14 - Zygote to Embryo 2 Flashcards
(37 cards)
What are the major events of the second week of development (7-12)?
Completion of implantation, amnion and umbilical vesicle formation, and extraembryonic mesoderm formation.
What is the big picture of implantation? What are the key players?
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.
What is the first phase of implantation?
Receptor mediated attachment of blastocyst to endometrium via selectins on surface of trophoblast on the side of the ICM (embryonic pole).
How does the uterus react to attachment of the blastocyst?
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.
What is the second phase of implantation?
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.
What happens are the blastocyst embeds further into the endometrium’s compact layer? Describe the name of the previous structure.
More syncytiotrophoblast is formed around the trophoblast.
Since the original trophoblast was cellular, it is referred to as the cytotrophoblast.
What happens within the syncytiotrophoblast?
The spaces form a lacunar network that maternal vessels and glands can empty into so that the embryo can absorb them via the microvilli.
At what point is the embryo fully embedded in the endometrium?
Day 10-12
How are pregnancy tests done? Why is this the case?
The syncytiotrophoblast surrounding the embryo secretes human chorionic gonadotropin, which is detected to confirm pregnancy.
Where is implantation of the blastocyst supposed to occur? Where can it occur that it’s not supposed to?
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
What can happen if implantation occurs near the cervix?
The mucosa in the cervix is insufficient for implantation, so detachment can occur too early.
Called placenta previa.
What is spontaneous abortion(aka miscarriage)? How common is this and what is the reasoning?
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.
What occurs after the embryo is embedded in the endometrium (day7-8/9)?
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.
What is the function of the amnion and amnionic fluids?
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
What occurs after the formation of the amnion membrane and cavity on days 8-13?
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.
What happens on day 13 after the primary yolk sac has formed? ?
Second group of flattened hypoblast cells migrate away from edges of embryonic disc to form a smaller secondary yolk sac.
What happens to the remained of the primary yolk sac after the secondary one is formed?
It atrophies but may persist as a small vesicle at the aembryonic pole of the blastocyst.
What is the function of the UV (yolk sac)? What occurs here?
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.
How does the extraembryonic mesoderm form on days 9-12?
The current opinion is that it’s formed by the delamination of the extraceolomic membrane that bounds the yolk sac.
Describe the characteristics of extraembryonic mesoderm?
It’s a mesenchyme:
- non-polar, free cell derivative of embryonic epithelium
- cells often motile and irregular in shape
- no cell junctions or layers
At the end of the second week of embryonic development, what are the two tissue types present? What is unique about this?
Epithelium that forms layers and mesenchyme (extraembryonic mesoderm) that are free cells.
During development one form can change into the other and vice versa.
Describe the embryo at the end of week 2?
- Extraembryonic mesoderm coats the inside of the trophoblast and the outside of the amnion and yolk sac.
- Split in the EEM forms the chorionic cavity.
- Primary chorionic villi form
- Extraembryonic mesoderm forms a bridge between the embryo and chorion - the connecting stalk (will become umbilical cord)
What are the two components of the chorion?
The extraembryonic mesoderm and the trophoblast.
What has occurred during week two?
- Completion of implantation: syncytiotrophoblast formed, erosion into endomet, decidual rxn, lacunae and early villi, established uteroplacental circulation.
- Continued development of bilaminar disc: amnion and umbilical vesicle (yolk sac)
- Formation of additional extraembryonic tissues: chorion, extraembryonic mesoderm, body stalk, primary chorionic villi.
