Placenta - Anatomy & Physiology

Diagramatic Representation of the various layers of the placenta (fetal side lower and maternal side upper)

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 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.

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

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 (although there is some variance across species). 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 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.

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.

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

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 the extent and number of these contact areas forms the basis for the classification of different types of placenta.

For example, horses and pigs have many small contacts spread over the entire surface of the fetal membranes and this form of placenta is termed a diffuse placenta. Ruminants have 15-120 button-like contact regions between the fetal membranes and the endometrium and this type of placenta is called a cotyledonary placenta. Predatory species have chorionic villi arranged in a circular band around the fetus, called the "zona placenta".

Microscopic Physiology

As well as physiologic differences in the macroscopic appearance of the placenta between species, the microscopic structure of the interaction between fetal and maternal tissues also differs between species. The interface between the chorion and uterus can consist of different numbers and arrangements of epithelial cells and basal laminae on both the fetal and maternal sides. Beneath the layers of basal laminae there are further layers of connective tissue that contain high densities of blood capillaries.

Epitheliochoreal

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.

This type of placenta is present in horses, pigs and ruminants.

Endotheliochoreal

In comparison to the epitheliochorial placenta, the endothelial placenta is slightly less complete in that the maternal epithelial cell layer (endometrial epithelium) and basal laminae are degraded such that the maternal capillaries present in the connective tissue traditionally found behind the basal laminae are in direct contact with the fetal membranes (chorionic epithelium) of the placenta. This reduces the length of the diffusion path between the maternal and fetal sides of the placenta.

This type of placenta is found in carnivores, including cats and dogs.

Exchange

In some types of placenta including the endotheliochorial, antibodies are able to cross the various layers between the maternal and fetal circulation. This allows the maternal blood to convey passive immunity against various infectious agents. However, in some species such as horses and farm species, there are some differences between the mechanisms used to facilitate exchange in different placentas. These species are not able to confer immunity via the placenta and instead rely on the passive transfer of antibodies via colostrum. This is the case for species with epitheliochorial placentas. The names and main differences in exchange characteristics are noted below;

Histotrophic Exchange

This type of exhange facilitates nourishment of the embryo prior to implantation, i.e. where no placenta exists. In ungulates, this type of exchnage is very important as there is a long period prior to implantation. For example, in the horse implantation can take up to 35 days, whilst in ruminants it can take between 15 to 20 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

Haemotrophic exchange is the main exchange mechanism utilised in most types of placenta. 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 fetus 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 fetal circulatory shunts that are related to the umbilical arteries and veins. The three major shunts are covered in more detail at 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.

The types of hormones and their effects are provided in detail at 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.

Freemartins occur in 1% of births and in 99% of cases, the female is rendered sterile. For more detailed information, please see bovine freemartinism.

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