Inside the Lab: Dear Diary, I’m Five Days Old

Day 1... Dear Diary Apparently I exist now

Dear Diary,

Big news.

Yesterday, an egg and a sperm met, and today someone in the lab is already checking up on me.

It has been roughly 16–17 hours since fertilisation, and the embryologist is looking closely at me under the microscope.

They are looking for two pronuclei inside me: one containing the genetic material that came from the egg and one containing the genetic material that came from the sperm.

They are also checking my polar bodies. Seeing two pronuclei and two polar bodies is the classic appearance of a normally fertilised zygote.

Or at least, that’s what I heard them saying while they were checking on me.

For a little while, the two parental sets of genetic material are still enclosed inside their own pronuclei.

Not for long.

The pronuclei will move closer together and eventually their nuclear envelopes will break down. Their chromosomes will then be organised for my first mitotic division. And that is when I will begin dividing into two cells.

For now though?

I am officially a zygote! One cell, still surrounded by my zona pellucida, the protective glycoprotein layer that has surrounded the oocyte since before fertilisation.

Inside that one cell, though, there are now two parental genomes preparing for the first division of a completely new embryo.

See you tomorrow.

Day 2... Dear diary, I’ve started dividing.

Dear Diary,

Well, that did not take long!

Yesterday I was one cell. Today, I have started dividing into smaller cells called blastomeres.

By the time the embryologist checks on me, I may already be around four cells, although I heard them saying that not every embryo follows exactly the same timetable.

Apparently, four cells is roughly what they hope to see around this point.

I heard them talking again.

Something strange is happening, though.

I am making more cells, but I am not really getting bigger.

I am still enclosed inside the same zona pellucida, so each time I divide, the cytoplasm I started with is simply being shared between smaller and smaller blastomeres.

More cells. Same little package. Meanwhile, the embryologist is watching very closely.

How many blastomeres do I have? Are they the size they would expect for this stage? Does each one appear to have a single nucleus?

They were also talking about something called fragmentation.

Apparently, fragments are small membrane-bound pieces of cytoplasm that can appear between my blastomeres while I am dividing.

A little fragmentation can happen, so it does not automatically mean something is wrong with me. What matters is how much there is, and what the rest of me looks like at the same time.

And that is not all they are checking.

They are also looking at the nuclei inside my blastomeres. Excuse me, madame. A little privacy, please!

Ideally, each blastomere should have a single visible nucleus. If one has more than one, that is called multinucleation.

So yes.

I am two days old, microscopic, and already being assessed on my cell number, cell size, nuclei and fragmentation.

Apparently... even embryos get performance reviews.

Okie dokie... No pressure...

Day 3... Dear diary, I’m starting to take over.

Dear Diary,

Things are getting busy.

By the time the embryologist checks on me today, I may be around eight cells. I had a quick look at my neighbour (the embryo in the dish next door) and apparently they are not quite at the same stage as me.

I was starting to feel a little competitive.

But then I heard the embryologists say that this can be completely normal. Embryos do not all develop at exactly the same speed, and a slightly different cell number at the same observation time does not automatically mean something is wrong.

Right. No comparing myself to the neighbours, then.

Eight cells is roughly what they expect to see around Day 3.

But today, something much bigger is happening than simply adding more blastomeres.

Until now, a lot of my early development has depended on RNA and proteins that were already stored inside the oocyte before I was even fertilised. Very thoughtful of the oocyte, really.

But those supplies were never meant to run things forever.

My own genes have actually shown some activity before now, but around the eight-cell stage, the major wave of embryonic genome activation gets underway and at the same time, many of the maternal RNA molecules I inherited from the oocyte are being broken down.

Scientists call this bigger handover the maternal-to-zygotic transition.

Basically, I am beginning to rely much more heavily on instructions coming from my own genome rather than mainly on molecular instructions that were packed into the oocyte for me. Quite a promotion for someone who is only three days old...

I am still going through cleavage divisions, still enclosed within my zona pellucida, and still not growing very much in overall size. I simply have more, smaller blastomeres inside the same little package.

And yes, the embryologist is still watching me. Cell number. Cell size. Fragmentation. How I have been dividing.

Apparently the performance reviews continue.

One more thing: if I am developing quickly, my cells may begin showing the very first signs of becoming more closely associated around this stage.

Compaction is usually associated with the transition toward the morula stage and becomes much more obvious later, so no dramatic transformation just yet.

For now, the biggest change is happening somewhere you cannot see just by looking at me under the microscope. Haha. Got you!

Day 4... Dear diary, we’re getting organised!

Dear Diary,

Okay... I look very different today.

Until now, the embryologist could pretty easily see my individual blastomeres.

Not anymore!

My cells are flattening against one another, increasing their contacts and pulling themselves into a much tighter structure.

Apparently this is called compaction.

It can begin from around the eight-cell stage, but by Day 4 it should be much more obvious as I make my way towards becoming a morula.

And here is the funny part.

Yesterday they were counting my cells. Today? Good luck!

Once most of my blastomeres become involved in the compacting mass, it becomes increasingly difficult to tell exactly where one cell ends and another begins. Embryologists therefore start paying more attention to how much of me has compacted rather than trying to count every individual cell.

But compaction is not just about making me look tidier.

My cells are also beginning to behave differently depending on where they are; Cells exposed on the outside start developing different polarity and contacts from cells positioned deeper inside me. These inside-versus-outside differences are some of the earliest steps toward the cell organisation that will become much more obvious when I form a blastocyst.

So no, I do not have a proper inner cell mass and trophectoderm that you can clearly identify yet.

That is tomorrow's drama. Ahhh I love being an embryo!

For now, I am becoming a tightly compacted morula, still safely enclosed inside my zona pellucida.

And if I am moving particularly quickly, a tiny fluid-filled space may even begin to appear as early cavitation starts.

I looked over at my neighbour again. Still not there yet.

Apparently that can be completely normal too. Some embryos reach these changes a little earlier, others a little later. Okayyyy... I need to stop comparing myself to the neighbour.

Current ESHRE/ALPHA guidance recognises both fully compacted morulae and morulae beginning early cavitation as expected Day-4 developmental appearances.

Day 5 — Dear diary, I’m a blastocyst.

Dear diary,

I have had a major upgrade.

Fluid has started collecting inside me, creating a cavity called the blastocoel. That means I am now at the blastocyst stage! Or... at least well on my way there.

Not every embryo reaches this stage at exactly the same time, but this is a common point for it to happen.

I no longer look like just a compact ball of cells.

Now, two main cell populations can be recognised.

The group of cells gathered on the inside forms the inner cell mass, which will give rise to the embryo proper.

The cells forming the outer layer are the trophectoderm, which will contribute mainly to extraembryonic tissues, including the fetal component of the placenta.

As I expand, the embryologist may start grading me by looking at how expanded I am, and at the appearance of both my inner cell mass and trophectoderm.

If I keep developing well, I may be transferred, vitrified for later use, or, when clinically indicated, a small number of my trophectoderm cells may be biopsied for PGT.

If I expand even more, my zona pellucida may begin to thin, and I may even start hatching.

Now I am a structured blastocyst with distinct cell populations and a fluid-filled cavity!

Oh... quite a week.

Dear diary, I think I’ll need a bigger page tomorrow.