic mesoderm forms a bridge between the embryo and chorion - the connecting stalk (will become umbilical cord)

Question 23

What are the two components of the chorion?

Answer 23

The extraembryonic mesoderm and the trophoblast.

Question 24

What has occurred during week two?

Answer 24

1. Completion of implantation: syncytiotrophoblast formed, erosion into endomet, decidual rxn, lacunae and early villi, established uteroplacental circulation.
2. Continued development of bilaminar disc: amnion and umbilical vesicle (yolk sac)
3. Formation of additional extraembryonic tissues: chorion, extraembryonic mesoderm, body stalk, primary chorionic villi.

Question 25

What are the major events that occur during week three of embryonic development (days 13-21)?

Answer 25

1. Trilaminar disc forms via gastrulation
2. Establishment of primary germ layers
3. Appearance of symmetry and asymmetry
4. basic body plan
5. beginning of organogenesis

Question 26

What is the primitive streak (PS) (day 13)? What does this lead to?

Answer 26

Transient thickening of epiblast with a midline groove due to converging cells.

Site of epithelial-mesenchymal transformation

Elongation of streak leads to embryo shape change from circle to oval.

Question 27

What happens during the epithelial-mesenchymal transformation at the primitive streak (day 13)?

Answer 27

Epithelial cells lose polarity, change shape, detach from one another, extend processes.

Question 28

What is the primitive node (day 16)? Why is this important?

Answer 28

Expansion at the cranial end of the PS that contains a pit.

Also site of epithelial-mesenchymal transformation.

Important signaling center for future structures.

Question 29

What happens to the first epiblast cells that transform into mesenchyme?

Answer 29

The cells use the primitive node as an escalator to go down between the hypo and epiblast to form the definitive endoderm.

Question 30

After the definitive endoderm is formed, what occurs?

Answer 30

Cells passing through the primitive node move to opposite end of disc to form the prechordal plate.

Cells begin passing through the streak to form the intraembryonic mesoderm.

Question 31

After the intraembryonic mesoderm is formed, what happens to the remaining cells in the epiblast?

Answer 31

They become the ectoderm.

Question 32

Describe the notochord and its formation (day 16-22)?

Answer 32

Central axis of embryo; solid cord from primitive node.

Cells invaginate from the primitive node and migrate cranially to the opposite end of the disc.

Question 33

What are the 2 places that no mesoderm migrates between the fused ectoderm and the definitive endoderm?

Answer 33

Oropharyngeal membrane at the cranial end of trilaminar disc: separates future mouth and foregut (forms day 15, rupture 4th week)

Cloacal membrane at causdal end: separates anus and hindgut (forms day 16, ruptures 7th week)

Question 34

What is allantois? What does this become in adults?

Answer 34

A diverticulum of the umbilical vesicle (yolk sac) which is a signaling area where vasculature begins to form.

Becomes mediaN umbilical ligament (fold).

Question 35

Describe the regression of the primitive streak (day 17-26)?

Answer 35

After more cells have gone through the PS, the embryo has expanded cranially and the streak is restricted to the tailbud.

As notochord grows cranially, the streak regresses caudally and the embryo lengthens along cranial/caudal axis.

Question 36

What is a teratoma?

Answer 36

A neoplasm (abnormal growth) of multiple cell types that’s unrelated to the attached structure.

Can be solid or cystic.

Usually sacrococcygeal and is the most common tumor in newborns.

Question 37

How is it possible that teratomas are composed of multiple cell types?

Answer 37

The primitive streak separates precursor cells into different germ layers; so if you have an accumulation of epiblast-like cells that don’t disappear, you have the potential to form various types of tissue.