The Placenta Development

The Placenta Development

The survival of the fetus depends on the integrity and efficiency of the placenta. It performs the function which the fetus is unable to perform for itself in-utero.

Development: The placenta originates from the tropholastic layerof the fertilized ovum which forms the chorionic villi. The chorionic villi become more profuse in the area which blood supply is richest. That is in the decidual basalis. This part is known as the CHORIONIC FRONDOSUM and it is what later develops into placenta. The capsular decidua later degenerate to form chorionic leave (bald chorium) from where the chorionic membrane is formed. These villi erode the maternal blood vessels opening them up to form a lake of maternal blood in which they float. Opened blood vessels are known as sinuses. Blood filled space is known as the intervilleous space. The maternal blood circulates around the villi slowly enabling it to absorb oxygen and nutrients and excrete waste into it. These are known as Nutritivevilli. A few villi are deeply attached to the decidua and are knownas Anchoring villi they stabilize the placenta. They lie between the maternal and the fetal blood vessels. Each villus originates from one single steam and it consists of 3 layers of cells mesoderm which contains the blood vessels, inner layer of cytotrophoblast and outer layer of syncytiotrophoblast; so it is impossible for the fetal and maternal blood to mix except when there is damage to the chorionic villi. Villi do not penetrate beyond the functional layer; it stopped by a layer of fibrinoid material in the decidua known as the layer of Nitabuch. By 10 weeks the placenta is completely formed and starts to function. It is initially a soft loose tissue. It becomes more compact as it matures

Placenta At Term

The placenta at term is a round flat organ about 20cm in diameter and 2.5cm thick at the centre. It weighs about 1/6^th of the baby’s weight at birth. It is made up of chorionic frondosum and blood vessel containing fetal blood and decidua Basalis. It has two surfaces the fetal and maternal surfaces.

The Fetal Surface

It is smooth, whitish and shiny covered by the amnion and chorion. The cord is attached to it at the centre and the fetal blood vessels can be seen radiating from the insertion of the cord to the edge. The chorion hangs from the edge of the placenta while an amnion can be peeled up to insertion of the cord.

The Maternal Surface: This is rough and bluish-red in color.It is made up of chorionic villi arranged in 20 cotyledons or lobes separated by sulci or furrous some small deposit of lime salt can be found on the surface which appear gritty in appearance. This has no clinical significance. The surface is covered by a layer of trophoblastic cells.

Functions Of The Placenta

1.           Respiratory: During intrauterine life no pulmonaryexchange of gases can take place. The fetus absorbs oxygen from the maternal haemoglobin by processes of simple osmosis and diffusion and gives off carbon-hydroxide into the maternal circulation similarly.

2.           Nutritive: All food nutrients required by the fetus for growthand energy are obtained from the mother’s blood in simplest form. Protein for building tissue, glucose for growth and energy, calcium & phosphorus for the bones and teeth, water, vitamins, electrolytes, iron and other minerals for blood formation, growth and various body processes. The Placenta does the selection. The placenta also does the metabolic function of glucose; it stores it as glycogen and converts it to glucose as required.

3.           Excretory: All waste products from the fetus are excretedinto the mother’s circulation through the placenta.

4.           Endocrine: Placenta produces some hormones.

·              Human Chorionic Gonadotrophin (HCG): This is a unique hormone in pregnancy produced by the langhans cells of the chorionic villi – cytotrophoblast from  its earliest day. It  makes the  corpus luteum to continue with production of progesterone and Oestrogen until the placenta takes over. It can be detected from about the 30^th day of conception and reaches its peak about 60-80 days of pregnancy. The peak drops at about the 12^th week and a low level is maintained throughout pregnancy. The high level persist longer in multiple pregnancy, trophoblastic tumour (hydatidiform mole). It is excreted in urine and form the basis for immunological test for diagnosing pregnancy. It also regulates the production of oestrogen by the placenta.

·              Progesterone: This is produced in the syncytial later bythe placenta from about the 3^rd month. It relaxes the smooth muscles and reduces exertibility tone e.g. uterus, stomach ureter and intestines. It is excreted in urine as pregnanediol. The level drops immediately before the onset of labor.

·              Oestrogen: Oestroil, oestradiol. It is produced by fetoplacenta unit from the 6^th week. It aids the growth of the uterine muscle and mobility of the nipple. The amount rises steadily until term and falls when the palcenta is expelled to allow prolactin to initiate lactation. The amount of the measured urine or serum eastroil indicates fetal well being.

·              Human placenta Lactogen (HPL): Aids thedevelopment and growth of the breast. Has generalized metabolic effect on carbohydrate and lipids. It has connection with the activity of the growth hormone. The level of it in the blood reflects placental function.

5.           Storage: It stores glucose in form of glycogen until the liverof the fetus is matured enough and capable of storage. Vitamins A & D and iron are also stored in the placenta.

6.           Protective: The placenta protects the fetus from someharmful diseases suffered by the mother e.g. malaria and T.B. Organisms can not pass through the placental barrier. But some bacteria and virus e.g. syphilis, rubella (German measles), small pox may, and cause congenital abnormalities and some drugs (morphine, Pethidine, heparin etc) can pass through and affect the respiratory centre. Penicillin and sulphonamides can also pass through but this serves as an advantage in syphilis. Antibodies, immunoglobulin G (IgG) confer immunity for the first 3 months of life.

