cell were injected directly into the heart of a human egg, the artificial hybridization would produce a viable, if ethically repulsive, embryo. But under the natural constraints of sexual reproduction, a chimpanzee sperm could not breach the forbidding zona pellucida of a human egg.
The zona also thwarts the entry of more than one sperm of its own kind. Before fertilization, its sugars are open and genial and seeking similar sugars on the head of a sperm. Once the zona has attached to the head of a sperm, it imbibes the sperm, and then it stiffens, almost literally. Its sugars turn inward. The egg is sated; it wants no more DNA. Any sperm that remain at its threshold soon will die. Still, the zona's task is not through. It is thick and strong, an anorak, and it protects the tentative new embryo during the slow descent down the fallopian tube and into the uterus. Only when the embryo is capable of attaching to the uterine wall, a week or so after fertilization, does the zona pellucida burst apart and allow the embryo to join its blood with mother blood.
The corona, cumulus, and zona all are extracellular, auxiliaries to the egg but not the egg. The egg proper is the true sun, the light of life, and I say this without exaggeration. The egg is rare in the body and rare in its power. No other cell has the capacity to create the new, to begin with a complement of genes and build an entire being from it. I said earlier that the mammalian egg is not like a bird's egg, insofar as it lacks the nutrients to sustain embryonic development. A mammalian embryo must tether itself to the mother's circulatory system and be fed through the placenta. But from a genetic perspective, the cytoplasm of a mammalian egg is complete, a self-contained universe. Somewhere in its custardy cytoplasm are factors proteins, or bits of nucleic acid that allow a genome to stir itself to purpose, to speak every word its
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species has ever spoken. These maternal factors have not yet been identified, but their skills have been showcased in sensational ways. When Scottish scientists announced in 1997 that they had cloned an adult sheep and named her Dolly, the world erupted with babble about human clones and human drones and God deposed. The endless exercises in handwringing resolved very little of the ethical dilemma that surrounds the prospect of human cloning, if dilemma there be. But what the sweet ovine face of Dolly demonstrates without equivocation is the wonder of the egg. The egg made the clone. In the experiments, the scientists extracted a cell from the udder of an adult sheep, and they removed the nucleus from the cell, the nucleus being the storehouse of the cell's genes. They wanted those adult genes, and they could have taken them from any organ. Every cell in an animal's body has the same set of genes in it. What distinguishes an udder cell from a pancreatic cell from a skin cell is which of those tens of thousands of genes are active and which are silenced.
The egg is democratic. It gives all genes a voice. And so the scientists harvested a sheep egg cell and enucleated it, taking the egg's genes away and leaving behind only the egg body, the cytoplasm, the nonyolk yolk. In place of the egg nucleus they installed the nucleus of the udder cell, and then they implanted the odd chimera, the manufactured minotaur, into the womb of another sheep. The egg body resurrected the entire adult genome. It wiped the slate clean, washed the milky stains from the dedicated udder cell, and made its old genes new again. Maternal factors in the egg body allowed the genome to recapitulate the mad glory of gestation to recreate all organs, all tissue types, the sum of sheep.
The egg alone of the body's cells can effect the whole. If you put a liver cell or a pancreatic cell into a uterus, no infant would grow of it. It has the genes to make a new being, but it has not the wit. Small wonder, then, that the egg is such a large cell. It must hold the
The zona also thwarts the entry of more than one sperm of its own kind. Before fertilization, its sugars are open and genial and seeking similar sugars on the head of a sperm. Once the zona has attached to the head of a sperm, it imbibes the sperm, and then it stiffens, almost literally. Its sugars turn inward. The egg is sated; it wants no more DNA. Any sperm that remain at its threshold soon will die. Still, the zona's task is not through. It is thick and strong, an anorak, and it protects the tentative new embryo during the slow descent down the fallopian tube and into the uterus. Only when the embryo is capable of attaching to the uterine wall, a week or so after fertilization, does the zona pellucida burst apart and allow the embryo to join its blood with mother blood.
The corona, cumulus, and zona all are extracellular, auxiliaries to the egg but not the egg. The egg proper is the true sun, the light of life, and I say this without exaggeration. The egg is rare in the body and rare in its power. No other cell has the capacity to create the new, to begin with a complement of genes and build an entire being from it. I said earlier that the mammalian egg is not like a bird's egg, insofar as it lacks the nutrients to sustain embryonic development. A mammalian embryo must tether itself to the mother's circulatory system and be fed through the placenta. But from a genetic perspective, the cytoplasm of a mammalian egg is complete, a self-contained universe. Somewhere in its custardy cytoplasm are factors proteins, or bits of nucleic acid that allow a genome to stir itself to purpose, to speak every word its
page_15
Page 15
species has ever spoken. These maternal factors have not yet been identified, but their skills have been showcased in sensational ways. When Scottish scientists announced in 1997 that they had cloned an adult sheep and named her Dolly, the world erupted with babble about human clones and human drones and God deposed. The endless exercises in handwringing resolved very little of the ethical dilemma that surrounds the prospect of human cloning, if dilemma there be. But what the sweet ovine face of Dolly demonstrates without equivocation is the wonder of the egg. The egg made the clone. In the experiments, the scientists extracted a cell from the udder of an adult sheep, and they removed the nucleus from the cell, the nucleus being the storehouse of the cell's genes. They wanted those adult genes, and they could have taken them from any organ. Every cell in an animal's body has the same set of genes in it. What distinguishes an udder cell from a pancreatic cell from a skin cell is which of those tens of thousands of genes are active and which are silenced.
The egg is democratic. It gives all genes a voice. And so the scientists harvested a sheep egg cell and enucleated it, taking the egg's genes away and leaving behind only the egg body, the cytoplasm, the nonyolk yolk. In place of the egg nucleus they installed the nucleus of the udder cell, and then they implanted the odd chimera, the manufactured minotaur, into the womb of another sheep. The egg body resurrected the entire adult genome. It wiped the slate clean, washed the milky stains from the dedicated udder cell, and made its old genes new again. Maternal factors in the egg body allowed the genome to recapitulate the mad glory of gestation to recreate all organs, all tissue types, the sum of sheep.
The egg alone of the body's cells can effect the whole. If you put a liver cell or a pancreatic cell into a uterus, no infant would grow of it. It has the genes to make a new being, but it has not the wit. Small wonder, then, that the egg is such a large cell. It must hold the
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