Animal Development—Part 3

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!
• Reptiles, birds and mammals: problem—they live on land and therefore must find a way to keep the embryo wet.
  • 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.
• 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.

Unknown source

Extraembryonic Membranes in Birds

• The embryo is shown in gray.
• 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.
• 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.

Figure 47.14, page 948, Campbell's Biology, 5th Edition

Extraembryonic Membranes in Reptiles and Mammals

	Reptilian Embryo		Mammalian Embryo
Membrane	Structure	Function	Structure	Function
Chorion	Membrane lining inside shell	Acts as respiratory surface; regulates exchange of gases and water between embryo and air	Fetal contribution to placenta	Provides surface for exchange of gases, nutrients, and wastes between embryo and mother
Amnion	Sac surrounding embryo	Encloses embryo in fluid	Sac surrounding embryo	Encloses embryo in fluid
Allantois	Sac connected to embryonic urinary tract; capillary-rich membrane lining inside of chorion, with blood vessels connecting to embryonic circulation	Stores wastes (especially urine); acts as respiratory surface	Provides blood vessels of umbilical chord	Carries blood between embryo and placenta
Yolk sac	Membrane surrounding yolk	Contains yolk as food; digests yolk and transfers nutrients to embryo; forms part of digestive tract	“Empty” membranous sac	Forms part of digestive tract

Human Development

• 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

Unknown figure, Purves's Life: The Science of Biology, 7th Edition; unknown source; figure 20.14, Purves's Life: The Science of Biology, 7th Edition

Pregnancy

• The embryo secretes hormones that tells the body of its existence, preventing the endometrium from sloughing off as it usually does each month.
• Key hormone: human chorionic gonadotropin (HCG)
  • Acts like LH, keeping the corpus luteum from degenerating.
  • 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.
• 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

Figure 43.14, Purves's Life: The Science of Biology, 7th Edition

Male Birth Control Pills

• Organon, a Dutch pharmaceutical company, has developed a male contraceptive pill.
  • The pill has proven to be 100 % effective in preliminary clinical trials.
• 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.

Figure 46.14, page 925, Campbell's Biology, 5th Edition

Reproductive Cloning of Mammals

• 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).
• 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.
• 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.

Figure 19.4, Purves's Life: The Science of Biology, 7th Edition

Therapeutic Cloning of Mammals

• 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.
  • 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
• Uses for embryonic stem cells:
  • These embryonic stem cells are totipotent—they can become any tissue.
  • 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.

Figure 19.6, Purves's Life: The Science of Biology, 7th Edition