Development of Lung, Liver, Digestive System and Body Cavities
By Rob Williams, 1998.
In this section I review the development of the endodermal derivatives—what are formally known as guts.
At the late embryonic stages (third and fourth weeks) the embyro roles up on itself both front-to-back and side-to-side. The ventral sheet of endoderm is transformed into an imperfect internal tube of endothelium. For semantic purposes we divide this tube up into three regions: a foregut that extends from the embryonic buccopharyngeal membrane; a midgut that is confluent with a still prominent yolk sac; and a hindgut that extends to the cloacal membrane. This relatively simple organization is retained pretty much intact in adult humans. But, the endoderm helps build numerous other organ systems.
Endodermal tissues at all levels have a propensity to produce outpockets, or evaginations. The endoderm always elopes with nearby mesenchymal mesoderm. These two tissues have inductive interactions that lead to the production of the following progeny (from rostral to caudal):
- Anterior pituitary—a midline outpocket from the roof of the pharynx
- Thyroid gland—a midline outpocket from the floor of the pharynx
- Middle ear cavity and auditory tube—bilateral outpockets associated with the 1st pharyngeal arch
- Tonsils—bilateral outpockets associated with the 2nd pharyngeal arch
- Thymus and inferior parathyroid gland—bilateral outpockets associated with the the 3rd pharyngeal arch
- The superior parathyroid gland—bilateral outpockets associated with the the 4th pharyngeal arch
- Lungs, trachea, and larynx—a ventral outpocket of the foregut
- Liver and gall bladder—an outpocket of endodermal epithelium into the transverse septum
- Pancreas—two separate outpockets from the region of the duodenum
- Appendix—an outpocket off of the cecum
- Allantois—an outpocket from the hindgut that extends into the umbilical stalk
- Renal and genital derivatives that we will defer for later
Don't feel obligated to memorize this right away. The interesting thing about this common pattern and process is that the surrounding mesoderm controls the path of tissue differentiation. If you remove a tracheal bud of endoerm and put it into the mesoderm of the transverse septum, you end up with tissue that looks just like another liver. In other words, the endoerm takes its cue from the surrounding mesoderm. A correlary is that mesoderm in different parts of the body must be differentiated at early stages. That differentiation can take the form of unique molecules embedded in the surface of mesodermal cells and the production of different diffusible molecules by these cells.
The Main Points
The digestive system is attached to the dorsal body wall by the mesentery
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For obvious reasons, the contents of the coelomic cavity need some freedom to move. They also need a blood supply. The mesentery is the flexible conduit that provides the hookup. In some cases, the mesentery fuses to the dorsal body wall and is obliterated; in other cases, the mesentery undergoes marked expansion (e.g., the ommentum).
The coelomic cavity is partitioned into three well sealed subunits.
- At the earliest stages (in the third week) the body cavity consists of a large rostral space surrounding the heart—the percardial cavity—and a peritoneal cavity that surrounds the gut. Both cavities arise as lateral splits in the middle of the mesoderm. The two cavities are confluent just dorsal to the liver (transverse septum). The pleural cavity expands as the lungs expand in this dorsal region above and liver and stomach. The pericardium and pleural spaces are separated by sheets of mesoderm that surround the common cardinal veins. The topology of this is hard to visualize. Have a look at Fig. 9.5B on page 178 of Moore. The pleural and peritoneal cavities are separated more gradually as the diaphagm closes in the sixth week.
The stomach rotates in two axes.
- The orginal dorsal surface (the greater curvature) of the stomach rotates to the left side of the body and then down. In this process the mesentery of the stomach (the dorsal mesogastrium is the proper term) gets bent out of shape. Dorsal to the stomach there is now a large and hidden peritoneal pocket called the ommental bursa.
References in Embryology
- Pictorial Human Embryolgy by S.G. Gilbert
- This is a stunning piece of art that every physican should have. It is about $20 from the University of Washington Press. ISBN 0295-96631-9 hardcover and ISBN 0-295-96632-7 in paperback. It doesn't cover any abnormalities, but it can't be beat for a concise visual and verbal description of normal human development.
- The Developing Human, Clinically Oriented Embryology, 5th ed
- This book by K.L. Moore and T. Persaud serves the market. All you need, and more, to pass the boards. If you want to know why developmental biology is intellectually exciting, you will have to search elsewhere.
- Ronan O'Rahilly's book