Reproductive Systems

WebLecture Topics

• Asexual Reproduction: Clones
• Non-Human Sexual Reproduction

Patterns of Reproduction

While our attention centers on human reproductive systems, we recognize that reproduction systems exist to support propagation of species other than our own. We'll deal with plant reproduction in later lectures. For now, let's take a look at how different animals reproduce.

Asexual reproduction

Budding

In budding, the adult body of a sponge or cnidarian such as a hydra creates a nodule or bud that can be broken off, and which will grow into a complete, new individual. Only the bud is capable of reproduction; other fragments of the body are not. Budding is not the same as binary fission, in which a single cell splits into two equal parts.

Fragmentation and Regeneration

Some simple organisms possess the ability to reproduce by regenerating lost body parts from any fragment. This differs from budding, where the fragment is designed for reproduction. For example, while planarians (a species of flatworms) do lay eggs, they can also split off a small piece of tail that will grow into an entire separate individual. Apparently differentiation of cells can be reversed to the point where development needs to start in order to form the individual organism.

Other examples of fragmentation and regeneration include echinoderms (starfish) and cnidarians (jellyfish). Failure to realize the regenerative capabilities of starfish nearly led to the demise of the oyster industry in Puget Sound, Washington State. In an attempt to rid areas around the oyster beds of predatory starfish, the oyster "farmers" hired divers to cut up any starfish they found. Unfortunately for the oysters, the pieces of starfish simple grew into whole starfish, greatly increasing the population of starfish.

The implications of regeneration for humans who have lost body parts due to injury or disease makes studies of these animals important. In April 2007, University of Florida researchers announced a $6 000 000 research project to attempt to find human "progenitor" cells and learn how they might be used to heal human injuries.

Parthenogenesis

Parthenogenesis occurs when an unfertilized eg develops into an adult animal. This has been documented in a number of species, including some mollusks, crustaceans, insects (aphids, ants and bees), and reptiles such as the Komodo dragon, when populations have declined below the ability of normal sexual reproduction to sustain the species. Lab experiments have also induced parthenogenesis in frogs and even rabbits, although in the case of these more complex vertebrates, the individuals that develop are usually abnormal. However, mammalian experiments indicate a possibility for human parthenogenesis, which itself raises huge ethical issues.

Non-human Sexual Reproduction

Reproductive systems in non-humans fall into two main categories: egg-laying species with external fertilization methods, and those marsupial and mammalian species who bear their young alive and require internal fertilization.

In most males, whether egg-laying or live-bearing species, organs like the testes produce sperm, which leave the male body by way of a system of ducts and reservoirs which may temporarily store sperm until the bird finds a mate. Reproductive cycles in the males are controlled by testosterone, as they are in humans. Roosters eject their sperm into the hens, which hold the sperm in "host glands" up to two weeks.

Female avian productive systems differ more from their human counterparts since birds not only produce eggs rather than live offspring, but also must produce more eggs at one time, given the rigors of being prey to large numbers of other birds, mammals, and reptiles. Birds have rapid ovulation cycles: hens lay every 48 hours, in clutches of 4-5 eggs each, while other species have more or fewer eggs; these eggs are produced in one ovary while the other atrophies. As with humans, the ovary releases a single ovum cell at a time (which can be up to 6" in the case of an osterich) from a follicle of the ovary, but after that, any similarity diminishes rapidly. Avian ovaries have no corpus luteum. After the egg leaves the ovary, it may meet sperm released from the female's host glands become fertilized, but in any case, it then receives albumen coats, mucous coats, and eventually a calcareous coat that becomes the eggshell as it passes through the oviduct and out of the vagina of the bird.

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