Lecture 10: Principles of Embryology

Question 1

what happens on day 1

Answer 1

• ovum and sperm meet in uterine tube (usually ampulla)
• penetration of corona radiata, then only 1 sperm penetrates zona pellucida
• fusion and 2nd meiotic division
• acrosome reaction makes ovum impermeable to other sperm
• results in zygote with 46 chromosomes

Question 2

what happens on days 2-3

Answer 2

• rapid process of mitotic divisions (cleavage
• by day 3 there is a 16 cell embryo, each cell being a blastomere
• as they replicate, blastomeres get closer and communicate though gap junctions
• whole solid sphere is called the morula

Question 3

what happens on days 4-5

Answer 3

• morula develops a cavity (blastoceole)
• outer layer thins out and becomes tropheoblast, which forms placenta later on
• rest of the cells are pushed up to form the inner cell mass (embryoblast)

Question 4

label day 4-5 diagram on T1 L10

Answer 4

check

Question 5

what happens on days 6-7

Answer 5

• bilaminar disc forms, as the embryo implants it forms 2 layers, epiblast and hypoblast in contact
• epiblast will make up the embryo
• hypoblast forms extraembryonic membranes
• amniotic cavity develops within epiblast mass and epiblasts that line this cavity are known as amnioblasts

Question 6

label day 6-7 diagrams on T1 L10

Answer 6

check

Question 7

what happens on days 6-8

Answer 7

• formation of the primary yolk sac within blastoceal cavity

- this supplies the nutrients before the embryo functions

Question 8

label diagram of day 6-8 on T1 L10

Answer 8

check

Question 9

what happens in week 2

Answer 9

• blastocyst has reached the uterine cavity and is ready for implantation
• corpus luteum produces progesterone to maintain endometrium
• implantation initiates decidual reaction, which causes maternal cells to contribute to the placenta

Question 10

what are the stages of implantation

Answer 10

1) shredding of the zona pellucida (hatching)
2) apposition until embryonic pole in contact with uterine wall
3) adhesions via molecular communication
4) invasion, including development of trophoblasts

Question 11

what is the trophoblast

Answer 11

• the outer layer of cells of blastocyst
• differentiate into cytotrophoblast (inner single layer of cells) and synctiotrophoblast (outer layer later known as synctium)
• secretes HCG hormone that maintains uterus and placenta

Question 12

what happens in week 3 onwards

Answer 12

• gastrulation occurs, where the bilaminar disc develops further into 3 distinct layers
• initiated by primitive streak
• epiblast cells move through primitive streak to become ectoderm
• hypoblast replaced with cells from epiblast and become endoderm
• invading epiblast cells make mesoderm
• hypoblast degenerates

Question 13

what happens in week 4 onwards

Answer 13

flat disc folds into 2 directions

• day 18: lateral folding brings amniotic cavity down, creating the future gut tube inside peritoneal cavity
• day 21: longitudnal folding brings head and tail closer

Question 14

what does the ectoderm go on to form

Answer 14

• nervous system
• eyes
• ears
• skin

Question 15

what does the mesoderm go on to form

Answer 15

• skeletal, muscular and renal system

- cardiovascular and gastrointestinal system

Question 16

what does the endoderm go on to form

Answer 16

cardiovascular and gastrointestinal system