Placentation

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Placentation

• Dr Ian Stewart
• i.stewart_at_abdn.ac.uk

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What is a placenta?What is the function of a
placenta?
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What is a placenta?

• Generally regarded as the site at which there is
  exchange of gasses (oxygen, carbon dioxide) and
  nutrients between maternal and foetal blood
• Site has separate maternal and foetal blood
  circulations which do not mix

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Placental function

• Placenta provides anchorage, establishes foetal
  vascular network in association with maternal
  blood supply but without connection
• Gaseous exchange
• Nutritional exchange
• Placenta provides hormonal support (endocrine
  gland)
• Immunological protection?
• Barrier to (maternal) blood borne pathogens, but
  not all

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Placenta as a filter/transfer organ

• Receives nutrients, oxygen, antibodies and
  hormones from the mother and passes out waste.
• Forms a barrier, the placental barrier, which
  filters out some substances which could harm the
  foetus.
• Many substances are not filtered out
• Alcohol and other social drugs
• Many prescription drugs
• Eg Thalidomide
• Some viruses
• Eg. Human cytomegalovirus
• Birth defects possible

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Placenta as an endocrine gland

• HCG (Human chorionic gonadotropin) - maintains
  ovary (corpus luteum)
• Progesterone maintains pregnancy (especially
  after 1st trimester)
• Sommatomammotropin (Placental lactogen
  increases maternal blood glucose and lipids
• Oestrogen
• Relaxin
• Prostaglandins

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Placenta Immune protection

• Foetus is an allograft
• Foetus will be rejected if exposed to maternal
  immune system
• Mother recognises foreign placenta but does not
  reject
• Placental cells immunoprotected

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Species variation - placental form

• The shape and exchanging surfaces of placental
  mammals varies according to species.
• Ruminants (sheep, cows) have cotyledonary
  placenta
• Many small placentas where the foetus' cotyledons
  interface with the dams' caruncle forming a
  placentome.
• Carnivores have a zonary placenta.
• Primates, Rodents have a discoid placenta (per
  embryo).

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Placental interface - features

• Large surface area
• Surface area increases as foetus grows
• High vascularity of both maternal and foetal
  circulations
• Separate but juxtaposed maternal and foetal
  circulations
• Principles common between species

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Placental nutrient interfaces

• Uterine (tube) milk
• From tubal/uterine secretions
• Histiotrophic
• From breakdown of tissues around site of
  implantation (glands, stroma)
• Haemotrophic
• From blood

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Human placentation

• Blastocyst implants uterine wall day 6-7 of
  gestation
• Pre-attachment phase
• Attachment phase
• Invasive phase
• Penetration of epithelium
• Stromal reaction (marked in primates and rodents)
• Interstitial implantation
• Histiotrophic phase
• Haemotrophic phase

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Human Placentation

• Singleton implantation usually
• Discoid placental shape
• Haemodichorial placenta
• Two trophoblast layers between blood systems

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Placental terminology

• Decidua
• Trophectoderm
• Trophoblast
• Cytotrophoblast
• Syncytiotrophoblast

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Embryological process
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Uterus

• Fundus above uterine tubes
• Body
• (Cervix)
• Fundus and body muscle endometrium
• (Cervix - dense connective tissue)

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Endometrium - Human
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Endometrium
Proliferative
Secretory
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Blastocyst

• Trophectoderm
• Placenta
• Secretion of HCG
• Gaseous exchange
• Nutrition exchange
• Attachment
• Immune protection
• Inner cell mass
• Embryo

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Attachment and implantation into uterine wall

• Blastocyst passes between uterine epithelial
  cells
• Trophectoderm around outside of blastocysts gives
  rise to placenta
• Inner cell mass to embryo
• Trophoblast cells (from trophectoderm)
  proliferate
• Migrate into uterine wall to establish placenta
• Uterine epithelium closes behind
• Early placental (trophoblast) cells secrete HCG
  feedback to maintain corpus luteum

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Implantation
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How does placenta form?

