Week 1

Fertilisation + early growth

Day 1

Egg meets sperm

On day one, the egg is invaded by a sperm, together forming the zygote. Consisting of male and female pronuclei, cytoplasm surrounded by the egg’s glycoprotein shell.

Zygote: from zugotos, ‘two things joined in a close relationship’, because sperm and egg have joined to become one cell.

Female and male pronuclei reside in a simplified egg. The blue represents the zona pellucida.

Day 2

Union!

First mitotic division of the zygote occurs around 30 hours, producing two blastomeres.

Blastomere: from blastós, ’sprout’, and méros, ‘part or segment’, because one blastomere is one segment of the developing human-sprout.

Two blastomeres, still inside the zona pellucida.

Day 3

2 -> 4 -> 8 -> 16

Division continues, once 16 blastomeres are present they are together known as the morula.

Morula: literally ‘mulberry’, because this bunch of cells visually resembles a mulberry.

Does it look like a mulberry?

Day 4

An internal cavity appears, and morula is now blastocyst.

Fluid enters the morula, forming an internal cavity. The morula is now known as blastocyst. From now on, we will stop drawing individual cells, and will just show the overall shapes of structures.

Blastocyst: from blastós, ’sprout’, and cyst, ‘small fluid-filled sac’, because the human-sprout now has fluid within it.

Day 5

The blastocyst 'hatches' from the zona pellucida.

By day 5, the blastocyst has shed its outer shell, a remnant from the egg. Cells have separated into two distinct areas: trophoblast, which develops into the placenta and will feed the embryo, and inner cell mass, which develops into the embryo proper.

Trophoblast: from trephein, ‘to feed’, because the trophoblast becomes the placenta which feeds the developing embryo.

Trophoblast surrounding inner cell mass, both cut in the coronal plane.

Day 6

Implantation begins.

Trophoblast cells break down barriers in the uterine epithelium, allowing entry of the blastocyst.

Day 7

Meanwhile, the trophoblast begins to transform into two separate parts: cytotrophoblast on the inside, syncytiotrophoblast on the outside. The latter is one cell with many nuclei.

Syncytiotrophoblast: from syn-, ‘together’, cyto-, ‘cell’, because it is not many separate cells, it is one ‘together-cell’.

As implantation progresses, the trophoblast becomes syncytiotrophoblast & cytotrophoblast.