Placenta - Anatomy & Physiology

Introduction

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.

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

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.

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 all types of placenta noted above, 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, there are some differences between the mechanisms used to facilitate exchange in different 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

Umbillical Arteries

• Arise from Iliac arteries along with vesicular arteries to the bladder.
• Form the ventral ligament of the bladder in the adult.
• Carry deoxygenated blood from the bladder.

Umbillical Veins

• Join the hepatic portal vein

Shunts

Vascular Dynamics

Protection

Physical

Teratogens

Endocrine

Endocrine Function