Freethought & Rationalism Archive

scigirl · 05-14-2003, 07:41 AM

Kidney evolution � biological and philosophical questions for creationists:

The human kidney develops in a rather bizarre way in utero. First, we form a pronephros, which does nothing except use up precious nutrients and then disappear. Then we form a mesonephric kidney, which works, sort of, but then also disappears (except for the duct system). Finally we form the �real� kidney � the metanephric kidney. Creationists - why does this occur? Read on for the evolutionary explanation:

http://www.auburn.edu/academic/class...4/Topic14.html

Quote:

The development of the kidney depends on the origin of these nephric tubules and where they form on the nephric ridge. So, we have a pronephros, a mesonephros, and a metanephros. The pronephros is the first stage. It has tubules anteriorly that combine to form a pronephric duct to the cloaca. The tubules often retain the duct into the coelom. This is the type of kidney possessed by hagfishes and lamprey larvae and a few adult fish. It is also present and functional in the larvae of the lower vertebrates; however, in most vertebrates, the pronephros regresses, is never functional, and is replaced by the mesonephros.
Tubules then form in the middle region of the kidney and they tap into the existing pronephric duct which is now called a mesonephric duct. The mesonephric kidney is present and functional in all embryos. If it is retained in the adult, additional tubules develop at the posterior part of the kidney and the kidney is termed opisthonephric. Opisthonephric kidneys are possessed by most fishes and amphibians.
Then what happens is that a diverticulum appears at the base of the mesonephric duct. This diverticulum stimulates the development of tubules from the posterior part of the kidney. The mesonephric duct is eventually lost and only the uretic diverticulum remains and it is now the ureter which is the tube between the kidneys and the urinary bladder. The mesonephric kidney regresses and only the metanephros is retained. This is the type of kidney possessed by all amniotes. As mammals, we further modify the kidney to make it better at its job.

Here's another web site which explains the development of the urogenital system:

http://www.leeds.ac.uk/chb/lectures/anatomy9.html

Quote:

During the process of evolution we quite often find that structures evolved to do one thing are no longer relevant. The appendix is a good example: useful in animals feeding on grass it has no function in man, has become small and may ultimately disappear. In other cases these relict organs may be taken over for quite another purpose. This is the case with the excretory and reproductive systems which are anatomically and evolutionary linked, and where parts of the excretory system have been taken over by the reproductive system.

The first site above had the following interesting comment:

Quote:

It always seem strange to talk about the urogenital system. Reproduction and urination cannot be further apart in terms of their duties, but the two systems are intricately intertwined in many different ways that one cannot speak of one without speaking of the other.

It appears that the intelligent designer of our bodies wasn�t that concerned with mixing the urinary and the reproductive functions. Which seems strange, considering how �sacred� reproduction is to many churches, yet taking a piss just really isn�t. However, the systems develop from the same tissue.

scigirl

Peez · 05-14-2003, 01:34 PM

Very nice, scigirl. This should be archived somewhere.

Peez

Nic Tamzek · 05-14-2003, 03:46 PM

Did someone say, "archived"?

There's just the place over at Wesley Elsberry's AE boards:

http://www.antievolution.org/cgi-bin.../ikonboard.cgi

lpetrich · 05-14-2003, 04:33 PM

A very interesting question here is the extent of deep homology. I did some searching for discussions of development-control genes for kidneys and comparable organs (insects' Malpighian tubules, etc.), and I did not find anything.

The main thing that urinary organs have in common across the phyla is a tubular structure; however, their exit location is very variable.

Earthworms have "kidneys" consisting of two long tubes (nephridia) per segment that exit at the sides of the segment.

Insects have "kidneys" consisting of the Malpighian tubules, which exit at the midgut/hindgut boundary of the digestive system.

Vertebrates' kidneys and their associated tubes exit at an opening near the anus.

This raises important questions of how urinary systems develop -- does the development start at the exit spot and go inward? If so, all that's necessary to produce this variation is to specify different exit spots.

lpetrich · 05-14-2003, 04:48 PM

Another curiosity is what the germ cells, the gamete precursors, do. They originate near the heart and then migrate to the gonads, which develop from excretory tubules.

This migration is shared by vertebrates and insects!

Here is stuff on fruit-fly gonad development.

