Have you ever wondered why the human body is shaped the way it is? The answer goes back to when a fertilised egg was growing in the mothers womb.

This series of articles explores why it is that our guts grow on the inside and our skin on the outside, and why this is important for you and me.

The previous article explored the morula; a ball of cells that implants in the uterus, and the two layers inside, which include a shell that goes on to help form the placenta (provides blood supply to the baby) and an inside group of cells that go on to make the end result – the human body.

To do so, the inside group of cells divide into two layers, to make an upper epiblast and lower hypoblast layers. A cavity also forms in the epiblast layer, known as the amniotic cavity, which fills with amniotic fluid. There’s also a lower cavity beneath the hypoblast layer, known as the chorionic cavity.

Later on, the two cavities fuse and line the outer region of the sac the baby develops in. When the mother is nearly ready to give birth, the “water breaks”; it is that sac and all the fluid inside that breaks.

At this point this sphere of cells, lodged into the mothers uterus, is divided along the equator by two layers of cells. The next step is really important: the top layer – the epiblast – folds inwards on itself into the middle, creating a new layer of cells in between the epiblast and hypoblast below. This process is called gastrulation, and turns the two cell layer into a three layer one.

This is a key point in embryology, the morula has now become the embryo and the rough outline of the shape of the tiny body can be seen.

You may well be thinking that only 3 layers of cells thick seems far too little, however, if you think of it in relation to an adult human being it makes more sense. Humans are divided into three layers; we have an outside- the skin, a middle – the bone, muscle, connective tissue, and all the inside parts- like the guts.

So, using this structure, it becomes clearer where each layer of the embryo goes: the most outside part of the embryo is the ectoderm, which forms the skin; the middle bit is the mesoderm which forms bone; the most inside bit is the endoderm, which forms the intestines and stomach. Cool, right?

As you can see this is a really important time for the embryo. Here, at three weeks, when the embryo is forming three layers inside the mother’s uterus, it is completely depend on the mother for food, oxygen… everything really.

All these things travel in the blood, and if the mother drinks any alcohol it travels to the embryo too. So what happens if the mother does drink alcohol?

The embryo begins to get ‘clumsy’, the three layers of cells get a little uncoordinated when trying to grow into our organs. This is very bad for the developing embryo because if the embryo grows the wrong way, there’s a chance it could be born with badly formed body parts.

It also worth noting here that at this stage the mother might not even know she is pregnant as pregnancy tests work around the same time that the two and then three layers of cells are forming.

Up next, the three layers of cells are about to divide uncontrollably into every body structure you possess. Viewed another way, that is a group of around one thousand cells that need to grow in the correct place, in the correct pattern, at the correct time to form the billions of cells in our bodies.

How on earth is that achieved? Look out for part 3 to find out.

Oliver Davis

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