What does the placenta consist of?

What does the placenta consist of?

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It includes the basal plate, the pool of blood between the chorionic plate and the basal plate and the chorionic plate, what else?

You could've easily googled this, but anyway…

Placenta is a composite structure made of embryonic and maternal tissues. The placenta is composed of: The chorion, which is the embryonic-derived portion of the placenta. Chorion contains the trophoblast cells (cells that make up the outer cell layer of the blastocyst). finger-like structures called chorionic villi are also present in the placenta. Lastly, the space in the placenta that surrounds the villi and the maternal blood is called the intervillous space.


The fetus could not grow and develop without oxygen and nutrients from the mother. Wastes from the fetus must also be removed in order for it to survive. The exchange of these substances between the mother and fetus occurs through the placenta.


The placenta is a temporary organ that begins to form from the trophoblast layer of cells shortly after implantation. The placenta continues to develop and grow to meet the needs of the growing fetus. A fully developed placenta is made up of a large mass of blood vessels from both the mother and fetus. The maternal and fetal vessels are close together but separated by tiny spaces. This allows the mother&rsquos and fetus&rsquos blood to exchange substances across their capillary walls without the blood actually mixing.

The fetus is connected to the placenta through the umbilical cord, a tube that contains two arteries and a vein. Blood from the fetus enters the placenta through the umbilical arteries, exchanges gases and other substances with the mother&rsquos blood, and travels back to the fetus through the umbilical vein.

The fetus and the placenta. Notice the fetus is attached to the placenta by the umbilical cord, made of two arteries and one vein.

Amniotic Sac and Fluid

Attached to the placenta is the amniotic sac, an enclosed membrane that surrounds and protects the fetus. It contains amniotic fluid, which consists of water and dissolved substances. The fluid allows the fetus to move freely until it grows to fill most of the available space. The fluid also cushions the fetus and helps protect it from injury.

Placental Mammals

Placental mammals are therian mammals in which a placenta develops during pregnancy. The placenta sustains the fetus while it grows inside the mother&rsquos uterus. Placental mammals give birth to relatively large and mature infants. Most mammals are placental mammals.

The Placenta

The placenta is a spongy structure. It consists of membranes and blood vessels from both mother and embryo (see Figure below). The placenta passes oxygen, nutrients, and other useful substances from the mother to the fetus. It also passes carbon dioxide and other wastes from the fetus to the mother. The placenta lets blood from the fetus and mother exchange substances without actually mixing. Thus, it protects the fetus from being attacked by the mother&rsquos immune system as a &ldquoforeign parasite.&rdquo

Placenta of a Placental Mammal (Human). The placenta allows the exchange of gases, nutrients, and other substances between the fetus and mother.

Pros and Cons of Placental Reproduction

The placenta permits a long period of fetal growth in the uterus. As a result, the fetus can become large and mature before birth. This increases its chances of surviving. On the other hand, supporting a growing fetus is very draining and risky for the mother. The mother has to eat more food to nourish the fetus. She also becomes heavier and less mobile as the fetus gets larger. As a result, she may be less able to escape from predators. Because the fetus is inside her, she can&rsquot abandon it to save her own life if she is pursued or if food is scarce. Giving birth to a large infant is also risky. It may even result in the mother&rsquos death.

Placenta: Structure and Functions (Explained with Diagram)

Placenta is a structure that establishes firm connection between the foetus and the mother.

From the outer surface of the chorion a number of finger like projections known as chorionic villi grow into the tissue of the uterus. These villi penetrate the tissue of the uterine wall of the mother and form placenta.

The placenta is a connection between foetal membrane and the inner uterine wall. Thus, placenta is partly maternal and partly embryonic. By means of placenta the developing embryo obtains nutrients and oxygen from the mother and gives off carbon dioxide and nitrogenous waste.

In the placenta, the foetal blood comes very close to the maternal blood, and this permits the exchange of materials between the two. Food (glucose, amino acids, lipids), water, mineral salts, vitamins, hormones, antibodies and oxygen pass from the maternal blood into the foetal blood, and foetal metabolic wastes, such as carbon dioxide, urea and warn pass into the maternal blood.

The placenta, thus, serves as the nutritive, respiratory and excretory organ of the foetus. The blood of the mother and foetus do not mix at all in the placenta or at any other place. The blood of the foetus in the capillaries of the chorionic villi comes in close contact with the mother’s blood in the tissue between the villi, Inn they are always separated by a membrane, through which substances must diffuse or lie transported by some active, energy requiring process.

The type of placenta in man is of described as deciduate (intimate contact between loetal and maternal tissue), discoidal (villi occur in the form of disc), haemo-chorial (chorionic epithelium in direct contact with maternal blood).

Functions of Placenta:

The placenta performs the following functions:

Food materials pass from the mother’s blood into the foetal blood through the placenta.

The trophoblast ol the placenta digest protein before passing them into foetal blood.

Through the placenta oxygen passes from the maternal blood to the foetal blood, and carbon dioxide passes from foetal blood to maternal blood.

