Embryology Biology 441 Spring 2008 Albert Harris
Key concepts and vocabulary from the first lecture:The "zona pellucida" is the stiff, gel-like spherical 'shell' that closely surrounds the egg cells of mammals. The equivalent structure is called the vitelline membrane surrounding amphibian eggs, echinoderm eggs, bird eggs, and almost every other kind of egg than mammals. It's just a historical quirk human eggs often have special vocabulary that applies just to them, or just to mammals! If embryologists had their way, we would probably abolish these special terms, and just call this structure the vitelline membrane of mammals and humans. But the medical textbooks use "zona pellucida", so we have to teach it. Even worse is the fact that fish embryologists use still another word! I will wait until later to tell you what word they use (& you won't believe what a misguided word they chose!).Also notice that "membrane" is somewhat misleading, as applied to these structures; they are nothing like the plasma membranes, nuclear membrane or membranes around vacuoles, which are lipid bilayers. But as late as the 1950s, scientists thought maybe they had the same sort of molecular structure. Another example of special words for mammal structures is that the upper end of their oviducts are called the Fallopian tubules (named after an Italian anatomist, Fallopius). The lower ends of the mammal oviduct become the uterus, which in normal human females is one structure, that formed by side-to side fusion of what had been two separate tubes. In many species, they don't fuse, and this occurs as a rare birth defect in humans - one of many examples of human birth defects that result from failure of fusion between separate tissues. The vitelline membrane is secreted by the oocyte itself, with maybe a little help from ovary cells. In many kinds of animals, more layers of various materials are secreted by the walls of the oviduct, around the outside of the vitelline membrane. Jelly layers are secreted around egg cells of frogs, salamanders and fish. The "white" of a bird's egg was secreted around the outside of its vitelline membrane by cells in the upper parts of its oviduct. The shell of a bird's egg is secreted around the outside of the egg white by cells of the lower part of the oviduct. This is sort of like two layers of paint being painted onto something in a production line. The shells of eggs of reptiles, platypuses and spiny anteaters are formed in this same way. Supposedly, if you surgically inserted a golf ball into the upper end of a bird's oviduct, then a normal layer of egg white and then a normal-looking shell would be secreted around that. In graduate school, we were told that somebody had actually done this, but it may be urban folk-lore: but it was the department chairman who told us, so we didn't argue. I mention it to clarify the physical situation. Sperm penetrate vitelline membranes by means of enzymes that they secrete from a special vacuole called the "acrosome". In birds and reptiles (and mammals), fertilization occurs in the upper end of the oviduct, before these other layers and shells are pasted onto the outside. This is in contrast to frogs, salamanders and (most) fish; their sperm have to digest through the jelly coats and also the vitelline membrane. One of my fellow graduate students earned his PhD in part by grafting parts of frog oviducts in reversed positions, so that different parts of jelly coats were secreted into their eggs in the wrong location (like, the outside layer in the inside, etc.). He did this in order to test some theories about fertilization. Maybe he got the idea from the chairman's lecture about golf balls in chickens? The points to understand are that these processes are MECHANISMS, and that it would not be unfair or inaccurate to describe much of experimental embryology as SURGICAL PRACTICAL JOKING. Whenever people have theories about some mechanism of embryonic development, they often have to think of some surgical operation to do on embryos, the result of which should be different depending on which theory is correct. Cutting whole embryos in two is not unusual! Often, scientists are astonished by what embryos do in response to experimental surgeries, and can't imagine ANY theory to explain what happens. It may help for embryologists to be a little crazy to begin with, because their experimental results may drive them crazy, anyway. After human and other mammal embryos have cleaved 5 or 6 times, their cells begin to adhere to each other in a process called "compaction", that also includes waves of active contraction (that never gets mentioned in textbooks, but you have seen in that movie, and that several very good researchers have studied). The word morula is used as a name for embryos at this stage, and not just in mammals, but also in amphibians, sea urchins and all kinds of animals that have such a stage (NOT insects). Mora is the Latin word for mulberry, and morula is the diminuitive form of that name. About this stage, mammal embryos secrete enzymes that digest the zona pellucida enough that the embryo can escape from it. The embryo then penetrates into the wall of the uterus, partly by enzymatic digestion, and partly by active "amoeboid locomotion" by cells at the surface of the embryo. The word "implantation" is used to refer to this process of attachment and penetration into the uterine wall. Chemical analogs of the steroid hormone called progesterone have the effect of preventing implantation. One such chemical analog was developed as a treatment for several kinds of cancer, and is named RU-486. This drug is widely used in Europe as a form of after-fertilization contraceptive, but for a long time it couldn't be imported into the US even for use as a cancer treatment. Now it is sold in this country. Mammal embryos will implant into whatever tissue they are next to when they digest their way out of the zona pellucida. All-too-often they implant into the wall of a Fallopian tubule. This results in what is called an "ectopic pregnancy" (ectopic meaning, basically, "in-the-wrong-place"). This is very bad because #1) The embryo doesn't have room to continue developing there, #2) It is incredibly painful for the woman, and #3) It results in the destruction of that Fallopian tubule, so that the woman is less likely ever to be able to have a baby. The chance that this will happen to a woman is greatly increased by past infections with germs (especially Chlamydia) that tend to cause scarring of the wall of Fallopian tubules, which slows down or blocks the movement of cleavage-stage embryos.
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