Placenta

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PLACENTA

  • Human placenta connects the foetus with the uterine wall of the mother.
  • It is a structure where maternal and foetal tissues come in direct contact without rejection.
  • Placental development begins at 3rd week and is completed by 12th week.
  • Placenta is attached to the upper part of the uterine body either at the posterior or anterior wall.

GROSS  FEATURES

  • Fleshy & Spongy to feel.
  • Weight- 500 gm
  • Diameter- 15-20 cm
  • Thickness- 2.5 cm
  • Occupies 30% of the uterine wall.
  • 4/5th of the placenta is of fetal origin and 1/5th is of maternal origin.
  • At full term the placenta is discoid, deciduate, haemo-chorial, villous type of labyrinthine organ.
  • Human placenta presents-
    • Foetal surface
    • Maternal surface
    • Peripheral margin

Foetal Surface :–

  • Covered by smooth and glistening amnion overlying the chorion.
  • Umbilical cord is attached at or near its center.
  • Branches of the umbilical vessels are visible beneath the amnion as they radiate from the attachment of the cord.
  • Divided into 60-100 cotyledons by the septa.
  • Each maternal cotyledon contains 3-5 foetal cotyledons.

Maternal Surface :–

  • Rough and spongy
  • Maternal blood gives it dull red colour
  • Remnants of the decidua basalis gives it shaggy appearance
  • Divided into 15-20 cotyledons by the septa

Peripheral Margin :–

  • Margins of the placenta are formed by fused fetal and maternal surfaces.
  • After delivery, placenta separates with the line of separation being through decidua spongiosum.

DEVELOPMENT

  • Placenta is developed from 2 components.
    • Foetal component
    • Maternal component

Foetal Component :–

·        It is derived from chorion frondosum.

·        It is composed of chorionic plate & chorionic villi.

Chorionic plate-

·        It forms the foetal surface.

·        After the implantation of blastocyst cytotrophoblast differentiates internally into extraembryonic mesoderm forming chorion.

·        From outside inwards chorionic plate consists of-

o   Syncytiotrophoblast

o   Cytotrophoblast

o   Extraembryonic mesoderm with branches of umbilical vessels

Chorionic villi-

·        Chorionic villi arise from chorionic plate and extend towards the basal plate.

Initially, lacunar spaces appear within syncytiotrophoblast around the chorion

 

Each lacuna communicates with adjacent one around cords of syncytial cells known as trabecula

 

Trabeculae start developing into chorionic villi

 

Lacunae enlarge and erode the branches of uterine arteries and veins

  

So that the maternal blood starts flowing in to the intervillous spaces

 

Thus, the utero-placental circulation is established. 

·        Stages of chorionic villi -

o   Primary chorionic villi- they appear as local masses of cytotrophoblasts lined by syncytiotrophoblasts.

o   Secondary chorionic villi- extraembryonic mesoderm grows into primary chorionic villi forming a core of loose connective tissue.

o   Tertiary chorionic villi- embryonic blood vessels grow into the core of secondary chorionic villi.

·        Types of chorionic villi -

o   Fixed / Stem / Anchoring villi- They are structural units of placenta, reach the decidua basalis from chorion frondosum.

o   Free / Branching / Terminal villi- they are functional units of placenta, branches of stem villi float in the blood of intervillous space.

·        Chorionic villi attached to embryonic pole of blastocyst proliferate more rapidly called as chorion frondosum.

·        Rest of the chorionic villi attached to abembryonic pole degenerate known as chorion leave.

·        From

Maternal Component :–

·        It is derived from decidua basalis.

·        It is composed of basal plate.

Basal plate-

·        It forms the maternal surface.

·        It is perforated by spiral arteries.

·        From outside inwards basal plate consists of-

o   Compact and spongy layers of decidua basalis

o   Outer Syncytiotrophoblast layer

o   Cytotrophoblastic shell

o   Inner Syncytiotrophoblast layer

PLACENTAL  BARRIER

·        Maternal and fetal blood are separated by placental membrane or feto-maternal barrier.

·        It intervenes between the foetal blood in the chorionic villi and maternal blood in the intervillous space.

