Pregnancy and Parturition - Horse Anatomy

Pregnancy

Mares are monotocious, meaning they normally produce a single offspring. Twin pregnancies do occur in the mare, but are rarely carried to full term and veterinary intervention is often used to prevent twin pregnancies in this species. Gestation length in the mare is approximately 330-340 days, however it is usually consistent within the individual mare from one pregnancy to the next.

Maternal Recognition of Pregnancy

The presence of a conceptus prevents luteolysis. In the presence of a conceptus, endometrial production of PGF2α is significantly reduced. The conceptus must migrate within the uterus from one uterine horn to the other 12-14 times a day during days 12, 13 and 14 of gestation in order to inhibit PGF2α. This migration is necessary, as the equine conceptus does not elongate, so there is less contact between the conceptus and the endometrial surface. It must move to distribute pregnancy recognition factors to the endometrial cells. The conceptus does produce proteins to effect the recognition of pregnancy, but specific roles are unknown. The luteolysin in the non-pregnanct cycle is thought to be PGF2α.

The embryo takes 6 days to traverse the oviduct (in other species this is normally ~4 days). It stops at various spots in the uterus, spending 5-20 minutes in each. The conceptus remains spherical in shape.

• Day 6-22: the trophectoderm secretes a glycocalyx, which hardens to form a capsule. This prevents attachment of the embryo to the uterine endometrium.
• 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.

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

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.

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.

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.

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.

Exchange

The mare is not able to confer immunity via the placenta and instead relies on the passive transfer of antibodies via colostrum.

Histotrophic Exchange

This type of exhange facilitates nourishment of the embryo prior to implantation, i.e. where no placenta exists. In horses, this type of exchnage is very important as there is a long period prior to implantation (up to 35 days). Nutrition is supplied by uterine secretions/debris, often referred to as 'uterine milk'. Uterine milk secretions are usually maintained by progesterone. Pinocytosis (cellular drinking) is the main exchange mechanism.

Haemotrophic Exchange

This type of exchnage utilises direct transfer of nutrients from the maternal to foetal blood via simple diffusion, facillitated diffusion, active transport and complex diffusion.

Placental Blood Supply and Drainage

Umbilical Arteries

The paired umbilical arteries arise from Iliac arteries along with vesicular arteries to the bladder. In the adult, the remnance of these vessels form the ventral ligament of the bladder. The umbilical arteries carry deoxygenated blood and waste products from the foetus to the placenta.

Umbilical Veins

A single umbilical veins runs from the fetus and joins the hepatic portal vein, effectively circumventing the liver which is not yet fully patent. The umbilical vein transports oxygen-rich and nutrient-rich blood from the placenta to the fetus.

Shunts

There are a number of foetal circulatory shunts that are related to the umbilical arteries and veins. The three major shunts are covered in more detail here 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 foetal circulatory shunts are the Ductus venosus, Foramen ovale and the Ductus arteriosus.