blastocyst dev

Question 1

when does the first cleavage division occur

Answer 1

as the conceptus is moving down the fallopian tube into the uterus

Question 2

how does the conceptus move to the uterus

Answer 2

• ciliated cells on fallopian tube
• smooth muscle contractions

Question 3

embryo transport requires

Answer 3

• progesterone
• oestrogen

Question 4

stages in preimplantation dev

Answer 4

• early cleavage divisions > smaller cells
• until morula (8-16 cell stage), then compaction - more cell-cell adhesion and highly polarised cells, leading to in/out localisation > differentiation and lineage segregation
• at blastocyst stage (32-64 cells), cavity develops internally. outer cells = trophoectoderm; inner cell mass = pluripotent, will form body and yolk sac

Question 5

function of ZP

Answer 5

• protects developing embryo so cells don’t fall apart
• prevents premature implantation

Question 6

describe blastocoel formation

Answer 6

• presence of tight junctions along basolateral membrane of external cells separates internal cells from environment
• also allows establishment of ion gradients for blastocoelic cavity; Na+ are pumped into space and H2O follows passively

Question 7

what happens to the trophoectoderm

Answer 7

• forms trophoblast cells of the placenta
• for nutrition/support

Question 8

inner cell mass - fate

Answer 8

• forms all 3 germ layers
• and extraembryonic mesoderm which contributes to the placenta

Question 9

what controls first few cleavage divisions

Answer 9

products of oogenesis, and therefore maternal genome

Question 10

when is embryonic transcription initiated

Answer 10

4 to 8 cell stage

Question 11

pattern of transcription in embryos

Answer 11

expression of genes restricted to certain cells due to position
- OCT4, NANOG, GATA6 - restricted to inner cells of morula and blastocyst
- CDX2, GATA3, EOMES - outer cells, for trophoectoderm

Question 12

how is restriction of gene expression achieved

Answer 12

by the kinase Hippo
- non-polarised: phosphorylates Yap, can’t enter nucleus, so cannot activate Cdx2
- Hippo suppressed in polarised cells so Yap + TEAD4 > Cdx2 & trophoectoderm cell fate

Question 13

where does fertilisation occur? characteristic? why?

Answer 13

• ampulla of fallopian tube
• oxygen-poor
• totipotent cells need low O2

Question 14

where does preimplantation embryo get nutrients from

Answer 14

• secretions from fallopian tube
• oviduct cells, then endometrial glands of uterus

Question 15

metabolic req of embryo at each stage of dev

Answer 15

2-cell: pyruvate + lactate (no net growth)
8-cell: glucose, essential amino acids, proteins & nucleic acid
blastocyst: simple sugars; 70% consumed O2 for ionic pumping (blastocoel)
later: placenta