PLACENTAL
FUNCTIONS
OUTLINE
INTRODUCTION
DEVELOPMENT
STRUCTURE
CIRCULATION
FUNCTIONS OF THE
PLACENTA
CLINICAL
CORRELATES
CONCLUSION
INTRODUCTION
Placenta=
intimate apposition or fusion of fetal organs to maternal tissues for
physiologic exchange.
The trophoblast
is the basic parenchyma of the placenta. When this becomes a membrane
penetrated by fetal mesoderm is called a CHORION.
The human
placenta is a villous hemichorial structure, which plays a critical role in
maternal fetal transfer and immunological acceptance of the fetal allograft.
DEVELOPMENT
Fertilization of
the ovum by spermatozoa occurs in the FT and enters the uterine cavity as a
MORULA.
The zona
pellucida is shed and converts itself into a BLASTOCYST.
The outer cell
layer of the blastocyst proliferates to form the prim. Trophoblastic cell mass
from which the cells infilterate btw those of the endometrial linings.
cont
The latter
degenerates and the trophoblast comes in direct contact with the endometrial
stroma. This is completed by the 10th or 11th post
ovulatory day.
In the 7th
day, the trophoblast differentiates into the inner cytotrophoblast and an outer
syncitiotrophoblast.
Btw the 19th-13th
post ovulatory days a series of intercommunicating spaces or lacunae develop in
the rapidly enlarging and dividing trophoblastic cell mass.
cont
The lacunae
become confluent and as the trophoblastic cells erode the maternal vessels,
become filled with bld form the intervillous spaces.
Groups of cytotrophoblasts
grow into the lumen of the spiral arterioles extending as far as the decidual
myometrial junction. These cells destroy the muscular and elastic layers of the
vessels wall which finally get replaced by a fibrinoid material which is
derived from the maternal bld and proteins secreted by the trophoblastic cells.
cont
The prim. Wave of
invasion weakens the spiral arteriolar wall, which dilates considerably and is
an important factor in augmenting bld flow to the placenta.
At the 4th
month there is a further proliferation of cytotrophoblasts and the second wave
of invasion involves myometrial portion of spiral arterioles extending up to
their origin from the radial arteries.
cont
The definitive
form of the placenta is achieved by the 4th month and continued
growth may occur up to term due to continuous arborization and formulation of
fresh villi.
STRUCTURE
Full term
placenta is discoid in shape, with a diameter of about 15-25 cm and 3cm thick.
Weighs btw
500-600gm (1/6 0f fetal wt.)
Occupies 1/3rd
of the uterine wall.
At birth it
separates from the uterine wall at the NITABAUCH~S layer and is expelled
approximately 30 mins after birth of the child.
Has 15-20
cotyledons when viewed from the maternal side. The fetal surface is smooth and
a number of arteries and veins are seen converging on the umbilical cord.
CIRCULATION
The cotyledons
receive bld supply through 80-100 special arterioles that enter the
intervillous space.
The placenta
membrane/ barrier separates maternal from fetal bld.
FUNCTIONS
NUTRITIVE
EXCRETORY
RESPIRATORY
BARRIER ACTION
ENDOCRINE
OTHERS
NUTRITIVE
Key function of
the placenta is to transport nutrient to the fetus.
Glucose is the
major metabolic fuel for the fetus, it crosses the placenta from the maternal
to fetal blood by FACILITATED DIFFUSION in accordance to glucose concentration
gradient.
Glucose is stored
in the placenta as glycogen and is used by the fetus for synthesis of fetal
tissues.
cont
Amino acids are
transferred across the placenta by active transport and used for protein
synthesis in the fetus.
Proteins ,despite
their large molecular size are transferred to the fetus by PINOCYTOSIS.
OTHERS: lipids,
vitamins and minerals are transferred to the fetus through the placenta
EXCRETORY
The placenta acts
as a FETAL KIDNEY excreting waste materials: uric acid,urea,creatinine and
salts into the maternal bld. These substances pass freely by simple diffusion.
The placenta also
transport waste material away from the fetus.
RESPIRATORY
The placenta acts
as the LUNGS of the fetus taking up oxygen from the maternal bld and giving up
carbondioxide.
In general gases
cross the placenta by simple diffusion.
BARRIER ACTION
The placenta can
prevent harmful substances from passing to the fetus from the mother.
This system is
imperfect because some organism can pass through:
Bacteria- typhoid
,TB, syphilis
Protozoa- malaria
Viruses- HIV,
Rubella, polio
ENDOCRINE
Produce
steroids needed for fetal development.
Progesterone,
oestriol, HCG, produced by the syncitiotrophoblast and excreted through
maternal urine.
OTHERS
Transmission of
maternal antibodies-IgG.
Transfer of heat.
Core temp. of human fetus is about 0.5oC above that of the maternal
core temp. and 0.2oC above that of amniotic fluid.
CLINICAL
CORRELATION
Most maternal
hormones do not cross the placenta.
Thyroxine crosses
at a slow rate.
Some synthetic
progestins can cross at a rapid rate and cause masculinization of the female
fetus.
The placental
barrier is protective, but several viruses can cross over; rubella, CMV,
varicella, measles,HIV, poliomyelitis,etc.
cont
Drugs and drugs
metabolites too can traverse the placental barrier and may cause serious damage
to the embryo.
Rhesus
incompatible fetuses, the antibodies produced in the rhesus negative mother
against the rhesus positive fetus in a previous pregnancy can cross over and
cause hemolysis in subsequent pregnancies resulting in IUFD or hemolytic
disease of the new born (HDN).
Absence of the
NITABAUCH`S layer results in morbidly adherent placenta.
cont
Absence of the
secondary wave of invasion of the placenta could cause vasospasm of the spiral
arterioles implicated in the
pathogenesis of PREECLAMPSIA/ECLAMPSIA and IUGR.
CONCLUSION
The placenta
provides the fetus with its essential nutrients, water and oxygen and a route
for clearance of fetal excretory products.
It also produces
a vast array of proteins, steroid hormones and factors essential for the
maintenance of pregnancy.
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July 14, 2015
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