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Normal Parturition - Anatomy & Physiology (view source)
Revision as of 09:50, 4 August 2008
, 09:50, 4 August 2008→Placental Expulsion
== Placental Expulsion ==
== Placental Expulsion ==
In most species, expulsion of foetal membranes quickly follows expulsion of the foetus.
* After the birth of the young, regular abdominal contractions largely cease.
* Myometrial contractions persist.
** Decreased amplitude, but become more frequent and less regular.
** Important for dehiscence and expulsion of foetal membranes.
* Waves of contractions from uterus to the cervix persist.
** Act in a peristaltic fashion in the cow and sow in the reverse direction.
=== Placental Changes ===
During the last 5 days of gestation, there are changes in the placenta.
* Collaginisation of the placentome.
* Flattening of maternal crypt epithelium.
* Leucocyte migration and increased activity.
* Reduction of binucleate cells in the trophectoderm.
* Weakening of the acellular protein layer between cotyledonary and caruncular epithelium.
=== Contractions ===
* Open endometrial crypts
* Foetal villi have shrunk due to the escape of blood from the foetal side of the placenta when the umbillical cord ruptures.
* Myometrial contractions aid exsanguination of the placenta.
* Separation of foetal membranes.
* Apex of the allantochorionic sac becomes inverted.
* As the sac is 'rolled' down the uterine horns, foetal villi are drawn out of the crypts.
* When a large portion becomes detached and inverted, it forms a mass in the maternal pelvis.
** Stimulates reflex contractions of abdominal muscles.
** Completes expulsion of the allantochorionic sac.
* In polytocious species, dehisence and expulsion of foetal membranes are interspersed with births of the young.
** Only expulsion of the last afterbirth stimulates abdominal contractions.
* The final stage of allantochorionic expulsion lasts 1 hour (mare) - 6 hours (cow).
* Domestic animals normally eat the afterbirth.
** Not the mare or camelids.
* Expulsion of the foetus requires that the chorionic villi become dislodged from the crypts of the maternal side of the [[Female_Reproductive_Tract_-Placenta_-_Anatomy_%26_Physiology|placenta]].
* Expulsion of the foetus requires that the chorionic villi become dislodged from the crypts of the maternal side of the [[Female_Reproductive_Tract_-Placenta_-_Anatomy_%26_Physiology|placenta]].
** Brought about by powerful vasoconstriction of arteries in the villi.
** Brought about by powerful vasoconstriction of arteries in the villi.