The First Three Weeks of Development

Question 1

time frame of embryo

Answer 1

week 1-8

Question 2

time frame of fetus

Answer 2

week 8-36

Question 3

What causes most deaths during the first year post-natal (infancy)?

Answer 3

developmental anomalies cause the most deaths during infancy

Question 4

morphogenesis

Answer 4

activities & processes that take place during embryology

Question 5

induction

Answer 5

tissue or cell tells another cell to do something (factor acts as intermediary)

Question 6

coelom

Answer 6

empty space inside organism not connected with outside

Question 7

zona pellucida

Answer 7

proteoglycan shell that keeps the egg substance together

Question 8

corona radiata

Answer 8

accumulation of cells around outside of egg

Question 9

Describe the formation of a zygote

Answer 9

• fusion of pronuclei –> first time in which there is the full complement of chromosomes in human
• happens in Fallopian tube

Question 10

blastomere formation

Answer 10

• zygote splits into two cells where the mass of the two cells equals the mass of the original cell (the daughter cells are half the size of the first)
• protoplasm being reduced

Question 11

morula formation

Answer 11

• zona pellucida still exists
• at 16 cell phase –> no longer dividing in sequence
• individual cells, especially cells around edge, develop tight junctions
• ring of cells that are no longer free to float around (morula)

Question 12

blastocyst formation

Answer 12

• cells on outside are pumping fluid into cavity and pushing inside cells to one end (compaction –> process developing inner cell mass)
• all cells are identical (have same potential) except for their position
• usually in the uterus, ready to embed into wall of uterus

Question 13

What is a possible result of blastocyst formation in the Fallopian tube?

Answer 13

• may try to embed in Fallopian tube leading to an ectopic pregnancy

Question 14

hatching

Answer 14

• zona pellucida starts to degenerate
• free to expand
• cells no longer cuboidal –> flatten and stretch out due to one way pumping by exterior cell group
• inner cell mass becomes embryo and exterior cell group supports embryo (trophoblast)

Question 15

blastocyst attachment to endometrium

Answer 15

• blastocyst rolls around slowly on wall of uterus inducing increased concentration of glands and blood vessels
• when end with inner cell mass gets close, trophoblast attaches to endometrium

Question 16

syncytiotrophoblasts

Answer 16

• produced by cytotrophoblasts just outside of attachment point to endometrium
• cells are multinucleate (tissue is called syncytium)
• produce enzymes that are inverse and break up ECM of maternal tissue
• cells move into empty space (maternal tissue also building up and embracing embryoblast)

Question 17

bilaminar embryo

Answer 17

• mass develops into upper (epiblast/ectoderm) and lower (hydroblast/endoderm) layer

Question 18

8 day embryo

Answer 18

• cells facing fluid-filled cavity (hypoblasts) develop into simple cuboidal epithelium
• cells facing maternal tissue (epiblasts) become pseudostratified columnar, make an empty space that becomes the amniotic cavity
• endometrium proliferating and completely encapsulating embryo

Question 19

What cells produce the amniotic fluid in the growing amniotic cavity?

Answer 19

• epiblastic derivatives

Question 20

Discuss the formation of Heuser’s membrane

Answer 20

• from hypoblasts, cells forming edge and creating lining for cavity
• cover cytotrophoblasts

Question 21

What function to the trophoblastic lacuna play in the developing embryo?

Answer 21

• trophoblastic lacuna are fluid filled spaces in the trophoblast
• maternal blood vessels will invade area and eventually dump blood into the lacuna but this blood does not come into contact with the embryo yet

Question 22

What function does the coagulation plug have?

Answer 22

• coagulation plug allows doctors to endoscopically identify where the embryo has embedded in the uterine wall

Question 23

What cells produce the fibrous acellular reticulum just outside Heuser’s membrane?

Answer 23

• cytotrophoblasts

Question 24

What cells produce the extraembryonic mesoderm?

Answer 24

• hydroblasts produce this subset of cells that move along Heuser’s membrane and along the cytotrophoblasts

Question 25

primary yolk sac

Answer 25

edge is continuous with edge of hypoblast (fluid of primary yolk sac touching top side of hypoblast

Question 26

Where does the chorionic cavity being to form?

Answer 26

- forms in the extramembryonic reticulum

Question 27

What layer of cells comes down and pinches primary yolk sac?

Answer 27

- extraembryonic mesoderm | - will cause the creation of the definitive yolk sac with the outer remnant being pushed away

Question 28

What function do the primary stem villi serve?

Answer 28

- embryo is getting big and needs to be anchored in the endometrium - cytotrophoblast make villi projections that extend all the way around outside of embryo to keep it fixed in endometrium

Question 29

connecting stalk

Answer 29

- holds embryo within the huge chorionic cavity | - becomes the umbilical cord

Question 30

gastrulation

Answer 30

- beginning is the formation of the primitive streak - epiblast producing mesoderm - epiblastic cells leave from primitive streak, turning into mesoderm, and moving into space between ectoderm and endoderm - cells repopulate middle AND bottom layers --> epiblastic derivatives move into middle and push hypoblasts out to become definitive endoderm

Question 31

primitive streak

Answer 31

- formation signals the beginning of gastrulation - long axis along endoderm - more toward tail end - little dimple with edge that surrounds --> lifting up causes groove

Question 32

What causes the dimples in the endoderm? What are they called?

Answer 32

- the dimples are caused from the fusing of embryonic ectoderm and endoderm - head region dimple = buccopharyngeal membrane (becomes mouth, etc.) - tail region dimple = cloacal membrane (becomes anus, etc.)

Question 33

What does gastrulation give rise to?

Answer 33

1. definitive endoderm | 2. intraembryonic mesoderm

Question 34

What cell layers are derived from the epiblasts?

Answer 34

All three (ectoderm, mesoderm, and endoderm)

Question 35

lateral plate mesoderm

Answer 35

group of epiblasts that move laterally to the primitive streak

Question 36

paraxial mesoderm

Answer 36

group of epiblasts that move up and out from primitive streak

Question 37

notochordal process

Answer 37

- group of epiblasts that move from dimple and move longitudinally - cells knock away endoderm and then they themselves leave - amniotic cavity and definitive yolk sac in contact for a short amount of time

Question 38

What are the neural crest cells?

Answer 38

- ectodermal derivative

Question 39

What causes the mesoderm to reorganize at the end of the third week?

Answer 39

- caused by the folding of the neural tube

Question 40

Describe the process by which the embryo develops primitive circulation

Answer 40

- secondary villus --> some of embryonic mesoderm - tertiary anchoring villus --> creates circulation (the blood vessels that are found in the umbilical cord) - maternal and embryonic circulations do not cross over at this point