13 - Zygote to Embryo 1

Question 1

The result of fertilization is the formation of a single diploid cell called ______.

Answer 1

A zygote.

Question 2

Formation of the zygote initiations the process of _______?

Answer 2

Prenatal embryonic development.

Question 3

We can collectively group the events of the first three weeks of embryonic development as __________?

Answer 3

Embryogenesis - the formation of an embryo.

Question 4

Human embryonic development is traditionally divided into two periods called _____ and _____.

Answer 4

The embryonic period

The fetal period.

Question 5

What occurs during the embryonic period?

Answer 5

Zygote (1 cell) becomes an embryo with a body plan, 3 layers, and 3 axis.

3 layers become 4 tissues, which become organs and organ systems.

Segmentation, head, link and trunk formation, and embryo folding occurs.

Question 6

How long is the embryonic period?

Answer 6

8 weeks, the end of which gets you to an embryo.

Question 7

Describe the fetal period of embryonic development?

Answer 7

Unborn baby - growth and maturation occurs.

Continued development of some systems.

Question 8

What is the perinatal period? Why is this a clinically important period?

Answer 8

Starts during fetal period and ends after fetal period.

From 22 weeks to 1 month after birth.

This is usually when premature births occur.

Question 9

What is the neonatal period?

Answer 9

The first month after birth (also perinatal period).

Question 10

How is human embryonic development divided in the clinics?

Answer 10

Into three 12 week trimesters.

Question 11

When does fertilization age start? What about the gestational age?

Answer 11

Begins at fertilization - this is the timeline that the embryo runs on.

Starts at last menstruation - two weeks later than fertilization age.

Question 12

What does it mean when you say someone is 12 weeks pregnant?

Answer 12

That is their gestational age.

Their fertilization (embryo) is actually only ten weeks.

Question 13

What occurs during the first week of embryonic developement?

Answer 13

Fertilization, cleavage, blastocyst formation, bilaminal embryonic disc.

Goes from one cell to solid ball of cells to fluid filled cyst with disk.

Question 14

What is the next step after zygote formation?

Answer 14

Cleavage.

Question 15

What is compaction?

Answer 15

Occurs after cleavage - it’s when the cells that divided in the cleavage stage loose their distinction.

Question 16

Describe the cell division during the cleavage stage?

Answer 16

Rapid; every 12-24 hours with NO G1 or G2.

Controlled and asynchronous.

No cell growth so the ratio of cytoplasm to nucleus goes from 600:1 to 3:1

Question 17

What controls the first cell division during cleavage? When is the embryo genome activated? What is needed next?

Answer 17

The maternal program.

Embryo genome activated at 2-4 (4-6?) cell stage.

New proteins needed for cleavage and blastocyst formation.

Question 18

Are all cleavage-stage blastomeres (cells formed by cleavage of zygote) the same? What is this knowledge the basis for?

Answer 18

Yes.

Basis for cloning. Nucleus of an adult cell can be removed and put into an oocyte with no nucleus to clone whatever animal the nucleus came from.

Question 19

In mammalian embryos, each blastomere up to about the 8 cell stage has the potential to form _____? Whose experiment determined this?

Answer 19

A complete embryo if isolated and allowed to develop - Spemann experiment.

But if it stars in place during development it only contributes to part of the embryo.

Question 20

How does a blastocyst form? When does this occur?

Answer 20

Day 4/5

1. 8 cell stage has two cell lineages: outer cells and inner cells
2. Compaction occurs to create a morula, a solid ball of cells
3. Cavity formation in uterine cavity creates a blastocyst:
   outer cells become the trophoblast and inner cells become the inner cell mass (ICM)

Question 21

The ______ becomes the first tissue of the embryo which is an ______ tissue. Describe the cells in this tissue.

Answer 21

Trophoblast.

Epithelium: cells joined by junction and form layers. Cells polarize. Cell layer lines cavity or covers surface.

Question 22

What type of tissue does the trophoblast form? What does this require?

