On Thu, 1 May 2003 15:04:48 +0000 (UTC), ragland37{at}webtv.net (Michael
Ragland) wrote:

>
>http://www.emc.maricopa.edu/faculty/farabee/ BIOBK/BioBookENDOCR.html 
>
>I was looking for information on the human endocrine system and I came
>across the above website. What I find interesting about the ordering of
>the cells which make up human embryonic development is the seemingly
>evolutionary sequence. For example, the endoderm is the inner layer of
>cells in embryonic development that gives rise to organs and tissues
>associated with digestion and respiration. These would be the most
>primitive cells. The mesoderm is the middle layer of cells in embryonic
>development that gives rise to muscles, bones, and structures associated
>with reproduction. Finally, there is the ectoderm which is that outer
>layer of cells in embryonic development that gives rise to the skin,
>brain, and nervous system. So if you think of a layered cake the
>ectoderm is the most recent evolutionary layer of embryonic cells.
>
>Perhaps biologists are aware of this but I was kind of excited when I
>learned about it. Is there an evolutionary ladder relationship between
>endoderm=mesoderm=ectoderm? There certainly seems to be. Since Darwinian
>evolution is so slow how do we know when endoderm cells evolved into
>mesodermic cells and when mesodermic cells evolved into ectodermic
>cells? Through fossils and comparative embryology?
>
>Obviously, these three embryonic cell layers work in conjuction with
>each other but it is clear we don't understand fully how they relate to
>each other. I'm particularly interested in the relationship between the
>mesoderm and the ectoderm.
>
>I'm also puzzled by hormones. The hypothalamus, pineal gland, adrenal
>glands, gonads, etc. produce hormones. I've never seen hormones refered
>to as genes. Hormones actually influence genes. Still, I would think
>there would be genes which are responsible for these hormones which are
>produced by the aforementioned. Is this too simplistic or is it that we
>just haven't discovered the genes for these hormones yet?
>
>Michael Ragland
>

You have an interesting idea, but unfortunately it is quite incorrect.
In fact, it is the ectoderm and endoderm which are evolutionarily the
oldest and the mesoderm which is more recent.  That is, all members of
the animal kingdom above the level of sponges have a particular form
of embryonic development which involves the production of ectoderm and
endoderm.  The Cnidaria (jellyfish, anemones, and Hydra) retain this
level of development.  Starting with the flatworms, all the remaining
animals also develop a mesoderm.  Check with any intro biology
textbook.

The endoderm is only associated with respiration in some animals.  Our
lungs are develop from swim bladders of fish, sacs that branch off the
gut.  Fish gills and the respiratory systems of other animals are not
endodermal in origin.

Your question about hormones is sort of strange.  We certainly do know
all the genes involved in producing the protein hormones and that are
involved in producing the enzymes that make the non-protein hormones.
Hormones are simply chemicals secreted by some cells that act as
signaling agents carried in the blood.  They are closely related to
other signaling chemicals -- neurotransmitters and local hormones
which also carry signals from one cell to another but are not
transported in the blood.  Signaling systems can, indeed, influence
gene transcription and signaling systems can also influence hormone
release.  Hormones are not referred to as "genes" because the two
concepts are entirely separate.
---
þ RIMEGate(tm)/RGXPost V1.14 at BBSWORLD * Info{at}bbsworld.com

---
 * RIMEGate(tm)V10.2áÿ* RelayNet(tm) NNTP Gateway * MoonDog BBS
 * RgateImp.MoonDog.BBS at 5/2/03 1:03:06 PM