Gastrulation - 7th to 10th Day

Axis formation in mammals is poorly understood and as in eg. xenopus there is an animal-vegetal pole and dorsal-ventral axis (which I won't cover in here) there is no such things in mammalian embryo. Only "axis" that can be said to exist is embryonic-aemryonic axis according to the Inner Cell Mass. But this axis is only geometrical, it has nothing to do with the future germ layers.

Blastocyst implants into the uterine wall and the trophectoderm gives rise to extra-embryonic ectoderm. The gastrulation involves cell movements which yield a three-dimensional embryo, where the germ layers, ectoderm, mesoderm and endoderm are in correct positions. The gastrulation is initiated by the formation of primitive streak and the cells move inside as individual cells, giving rise to endoderm and mesoderm.



The embryo is now 10 days old.

This part of the embryo development wasn't covered in our course, only the gastrulation of sea urchins and xenopus was covered and as we are not interested on them, I will not go through them ;)

Cleavage - First Week

The cleavage include rapid cell divisions and during these cell divisions the size of the embryo is no increasing. It is still the sized of an egg cell. The egg cell has nutrients and the embryo needs to rely on these nutrients during the first fast cell divisions. Normally cell division (mitosis) includes several steps and it lasts about 16 hours but during cleavage the cells skip most of the steps and and there is no transcription of the genes, thus the cell division lasts only 30 minutes (in mice). After the first few cell divisions the embryo is called morula.

At this stage the cells become compacted when the blastomeres maximize the cell contacts. The cells become polarized and compaction yields inner and outer cells. Some of the Inner Cell Mass (ICM) form gap junctions and these cells give rise to the embryonic tissues.

The outer cells become trophoblast and give rise to extra embryonic membranes. The trophoblast also secretes fluids into the morula and forms blastocoel in the late cleavage. The developing embryo becomes blastocyst (hollow ball of cells).

After the formation of blastocyst, the embryo is ready to go to gastrulation to form multilayered embryo.

Here's the time window for these events:

• Day 1: First cell division
• Day 2: Second cell division (4-cell stage)
  
• Day 3: 6-12 cell stage
• Day 4: 16-32 cell stage and the embryo becomes morula
• Day 5: Embryo forms blastocyst
• Days 6-7: Embryo implants into the uterus wall