Biology 441, Spring 2013: Lecture notes, February 25
A U.S. Representative from Georgia said last fall: "All that stuff I was taught about evolution, embryology, the Big Bang theory, all that is lies straight from the pit of hell."
He is taking a brave stand against confusing complexities of reality. And who among us does not sympathize with this noble battle.
Why not add Organic Chemistry to the list of demonic subjects?
Embryological development of the ovary and testes.The two "Genital Ridges" develop into the (right and left) ovaries in females and testes in males.
"Gonad" is the word meaning either ovary or testis.
For example: "A male gonad is called a testis"
The genital ridges are a pair of thickenings form in small parts of the lateral plate mesoderm, a short distance farther to each side than where the kidneys develop, and at an anterior-posterior region next to the middle of the mesonephros.
There are several odd facts about embryonic development of the gonads: (Which include big differences between male and female gonads).
ONE) The internal histological structure of testes consists of seminiferous tubules (Geometrically much like spaghetti!). The sperm cells differentiate from cells in the walls of these seminiferous tubules, and when differentiation is complete sperm move (swim?) down the seminiferous tubules to the Wolffian Duct (which is another word for the Pronephric duct), and down that duct to the Cloaca (and in mammals to the Urethra, which is the tube through which sperm and urine leave the body).
TWO) A special kind of cell called the "Sertoli Cell" also is a major part of the wall of seminiferous tubules. Sertoli cells partially phagocytize the anterior ends of differentiating sperm, and provide nourishment. The differentiating sperm need this nourishment because the have thrown away (budded off) 90% of their cytoplasm. (That is a rough estimate of the fraction of cytoplasm budded off, it includes most of the mitochondria, and almost everything but the nucleus. The nuclei of sperm cells shrink greatly, pack their chromosomes tightly together, which represses gene transcription.
After fertilization, sperm nuclei re-expand in the oocyte cytoplasm, and are called "Male Pronuclei". Guess what is meant by the term "Female Pronucleus".
THREE) The part of the Wolffian Duct between the testis and the cloaca becomes the Vas Deferens (the anatomical name for the sperm duct).
The Wolffian Duct degenerates in females.
In both sexes the urine duct becomes the Ureter, which is a synonym for the metanephric duct.
It is easy to lose track of which names are different names for the same duct, in contrast to which names refer to completely different structures. The ureters are completely different structures from the pronephric duct.
Pronephric Duct = Wolffian Duct = Mesonephric Duct = Vas Deferens = Sperm Duct
Metanephric Duct = Ureter = Urine Duct (after embryonic development)
(During embryonic and fetal development, the urine is carried by the Wolffian Duct, which is also called….what? Are you getting this? It's important?)
It is different in teleost fish! But I am going to leave that out of this course, unless students petition me: "We demand to learn about fish sperm and urine ducts!" A written petition on this subject will be passed around in class, maybe.
Many species of teleost fish often change from being males to being females as they get older. Other species of teleost fish change from being females to being males. Do you see why this requires the sperm ducts and the oviducts to be different from what is described above? Such complexities confuse students (and me too). They seem to have driven the Georgia Politician around the bend, and I sympathize with his mental problem.
FOUR) The internal histological structure of ovaries consist of large oocytes scattered through a continuous mass of a special kind of cell called Follicle Cells. These provide nutrition to the oocytes, which are much bigger than them. Follicle cells are not arranged as epithelia. There is nothing analogous to the seminiferous tubules in ovaries. To get out of the ovary, blister-like follicles inflate with liquid and rupture. The oocytes burst out of the outer surface of each ovary. Normally, they then get drawn into the funnel like upper end of the oviduct, and go down the oviduct to the outside of the body.
In mammals, the lower end of the oviducts enlarge, fuse (right with left) and become the thick-walled Uterus
(Please notice the analogy to the formation of the heart by fusion of right and left masses of anterior lateral plate cells. A further similarity is that gonads are formed from lateral plate mesoderm.)
[Please also notice that these right-left fusions to form single organs are analogous to putting two one-cell-stage embryos in close contact, so that they merge to form a single body. This can be done with genetically different embryos, in which case the resulting animal is an example of a "Chimera". ]
I don't know whether Hans Driesch thought about what it means for two embryos to fuse; which you can think of as either the same as, or the opposite of, splitting apart the first two, or the first four, cells of an embryo of a regulative species of animal, like a sea urchin 2 cell stage.
Did Driesch worry about fusing embryos? Would the enteleches play well together!? Especially if they were from different species? Did Driesch see the analogy of the heart and uterus forming by fusion of right and left tissues? Driesch probably wrote about this somewhere, because he was very intelligent, but just slightly crazy. He was unhinged by the various miracles of normal embryonic development. I sympathize with him.
Entelechies that tell each cell what to do seem to be an early version of "Positional Information". "Deus ex machine" (look it up) solutions to the central questions of embryology
But, I digress...
The upper ends of the oviducts are called the Fallopian Tubules. (They are named after an early embryologist, who discovered them.