Nitrogenous wastes such as urea pass from foetal blood into maternal blood through placenta and are filtered out by the kidneys of the mother.

The placenta stores glycogen, fat etc. for the foetus before liver is formed.

Placenta functions as an efficient barrier (defensive wall) and allows useful: aerials to pass into the Social blood. Harmful substances such as nicotine from cigarette and addictive drugs such as heroin can pass through placenta. Therefore, pregnant women should avoid cigarette and drugs. Viruses and bacteria can pass through placenta.

Placenta functions as an endocrine gland it secretes hormones such as oestrogen, progesterone and human chorionic gonadotropin (HCG).

Parturition (Birth):

The gestation period or the baby carrying period is the time from conception to birth. In human being, it is approximately 280 days. After a complete period of gestation the child birth takes place. It is called as parturition. Before child birth, there is a long series of involuntary contractions of uterus called as “labour pains”. Umblical cord is a tube containing blood vessels which connect the abdomen of the developing embryo with the placenta of the mother. Its position in the baby is shown by navel.

The signals for parturition orginate from the fully developed foetus and the placenta which induce mild uterine contractions called foetal ejection reflex. Parturition is induced by a complex neuroendocrine mechanism. When time comes for the baby to be delivered, pituitary gland secretes adrenocortico tropic hormone (ACTH) which stimulates the adrenal glands to secrete steroids.

These steroid hormones stimulate placenta to produce Prostaglandins. Hormone oxytocin is secreted from pituitary. These two hormones cause uterus to begin powerful muscular contractions which becomes stronger and stronger over a period of 2 to 18 hours. During that time, the cervix becomes fully dilated and amniotic sac ruptures.

The baby is expelled out of the uterus through the birth canal. Soon after, the placenta is also expelled out of the uterus. Labour pain can be induced artificially by injection of oxytocin or insertion of prostaglandin into the vagina. When the woman is incapable of taking the labour pain, she can have a surgical procedure for child birth called caesarean.


The production and release of milk is called lactation (L. lactare = to suckle). Prolactin, a hormone of anterior pituitary stimulates lactation after parturition. High levels of estrogen act directly on mammary glands and can block the stimulation by prolactin. The mother produces thick, yellowish, high protein fluid called colostrum for 2-3 days after parturition.

Colostrum contains a great amount of maternal antibodies and helps in strengthening the baby’s immune system. Colostrum also acts as a laxative, removing foetal wastes, called meconium, retained in the intestine. The newborn’s suckling stimulates the pituitary to release oxytocin as well as prolactin. Oxytocin triggers milk release from the mammary glands. Breast feeding during the period of infant growth is always recommended for bringing up a healthy baby.

Usually there are mild contractions (sometimes there may not) that could help the placenta to separate from the uterine wall and move through the birth canal.

In a vaginal delivery, the third stage of labor begins with childbirth and ends with placental delivery. Your practitioner may inject Pitocin (oxytocin) into your body to induce uterine contraction and speed up placenta expulsion (20).

In a C-section, your practitioner physically removes the placenta before closing the incision. The remaining fragments are removed to prevent infection and bleeding (21).

When does the placenta form?

About 10 days after conception, as soon as the fertilized egg implants in the uterus, the chorion forms. The chorion is an embryonic organ that precedes the placenta’s development.

The placenta fully develops by weeks 18–20 of pregnancy, but it continues to provide the baby with oxygen, nutrition, and immunity. The mother’s blood supply is fully connected to the developing placenta by week 14 of pregnancy.

The anatomy of the placenta consists of two components:

  • Maternal placenta – this part of the placenta develops from the mother’s uterine tissue at around 7–12 days after conception.
  • Fetal placenta – this piece forms when the outer cells of the blastocyst (the earliest form of the embryo) divide and burrow deep into the uterus to connect to the mother’s blood supply. It starts forming 17–22 days after conception.

Formation of Placenta | Embryology

The embryo, specially in eutherian mammals, becomes implanted to the ute­rine wall. The process of implantation in­volves tissue interaction and establishment of connection between the uterine wall and the extraembryonic membranes. The region of attachment between the embryo­nic tissue and the uterine wall is called the placenta and the process involved in im­plantation is called the placentation.

The placenta is usually defined as an apposi­tion or fusion- between uterine and em­bryonic tissues for physiological exchange of materials. Human placenta is a round flattened mass from which the name pla­centa is derived. The name placenta has been derived from the Greek word meaning a flat cake.

Types of Placenta:

The Placenta is divided into several types:

A. Depending on the Involvement of Embryo­nic Tissue:

When the midgut extension of the splanchnopleure enclosing the yolk fuses with the extraembryonic somatopleure to make embryonic contact with the uterine wall. Examples: Mustelus.

(ii) Chorio-Allantok Placenta:

The allan­toic evagination of the hindgut unites with the extraembryonic somatopleure to make contact with the uterine tissue. Examples: Eutherian mammals and a lizard, Chalcides.

B. Depending on the Distribution of Villi:

(i) Diffused Placenta:

The villi are nume­rous and distributed uniformly over the whole of chorion. Examples: Ungulates, Cetacea.

