I have a question about embryology?

Question

Which are the first body systems to develop in a human embryo and why are they the first? Which are the last to develop and why are they last?

Answer 1

Hmm, well there aren't really firsts and lasts with development. Human development occurs in a series of layers. First you have just a bunch of undifferentiated cells. Then you have three distinguishable layers. And then each of those layers subdivides and differentiates further, etc, etc. So a lot of systems are all developing simultaneously.

One thing that does happen relatively early, and its probably the first 'system' is the formation of the neural tube, and neural crest cells. The neural tube, and some of the neural crest cells, will go on to form the nervous system. However, by the time it even resembles nervous tissue, you've also got the heart forming, blood vessels, kidneys, etc.

From what I remember, bones are some of the last things to really finish developing. In fact, several continue to form after birth. The lungs are rather underdeveloped until birth as well, for a clear reason; they're not used until the baby is born.

Answer 2

Stangely enough (or maybe not so strange), the nervous system is the first to develop and also one of the last to finish developing. This obviously indicates that it takes a long time to develop and is very complicated. Also, the production of surfactant, necessary for the development of the respiratory system is also one of the last to develop. This maybe because the fetus doesn't need it until after birth and if it develops earlier it doesn't provide an advantage because the fetus probably wouldn't have survived an earlier birth. Nowadays, fetus can survive an earlier birth, but only within the last 50 years. It hasn't evolutionarily caught up.