·        Up to the 3rd month of pregnancy the barrier consists of 4 layers-

o   Endothelium of fetal capillary on a basement membrane

o   Mesodermal cells

o   Cytotrophoblast on a basement membrane

o   Syncytiotrophoblast

·        From 4th month onwards the barrier in terminal villi consists of 2 layers-

o   Endothelium of fetal capillary on a basement membrane

o   Syncytiotrophoblast

·        Placental barrier in terminal villi shows fusion of syncytium with capillary wall forming membranous areas known as alpha zones, which are the sites of materno-foetal exchanges.

PLACENTAL  CIRCULATION

Maternal Blood in the Intervillous Space :–

·        Spiral branches of uterine arteries open into intervillous space by piercing the basal plate.

·        Venous blood of intervillous space is drained by uterine veins.

·        From intervillous space syncytial sprouts are carried into maternal circulation via uterine veins.

·        Short-circuit of blood between the spiral arteries and veins is prevented by-

o   Higher perfusion pressure of endometrial arteries

o   Contraction of uterine musculature

o   Foetal pulse in villi

·        Blood pressure in the intervillous space is 15 mm of Hg.

Foetal Blood in the Villi :–

·        Branching villi arising from tertiary stem villi ramify in a “tambour system”.

·        The first order of branching villi breaks up into second order of branches which run parallel to chorionic plate.

·        Second order of branching villi divide into third order of branches which pass towards the basal plate in a circular fashion around a cylindrical core within the intervillous space.

·        There is one spiral arterial inlet pouring the maternal blood into villus free central area of a tambour system.

·        Maternal and foetal blood streams flow side by side but in opposite direction.

·        Blood pressure in the foetal capillaries is 30 mm of Hg.

FUNCTIONS  OF  PLACENTA

  • Placenta a ts in the exchange of gaseous and metabolic products between maternal and foetal blood streams by simple diffusion or active diffusion process.
  • Foetal and maternal red blood cells may cross the placental barrier.
  • Transfer of maternal antibodies (IgG) may gain entrance to the foetus by pinocytosis of syncytiotrophoblasts.
  • Placental barrier prevents the entry of pathogenic organisms from mother to foetus by the presence of Hofbauer cells.
  • Placenta acts as a storage organ for glucose, calcium and iron.
  • Placenta synthesizes prostaglandins and hormones like human chorionic gonadotropin (HCG), oestrogen, progesterone and human placental lactogen (HPL).

CLASSIFICATION / TYPES  OF  PLACENTA

According to the Shape of Placenta :–

Lobed placenta-

  • It may exhibit two or more lobes.

Placenta membranacea-

  • It is diffuse and thin.
  • Chorionic villi project from the entire blastocyst cavity.

Circumvallate placenta-

  • The peripheral margin of placenta is surrounded by a sulcus and overlapped by a circular fold of decidua.

Accessory Placenta :–

Placenta succenturiata-

  • An accessory lobe of placenta is connected tot eh main mass by foetal membrane.

According to the Attachment of Umbilical Cord :–

Battle-dore placenta-

  • Umbilical cord is attached close to the margin of placenta.

Velamentous placenta-

  • Umbilical cord is attached to the periphery of placenta.

According to the Distribution of Umbilical Arteries :–

Disperse type-

  • Umbilical arteries divide in dichotomous manner and undergo successive reduction in calibre.

Magistral type-

  • Umbilical arteries maintain a uniform calibre up to the periphery of placenta and give off smaller branches.

According to the Site of Implantation :–

Placenta praevia-

  • Blastocyst is implanted in the lower part of uterine cavity overlapping the internal os of cervix.

According to the Degree of Adhesion or Penetration :–

Placenta accreta-

  • Placenta is adhered pathologically to the decidua basalis.

Placenta increta-

  • Placenta penetrates myometrium.

Placenta percreta-

  • Placenta penetrates the entire uterine wall.

According to the Tissues Forming Placental Barrier :–

Epithelio-chorial-

  • Chorion is separated from the maternal blood by endometrial epithelium, endometrial stroma and endothelium of maternal capillaries.
  • No part of decidua is shed off at full term.
  • It is also called as non-deciduate placenta.

Syndesmo-chorial-

  • Chorion is separated from the maternal blood by endometrial stroma and endothelium of maternal capillaries.

Endothelio-chorial-

  • Chorion is separated from the maternal blood by endothelium of maternal capillaries.

Haemo-chorial-

  • Chorion and its villi directly come in contact with maternal blood.

Haemo-endothelial-

  • Only endothelium of foetal vessels intervenes between the maternal and foetal blood.

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