Coelomic Cavities

I hardly remember this lecture, and Hazel keeps saying that we should rewatch it now that we've learned some other stuff. But ehhh, let's just see how much I can rip out of the lecture slides first.

Definition and Adult Derivatives

Coelomic spaces develop within the mesoderm during embryological development. I've mentioned this before, but a quick recap- spaces that form within the extraembryonic mesoderm join up to form the extraembryonic coelom, and spaces that form within the intraembryonic lateral plate mesoderm join up to form the intraembryonic coelom. The intraembryonic coelom also separates the two layers of the lateral plate mesoderm. Eventually this coelom goes on to form the pleural, pericardial and peritoneal cavities of the body. (These cavities are also known as serous cavities.)

Structure of Serous Cavities

So, a bit more on serous cavities. Serous cavities are lined by serous membrane, which has connective tissue on one side and a special kind of epithelium on the other. This special kind of epithelium is called "mesothelium." It is made of simple squamous cells which exude serous fluid so that this surface is non-stick. The connective tissue, on the other hand, is what sticks to adjacent tissues (probably to anchor stuff down).

Even though the serous fluid is non-stick, there are places where the mesothelium sticks to itself in a process called zygosis. This can occur as part of an infection, or as part of the normal process of organs sticking to the body wall (I'll go into this in some more detail later).

When organs grow, they kind of push into the serous cavities so that the cavities and their linings "wrap around" the organs (the organs don't grow directly inside the cavities). The bits where the organs touch is known as the visceral layer, the bits where the body wall touches is the parietal layer and other random connecting bits are mesenteries.

Development of Serous Cavities

First I'm going to talk about general serous cavity development before I go into the details of each type of cavity.

As mentioned before, the intraembryonic coelom develops within the lateral plate mesoderm. As not mentioned before, the coelom is horseshoe shaped. The ends of the "horseshoe" poke outside, so the intraembryonic coelom is actually originally continuous with the extraembryonic coelom. The middle of the arch of the horseshoe passes between the cardiogenic area and the oral membrane.

After folding, however, the coelom becomes somewhat M-shaped (the dip in the middle is from the arch of the horseshoe folding over). Thus you end up with a pericardial cavity in the middle, and two pleuroperitoneal cavities on the sides.

Pericardial Cavity

As just mentioned, the pericardial cavity pretty much already becomes its separate thing after folding. This separation is also assisted by the phrenic nerve coming down, sorta like using a thread to cut through something soft (okay, shit analogy, but hopefully it makes sense).

Pleural Cavities

The lungs develop from lung buds, which arise from the foregut. The lung buds grow into the pleuroperitoneal cavities which "wrap around" them (in the process I mentioned earlier in this post). The pleural cavities are then separated by the peritoneal cavity by the formation of the diaphragm. The diaphragm forms from the septum transversum in the centre, ingrowth of the body wall laterally and pleuroperitoneal folds posteriorly. Sometimes the pleuroperitoneal folds fail to close, which means that the diaphragm doesn't completely seal off the abdomen. This allows abdominal organs to herniate into the thorax.

Gut Tube

You might have noticed that there are two peritoneal cavities, when we only really have one in the adult. That's because embryology is complicated and other stuff happens.

Anyway, left and right peritoneal cavities with the gut tube right in the middle. The gut tube is connected to the body wall anteriorly and posteriorly by ventral and dorsal mesogastrium, respectively (they're mesenteries). Level with the foregut, which is the area that goes on to be supplied by the coeliac artery, the ventral mesogastrium develops into the liver and ventral pancreas whereas the dorsal mesogastrium develops into the spleen (which is my favourite word to say!) and the dorsal pancreas. (Remember, embryos start off with two pancreases which eventually fuse to become one.) The rapid growth of the liver eventually forces the stomach to the left.

Below the level of the foregut, the ventral mesentery disappears. (Unfortunately, they didn't go into detail as to how it happens, so I guess we just have to imagine that it goes "POOF!" and disappears.) From the midgut (supplied by the superior mesenteric artery) down, you only have one peritoneal cavity. The midgut grows really fast, which is why embryos often have a "midgut hernia" which is where bits of their gut poke through into the umbilicus (I think...). The gut also rotates when this happens, which is why our small intestines are so twisty.

As for the hindgut (supplied by the inferior mesenteric artery)? Not as much to say here, aside from that near here, the cloacal membrane gets split up into the rectum and bladder. This occurs when the urogenital septum grows, cutting through the cloaca. That's why our genitals are between the bladder and the rectum.

Retroperitoneal vs. Intraperitoneal Organs

I'm not sure how well I can explain this, but I'll try. Retroperitoneal organs are those that are against the body wall and covered by peritoneum. There are some organs which are secondarily retroperitoneal, but become retroperitoneal by the mesentery sticking to itself and eventually fusing with the body wall (a process called zygosis, as mentioned above somewhere). There are other organs that aren't against the body wall, but are held in place by mesenteries. These organs are called intraperitoneal.