Implantation, placentation and establishment of basic body plan Flashcards
What is the first step in implantation?
Disappearance of the zona pellucida
What do trophoblastic cells do when implantation starts?
Trophoblastic cells over the embryoblast pole being to penetrate between the epithelial cells of the uterine mucosa on about the 6th day
What are the mediators of initial attachment of the blastocyst to the uterus?
*L-selectin on trophoblast cells
*Its carbohydrate receptors on the uterine epithelium
What is the endometrial receptivity window?
The 6-10 day period after fertilization during which implantation must occur.
What happens during Week 2 Day 8?
The blastocyst is partially embedded in the endometrial stroma. The trophoblast and embryoblast differentiate into their two layers.
What two layers does the trophoblast differentiate into?
1) Cytotrophoblast: an inner layer of mononucleated cells
2) Syncytiotrophoblast: an outer multinucleated zone without distinct cell boundaries
What two layers does the embryoblast differentiate into?
1) Hypoblast layer: a layer of small cuboidal cells adjacent to the blastocyst cavity
2) Epiblast layer: a layer of high columnar cells adjacent to the amniotic cavity
What are the two new cavities formed during implantation?
Blastocyst cavity and amniotic cavity
What happens during Week 2 Day 9?
Blastocyst is more deeply embedded in the endometrium. The penetration defect in the surface epithelium is closed by a fibrin coagulum.
What occurs during the lacunar stage?
Vacuoles (lacunae) appear in the syncytium
When does the Heuser membrane originate from the hypoblast?
Week 2 Day 9
The blastocyst cavity is also known as…
Exocoelomic cavity or primitive yolk sac
What happens during Week 2 Days 11 and 12?
Blastocyst is completely embedded in the endometrial stroma. Surface epithelium almost entirely covers the original defect in the uterine wall. There is establishment of the uteroplacental circulation.
What is the extraembryonic mesoderm?
- A new population of cells derived from the yolk sac
- Located between the inner surface of the cytotrophoblast and the outer surface of amnion and the exocoelomic membrane
What is the extraembryonic/chorionic cavity?
Large cavity that develops in the extraembryonic mesoderm
What are the subdivisions of the extraembryonic mesoderm?
- Connecting stalk (which becomes the umbilical cord in the future)
- Extraembryonic somatic mesoderm
- Extraembryonic splanchnic mesoderm
What occurs on Week 2 Day 13?
Bleeding occasionally occurs at the implantation site as a result of increased blood flow into the lacunar spaces.
This bleeding occurs near the 28th day of the menstrual cycle and can be confused with menstrual bleeding.
What is the trophoblast characterised by in Week 2 Day 13?
Villous structures
What is the chorionic plate?
The extraembryonic mesoderm lining the inside of the cytotrophoblast
When does gastrulation occur?
Week 3
What is gastrulation?
The most characteristic event in embryogenesis which establishes the three germ layers; ectoderm, mesoderm and endoderm.
What does gastrulation begin with?
Formation of the primitive streak on the surface of the epiblast (visible in a 15-16 day embryo)
How is endoderm formed?
When epiblast cells migrate through the primitive streak into the primitive node and reside in the hypoblast. They replace the hypoblast cells and become a newfound endoderm.
How is mesoderm formed?
Epiblast cells migrate to the region between the endoderm and epiblast and will become the mesoderm.
How is ectoderm formed?
Cells that remain in the epiblast form the ectoderm.
What are the oropharyngeal and cloacal membranes?
Cranial and caudal ends with no mesoderm apparent.
What are the body planes in embryos?
- Anterior/cranial/cephalic/rostral: towards the head
- Posterior/caudal: towards the tail
- Ventral: towards the front of the body
- Dorsal: towards the back of the body
What is placentation?
Further development of the trophoblast following gastrulation
What are the three types of villi that characterise placentation?
1) Primary placental villi
* beginning of third week
* consist of a cytotrophoblastic core covered by a syncytial layer
2) Secondary placental villi
* Middle of third week
* Formed when mesodermal cells penetrate the core of primary villi and grow toward the decidua
3) Tertiary or definitive placental villi
* end of third week
* Formed when mesodermal cells in the core of the villus begin to differentiate into blood cells and small blood vessels
* Establish contact with the intraembryonic circulatory system, connecting the placenta and the embryo
What is the morphological classification of placental villi?
1 - stem (anchoring) villi
2 - free (terminal) villi
What is chorion frondosum?
When villi covers the entire surface of the chorion at the embryonic pole
What is chorion laeve?
When the villi on the abembryonic pole degenerates by the third month
What is the decidual reaction?
The reaction of endometrium to implantation
What does the decidual reaction involve?
- Cells of the endometrium become polyhedral and loaded with glycogen and lipids (called decidual cells)
- Intercellular spaces are filled with extravasate
- Tissue becomes edematous
What is the decidua?
Functional layer of the endometrium which is shed during parturition
What is the decidua composed of?
1) Decidua basalis/decidual plate
* the decidua over the chorion frondosum
* tightly connected to the chorion
2) Decidua casularis
* the decidual layer over the abembryonic pole
3) Decidua parietalis
* the decidua lining the main cavity of the pregnant uterus elsewhere than the site of decidua basalis and decidua capsularis
What happens to the decidua with growth of the amniotic cavity?
