Functions &abnormalities
*. The placenta a mateno-fetal organ which begins developing at implantation of the blastocyst and is delivered with the fetus at birth.
Dilation of the distal segment of the spiral artery in normal pregnancies will reduce the velocity of incoming bloodTransit time to the uterine vein is estimated to be in the order of 25–30 s, allowing adequate time for oxygen exchange.
implantation
of the fertilized ovum occurs on the 7th- 10th day following conception. The layer of cells forming the surface of the blastocyst develops into the chorionic membrane, and cytotrophoblast cells proliferate from it. Trophoblast cells already are visible and functioning in the invasive process of implantation. Syncytiotrophoblast cells are multinucleated aggregates of cytotrophoblast cells and are formed continually from them. These cells, plus the villi, are the characteristic and unique features of the human placenta.*
At first the villi are formed over all the surface of the gestational sac (at 4th week) Between 12th-16th week the villi on the capsular surface degenerate & become smooth called the chorion leave. In compensation the villi on the decidua basalis undergo great hypertrophy called chorion frondosum . After 20 w the cytotrphoblast disappear &finally only thin layer of syncytiun remain
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Structural characteristics of the human placenta
*haemomonochorial
This type of placentation is termed haemomonochorial since on the maternal side there is only blood and no longer blood vessels (haemo) and on the fetal side there is only one layer of trophoblast (monochorial) between maternal blood and the fetal capillaries
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The placenta at term
The functional unit of the placenta is fetal cotyledon & the mature placenta has about 120 cotyledons, which are grouped into visible lobes*
Each cotyledon contains a primary villus stem arising from the chorionic plate, which is divided to form 2ry &3ry system from which terminal villi arise, where the fetal- maternal exchange takes place
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Macroscopic features of the term placenta
MeasuresThe placenta at term displays a round disc - like appearance, with insertion of the umbilical cord in a slightlyeccentric position on the fetal side of the placenta. The average measures of a delivered placenta at term are as follows: diameter 22 cm, central thickness 2.5 cm, andweight 450 – 500 g. **
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Normal placentation
The maternal flow to the placenta increases from 50mL/min in the 1st trimester to 600mL/min at term.*
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All these are clinical manifestations of total or patchy failure of trophoblast invasion of the myometrial segments of the spiral arteries.
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All these result in a small placenta with gross morphological changes which are : Infracts represents an area of ischemic necrosis of cotyledon resulting from spiral a. occlusion, usually by thrombosis Basal haematomas consist of a mass of blood in the centre of the cotyledon due to the rupture of the damaged spiral a. these pathological condition associated with increased perinatal mortality.
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Other abnormalities of the placenta
Anomalies in weight In cases of diabetes & haemolytic disease of the newborn the placental wt may increase to up to half the wt of the fetus. Site of implantation of the placenta The placenta usually attached to the uterine wall near the fundus, to either the anterior or posterior surface.*
In about 1 in 250 pregnancies the placenta is implanted wholly or partially on the lower segment of the uterus (placenta previa). Bilobate & trilobate placenta Instead of a single disc , it may consist of2or 3 lobes partly fused( of no clinical importance).
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Placenta succenturiata This is not uncommon. One or more accessory lobes of placenta are found on the chorion at a distance from the edge of the main placenta,
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Placenta circumvallata Where the original area of attachment of the chorionic plate to the uterine wall is small & placental growth has continued beyond its margin, a fibrous ring is seen on the fetal surface of the placenta the placenta continue to function normally.
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Morbid adherence of the placenta In 3rd stage of labour the placenta normally separates through the stratum spongiosum of the maternal decidua ( the superficial part of the decidua comes away with the placenta & the deeper part remains on the uterine wall), normally the chorionic villi only penetrate as far as this distance
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Morbid adherence of the placenta results from increased penetration of the decidua & myometrium by the villi.The degree of morbidity is determined by the depth of invasion. Placenta accreta- the placenta is partially or completely adherent to the uterus with penetration of villi into the superficial part of the myometrium. Placenta increta- the villi penetrate deeply through the decidua into the myometrium. Placenta percreta- .penetration can even be seen on the serosal surface.
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Tumours of the placenta Apart from choriocarcinoma, tumours of the placenta is rare like vascular tumours known as haemangiomas or chorangiomas Hydropic placenta In sever cases of isoimmunization (hydrops fetalis) the placenta show the same changes of fetus, being enlarged, pale & odematous with a marked increase in wt.
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The umbilical cord
Abnormal length The usual length, same as fetus at term 50cm. Excessive length predispose to prolapse of the cord, formation of loops round some part of the fetus may cause IUD in very rare cases. Short cord , delay in 2nd stage of labour, premature separation of the placenta , inversion of uterus are theoretical accidents.*
Knots in the cord These may be formed by fetal movement, knots are rarely tight enough to obstruct the circulation, but they do occasionally cause IUD. Abnormal insertion of the cord The cord usually attached to the centre of the placenta, but sometimes attached to the edge of placenta ( squash racket placenta) of no clinical importance.
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In very rare cases the cord is attached to the membrane at some distance from the edge of the placenta, at this point the vessle may divide into branches which run on the membrane before reaching the placenta (velamentous insertion of the cord ). This can be dangerous to the fetus if the vessels happen to pass across part of the chorion that lies below the presenting part ( vasa previa), as a branch may be torn when the membrane rupture, leading to fetal blood loss.
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Single umbilical artery This is uncommon, but can be associated with other abnormalities of the fetus, notably those of the kidneys, ureters or bladder.
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