Interestingly, earthworms' gonads are in some forward segments, around the location of their 5 hearts (one per segment).

So why do they migrate? Here's a possible scenario.

Nephridia empty into coelom, which empties through pores in the body wall. Gametes are also released, and they exit through those pores.

Nephridia empty through body sides; gametes find their way into nephridia and use them to exit.

Nephridia reorganized in various ways.

Germ cells have to migrate to reach suitable nephridia.

Gonads develop as residences for the germ cells.

So this germ-cell migration is a vestigial feature.

manderguy · 05-15-2003, 06:41 AM

Very cool.

You rock, scigirl.

Thanks.

pz · 05-15-2003, 07:02 AM

Quote:

Originally posted by lpetrich
Another curiosity is what the germ cells, the gamete precursors, do. They originate near the heart and then migrate to the gonads, which develop from excretory tubules.

This migration is shared by vertebrates and insects!

Hm? Not in Drosophila. Pole cells (the germ cells) are set aside very early in development, as the syncytial blastoderm forms. They are found at the posterior end of the animal, and migrate with the end of the abdomen as it extends and wraps around the egg; they then migrate inward to the ovaries/testes.

lpetrich · 05-15-2003, 09:37 AM

You are partially correct; the pole cells migrate forward toward the rear of the midgut, and then backward again to the gonads, whose somatic cells migrate forward.

However, why toward the rear of the midgut? That's where the Malpighian tubules exit into the gut, so there may be some urogenital connection here also.

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05-14-2003, 01:34 PM	#2
Peez Veteran Member Join Date: Apr 2001 Location: Canada Posts: 5,504	Very nice, scigirl. This should be archived somewhere. Peez

05-14-2003, 03:46 PM	#3
Nic Tamzek Senior Member Join Date: Jan 2002 Location: California Posts: 646	Did someone say, "archived"? There's just the place over at Wesley Elsberry's AE boards: http://www.antievolution.org/cgi-bin.../ikonboard.cgi

05-14-2003, 04:33 PM	#4
lpetrich Contributor Join Date: Jul 2000 Location: Lebanon, OR, USA Posts: 16,829	A very interesting question here is the extent of deep homology. I did some searching for discussions of development-control genes for kidneys and comparable organs (insects' Malpighian tubules, etc.), and I did not find anything. The main thing that urinary organs have in common across the phyla is a tubular structure; however, their exit location is very variable. Earthworms have "kidneys" consisting of two long tubes (nephridia) per segment that exit at the sides of the segment. Insects have "kidneys" consisting of the Malpighian tubules, which exit at the midgut/hindgut boundary of the digestive system. Vertebrates' kidneys and their associated tubes exit at an opening near the anus. This raises important questions of how urinary systems develop -- does the development start at the exit spot and go inward? If so, all that's necessary to produce this variation is to specify different exit spots.

05-14-2003, 04:48 PM	#5
lpetrich Contributor Join Date: Jul 2000 Location: Lebanon, OR, USA Posts: 16,829	Another curiosity is what the germ cells, the gamete precursors, do. They originate near the heart and then migrate to the gonads, which develop from excretory tubules. This migration is shared by vertebrates and insects! Here is stuff on fruit-fly gonad development. Interestingly, earthworms' gonads are in some forward segments, around the location of their 5 hearts (one per segment). So why do they migrate? Here's a possible scenario. Nephridia empty into coelom, which empties through pores in the body wall. Gametes are also released, and they exit through those pores. Nephridia empty through body sides; gametes find their way into nephridia and use them to exit. Nephridia reorganized in various ways. Germ cells have to migrate to reach suitable nephridia. Gonads develop as residences for the germ cells. So this germ-cell migration is a vestigial feature.

05-15-2003, 06:41 AM	#6
manderguy Senior Member Join Date: May 2002 Location: Happyville, MI Posts: 751	Very cool. You rock, scigirl. Thanks.

05-15-2003, 09:37 AM	#8
lpetrich Contributor Join Date: Jul 2000 Location: Lebanon, OR, USA Posts: 16,829	You are partially correct; the pole cells migrate forward toward the rear of the midgut, and then backward again to the gonads, whose somatic cells migrate forward. However, why toward the rear of the midgut? That's where the Malpighian tubules exit into the gut, so there may be some urogenital connection here also.

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