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  1. Placenta is a temporary organ formed during pregnancy. It forms an important circulatory link between mother and fetus. The placenta consists of two major portions:-
    1. Maternal part of placenta (derived from decidua).
    2. Fetal part of placenta (derived from chorion).
  2. Decidua refers to functional layer of endometrium after implantation of the blastocyst stage of fertilized ovum (gravid endometrium). It is divisible into three parts:-
    1. Decidua basalis (Decidual plate) is the part of endometrium related to embryonic pole of conceptus and forms the maternal part of placenta.
    2. Decidua capsularis is the superficial part of decidua covering abembryonic pole of conceptus. Decidua parietalis is the part of decidua lining the rest of uterine cavity.
  3. Chorion is the triple layered membrane made up of somatopleuricextraembryonic mesoderm, cytotrophoblast and syncytiotrophoblast. It is divisible into 2 parts:-
    1. Chorionic frondosum developing at the embryonic pole contributing to placenta.
    2. Chorionic laeve developing at abembryonic pole and related to decidua capsularis.
Structure of placenta

Placental tissues are arranged as:-
  1. Chorionic plate (on fetal side): It is covered by amnion and is made up of
    1. Somatopleuric layer of extraembryonic mesoderm,
    2. Cytotrophoblast and
    3. Syncytiotrophoblast.
  2. Basal plate (on maternal side): It is made up offollowing layers from fetal to maternal aspect
    1. syncytiotrophoblast (forming outer layer of intervillous space),
    2. Rohr's fibrinoidstria,
    3. cytotrophoblastic shell,
    4. Nitbuch'sfibrinoidstria, and
    5. decidua basalis.
  3. Chorionic villi: These are three types:-
    1. Primary chorionic villi develop when a column of cytotrophoblast cells grow through syncytiotrophoblastfrom chorion towards decidua.
    2. Secondary chorionic villi develop when extraembryonic mesoderm extends into the core of primary villi. Tertiary chorionic villi develop when fetal blood vessels develop in the extraembryonic mesodermal core.
  4. Intervillous space: After development of chorionic villi, the lacunar spaces of syncytiotrophoblast are transformed into intervillous spaces.
Placental membrane
  1. Placental membrane is a composite structure separating maternal and fetal blood and permits fetomaternal exchange.
  2. Placental membrane is made up of following layers (from fetal to maternal side):-
    1. Endothelium of fetal capillaries and its basement membrane.
    2. Surrounding Mesenchymal tissue (connective tissue).
    3. Cytotrophoblast (Langerhans slayer).
    4. Syncytiotrophoblast

Umbilical Cord

  1. Umbilical cord develops from connecting stalk, which extends from the caudal end of the embryo to the chorion. Umbilical cord is covered by amniotic epithelium and contains a core of mesenchyme known as whartan's jelly.
  2. Contents of umbilical cord are:
    1. Wharton's jelly,
    2. Two umbilical,
    3. One (left) umbilical
    4. Remnant of yolk sac, and
    5. Remnant of allan to-enteric diverticulum.
  3. Initially 2 umbilical arteries and 2 umbilical veins develop in mesoderm, but right umbilical vein disappears. Left umbilical vein persists along with 2 umbilical arteries.
  4. At full term, the umbilical cord measures 1-2 cm in diameter, and 50-55 cm in length and generally attached to the center of fetal surface of placenta

Fetal Membranes

Amnion is formed early in 2nd week by delamination from the cytotrophoblastand closes a cavity called amniotic cavity. Amnion is continuous with epiblast layer of bilaminar germ disc. After folding of the embryo the amniotic cavity enlarges and comes to surround the embryo on all sides and the amnion also forms covering of umbilical cord. Amnion also covers the fetal surface (chorionic plate) of placenta.
Chorion is formed in the end of 2nd week by fusion of extraembryonic mesoderm (somatopleuric layer) with overlying cytotrophoblast. It initially encloses the embryo proper and all other extraembryonic membranes. Later extraembryonic coelom expands and forms a large cavity called chorionic cavity. Chorion forms the fetal surface of placenta.
Amniochorionic membrane
Amniochorion membrane is formed by fusion of amnion with chorion in 10th week of gestation. At this stage fetus in the amniotic membrane is bounded by amniochorionic membrane bounded by decidua capsularis.
Amnio-chorio-decidual membrane
Further expansion of amniotic cavity leads to obliteration of uterine cavity by fusion of decidua capsularis with decidua parietalis at 4th month of gestation. From this stage onwards the fetus in amniotic cavity is bounded by Amnio-chorio-decidual membrane.

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