Development of the digestive system and body cavities

Definitions & Key takeaways

https://osmosis.org/learn/Developmentofthedigestivesystemandbody_cavities The primitive digestive system starts developing at around week 3 in the embryo. The primitive gut starts to form when the yolk sac becomes incorporated into the embryo. It then forms a blind-ended tube, which becomes the foregut on the cephalic end, and the hindgut on the caudal end. The middle part of the midgut also forms but remains temporarily attached to the yolk sac via the vitelline duct (yolk stalk). The accessory organs of the digestive system form as outpouchings alongside the primitive gut tube.

Watch video only
During the third week of development, the embryo is a flat, three layered disc, with each layer containing germ cells that give rise to the organs and tissues of the body.
The ventral, or bottom germ layer is called endoderm, and it forms the lining of the gut tube and the respiratory system.
The middle germ layer is called mesoderm and it forms connective tissues like muscles and bones. And the dorsal or top germ layer is called ectoderm, and it gives rise to the sensory organs like skin, the eyes, and the central nervous system.
The mesoderm differentiates into paraxial mesoderm, intermediate mesoderm, and lateral plate mesoderm, and around day 19, a space forms in the lateral plate mesoderm.
This space, called intraembryonic coelom or cavity, separates the lateral plate mesoderm into dorsal and ventral layers.
The dorsal layer is the parietal mesoderm layer, and its cells adhere with cells from the overlying ectoderm and wrap around the amnion.
The ventral layer is called the visceral mesoderm layer and its cells adhere to cells from the underlying endoderm which covers the yolk sac.
Parietal mesoderm give rise to the serous membranes that line the different cavities in the body, and visceral mesoderm becomes the serous membrane that covers the lungs, heart and abdominal organs, and these are really important because they prevent the organs from getting injured as they rub up against each other or against the body wall.
During the fourth week of development, the embryo undergoes structural changes, transitioning from a trilaminar disc into a more cylindrical shape.
The combined visceral mesoderm and endoderm layer which line the yolk sac, fold rostral and caudally creating a primitive gut tube out of the yolk sac, leaving the remainder of the yolk sac in the middle section of the gut.
While that’s happening, the combined parietal mesoderm and ectoderm folds down with the amnion forming the lateral body folds which eventually merge and become the anterior body wall of the embryo - what you might think of as the chest.
This creates a gut tube inside a body tube, or more simply a tube inside a tube in the rostral and caudal parts of the embryo, but there’s still the yolk sac poking out in the mid-section.
At this point, the dorsal portion of the yolk sac has become a part of the gut tube while the connecting stalk between the yolk sac and gut tube is renamed the vitelline duct.
A ridge of double-layered visceral mesoderm forms the dorsal mesentery and it suspends the gut tube from the body wall as it dangles in the embryonic cavity.
This pendulous flap of tissue was only recently classified as an organ, and it provides a pathway for blood vessels, nerves and lymphatics to reach the various body organs.
Of course, the body doesn’t have one continuous cavity. Around day 22, a thick plate of visceral mesoderm called the septum transversum forms just cranial to the vitelline duct, and it starts to divide the thoracic cavity and abdominal cavity.
Around the fifth week of development, two flaps of tissue called the pleuropericardial folds, grow out of the lateral body wall and fuse together to divide the thoracic cavity into two pleural cavities, that house the lungs, and a pericardial cavity that houses the heart.
In the seventh week of development, two more flaps of tissue - this time pleuroperitoneal membrane tissue, grow ventrally from the body wall and fuse with the septum transversum to completely seal off the thoracic cavity from the peritoneal cavity.
At that point, there are pericardial and pleural cavities in the thorax and the peritoneal cavity in the abdomen. There’s also the diaphragm, which is the large muscular organ that separates the abdominal and thoracic cavity.
Mesodermal cells from the third, fourth, and fifth pairs of somites penetrate the pleuroperitoneal membranes and form the muscular portion of the diaphragm.
Meanwhile, the two crura of the diaphragm, which are the two main tendons that tether it to the vertebral column, come from the mesoderm of the lumbar region.
All right, as a quick recap… By the end of week 3, the lateral plate mesoderm divides into parietal and visceral mesoderm and they form the serous membranes for the developing organs and also establish the pleural, pericardial, and peritoneal cavities.
Finally, mesoderm from the third, fourth, and fifth somites help give rise to the muscular portion of the diaphragm.