(ii) Cotyledonary placenta:

The villi become aggregated in special regions to form small tufts. Examples: Ruminants.

(iii) Zonary Placenta:

The villi are confined to an annular zone on the chorion. Exam­ples: Carnivora (Pinnipedia).

(iv) Discoidal Placenta:

The villi become restricted to a discoidal area as seen iruro- dents and insectivores. In apes and man, the placenta is of metadiscoidal type.

C. Based on the Relationship of Villi with the Uterine Wall:

(i) Deciduate Placenta:

The villi become intimately connected with the mucous membrane of the uterine wall which comes out with the embryo at the time of birth.

(ii) Indeciduate or Adeciduate Placenta:

The villi are loosely united with the uterine walls which separate from the uterus at birth.

D. Based on the Degree of Involvement of Foetal and Maternal Tissues:

(i) Epitheliochortal Placenta:

The epithe­lium of uterus remains in simple apposition with the chorion of the embryo. Examples: Pig and Horse.

(ii) Syndesmochorial Placenta:

The epithe­lium of the uterus disappears and the chorion comes in direct contact either with the glandular epithelium or endometrium of the uterus. Example: Sheep.

(iii) Vasochorial or Endotheliochorial Placenta:

Both the glandular epithelium and the. endometrium disappear and the chorion comes in close contact with the endothe­lium of the uterine capillaries. Examples: Dogs and Cats.

(iv) Haemochorial Placenta:

The glandular epithelium, endometrium and endothelium of the capillaries disappear and the cho­rion is bathed with circulating maternal blood. Example: Man.

(v) Haemoendothelial Placenta:

Like that of haemochorial type of placenta, the glan­dular epithelium, endometrium and the endothelium of the maternal blood capil­laries disappear. With the disappearance of these maternal structures, the tropoblastic epithelium (outer layer of the blas­tocyst) of the foetus also disappears, as a result the foetal endothelium separates the maternal and foetal circulating blood stream. Examples: Many rodents.

Organogenesis of Human Placenta:

In human females, implantation of the developing embryo occurs in the early luteal phase when the endometrium of the uterus remains in optimum condition. The deve­loping egg reaches the uterus in blastocyst condition with a greatly enlarged blastocoelic space.

The placental organogenesis is described under two broad aspects:

a. Previllous period (6th-13th day).

b. Villous period (14th day to term).

The implantation of human embryo takes place about 6th to 9th days after fer­tilization (Figs. 5.44, 5.45).

The end of the blastocyst containing the developing ger­minal disc attaches itself to the uterine wall (Fig. 5.46A). The uterine epithelium is eroded at the region of contact. The trophoblast tissue increases in thickness in this contact area due to the division of epithelial cells of the trophoblast layer.

The implanted embryo, consisting of a bilaminar disc, is protruded into a cavity (lecithocoel). This cavity is enclosed by the trophoblast. Profound cytological changes occur in the tropho­blast layer. The inner trophoblast cells remain cellular and are designated as the cytotrophoblast while the outer cells fuse together to form a syncytium called the syncytiotrophoblast.

The syncytiotrophoblast serves as the invading tissue of the embryo into the uterine wall. The growth of this syncytiotrophoblast is caused by differen­tiation of the cytotrophoblast and by ami­totic division of the syncytial nuclei.

Lacunar stage (10th to 13th days of Pla­cental development):

As the syncytiotro­phoblast invades and increases in quantity, irregular spaces are produced in the syn­cytiotrophoblast. These spaces are called trophoblastic lacunae. (Fig. 5.46B).

During the period between 14 and 18 days, the trophoblastic lacunae merge with one another to form large cavities bor­dered by syncytiotrophoblast. Such a cavity is named as the intervillous space and the primary villi are formed through the proliferation of the cytotrophoblastic ele­ments into the syncytial trabeculae.

The primary villi, when first formed, lack meso­dermal core. The mesoderm of the somatopleure invade into them to form the secondary villi (Fig. 5.46D).

With the forma­tion of the blood islands and appearance of blood vessels, the secondary villi transform into the definitive tertiary villi. At this time, some endometrial tissue including blood vessels near to the invading chorionic vesicle break down to produce liquefied areas called the embryotroph. The liquefied

material from the embryotroph is assi­milated by the syncytiotrophoblast for the growth of the embryo. This particular type of. nutrition is called the histotrophic nutrition.

From the physiological point of view the foetal villi developing from chorionic plate can be divided into three categories:

(i) Chorionic villi, through which phy­siological exchange of materials taking place between the foetus and the mother

(ii) Anchoring villi for mechanical anchor­age of foetus and

The developing chorionic vesicle grows and invades the endometrium of the uterus. The uterine mucosa extends over the invading vesicle. The endometrial tissue covering the chorionic vesicle is called the decidua capsularis while the endo­metrial portion which is not concerned with the closure of such vesicle is called the decidua parietalis or decidua vera.

The endo­metrial portion lying between the mus­culature of the uterine wall and the invad­ing villi is called decidua basalts.