1 - Fusion of the amnion and chorion at the end of the third month
2 - Form the amniochorionic membrane
3 - Obliteration of chorionic cavity
What happens to amniochorionic membrane during labour?
Ruptures (breaking of the water)
What is the placenta?
Organ that facilitates nutrient and gas exchange between maternal and fetal compartments
Its increase in surface area roughly parallels that of the expanding uterus
Covers approximately 15-30% of the internal surface of the uterus throughout pregnancy
What are the 4 key functions of the placenta?
1) Exchange of gases
2) Exchange of nutrients and electrolytes
3) Transmission of maternal antibodies
4) Hormone production
What is the structure of the placenta (2 portions)?
1) Fetal portion:
* Formed by the chorion frondosum
* Bordered by the chorionic plate
* Covered by trophoblast
2) Maternal portion
* Formed by the decidua basalis
* Bordered by the decidua basalis (decidual plate)
What are intervillous spaces in the placenta?
Lie between the chorionic and decidual plates.
Derived from lacunae in the syncytiotrophoblast
Lined with syncytium of fetal origin
Filled with maternal blood
How are compartments/cotyledons formed in the placenta?
These are formed when decidual septa project into intervillous spaces but do not reach the chorionic plate
What is the structure of the septa in the placenta?
Core: maternal tissue
Surface: a layer of syncytial cells
What is the placental membrane/barrier?
A barrier that separates maternal and fetal blood
What are the initial components of the placental barrier?
1) Endothelial lining of fetal vessels
2) Connective tissue in the villus core
3) Cytotrophoblastic layer
4) Syncytium
Which components of the placental barrier remain in month 4?
1) Endothelial lining of fetal vessels
2) Syncytium
What is the primitive umbilical ring?
The oval line of reflection between the amnion and embryonic ectoderm (amnio-ectodermal junction)
Which structures pass through the umbilical ring?
1) Connecting stalk: containing the allantois and the umbilical vessels
2) Yolk stalk: accompanied by the vitelline vessels
3) Canal connecting the intraembryonic and extraembryonic cavities
When is the definitive umbilical cord formed?
When the allantois and the vitelline duct and its vessels are obliterated
What does the definitive umbilical cord contain?
- Umbilical vessels
- Surrounding wharton jelly
What are the contents of the umbilical cord?
- more proximally: some intestinal loops and the remnant of the allantois
- distally: yolk sac stalk and umbilical vessels
What is physiological umbilical hernia?
When the abdominal cavity is temporarily too small for the rapidly developing intestinal loops, some of them are pushed into the extraembryonic space in the umbilical cord. At approx end of third month, the loops are withdrawn into the body of the embryo.
What are the 3 umbilical vessels?
2 arteries: carry the low oxygenated blood from the fetus to the placenta
1 vein: carries the oxygenated blood from the placenta to the fetus
What is amniotic fluid and what produces it?
Fluid that fills the amniotic cavity. Produced in part by amniotic cells and primarily from maternal blood.
How does the amount of amniotic fluid progress? (values)
10 weeks gestation: 30mL
20 weeks gestation: 450mL
37 weeks gestation: 800-1000mL
What is the function of amniotic fluid?
1) Absorbs jolts
2) Prevents adherence of the embryo to the amnion
3) Allows for fetal movements
What are the measurements of the full-term placenta?
Shape: discoid
Diameter: 15-25cm
Thickness: 3cm
Weight: 500-600g
What are amniotic bands?
A clinical abnormality formed by tears in the amnion. Consequences include amputations, ring constrictions and craniofacial deformations.
How are dizygotic/fraternal twins formed?
Simultaneous shedding of two oocytes and fertilization by different spermatozoa. The zygotes implant individually in the uterus.
What are fetal membranes like in dizygotic/fraternal twins?
Usually each develops its own placenta, amnion and chorionic sac. Sometimes the two placentas are so close together that they fuse. Similary, the walls of the chorionic sacs may also come into close apposition and fuse.
How do monozygotic or identical twins form?
Develop from a single fertilized ovum. Result from splitting of the zygote at various stages of development.
What are the three stages at which the zygote can split in monozygotic twins?
1) At the two-cell stage
2) At the early blastocyst stage
3) At the bilaminar germ disc stage
What occurs at the two-cell stage?
- Two separate zygotes develop
- Blastocysts implant separately
- Each embryo has its own placenta and chorionic sac
- Arrangement of the membranes of these twins resembles that of dizygotic twins
What happens at the early blastocyst stage?
- The inner cell mass splits into two separate groups of cells within the same blastocyst cavity
- The two embryos have a common placenta and a common chorionic cavity but separate amniotic cavities
What happens at the bilaminar germ disc stage?
Single placenta and a common chorionic and amniotic sac
What are the different forms of prenatal screening and diagnosis?
1) Chorionic villus samples: sampling of chorionic membrane or placenta
2) Amniocentesis: analysis of fetal cells from amniotic fluid
3) Ultrasound: non-invasive diagnostic technique