Answer 22

Extra-embryonic tissue: membranes that surround the embryo but don’t form the parts of the embryo itself.

Require paternal derived alleles (genomic imprinting)

Question 23

What happens to the inner cell mass?

Answer 23

Most of these cells become part of the embryo.

ICM remodels into bilaminar disc containing the epiblast (primitive ectoderm) upper layer and the lower hypoblast (primitive endoderm) layer.

Question 24

What shape are cells in the epiblast layer? What about the hypoblast layer?

Answer 24

Epiblast: taller, columnar-like

Hypoblast: flatter, squamous-like

Question 25

What is the last stage of cleavage? What are the functions of this?

Answer 25

“hatching” from the zona pellucida.

ZP keeps blastomere together, prevents premature implantation, and is an immunological barrier.

Question 26

What is regulation as it pertains to an embryo in the cleave stage?

Answer 26

The ability of an embryo/organ to produce a normal embryo/structure when parts have been removed or added.

Question 27

How does the embryos ability for regulation change?

Answer 27

It decreases with age of the embryo: 0-8 cells are totipotent, while 16+ cells only ICM cells are pluripotent.

Question 28

What is an example of regulation by an embryo?

Answer 28

You can destroy some blastomeres and still have normal development occur because the embryo can compensate for what is lost.

Question 29

What is a chimeric embryo? Is it viable?

Answer 29

Embryo composed of cells from different zygotes that can be generated during cleavage.

Still possible to get a normal embryo.

Question 30

How do blastomeres begin to exhibit cell diversity? What are factors that are believed to mediate cell diversity.

Answer 30

Certain blastomeres have different fates such as trophoblast, epiblast, and hypoblast.

Factors: cell position, number of cell divisions, cytoplasmic determinants, cell-cell interactions.

Question 31

Describe the levels of potency during embryonic development?

Answer 31

Totipotent: has all developmental pathway options

Pluripotent: several developmental pathway options

Bipotent: 2 developmental pathway options

Unipotent: 1 option (terminally differentiated).

Question 32

The term differentiate or differentiation are often used to describe all phases in the process of ____?

Answer 32

Limiting a cell’s potency.

Question 33

What is a characteristic of terminally differentiated cells?

Answer 33

They have stable phenotypes and cell divisions result in daughter cells with the same phenotype.

Terminal differentiation does NOT mean it’s post-mitotic.

Question 34

Define stem cells?

Answer 34

Differentiated cells which retain some pluripotent capabilities under appropriate influence.

Question 35

Is gene expression during embryonic development random?

Answer 35

No. It is patterned and follows a specific plan.

Question 36

Describe gene expression during embryonic development.

Answer 36

Results in recognizable morphological arrangements.

Occurs at different levels: molecular-organism

Occurs along axes

Controlled by several families of TFs.

Question 37

What are homeotic mutations an example of? What is an example?

Answer 37

Abnormal patterning during development.

Differential expression of HOX genes control regional patterning in embryos.

Question 38

Very rarely, division can occur after the formation of the embryonic disc. What does this result in?

Answer 38

Formation of conjoined (parasitic) twins.

Question 39

How can multiple embryos be formed (ie twins) ?

Answer 39

If the ZP opens up and multiple embryos grow.

Question 40

What are the stages in which twins can form? What percentage is each?

Answer 40

65% occur from divisions at the blastocyst stage: two inner cell masses instead of one. They have the same placenta but different amnions.

35% occur from divisions between 2-cell stage to morula. Independent embryos with the same genome. Separate placentas and amnions.

Question 41

What are the three basic steps of development during week one?

Answer 41

1. Fertilization: 2N restored, genetic sex determined, cleavage initiated
2. Cleavage: multicellularity, blastocyst formation, first recognizable cell lineages (trophoblast, ICM), bilaminar disc formation.
3. Implantation initiated (bridges week 1 and 2): blastocyst attachment to uterine endometrium