Although the chorionic villi are formed over the entire chorionic vesicle, only the villi in relation to decidua basalis are re­tained and those grown to decidua parie­talis are reabsorbed to form a smooth area named as the chorion leave. The villi within decidua basalis become greatly enlarged to serve the main role of physiological interchange (Fig. 5.46E).

This area of chorionic vesicle with the villi is the chorion frondosum which together with the tissue of decidua basalis forms the actual placenta. Thus the placenta is composed of decidua basalis (maternal placenta) and chorion frondosum (foetal placenta) (Fig. 5.46F).

The villi, at the early phase of develop­ment, consist of blood capillaries within mesodermal core covered over by cytotrophoblast and syncytiotrophoblast on the outer side. As development goes on the blood capillaries grow enormously and the cytotrophoblast layer is extremely reduced to few scattered cells below syncytiotro­phoblast.

The villi are aggregated into groups call­ed the cotyledons which are separated by incomplete placental septa. The villi in each cotyledon remain surrounded by a pool of maternal blood and by this way a hemochorial type of placenta is established.

Organogenesis of Placenta in Rabbit:

In rabbit, as in other eutherian mam­mals, placentation involves an interaction and attachment between the uterine wall and the extraembryonic membranes (chorion). The initial contact of the em­bryonic tissues with the uterine wall forms an epitheliochorial relationship in rabbit.

Sub­sequently in course of development, it chan­ges into a hemochorial condition in which the endothelial tissue of the uterine blood vessels is destroyed by the erosive action of the foetal tissue. As a consequence, the chorionic epithelium of the embryonic part of the placenta comes in direct con­tact with the maternal blood.

During the later stages of pregnancy, even the chorio­nic epithelium also disappears thus leaving the endothelial lining of the foetal blood vessels in contact with the maternal blood. This type of placental relationship is called the hemoendothelial type (Fig. 5.47) and is regarded to be the most intimate grade of placental contact in animal kingdom.

De­pending upon the distribution of villi, the placenta in rabbit is categorized as the discoidal type. At the onset, the chorion becomes uniformly covered with villi, but at late stage the villi remain only on one side. The villi continue to develop on the side which turned away from the restricted lumen of the uterus, while the villi on the other parts of villi become atrophied.

Events in Placentation in Rabbit:

The egg of rabbit, after fertilization, comes to lie in one of the crypts of the uterine wall (Fig. 5.48). As soon as the fertilization is completed the corona radiata (covering of the egg) breaks down to form a frothy substance (histotrophy which surrounds the entire fertilized egg.

The histotroph, around the egg, provides nutrition for further growth and develop­ment. The egg starts development and grows vigorously at the expense of the histotroph and soon completes the gastru­lation stage to form the three primary germinal layers (viz., ectoderm, mesoderm and endoderm).

With the differentiation of the primary germinal layers in the embryo, remarkable physiological changes occur in the uterine wall of the mother. The region of the uterine wall where the embryo touches, continues to bleed due to some special enzymatic action.

The ‘maternal site of implantation’ becomes subsequently thick, spongy and vascularised. The superficial contact is made more intimate by the development of minute finger-like projections called the villi.

The villi develop as outgrowths from the extraembryonic region of the foetus and penetrate into depressions in the wall of the uterus. These outgrowths (villi) are initially formed by the trophoblast but the connective tissue and blood vessels enter into the outgrowths later on. They are designated as the ‘chorionic villi’ and the blood vessels are actually the ramifications from allantoic blood vessels.

The region of the uterine wall which has passed the preparatory stage (vascularised) is called the trophospongia. The trophoblast and tro­phospongia come to establish intimate physiological relationship during placenta­tion.

The villi of the trophoblast penetrate into trophospongia and ramify extensively deep into the maternal tissues. During this action, the villi are constantly bathed by the maternal blood vessels because of excessive vascular state of trophospongia.

After coming into close physiological unition, the trophoblast and trophospongia form a nutritional bridge where the ma­ternal blood will bring nutrient materials for the developing embryo throughout the entire period of gestation. In rabbit, the epithelium of the chorion may disappear in some regions during the later stages of gestation—thus permitting an exposure of foetal blood vessels to the maternal blood.

The placenta is represented jointly by the trophoblast (chorion with its villi) and the trophospongia (uterine wall). At the time of parturition the chorionic villi are simply drawn out from the depressions in the uterine wall and the foetal tissues are separated without causing damage to the uterine wall and without causing any bleeding.

Functions of Placenta:

The functions of the placenta are many- fold.

(i) Adhesion or anchorage of the deve­loping embryo with the uterine wall.

(ii) Nutritional role—the food materials from the maternal blood circulation reach the blood stream of the embryo for supply­ing nutrition.

(iii) Excretory role—the waste products from embryonic circulation are eliminated to the maternal blood stream.

(iv) Respiratory role—it serves as the external respiratory surface for the deve­loping embryo.

(v) Elaborate endocrine functions: Two ovarian hormones—estrogen and proges­terone together with chorionic follicle- stimulating and luteinizing hormones are functionally elaborated by the placenta.

