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Placenta - Anatomy & Physiology (view source)
Revision as of 18:28, 3 July 2012
, 18:28, 3 July 2012no edit summary
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[[File:Gray39.png|right|400px|thumb|Diagramatic Representation of the various layers of the placenta (fetal side lower and maternal side upper)]]
==Introduction==
==Introduction==
The placenta signifies the "second" or "embryonic" period of pregnancy (after the implantation period) and describes the establishment of a fully functional placenta. The placenta is an apposition of foetal and parental tissue for the purposes of physiological exchange. There is little mixing of maternal and foetal blood, and for most purposes the two can be considered as separate.
The formation of the placenta is referred to as the "second" or "embryonic" period of pregnancy (after the implantation period) and describes the establishment of a fully functional placenta. The placenta is an apposition of foetal and parental tissue for the purposes of physiological exchange. There is little mixing of maternal and foetal blood, and for most purposes the two can be considered as separate.
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The placenta can be thought of as a "symbiotic parasite", unique to mammalia. The placenta provides an interface for the exchange of gases, food and waste. It also facilitates the de novo production of fuel substrates and hormones and filters potentially toxic substances.
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The placenta has two distinct seperate compartments; the fetal side consisting of the trophoblast and chorionic villi and the maternal side consisting of the decidua basalis.
==Formation==
==Formation==
* Endothelium of the fetal placental capillary in the teriary villus
* Endothelium of the fetal placental capillary in the teriary villus
===Placental Blood Supply===
===Placental Blood Supply===
Fetal blood enters the placenta via a pair of umbilical arteries which have numerous branches within the placenta to the chorionic plate. Oxygen and nutrient rich blood returns to the fetus via the umbilical vein. Maternal blood is supplied to the placenta via 80-100 spiral endometrial arteries which allow the blood to flow into intervillous spaces facilitating exchnage. The blood pressure within the spiral arteries is much higher than that found in the intervillous spaces resulting in more efficient nutrient exchange within the placenta.
Maternal blood carrying oxygen and nutrient substrate to the placenta must be transferred to the fetal compartment and this rate of transfer is the rate limiting step in the process. Therefore the placenta has a significant blood to facilitate improved exchange.
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Fetal blood enters the placenta via a pair of umbilical arteries which have numerous branches resulting in fetal chorionic villi within the placenta, terminating at the chorionic plate. The fetal chorionic villi are then surrounded by maternal tissues. This physiology is referred to as "invasive decidualisation" as the fetal chorionic villi effectively invade the maternal tissues. Invasive decidualisation is not present in pigs or sheep.
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Oxygen and nutrient rich blood returns to the fetus via the umbilical vein. Maternal blood is supplied to the placenta via 80-100 spiral endometrial arteries which allow the blood to flow into intervillous spaces facilitating exchnage. The blood pressure within the spiral arteries is much higher than that found in the intervillous spaces resulting in more efficient nutrient exchange within the placenta.
==Types of Placenta==
==Types of Placenta==
=== Shunts ===
=== Shunts ===
There are a number of fetal circulatory shunts that are related to the umbilical arteries and veins. The three major shunts are covered in more detail at [[Foetal_Circulation_-_Anatomy_%26_Physiology#Circulatory_Changes_at_Birth|Fetal Circulation]] but are important to ensure that organs are always supplied with oxygen and nutrient rich blood, to prevent waste accumulation and protect organs that are not yet fully patent. The main fetal circulatory shunts are the '''Ductus venosus''', '''Foramen ovale''' and the '''Ductus arteriosus'''.
== Endocrine Placenta ==
The placenta also plays an important role in the endocrinological management of the fetus and the activities of the maternal side. The endocrinological functions are complex and differ across species. In essence, the placenta attempts to perform the endocrine function of other organs that are either not yet able or not yet formed such as the pituitary gland and the ovary.
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The types of hormones and their effects are provided in detail at [[Placenta Endocrine Function - Anatomy & Physiology|Placental Endocrine Function]].
==Placental Problems==
===Freemartinism===
Freemartinism is a condition usually found in bovine species (although it is found in other species) and occurs in the female offspring of dizygotic twins in a mixed sex pregnancy, i.e. a male and female pair of twins. Placental fusion between the male and female fetuses occurs and this permits the exchange of fetal cells and hormones. Testicular hormones from the male can result in the androgenisation of the female fetus resulting in a "chimeric" female (XX/XY)which will be sterile.
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Freemartins occur in 1% of births and in 99% of cases, the female is rendered sterile. For more detailed information, please see [[Freemartinism|bovine freemartinism]].
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[[Category:Pregnancy and Parturition]]
[[Category:Pregnancy and Parturition]]