Pre-Implantation Development

Question 1

Define the start and end points of the pre-implantation embryo

Answer 1

The start point is an egg after fertilisation - a zygote. The end point is a blastocyst.

Question 2

Describe the structure of a zygote

Answer 2

A large cell containing protein, DNA, and RNA. It is a diploid cell.

Question 3

Describe the structure of a blastocyst

Answer 3

A blastocyst is made up of 3 different cell lineages. Around the outside of the blastocyst are trophoectoderm cells (TE) - these go on to form the placenta. At the superior pole of the cell are two lineages, epiblast (EPI) and primitive endoderm (PE). EPI cells are inbetween the TE and PE, and they go on to form all the tissues of the embryo in humans, in mice they make part of the placenta and first layer of yolk sac. The PE cells wrap around the EPI cells next to the blastocoel and form the yolk sac. The EPI and PE are collectively referred to as the Inner Cell Mass - ICM.

Question 4

Outline the steps that take place when the zygote turns into the blastocyst

Answer 4

Syngamy - The 2 pronuclei from sperm and egg fuse together, allowing the first cell division to proceed
Cleavage - Early cell divisions are cleavage divisions. The embryo stays the same size - there is no growth. At this stage, the cells are equivalent - called blastomeres.
Compaction - At the 8 cell stage, the cell boundaries become harder to see - morula (lumpy ball shape). Tight junctions form between the cells, and they have an inside and an outside - they become polar.
Inside vs Outside - The first decision the cells face. The cells on the outside become TE, inside cells become ICM. The decision is influenced by many factors.
Blastocoel formation - Developing TE cells secrete fluid which results in the formation of the blastocoel cavity
Early Blastocyst - The second cell choice, ICM —> EPI PE. Happens 24hr after the TE/ICM decision.
Late Blastocyst - In mice, the pre-implantation blastocyst has >100 cells in 4.5 days, 6-8 days in humans.