Changes

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.

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;

Blood begins to circulate through the embryonic and cardiovascular system, and therefore into the placenta, at approximately 21 days. Seperation of the maternal and foetal blood is referred to as the "placental barrier". The placental barrier is made up of a number of layers;

* Syncytiotrophoblast

* Syncytiotrophoblast

* Discontinuous inner cytotrophoblast layer

* Discontinuous inner cytotrophoblast layer

* Connective (mesenchymal) tissue of the villus

* Connective (mesenchymal) tissue of the villus

* Basal lamina of the endothelium

* Basal lamina of the endothelium

* Endothelium of the fetal placental capillary in the teriary villus

* Endothelium of the foetal placental capillary in the teriary villus

====Macroscopic Physiology====

====Macroscopic Physiology====

The physical contact surfaces used within the process of circulatory exchange are the fetal membranes and the endometrium and this exchange takes place via microscopic chorionic villi that invade the endometrium. These chorionic villi are covered by epithelium.  Horses have many small contacts spread over the entire surface of the fetal membranes and this form of placenta is termed a '''diffuse placenta'''.

The physical contact surfaces used within the process of circulatory exchange are the foetal membranes and the endometrium and this exchange takes place via microscopic chorionic villi that invade the endometrium. These chorionic villi are covered by epithelium.  Horses have many small contacts spread over the entire surface of the fetal membranes and this form of placenta is termed a '''diffuse placenta'''.

====Microscopic Physiology====

====Microscopic Physiology====

The horse has an '''epitheliochereal''' placenta. This type of placenta can be said to be the most complete form, where the interface between the chorion (chorionic epithelium)and uterus (endometrial epithelium) consists of intact layers of epithelial cells with a basal laminae on each side. Both sides of the placenta have supporting connective tissue and a high density of blood capillaries.

The horse has an '''epitheliochereal''' placenta. This type of placenta can be said to be the most complete form, where the interface between the chorion (chorionic epithelium)and uterus (endometrial epithelium) consists of intact layers of epithelial cells with a basal laminae on each side. Both sides of the placenta have supporting connective tissue and a high density of blood capillaries.