(vi) Protective role: Acts usually as barrier against the transportation of micro­bes into the embryo but the viruses and antigen do pass through the placenta.

(vii) Storage function: Glycogen, fats and some inorganic salts are stored in the pla­centa.

(viii) Source of nourishment for mother: Female of many mammals takes the pla­centa and after-birth tissues which provide nourishment.

Placenta: Meaning, Types and Function

In mammals although the fertilized ovum develops in the body of the mother, the extra embryonic membranes are formed in similar fashion like that of the birds. The extra-embryonic somatopleure contributes to the formation of amnion and chorion while the splanchnopleure forms the yolk sac and allantois.

The allantois grows out of the hindgut of the embryo and expands into the extra-embryonic coelom. It later fuses with the chorion. Although no yolk is present in the mammalian ovum, the yolk sac is esta­blished in a similar manner like that of the birds. However, with the enlargement of the allantois, the yolk sac rapidly declines and becomes a shriveled remnant.

At the same time, the endometrium of the mother’s uterus has nearly completed its preparation to receive the embryo. The uter­ine stromal cells undergo a pronounced transformation where its cytoplasm becomes filled with glycogen and lipid droplets. These transformed stromal cells are called decidual cells. The endometrium containing these cells will contribute to the formation of an entity called placenta.

The word placenta is derived from a Greek word meaning a “flat cake”. Placenta may be defined as a temporary structure formed by the association or fusion between the extra-embryonic membranes of the foetus and the endometrium of mother for the pur­pose of physiological exchange of materials.

Therefore, the placenta from its origin point of view consists of two parts a foetal placen­ta furnished by the extra-embryonic mem­branes and a maternal placenta contributed by the uterine endometrium. The method of formation and fusion of the foetal placenta to the uterine wall is called placentation.

From the maternal side only a single component, the endometrium, is involved, but from the foetal side there are four prospective elements — amnion, chorion, yolk sac and allantois. The amnion, being the inner most membrane, does not directly con­tribute to the making of the placenta.

The chorion because of its external position is the membrane that makes immediate contact with the endometrium. The chorion (as seen in the chick embryo) plays its role by way of a vascular supply. In mammals there are two possible sources of chorionic vascularization — the vitelline circulation provided by the yolk sac and the allantoic circulation provi­ded by the allantois.

Types of Embryonic Tissues Involved in Placentation:

In mammals, depending upon the types of embryonic tissues involved in placenta­tion, there exists two basic types of placenta, which are related to the two different sources of chorionic vascularization.

i. Choriovitelline Placenta (Yolk-sac Pla­centa):

In some mammals, particularly in most marsupials (Didelphys, Macropus), the allan­tois remains relatively small and never makes contact with the chorion. The yolk sac on the other hand becomes very large and fuses with the chorion (Fig. 5.50).

In these mammals the chorion receives its blood sup­ply from the yolk sac (vitelline circulation) and the placenta is thus called chori­ovitelline placenta. In marsupials, although only a portion of the yolk sac (and thus the chorion) is provided with vascular meso­derm (Fig. 5.50), it is still referred to as yolk- sac placenta. The chorion, however, never advances beyond a smooth membrane, applied closely with the endometrium.

Among eutherian mammals, many car­nivores, rodents and insectivores, a similar type of placenta may exist either temporari­ly or permanently. In those where the yolk sac placenta exists temporarily, the yolk sac provides the initial vascular supply.

It then gradually regresses, while the developing allantois reaches the chorion and vascula­rizes it. In the other type the yolk sac shares with the allantois the task of vascularizing the chorion.

ii. Chorioallantoic Placenta:

In most eutherian mammals and in some marsupials (Parameles, Dasyurus), the yolk sac remains rudimentary, while the allantois becomes well developed, fuses with the chorion and provides the chorionic circulation. This type of foetal placenta is called chorioallantoic placenta (Fig. 5.51). Here the chorion possesses finger-like vascular pro­cesses, the villi, which grows out into the adjacent maternal tissue.

Functions of Placenta:

Histologically the placenta consists of barriers that prevent the blending of blood of the foetus and mother. From the maternal side the blood, enters into the inter-villous spaces or crypts through about 30 spiral arteries and at high pressure.

The arterial blood rich in oxygen, nutrients etc. passes over the villi in small fountain like streams and then under reduced pressure settles down at the maternal base of the placental compartment from where it is removed by open-ended uterine veins (Fig. 5.56).

On the foetal side blood enters the villi through the branches of umbilical arteries. Although arte­rial, the blood is poor in oxygen and high in carbon-dioxide and other waste products. The foetal vessels at the terminal end of the villi form capillary network and at this region bulk of the placental exchange (Fig. 5.57) takes place. The blood now richer, is placental villus drained back to the foetus via the umbilical vein.

Exchange of substances from one blood stream to the other, takes place by various transfer mechanisms such as:

(iv) Leakage (i.e., by breakage of placental membrane).

The functions of placenta are many fold and are as follows:

Placenta serves as adhe­sion or anchorage of the developing embryo with the uterine wall.

