The Stark Reality: Embryos Need Water
- All organisms, including vertebrates, need an aqueous environment for their developing embryos.
- Fish and amphibians: no problem—they live in water!
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Reptiles, birds and mammals: problem—they live on land and therefore must find a way to keep the embryo wet.
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Two different strategies have evolved:
- Shelled egg: reptiles and birds
- Uterus: mammals
- In all cases, the embryo is surrounded by fluid within a sac formed by a membrane called the amnion.
- Therefore, vertebrates in these three classes are called amniotes.
Development in Birds
- After a bird's egg is fertilized, it undergoes cleavage, in which cell division occurs in a small region on top of the yolk.
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Gastrulation is much more complicated than we observed in the frog embryo.
- When this complex process is finished, a very strange looking structure has been built.
Extraembryonic Membranes in Birds
- The embryo is shown in gray.
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Four extraembryonic membranes are created to support the development of the embryo:
- Yolk sac
- Amnion
- Chorion
- Allantois
- These membranes help provide an aqueous environment for the developing embryo.
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A similar process occurs in mammals, except that instead of being inside a shelled egg, the mammal is inside a uterus.
- In both cases the embryo is surrounded by the amnion, which contains amniotic fluid.
- This substitutes for a watery environment and is birds and mammals are all called amniotes.
Extraembryonic Membranes in Reptiles and Mammals
Human Development
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By the 32-cell stage, a blastocyst is formed.
- This is different from the blastula in other animals.
- An inner cell mass of tightly compacted cells will become the embryo.
- A group of cells called the trophoblast surround the blastocoel and will become part of the placenta.
- Symbryo: Simulated Embryo
Pregnancy
- The embryo secretes hormones that tells the body of its existence, preventing the endometrium from sloughing off as it usually does each month.
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Key hormone: human chorionic gonadotropin (HCG)
- Acts like LH, keeping the corpus luteum from degenerating.
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This allows the corpus luteum to continue to secrete estrogen and progesterone.
- These two hormones tell the uterine wall (the endometrium) to stay in place.
- This is the hormone that is detected in pregnancy tests.
Female Birth Control Pills
- Synthetic estrogens and progesterone-like hormones.
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Act by negative feedback to stop the release of GnRH by the hypothalamus, as well as FSH and LH by the anterior pituitary gland.
- Blocking of LH: no ovulation
- Blocking of FSH: no follicle development
Male Birth Control Pills
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Organon, a Dutch pharmaceutical company, has developed a male contraceptive pill.
- The pill has proven to be 100 % effective in preliminary clinical trials.
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The pill contains desogestrel, a synthetic hormone that is the main component in the female pill, in addition to testosterone.
- This combination blocks the production of sperm while maintaining male characteristics and sex drive.
- As with the female contraceptive pill, it must be taken daily.
Reproductive Cloning of Mammals
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Remove a cell from mammary gland of a Dorset sheep (hence the eventual name “Dolly” after Dolly Parton).
- This is the organism that is being cloned.
- These cells are deprived of nutrients to prevent them from going through the S phase in the cell cycle (DNA replication).
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Remove an egg from a Scottish Blackface sheep.
- Using a micropipette, extract the nucleus from this egg and discard this nucleus.
- This egg is now enucleated (i.e. it has no nucleus).
- Fuse the mammary gland cell from the animal that is to be cloned (Dorset sheep) to the enucleated egg cell (from the Blackface sheep).
- Use electrical shock or chemicals to stimulate this new cell to begin dividing mitotically.
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Implant this early embryo in a third sheep (Blackface sheep).
- The embryo develops and is born.
- It is genetically identical to the original Dorset sheep.
Therapeutic Cloning of Mammals
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If an organism is cloned (as above) but is not implanted in a female (i.e. it is left in a petri dish to form an early embryo), then cells from this cloned embryo can be used to produce embryonic stem cells.
- This is called therapeutic cloning and must be distinguished from reproductive cloning.
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Reproductive cloning: cloning an entire new organism
- Intense moral and ethical issues with regard to humans.
- However, it is very similar to the production of identical twins.
- Therapeutic cloning: cloning an organism to produce embryonic stem cells for therapeutic reasons
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Uses for embryonic stem cells:
- These embryonic stem cells are totipotent—they can become any tissue.
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It is hoped that they can be used to treat diseases in which cells have been destroyed and need to be regenerated:
- Diabetes
- Alzheimer's Disease
- If you clone cells of the diseased person, then the cells will not be immunologically rejected.