The Membranes

These are the sacs that contain the fetus and the amniotic fluid. The Chorion: Is a thick, rough, opaque and fragile membranecontinuous with the placenta at its edge. It lines the decidua vera of the uterine cavity. It is derived from chorion leave of the trophoblast and continuous with the chorionic plate. It ruptures easily and can be retained during the delivery of the placenta.

The Amnion: It forms the sac that contains the fetus, the amnioticfluids and the cord. It lies in contact with the chorion. It is smooth, translucent and tough. It is derived from the inner cell mass. It is thought to have a role in the formation of the amniotic fluids. The amnion is much stronger than the chorion and hardly retained. It can be stripped off up to the insertion of the cord.

The Amniotic Fluid: It is straw-colored fluid, alkaline in reaction.It is secreted from amniotic membranes, exudates from the decidua and placenta vessels and from fetal urine. The volume is 400-1,500mls in normal cases. It increases at the rate of about 30mls per week but decrease at term as the baby fills the uterine cavity. It reduced to about 1 litre near term (38wk). The reduction in volume may be partly due to the fetus swallowing it at term. It is most abundant in mid-trimester. It has the specific gravity of 1010, 99% water. The 1% solid matter is composed of lanugo, hair, epithelia cells, vernix caseosa, protein, glucose sodium, potassium and calcium. It has pH of 7.0 – 7.5.

Less than 300mls is regarded as Low volume – oligoh ydramnios More than 1,500mls is regarded as High volume – Pol yhydramnios

Functions of Amniotic Fluid

1.           Provides protective medium for the fetus against injuries.

2.           Acts as shock absorber

3.           Equalizes the pressure by uterine contraction over the fetus and cord.

4.           It permits free movement of the fetus in utero.

5.           Maintains the temperature of the fetus.

6.           It flushes the birth canal at and before the delivery of the baby

7.           Provides nutritive material

8.           Help impede the entering of bacterial into the uterus.

9.           Aids effacement and dilatation where there is poor application of the presenting part.

The Umbilical Cord Or Funis

This forms the connection between the fetus and the placenta. It is composed of a jelly-like material known as the Wharton’s jelly covered with a single layer of amniotic epithelium and stratified cubical cells. It contains one large umbilical vein which carries oxygenated blood from placenta to the fetus. Two arteries which is a continuation of the hypogastric arteries, wind round the vein and carry deoxygenated blood from the fetus to the placenta. The cord is about 50cm in length regarded short if less than 40cm but the length varies greatly. It may be as short as 7.55cm or as long as 2m. It is not of uniform thickness but is as thick as the little finger. There may be excessive collection of Wharton’s jelly known as “False knot”. The cord is attached to the placenta at the centre.

The false knot is not harmful but True knot which is very uncommon can be very dangerous. It results from excessively long cord & excessive movement. Short cord can cause delay in descent of the presenting part and premature separation of placenta. Excessively long cord can predispose to cord round the neck, body of the fetus, cord prolapse or True knot.

Anatomical variations of the placenta & the Cord

1.           Succenturiate Placenta: An accessory lobe of the placentais attached to the membranes, blood vessels run through the membrane to it. If retained may lead to PPH. It can be diagnosed with a hole in the membrane with blood vessels running into it.

2.           Circumvallate Placenta: this is a situation where chorionand Amnion form a double layer. It is seen as an opaque ring on the fetal surface. It is of no significance.

3.           Battledore Insertion of the Cord: the cord is attached to theedge of the placenta. Looks like a table tennis bat.

4.           Velamentous insertion of the cord: The cord is inserted intothe membranes some distance away from the edge of the placenta. The umbilical vessels run through membranes from the cord to placenta. It causes no harm to the fetus in a normally situated placenta, but may separate during the active management of the third stage of labor. 

5.           Bipartite placenta: The placenta divides into two separatesegments the cords join together shortly after the segments. When it divides into three, it is known as Tripartite placenta.

6.           Placenta Accrata: The placenta embeds beyond the normallevel. Separation becomes impossible.

7.           Placenta Fenestrate: Abnormal hole appear in the middle ofthe placenta it may be wrongly taken for missing lobe.

8.           Vasa Praevia: If the placenta is low-lying the vessels maypass across the OS. In this case there is danger to the fetus as the vessel can be torn when the membranes rupture (e.g. artificial rupture of membrane) leading to severe and rapid haemorrhage and rapid exsanguinations of the fetus. This is suspected when onset of haemorrhage coincides with rupture of the membranes.

Diseases of the Placenta

1.           Hydatidiform mole: A poliferative cystic degradation of the chorionic villi.

2.           Calcareous degeneration: Associated normal degerative process of the placenta. The maternal surface is rough to touch and white gritty substance like broken egg shells form opaque on it.

3.           Infarcts: Small whitish area of dead tissue found on the maternal surface. It results from necrotic chorionic villi. It can be found in cases of Pre-Eclampsia Essential hypertension and prolonged pregnancy.

4.           Oedematous placenta: This condition is found in hydrops fetalis when the placenta is large, pale with fluid oozing out from it.

5.           Syphilitic placenta: the placenta is greasy –lookin g and may weigh as much as one quarter of the weight of the fetus.