The foetus gets its nutrition from the maternal blood. Monosaccharide’s, lipids, amino acids, vita- mines and hormones pass by diffusion or active transport. Macromolecules of polysac­charides, lipids and proteins are absorbed by the trophoblast cells by pinocytosis. Water and electrolytes such as chlorides and phos­phates of sodium, potassium and magnesium pass by diffusion from mother to foetus.

Gaseous exchange takes place by diffusion across the foetal membrane. Oxygen diffuses from maternal blood into the foetal blood, while reverse dif­fusion takes place in case of carbon dioxide.

Waste products like urea, uric acid and creatinine are eliminated via placenta, from the embryonic blood to the maternal blood stream by diffusion. The kid­ney of mother removes these wastes of foetal metabolism along with her own waste pro­ducts.

Glycogen, fats and some inorganic salts are stored in the placenta to be utilized when diet of the foetus is inadequate.

Placenta produces various enzymes such as diamine oxidase, oxytocinase and phospholipase-A2, which protects the foetus.

Placenta acts temporarily as an endocrine organ. It secretes many hormones such as estradiol, proges­terone, chorionic gonadotropin in most mammals and also placental lactogen in human female. In some animals, such as rabbit, human females etc., the placenta is a significant source of relaxin, that relaxes the pelvic liga­ments to facilitate child birth.

Placenta acts as a barrier against the transportation of microbes into the embryo. However, antibo­dies which have developed in the blood of a mother who has acquired immunity against certain diseases like diphtheria, scarlet fever, small pox and measles are passed on to the foetus, who become passively immunized to these illness in the first period after birth.

i. Destructive Function:

Certain pathogenic organisms can penetrate through the placental barrier and infect the foetus. This occurs if the mother is infected by those pathogens causing syphilis, small pox, chick­en pox, measles and rubella. Similarly any drug used during pregnan­cy can cross the placental barrier and cause disastrous effect on the foetus.

The occurrence of a placental lake is a normal feature, and seldom a point of concern. Experts state that placental lakes have little to no clinical significance (5). Research has found no difference in the pregnancies of women with placental lakes and those without placental lakes. There was no adverse event during the pregnancy due to placental lakes. No anomalies in the baby’s gestational age and birth weight were observed (6).

Placental lakes can be of concern in the following situations (4) (7).

  • They occur early, in the first trimester or early second trimester
  • Presence of more than three placental lakes
  • The diameter is more than two centimeters
  • Large placental lakes with a diameter greater than five centimeters

Complications of placental lakes

Placental lakes can be a complication only if they are affecting the growth of the fetus.

The problems caused by them may or may not happen, and you may even give birth to a healthy baby. Therefore, do not panic if you have placental lakes. Any complication related to the pregnancy is quite likely to be detected and treated early if you visit the doctor for regular ultrasound examination.


Although all mammalian placentae have the same functions, there are important differences in structure and function in different groups of mammals. For example, human, bovine, equine and canine placentae are very different at the both gross and the microscopic levels. Placentae of these species also differ in their ability to provide maternal immunoglobulins to the fetus. [5]

Placental mammals, such as humans, have a chorioallantoic placenta that forms from the chorion and allantois. In humans, the placenta averages 22 cm (9 inch) in length and 2–2.5 cm (0.8–1 inch) in thickness, with the center being the thickest, and the edges being the thinnest. It typically weighs approximately 500 grams (just over 1 lb). It has a dark reddish-blue or crimson color. It connects to the fetus by an umbilical cord of approximately 55–60 cm (22–24 inch) in length, which contains two umbilical arteries and one umbilical vein. [6] The umbilical cord inserts into the chorionic plate (has an eccentric attachment). Vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells. This results in the formation of villous tree structures. On the maternal side, these villous tree structures are grouped into lobules called cotyledons. In humans, the placenta usually has a disc shape, but size varies vastly between different mammalian species. [7]

The placenta occasionally takes a form in which it comprises several distinct parts connected by blood vessels. [8] The parts, called lobes, may number two, three, four, or more. Such placentas are described as bilobed/bilobular/bipartite, trilobed/trilobular/tripartite, and so on. If there is a clearly discernible main lobe and auxiliary lobe, the latter is called a succenturiate placenta. Sometimes the blood vessels connecting the lobes get in the way of fetal presentation during labor, which is called vasa previa.

Gene and protein expression Edit

About 20,000 protein coding genes are expressed in human cells and 70% of these genes are expressed in the normal mature placenta. [9] [10] Some 350 of these genes are more specifically expressed in the placenta and fewer than 100 genes are highly placenta specific. The corresponding specific proteins are mainly expressed in trophoblasts and have functions related to female pregnancy. Examples of proteins with elevated expression in placenta compared to other organs and tissues are PEG10 and the cancer testis antigen PAGE4 and expressed in cytotrophoblasts, CSH1 and KISS1 expressed in syncytiotrophoblasts, and PAPPA2 and PRG2 expressed in extravillous trophoblasts.

Development Edit

The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium. The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This outer layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer. The syncytiotrophoblast is a multinucleated continuous cell layer that covers the surface of the placenta. It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development. The syncytiotrophoblast (otherwise known as syncytium), thereby contributes to the barrier function of the placenta.

