Friday, May 12, 2006

PLACENTAL AND MARSUPIAL MODES OF REPRODUCTION

The placenta is a temporary organ which is present in females especially during pregnancy (2). This name is given to the mammal that have placenta, that connects the growing embryo within uterus to the mother. Placenta is composed of two parts the genetically and biologically part of the fetus and the other part of the mother. The fetus is implanted in the wall of the uterus where it receives nutrients and oxygen from the blood of the mother and passes out waste. (2)

While Marsupials can regard as non-placental mammal, they reproduce by lying eggs. In some marsupials there is rudimentary placenta that functions on for short period. Its reproduction is different to that of placental mammals; it develops a yolk sack in the womb which delivers nutrients to the embryo. The embryo of marsupials born at an early stage of development, it crawls up to the belly of its mother and attaches itself to a nipple. It stays at the nipple for a period. Then the offspring passes a stage where it can leave its mothers pouch temporarily and returns for warmth and nourishment (1).

Birth in marsupials occurs very fast depending on the egg yolk type, and also the type of reproduction speed up the birth process. It only takes twelve days for marsupial embryo in the reproductive track, whereas in placental mammals’ gestation period takes long, because the offspring will need to be fully developed before birth. It takes nine months for placental mammal to give birth.

During reproduction placental mammals develops a placenta that protect the fetus from the mothers’ immune system, while in marsupials they develop a complex fetus. In placental mammals, if they give birth before times the baby or fetus become prematurely and it will need care the same way the marsupial do to their embryo, carrying it to the belly for a period.

When the placental mammals are pregnant the process of oestrus cycle are likely to happen until the end of pregnancy. Whereas in marsupials the oestrus cycle does not stop it takes place. In marsupial the ovarian inhibition is mediated by lactation or suckling stimulus. These regulatory modifications are important because the baby will no longer be carried internally so negative feedback stimulus from the presence of the babies must come from nursing activity.

Differences in these two modes of reproductions are the time that the offspring take in the uterus. Also the development of the embryo is different to each other. As in marsupial the embryo is separated from the body of its mother. While in the placental is connected to the mother. Mating system in marsupial is different through out the year. Some species are single and they only come together to mate and this pattern of social behaviour reflect promiscuous mating systems.

In conclusion mode of reproduction differs with type of species and also the size of the species involve to the way in which species reproduce. Species that reproduce by placental must have big body as they have to carry the embryo for couple of months, while those with small bodies do not carry embryo in their belly they reproduce by marsupials process.

Reference:

1. Ashcraft.C.W. Marsupial Evolution And Post Flood Migration.[Internet]cited 2006-May-10] Available from: http://www.nwcreation.net/marsupials.html

2. Opossum Reproduction And Life Cycle [Internet][cited 2006-May-10]Available from: http://www.opossumsocietyus.org/opossum_reproduction_and_life_cycle.htm

3. Wikipedia contributors. Marsupials [Internet]. Wikipedia, the Free Encyclopedia; 2006 May 9, 17:16. PTA [cited 2006 May 10]. Available from: http://en.wikipedia.org/wiki/Marsupials

4. Wikipedia contributors. Placenta [Internet]. Wikipedia, the Free Encyclopedia; 2006 May 7, 18:21. PTA [cited 2006 May 10]. Available from: http://en.wikipedia.org/wiki/Placenta

5. Wikipedia contributors. Umbilical cord [Internet]. Wikipedia, the Free Encyclopedia; 2006 May 8, 18:02. PTA [cited 2006 May 10]. Available from: http://en.wikipedia.org/wiki/Umbilical_cord

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