Early Mammalian Development

Question 1

After the one cell stage the embryo divides into 2-4 cells.

What are the important properties of the embryo at this next stage?

Answer 1

1. cells at this stage are called Blastomeres
2. Zona pellucida present.
3. First cleavage occurs at about 24 hours.

Question 2

What happens at 4-16 cell stage?

Answer 2

cells at the 8 cell stage are called Morula go from uncompacted to compacted
Zona pellucida is present.

Question 3

What is compaction?

Answer 3

process in which multiple cells are pressed together to get them ready for differentiation.

Question 4

What happens at 16 - 40 cell stage?

Answer 4

1. Blastocyst begins to form because cells begin to differentiate into ICM and TE. evident from the blastocoel that begins to appear.
2. Zona pellucida present.

Question 5

what happens next?

Answer 5

Blastocyst hatches as it looses its Zona Pellucida

Question 6

What happens next?

Answer 6

blastocyst attaches to the uterus and is now implanted - first evidence of embryonic endoderm cells covering the blastocoelic surface of the inner cell mass [which means epiblast begins to form].

Question 7

What happens next? Stage: Implantation

Answer 7

1. blastocyst takes the shape of an egg cylinder
2. Ectoplacental cone appears.
3. Rapid increase in the number of inner cell mass cells leading to the formation of the epiblast and hypoblast.
4. Hypoblast lines the mural trophectoderm.

Question 8

What happens next? Stage: blastocyst egg cylinder differentiates.

Answer 8

egg cylinder differentiates into embryonic and extra-embryonic regions and the formation of the pro-amniotic cavity begins.

Question 9

What happens next? Stage: Advanced egg-cylinder stage

Answer 9

1. embryonic axis begins to form.
2. embryonic and extra-embryonic ectoderm have fully formed.
3. This is the stage just before Primitive Streak forms (pre-streak)

Question 10

What happens next? Stage: Early streak

Answer 10

1. first mesodermal cells are produced that are slowly forming primitive streak which will mark the site of gastrulation.

Question 11

What happens next? Stage: Mid streak

Answer 11

1. Amnion begins to form –

2. Tissue at the posterior end of the primitive streak bulges into the pro-amniotic cavity and forms the amniotic fold

Question 12

What happens next? Stage: MS - LS (mid-streak to late streak)

Answer 12

In the mesoderm of the posterior amniotic fold small cavities coalesce to form a single cavity - the exocoelom

Question 13

What happens after the primitive streak forms?

Answer 13

1. The allantoic bud first appears
2. After primitive streak forms gastrulation begins
3. cells from PS and epiblast undergo EMT and ingress at PS
4. Germ layers form and the node becomes visible.

Question 14

What happens after gastrulation ends? [Stage 11A]

Answer 14

1. Organogenesis begins
2. Neural Plate forms and embryo prepares to generate somites.
3. The amniotic cavity is now sealed off into three distinct cavities - the amniotic cavity, the exocoelom and the ectoplacental cleft.
4. The neural plate is defined anteriorly and the head process is developing. In the midline, subjacent to the neural groove, the notochord plate is visible.