The placenta grows throughout pregnancy. Development of the maternal blood supply to the placenta is complete by the end of the first trimester of pregnancy week 14 (DM).

Placental circulation Edit

Maternal placental circulation Edit

In preparation for implantation of the blastocyst, the endometrium undergoes decidualization. Spiral arteries in the decidua are remodeled so that they become less convoluted and their diameter is increased. The increased diameter and straighter flow path both act to increase maternal blood flow to the placenta. There is relatively high pressure as the maternal blood fills intervillous space through these spiral arteries which bathe the fetal villi in blood, allowing an exchange of gases to take place. In humans and other hemochorial placentals, the maternal blood comes into direct contact with the fetal chorion, though no fluid is exchanged. As the pressure decreases between pulses, the deoxygenated blood flows back through the endometrial veins.

Maternal blood flow is approximately 600–700 ml/min at term.

This begins at day 5 - day 12 [11]

Fetoplacental circulation Edit

Deoxygenated fetal blood passes through umbilical arteries to the placenta. At the junction of umbilical cord and placenta, the umbilical arteries branch radially to form chorionic arteries. Chorionic arteries, in turn, branch into cotyledon arteries. In the villi, these vessels eventually branch to form an extensive arterio-capillary-venous system, bringing the fetal blood extremely close to the maternal blood but no intermingling of fetal and maternal blood occurs ("placental barrier"). [12]

Endothelin and prostanoids cause vasoconstriction in placental arteries, while nitric oxide causes vasodilation. [13] On the other hand, there is no neural vascular regulation, and catecholamines have only little effect. [13]

The fetoplacental circulation is vulnerable to persistent hypoxia or intermittent hypoxia and reoxygenation, which can lead to generation of excessive free radicals. This may contribute to pre-eclampsia and other pregnancy complications. [14] It is proposed that melatonin plays a role as an antioxidant in the placenta. [14]

This begins at day 17 - day 22 [15]

Birth Edit

Placental expulsion begins as a physiological separation from the wall of the uterus. The period from just after the child is born until just after the placenta is expelled is called the "third stage of labor". The placenta is usually expelled within 15–30 minutes of birth.

Placental expulsion can be managed actively, for example by giving oxytocin via intramuscular injection followed by cord traction to assist in delivering the placenta. Alternatively, it can be managed expectantly, allowing the placenta to be expelled without medical assistance. Blood loss and the risk of postpartum bleeding may be reduced in women offered active management of the third stage of labour, however there may be adverse effects and more research is necessary. [16]

The habit is to cut the cord immediately after birth, but it is theorised that there is no medical reason to do this on the contrary, it is theorised that not cutting the cord helps the baby in its adaptation to extrauterine life, especially in preterm infants. [17]

Microbiome Edit

The placenta is traditionally thought to be sterile, but recent research suggests that a resident, non-pathogenic, and diverse population of microorganisms may be present in healthy tissue. However, whether these microbes exist or are clinically important is highly controversial and is the subject of active research. [18] [19] [20] [21]

Nutrition and gas exchange Edit

The placenta intermediates the transfer of nutrients between mother and fetus. The perfusion of the intervillous spaces of the placenta with maternal blood allows the transfer of nutrients and oxygen from the mother to the fetus and the transfer of waste products and carbon dioxide back from the fetus to the maternal blood. Nutrient transfer to the fetus can occur via both active and passive transport. [22] Placental nutrient metabolism was found to play a key role in limiting the transfer of some nutrients. [23] Adverse pregnancy situations, such as those involving maternal diabetes or obesity, can increase or decrease levels of nutrient transporters in the placenta potentially resulting in overgrowth or restricted growth of the fetus. [24]

Excretion Edit

Waste products excreted from the fetus such as urea, uric acid, and creatinine are transferred to the maternal blood by diffusion across the placenta.

Immunity Edit

The placenta functions as a selective barrier between maternal and fetal cells, preventing maternal blood, proteins and microbes (including bacteria and most viruses) from crossing the maternal-fetal barrier. [25] Deterioration in placental functioning, referred to as placental insufficiency, may be related to mother-to-child transmission of some infectious diseases. [26] A very small number of viruses including rubella virus, Zika virus and cytomegalovirus (CMV) can travel across the placental barrier, generally taking advantage of conditions at certain gestational periods as the placenta develops. CMV and Zika travel from the maternal bloodstream via placental cells to the fetal bloodstream. [25] [27] [28] [29]

Beginning as early as 13 weeks of gestation, and increasing linearly, with the largest transfer occurring in the third trimester, IgG antibodies can pass through the human placenta, providing protection to the fetus in utero. [30] [31] This passive immunity lingers for several months after birth, providing the newborn with a carbon copy of the mother's long-term humoral immunity to see the infant through the crucial first months of extrauterine life. IgM antibodies, because of their larger size, cannot cross the placenta, [32] one reason why infections acquired during pregnancy can be particularly hazardous for the fetus. [33]

