Week 2

Bilaminar disc + internal cavity formation

Where we left off on day 7.

To simplify things, and match the imagery in most textbooks, we'll remove the endometrium now, and rotate our view 180˚, like so:

Day 8

The inner cell mass, in red. Bear in mind the scale of these images is wilfully inaccurate.

The inner cell mass differentiates into hypoblast and epiblast. A space develops in the epiblast - the amniotic cavity.

Hypoblast: from hypo-, ‘below’, blastós, ‘sprout’

Epiblast: from epi-, ‘above’, blastós, ‘sprout’

Amniotic: from amnion, Latin for “membrane around a foetus”.

Epiblast in light blue, with amniotic cavity inside it. Hypoblast in yellow.

Day 9

Hypoblast cells migrating across the interior of the cytotrophoblast.

Meanwhile, cells of the hypoblast have migrated to replace the blastocyst cavity with the primitive yolk sac. Hypoblast and epiblast together are here known as the bilaminar disc.

Bilaminar: means 'two-layered'

The hypoblast sits atop the primitive yolk sac. The epiblast sits at the base of the amniotic sac.

Day 10

The vessel walls of sinusoids are broken down by the syncytiotrophoblast.

By day 10, the blastocyst is now firmly implanted in the endometrium. The syncytiotrophoblast breaks into maternal capillaries known as sinusoids, effectively co-opting their blood for the developing embryo.

Maternal blood flows in to provide nourishment for the developing embryo.

Day 11

In orange, the extra-embryonic mesoderm.

On the inner surface of the cytotrophoblast, tissue known as extra-embryonic mesoderm has developed. This eventually develops cavities which form the extra-embryonic coelom. The result is extra-embryonic mesoderm adhering to cytotrophoblast and amniotic sac, where it is known as somatopleuric, and to the yolk sac, where it is known as splanchnopleuric.

Splanchnopleuric: from splánkhna, “viscera” & pleurá, “a side of something”, because this mesoderm adheres to the viscera-side (the yolk sac becomes the gastrointestinal system).

Somatopleuric: from sôma, “body”, & & pleurá, “a side of something”, because this mesoderm adheres to the ‘body-side’.

Between the cytotrophoblast and developing embryo now lies the extra-embryonic coelom.

Day 12

The exocoelomic cyst separates from the primitive yolk sac.

The yolk sac pinches off part of itself, leaving us with a remnant of the primitive yolk sac: the exocoelomic cyst; and the secondary yolk sac, which we’ll from now on just call the yolk sac.

Exocoelomic cyst: meaning ‘fluid filled sac outside the main cavity’

The primitive yolk sac is now referred to as the secondary yolk sac, or simply, the yolk sac.

Day 13

Highlighted in blue is the chorionic cavity.

The space known as the extra-embryonic coelom has expanded, and is now known as the chorionic cavity. The extra embryonic mesoderm linking the cytotrophoblast to the embryo is known as the connecting stalk, which will eventually become the umbilical cord.

Chorionic: from chorion, meaning ‘outer membrane of the foetus’.

Highlighted here is the connecting stalk.

Day 14

Cranial to the left, caudal to the right.

Taking a closer look at the bilaminar disc now.

Cells in the epiblast begin to differentiate and burrow underneath. This looks like a burrow down the caudal end of the epiblast. It’s known as the primitive streak, and seeing that marks the beginning of gastrulation.

Gastrulation: means ‘becoming a gastrula (a trilaminar embryonic disc - more on that in week 3)

The primitive streak, to the right.