Week 3 Stages in Development from fertilized egg to Later blastocyst

Question 1

Stages (without elaboration)

Answer 1

A. Fertilized egg
B. 2 cell stage
C. 4 stage stage
D. 11 cell stage morula, uneven development observed
E. Blastocyst stage consisting of an ICM and a TR
F. Later blasyocyst

Question 2

Post-Fertilization Events

Answer 2

Zygote becomes metabolically active
Zygote becomes mitotically active, begins to undergo cleavage
Zygote is transported down uterine tube to uterus
Journey takes several days
Zygote loses Zona pellucida prior to implantation
Implantation

Question 3

Cleavage Events

Answer 3

Zygote —> 8 cell stage embryo

Blastomeres adhere via gap junctions and appear to love their individual identity

involves E-cadherins and other calcium-dependent CAMs

Zona Pellucida is still intact

Question 4

How does blastocoel form?

Answer 4

occurs 16 cell morula stage

Water is transported into the ball of cells via Na and K-ATPase transporters

blastocoel forms as a result of cavitation

Question 5

Blastocyst = what number of cells?

Answer 5

58 cells, has blastocoel, has zona pellucida

Question 6

Blastocyst Features

Answer 6

Surrounded by Zona Pellucida
Large, eccentrically placed blastocoel
consists of ICM and TB

TB forms placenta and extraembryonic structures
ICM forms embryo plus some extraembryonic structures

Blastocyst is polarized
Embryonic pole marks pole of the blastocyst where the inner cell mass is located

Abembryonic pole marks the opposite pole

Question 7

Genetic control of cleavage

Answer 7

maternal gene products are produced but usually degraded by the 2 cell stage

by 4 cell stage, most transcription is via embryonic genome

Question 8

Epigenetic regulation and methylation cycles

Answer 8

methylation occurs when growth isn’t occurring or stalled: demethylation activates development (broadly speaking)

1. DNA of mature eggs and sperm are methylated
2. shortly after fertilization demethylation occurs
3. Remethylation of inner cell mass occurs until late blastocyst state (32-64 cell stage)
4. methlyation levels fall after primordial germ cells enter genital ridges
5. remethylation occurs later during gametogenesis and may lead to maternal/paternal imprinting

Question 9

between fertilization and 8 cell embryo….

from the morula (8-16 cell) stage until blastyocyst formation…..

Answer 9

the embryo’s genome is demethylated

methylation increases