Normally, mammal's embryos implant in the thick wall of the uterus.
All too often, early blastocyst stages have not yet reached to uterus at the time their trophoblast cells become invasive and therefore implant into some part of the inside of a Fallopian Tubule.
This is called a tubal pregnancy. It is extremely painful, and requires surgical removal of the embryo, which cannot develop successfully when located inside a Fallopian Tube.
This removal destroys that Fallopian Tubule, which will result in sterility if it happens to both Fallopian Tubules.
Tubal pregnancies become more likely if a woman's fallopian tubes are physically damaged in any way, such as by untreated infection with the STD bacteria Chlamydia. Everybody should be aware of this.
(No connection to Chlamydomonas, which is a green alga)
POINT FIVE Back to GONAD DEVELOPMENT)
The Sperm Cells and Oocytes DO NOT develop from cells originally located at the genital ridge.
Instead, Sperm Cells and Oocytes develop from PRIMORDIAL GERM CELLS which arise at some entirely different location in the body, and migrate by a form of amoeboid locomotion to the gonads.
For example, in birds, the primordial germ cells start in a crescent shaped area of extra embryonic tissue out in front of the head. From there, they crawl to the gonads, with many P.G.C.s penetrating into blood vessels and being carried along in flowing blood, until they eventually stick at the gonad site, and leave the blood vessel.
Notice how similar that is to the metastasis of cancer cells from one part of the body to another.
Embryologists have done experience in which the blood vessels of two genetically different strains of chickens are fused with each other (so their blood is freely inter-mingled). The eggs laid by the chickens some have one genotype and others have a different genotype! If this anterior crescent of tissue is surgically removed or damaged by heavy doses of X-rays, them the chicken develops with sterile gonads, that is with Sertoli cells and seminiferous tubules, or with follicle cells, but no egg cells or sperm cells.
In flies and nematode worms, the Primordial Germ Cells are formed at the rearmost part of the embryo, at a place where special granules are pre-positioned in the cytoplasm. If you remove or X-ray these granules --> sterile animals If you transplant the granules to a different part of the animal, then Primordial Germ Cells develop at that other location. Lots of research has been done on this.
Mammal and Amphibian PGCs start out in what would be extreme ventral endoderm.
Somebody told me that a kind of animal may have been found in which the PGCs start at the same place as the gonads, and if anybody knows of an example, please tell me!
As our textbook mentions, the best guess is that PGCs are (in some sense!) "protected from" being caused to differentiate (like into liver, or kidney, or something) irreversible. That's supposed to be the reason they develop far away from the centers of inductive stimulation; and also why it doesn't matter exactly which peripheral area they start from (crescent in front of the head, extreme ventral endoderm, rearmost "germ-plasm" - just anywhere distant from where the action is. That's the idea.
Nobody has really tested this explanation however! For example, what if chicken cells were taken from that anterior crescent area and surgically grafted right next to the middle of the primitive streak of an embryo of a genetically different strain of chicken? What would you predict? What result would prove this hypothesis about protecting PGCs from abnormal differentiation? What results would/could DIS-prove it, in your opinion.
Please give serious thought to these questions, and be ready if questions about such an experiment show up on the next hour exam or on the final exam.
If you did this experiment, and achieved a clear-cut result (that proved or disproved a theory fairly decisively) then that's exactly the kind of discovery Nature and Science magazines are looking for, and that would get your name into the textbooks, and get you hired as a professor, that most exalted of all goals.
Point SIX: The last part of today's lecture:
Yet another question that hasn't been solved is what mechanism guides PGCs to the gonads:
Selective Adhesion? Meaning "rap Action" (wander randomly, until reaching the gonad, then stopping.)
Chemotaxis? And remember there are two very different mechanisms, both called chemotaxis.
Temporal Chemotaxis (As in chemotaxis by bacteria and ciliate protozoa)
This works by going approximately straight in the same (random) direction as long as the concentration of the attractant substance increases.
And randomly changing to some other direction in response to detecting a decreasing concentration of the "attractant" substance.
Spatial Chemotaxis (As by white blood cells, and maybe some nerve growth cones)
Each cell can "measure" or "compare" the attractant concentration on the right versus the left; They turn right if the "attractant" concentration is higher on the right side. They turn left if the "attractant" concentration is higher on the left side.
(Textbooks then say that chemotactic gradients "pull" on cells, or "push" on them in the case of negative chemotaxis)
Another equivalent to chemotaxis works by chemical induction of cell detachment and retraction, caused by bonding of a special protein (Named EPHRIN, or Ephrin. It's an acronym for Erythropoitin Hepatocyte something-something, and was discovered being secreted by some random line of cancerous cells.
The more Ephrin a cell has on its surface, the more sensitive it will be to induction of detachment and retraction in response to a given concentration of Ephrin Receptor.
Gradients of concentrations of ephrin and ephrin receptors turn out to be the guidance control mechanism for some (many?) nerve growth cones (in the optic nerve, innervating the brain) and also the guidance of endothelial cells (that form the walls of blood vessels.
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