A. Embryological period:
1.

Here we have another organ system that develops during the embryonic and foetal period and ready to operate after birth.

2.
As the endoderm folded, as discussed in a previous section, a tube forms running through the embryo.
3.

Very soon, three regions can be discerned in this tube:

  1. the foregut
  2. the midgut
  3. the hindgut
4.

The foregut ‘runs’ from a closed membrane (buccopharyngeal membrane) to the midgut. It will later open as the mouth and part of the pharynx (as you may have noticed in its name; pharynx!).

5.

The hindgut runs from the midgut towards another closed membrane (cloacal membrane). This membrane will later open as the anal canal.

6.
The midgut connects both caudal and distal guts to each other and to the yolk sac.
7.

The foregut will later develop into the pharynx, oesophagus, stomach and the first part of the duodenum.

8.

The midgut will develop into the second part of the duodenum, the small intestine and the first part of the large intestine.

9.

The hindgut will develop into the rest of the large intestine, the rectum and anus.

B. Foetal period:
1.

A few weeks later, several buds develop along the three guts, gradually leading to several new organs, both of the GI-system but also other organs such as the lungs and the liver.

2.

Some of these (sub)organs develop quite straightforward such as the oesophagus. It started as a straight tube; it remains a straight tube.

3.

But the stomach, for example, develops gradually into a more complex organ, becoming broader in some regions and twisting around in another part.

4.
Even more complicated is the development of the small and large intestines. This development requires a lot of turning, twisting and increase in length to reach its final size and structure.
5.

As with the other organ systems, the foetal digestive system must also ‘exercise” as it grows and develop during the foetal period.

6.

It does this by regularly ingesting large quantities of amniotic fluid. This fluid is then propelled through the system. Part of the fluid is absorbed by the GI system.

7.

At the same time, the digestive system also exports fluid and some mucus from its epithelium and all this is excreted, as meconium, through the anal canal into the amnion space.

C. Associated Organs:
1.

And, finally, the digestive system also consists of several other organs, in various ways linked to the stomach and the intestines.

2.

These are the:

  • liver
  • spleen
  • pancreas
  • gallbladder
3.

These organs arise, gradually, from several buds emerging from the fore, mid or hindgut.

4.

Interestingly, another bud, high in the foregut, develops into the lungs! Although this will become a very different organ, its connection to the foregut is crucial as this will develop into the trachea connected to the oesophagus.

5.

The other associated organs also develop gradually during the foetal period, and thereby start performing what they are supposed to do in the new-born baby.

6.

The spleen for example will start creating red and white blood cells in the second trimester. Btw, after birth, it will stop creating red blood cells as this task is (gradually) taken over by the bone marrow.

7.

The development of the liver is (also) quite complex as it develops into different layers, structures, including the creation of the gallbladder and the connection, through the ampulla of Vater, with the pancreas.

8.
The pancreas develops from two buds, located close to each other, and again forms into an organ that, in the fourth month, starts producing insulin, amongst other hormones.
9.
 To be honest, the development of these (additional) organs is very, very, complicated, and, in my opinion, the details are not relevant at this basic level. This is really embryology science at its highest level!

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