Embryology   Biology 441   Spring 2011   Albert Harris


Tissues and organs that develop from endoderm



What parts of the body develop from endoderm?

Salivary glands: (note, these develop partly from stomodeum epithelium; and stomodeum is traditionally regarded as part of the ectoderm)

Pharyngeal pouches: : "gill slits"

Our very distant ancestors were fish, and had gills.
Land animals, including humans, develop embryonic structures that look like the structures from which fish embryos develop gills.

Except, of course, in mammals, birds etc. these gill-like structures develop into organs with other functions.
The most anterior pair become the eustachian tube, and the canal through which sound waves reach the inner ear. The second pair of "gill slits " pharyngeal pouches develop into your tonsils.

The third and fourth pair develop partly into the parathyroid gland (which is an endocrine gland that secretes parathyroid hormone)

And other parts of the third and fourth pharyngeal pouches develop into the thymus "gland", which is a very important part of the immune system (rather than a gland).

Its true function was discovered over the last few decades, and more will be learned.

Mutations that cause the third and fourth pharyngeal pouches not to develop result in animals with very weak immune systems.

The best example are "nude mice". For some unknown reason, their hair does not develop, in addition to failure of development of the third and fourth pharyngeal pouch. It is not yet known whether this is two effects of the same gene, or whether two separate, but very closely-linked, genes control development of hair and of those pharyngeal pouches.

Thyroid gland forms as an outpocketing of the floor or the endoderm behind the throat, that then disconnects from the surface, but remains as a series of hollow sacks, full of mucus.

trachea; bronchi; alveoli (in mammals) (air capillaries in birds)

Hepatic diverticulum (--> liver) gallbladder


Cloaca --> Bladder (these separate in mammals)
--> Rectum

Birds, reptiles & amphibia continue to have cloacas

In amphibian embryos, the anus develops from the blastopore.
But in reptiles birds and mammals, there is no blastopore.

Instead, they form a posterior infolding called the proctodeum
This fuses with and then opens into the rear of the archenteron

The proctodeum is an infolding of the somatic ectoderm, equivalent to the stomodeum at the other end.



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