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First weeks of development

35 important questions on First weeks of development

What are pluripotent stem cells?

Cells that can make all cell types of a human body, but not all cells needed for an embryo (for example, they can not become cells of the placenta).

When cells of an embryo are taken from the tissue where they are in and implanted into an other tissue, what do they become?

- That depends on the stage of development the tissue is in,
- in a very early state (when the (ICM) cells can become almost everything) the cell will become like the cells of the tissue in which it is implanted.

What are two transcription factors that are essential for the formation of the ICM, in which stage are they expressed, and what happens when they are blocked?

- Oct4 and Nanog
- expressed in the morula stage
- when they are blocked, no ICM will form, only trophoblasts are formed

What are the transcription factors that block the expression of Oct4 and Nanog in trophoblast cells, and what happens if they are blocked?

- Cdx2 and Eomes (only expressed in trophoblast cells),
- if they have a loss of function, no differentiation is made between trophoblast cells and ICM cells, because Oct4 and Nanog are not downregulated.

What is the major site of hematopoiesis in this stage of embryonic development?

The extraembryonic mesoderm of the yolk sac wall.

Why is the process of placentation (= formation of placenta) essential?

As the embryo gets larger, it needs more nutrients, so simple diffusion is not enough anymore.

When does placentation start, and with what?

- On day 9 (which is right after implantation!)
- it starts with the trophoblastic lacunae (filled with the mother's blood

What happens with the maternal blood vessels and the trophoblastic lacunae?

Some maternal blood vessels will rupture and form sinusoids that anostosome with trophoblastic lacunae.

What happens in week 3 of the embryonic development in the process of placentation?

- Secondary chorionic stem villi form (cytotrophoblast cells and the extraembryonic mesoderm have grown into the syncytiotrophoblasts)
- Tertiary chorionic stem villi form (containing differentiated blood vessels).

What are made by day 21 in the formation of the villi?

Both the connective tissue and blood vessels.

What are the 5 types of hormones the placenta secretes, and what are their functions?

Trophoblast cells secrete hCG, which signals the corpus luteum to enlarge and produce progesterone
Placental lactogen: prepare mammary glands: milk production
Relaxin: increases flexibility of pelvis during birth, suppresses oxytocine to delay onset of labour
Progesterone & estrogen: maintain endometrium

What are the two structures the placenta develops from?

The embryonic chorion
Maternal uterine tissue

What structure is fromed from the chorion, and what connects the embryo and the placenta?

- Chorionic villi are formed from the chorion, they project into the endometrium of the uterus
- the umbilical cord connects the embryo and the placenta.

What is the function of the yolk sac during the process of placentation?

Blood cell formation.

Does does the embryo develop in week 4?

The head and the tail are formed.

What happens in the development of the extraembryonic membranes in week 5?

- The endometrium has no role in nutrient supply anymore
- Body stalk and yolk stalk fuse to form the umbilical stalk
- Placenta is formed

When is the placenta fully grown?

At 10 weeks

In the placenta, where does the maternal blood come from?

From ruptured blood vessels.

How is it possible that oxygen of the mother starts to bind to the blood of the fetus?

Because the type of hemoglobin the fetus has, has a higher affinity for oxygen than the type of hemoglobin the mother has, making exchange possible.

When are the genes that encode for the fetal hemoglobin type, active?

Only during fetal life.

What is the first type of hemoglobin an embryo has, and where is it produced?

- The Hb-ζε (made out of two ζ-chains encoded within the alfa cluster, and two epsilon chains encoded within the beta cluster)
- produced in the yolk sac in the first weeks of embryonic development.

When does the production of the Hb-ζε type start to decline?

- The synthesis changes from the yolk sac to the liver after 6-8 weeks,
- and with this change the Hb-ζε production declines.

What is a type of hemoglobin of which the synthesis starts during fetal development, and which stays throughout life?

The α-globin, expressed from the α-clusters and made out of two identical proteins.

What kind of genes for hemoglobin are expressed after the embryonic genes are turned off?

The β-globin genes, identified as the γ-genes (Aγ and Gγ).

What kind of swith takes place shortly before birth?

A switch from fetal γ-globin gene expression to adult β-globin gene expression.

What kind of unwanted things does the placenta allow to pass from mother to fetus?

Viruses as HIV
Drugs
Alcohol
Chemicals in tobacco smoke

What does the placenta also allow to pass form mother to fetus?

Protective antibodies

What happens when a blastocyst does not implant in the uterus, but somewhere else?

The embryo is not viable, and this is also life-threatening to the mother.

How is it called when the blastocyst implants somewhere else?

Ectopic implantation.

What are places where the blastocyst can implant, outside of the uterus?

The ovarian tube
An ovary
The abdomen

What are risk factors for ectopic implantation?

Infections
Surgical interventions in the pelvis
Tobacco misuse
IVF

What can implantation at the internal os lead to?

Placenta previa (PP), which can lead to bleedings during pregnancy and detachment of placenta before the fetus is born.

What kind of change happens when the blastocyst implants in the uterus, and what are the results of this?

- There is a change in the mother's immune system (it works a little less) to tolerate the 'foreign body',
- which causes additional risk for the mother (for example for infectious diseases),
- and for some auto-immune diseases the condition shows improvement.

In human development and life, what are the three different sites for hematopoiesis?

Yolk sac,

after that,

Liver,

after that,

Bone marrow (after birth)

What are two types of risk factors for ectopic implantation?

Uterine factors: scars, cesarian delivery, miscarriage
Placental factors: compensation of the placenta for low nutrients (smoking, high altitude)

The question on the page originate from the summary of the following study material:

Lectures Human Development

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