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Pregnancy and Parturition - Horse Anatomy (view source)
Revision as of 17:17, 26 November 2012
, 17:17, 26 November 2012→Pregnancy
* Day 7-17: Peristaltic contractions of the uterine myometrium move the embryo around the uterus. The conceptus begins to secrete oestrogens, but their role is unknown. If pregnant, upregulation of oxytocin receptors between day 10-16 is inhibited
* Day 7-17: Peristaltic contractions of the uterine myometrium move the embryo around the uterus. The conceptus begins to secrete oestrogens, but their role is unknown. If pregnant, upregulation of oxytocin receptors between day 10-16 is inhibited
* Day 17: The myometrium clamps the embryo in position at the base of the uterine horns, preventing movement.
* Day 17: The myometrium clamps the embryo in position at the base of the uterine horns, preventing movement.
===Placenta===
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 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.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.
The placenta consists of a foetal portion, formed by the chorion, and a maternal portion formed by the decidua basalis. The uteroplacental circulatory system begins to develop from approximately day 9 via the formation of vascular spaces called "trophoblastic lacunae". Maternal sinusoids develop from capillaries of the maternal side which anastamose with these trophoblastic lacunae. The differential pressure between the arterial and venous channels that communicate with the lacunae establishes directional flow from the arteries into the veins resulting in a uteroplacental circulation.
Blood begins to circulate through the embryonic and cardiovascular system, and therefore into the placenta, at approximately 21 days. Seperation of the maternal and fetal blood is referred to as the "placental barrier". The placental barrier is made up of a number of layers;
* Syncytiotrophoblast
* Discontinuous inner cytotrophoblast layer
* Basal lamina of the trophoblast
* Connective (mesenchymal) tissue of the villus
* Basal lamina of the endothelium
* Endothelium of the fetal placental capillary in the teriary villus
====Placental Blood Supply====
Maternal blood carrying oxygen and nutrient substrate to the placenta must be transferred to the foetal 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. Foetal blood enters the placenta via a pair of '''umbilical arteries''' which have numerous branches resulting in '''foetal chorionic villi''' within the placenta, terminating at the '''chorionic plate'''. The foetal 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. <br />
Oxygen and nutrient rich blood returns to the foetus 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.
==Parturition==
==Parturition==