Endocrine function Edit

  • The first hormone released by the placenta is called the human chorionic gonadotropin hormone. This is responsible for stopping the process at the end of menses when the Corpus luteum ceases activity and atrophies. If hCG did not interrupt this process, it would lead to spontaneous abortion of the fetus. The corpus luteum also produces and releases progesterone and estrogen, and hCG stimulates it to increase the amount that it releases. hCG is the indicator of pregnancy that pregnancy tests look for. These tests will work when menses has not occurred or after implantation has happened on days seven to ten. hCG may also have an anti-antibody effect, protecting it from being rejected by the mother's body. hCG also assists the male fetus by stimulating the testes to produce testosterone, which is the hormone needed to allow the sex organs of the male to grow. helps the embryo implant by assisting passage through the fallopian tubes. It also affects the fallopian tubes and the uterus by stimulating an increase in secretions necessary for fetal nutrition. Progesterone, like hCG, is necessary to prevent spontaneous abortion because it prevents contractions of the uterus and is necessary for implantation. is a crucial hormone in the process of proliferation. This involves the enlargement of the breasts and uterus, allowing for growth of the fetus and production of milk. Estrogen is also responsible for increased blood supply towards the end of pregnancy through vasodilation. The levels of estrogen during pregnancy can increase so that they are thirty times what a non-pregnant woman mid-cycles estrogen level would be. is a hormone used in pregnancy to develop fetal metabolism and general growth and development. Human placental lactogen works with Growth hormone to stimulate Insulin-like growth factor production and regulating intermediary metabolism. In the fetus, hPL acts on lactogenic receptors to modulate embryonic development, metabolism and stimulate production of IGF, insulin, surfactant and adrenocortical hormones. hPL values increase with multiple pregnancies, intact molar pregnancy, diabetes and Rh incompatibility. They are decreased with toxemia, choriocarcinoma, and Placental insufficiency. [34][35]

Immunological barrier Edit

The placenta and fetus may be regarded as a foreign body inside the mother and must be protected from the normal immune response of the mother that would cause it to be rejected. The placenta and fetus are thus treated as sites of immune privilege, with immune tolerance.

For this purpose, the placenta uses several mechanisms:

  • It secretes Neurokinin B-containing phosphocholine molecules. This is the same mechanism used by parasiticnematodes to avoid detection by the immune system of their host. [36]
  • There is presence of small lymphocytic suppressor cells in the fetus that inhibit maternal cytotoxic T cells by inhibiting the response to interleukin 2. [37]

However, the Placental barrier is not the sole means to evade the immune system, as foreign fetal cells also persist in the maternal circulation, on the other side of the placental barrier. [38]

Other Edit

The placenta also provides a reservoir of blood for the fetus, delivering blood to it in case of hypotension and vice versa, comparable to a capacitor. [39]

Numerous pathologies can affect the placenta.

    , when the placenta implants too deeply, all the way to the actual muscle of uterine wall (without penetrating it) , when the placement of the placenta is too close to or blocks the cervix /abruptio placentae, premature detachment of the placenta , inflammation of the placenta, such as by TORCH infections.

The placenta often plays an important role in various cultures, with many societies conducting rituals regarding its disposal. In the Western world, the placenta is most often incinerated. [40]

Some cultures bury the placenta for various reasons. The Māori of New Zealand traditionally bury the placenta from a newborn child to emphasize the relationship between humans and the earth. [41] Likewise, the Navajo bury the placenta and umbilical cord at a specially chosen site, [42] particularly if the baby dies during birth. [43] In Cambodia and Costa Rica, burial of the placenta is believed to protect and ensure the health of the baby and the mother. [44] If a mother dies in childbirth, the Aymara of Bolivia bury the placenta in a secret place so that the mother's spirit will not return to claim her baby's life. [45]

The placenta is believed by some communities to have power over the lives of the baby or its parents. The Kwakiutl of British Columbia bury girls' placentas to give the girl skill in digging clams, and expose boys' placentas to ravens to encourage future prophetic visions. In Turkey, the proper disposal of the placenta and umbilical cord is believed to promote devoutness in the child later in life. In Transylvania, and Japan, interaction with a disposed placenta is thought to influence the parents' future fertility. [ citation needed ]

Several cultures believe the placenta to be or have been alive, often a relative of the baby. Nepalese think of the placenta as a friend of the baby Malaysian Orang Asli regard it as the baby's older sibling. [44] Native Hawaiians believe that the placenta is a part of the baby, and traditionally plant it with a tree that can then grow alongside the child. [40] Various cultures in Indonesia, such as Javanese, believe that the placenta has a spirit and needs to be buried outside the family house.

In some cultures, the placenta is eaten, a practice known as placentophagy. In some eastern cultures, such as China, the dried placenta (ziheche 紫河车, literally "purple river car") is thought to be a healthful restorative and is sometimes used in preparations of traditional Chinese medicine and various health products. [46] The practice of human placentophagy has become a more recent trend in western cultures and is not without controversy its practice being considered cannibalism is debated.

Some cultures have alternative uses for placenta that include the manufacturing of cosmetics, pharmaceuticals and food.