• As blastocyst develops into embryo it gets bigger
• Must establish system for delivery of oxygen,
  nutrients etc
• Initially proliferation of trophectoderm on
  embryonic pole
• Differentiates into outer layer of
  syncytiotrophoblast and inner cytotrophoblast
• Lacunae develop on syncytiotrophoblast layer into
  which maternal blood passes
• Villi of syncytiotrophoblast with inner core of
  cytotrophoblast develop

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Early placenta formation

• Two types villi form
• Anchoring villi hold on to uterine wall on part
  of endometrium now known as decidua basalis or
  basal plate/decidual plate
• Free/floating villi branch out from stem villi

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Floating villi

• Primary to secondary to tertiary villi by end of
  3rd week of gestation

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Placental villi

• Primary to secondary to tertiary villi by end of
  3rd week of gestation
• Blood vessels in villi continuous with umbilical
  vessels and fetal circulation
• Maternal blood bathes external surface of
  syncytiotrophoblast (of villi)
• Ready for action end week 3-4 (when
  cardiovascular system begins to develop)
• Get thinner with advancing gestation and more
  extensive network as demands for nutrients
  increases

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Placenta
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Placenta formation
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Placental tertiary villi
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Human Placenta
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Rodent placentation

• Human
• 38 weeks gestation
• Uterine milk, histiotrophic, haemotrophic phases
• Interstitial attachment
• Discoid placenta
• Two trophoblastic layers
• Haemodichorial

• Mouse
• 3 weeks gestation
• Uterine milk, histiotrophic, haemotrophic phases
• Eccentric attachment
• Discoid placenta
• Three trophoblastic layers
• Haemotrichorial

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Mouse placentation

• Multiple implantation sites
• Eccentric implantation
• Blastocyst does not penetrate epithelium
• Epithelium breaks down
• Stromal reaction before epithelium breaks down
• Epithelium plus adjacent stroma provides
  histiotrophic support
• Maternal placenta forms
• Increased vascularisation

Day 5
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Mouse placentation

• 3 week gestation
• By day 7 some embryo growth (E)
• By day 7 much decidual development including
  vascularisation
• Little foetal placenta formation
• Histiotrophic support

E
Day 7
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Mouse placentation

• 3 week gestation
• By day 10 decidua well vascularised
• Foetal placenta (F) forming
• Haemotrophic support beginning
• Embryo growing (E)

F
E
Day 10
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Mouse placentation

• 3 week gestation
• By day 14 foetal placenta (F) formed
• Haemotrophic support
• 3 trophoblast layers plus endothelium separate
  maternal and foetal blood
• Foetus growing fast

F
E
Day 14
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Mouse placentation

• 3 trophoblast layers plus endothelium separate
  maternal and foetal blood
• Haemotrichorial placenta
• Cytotrophoblast sysncytiotrophoblast
• Labyrinthine placenta
• Human villous placenta

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Mouse placentation
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Ruminant placenta

• Implantation
• Implantation in ruminants is non-invasive.
• There is close attachment between embryonic
  membranes and the endometrium overlying caruncles
  at 5 weeks in cattle and 3 weeks in sheep.
• Shortly thereafter, the placenta is established.
• Gross Structure of the Placenta
• Ruminants have a cotyledonary placenta. Instead
  of having a single large area of contact between
  maternal and fetal vascular systems, these
  animals have numerous smaller placentae. The
  terminology used to describe ruminant
  placentation is
• Cotyledon the foetal side of the placenta
• Caruncle the maternal side of the placenta
• Placentome a cotyledon and caruncle together

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Ruminant placenta

• Placentome a cotyledon and caruncle together
• Pregnant sheep, goats and cattle have between 75
  and 125 placentomes.

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Microscopic Structure of the Ruminant Placenta
A prominent feature of the ruminant placenta is
the presence of large numbers of binucleate
cells. These cells arise early as part of the
foetal trophoblast from cells that fail to
undergo cytokinesis following nuclear division.
They invade and fuse with caruncular epithelial
cells to form small syncytia. Binucleate cells
secrete the hormone placental lactogen.
Ruminants basically have an epitheliochorial
placenta, but because the uterine epithelium is
modified by invasion and fusion of binucleate
cells, its structure is generally referred to as
synepitheliochorial. Prior to detailed study of
these structures, it was thought that the
maternal epithelium was eroded away, leaving
trophoblast in contact with maternal connective
tissue.
T
E
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Ruminant placental transport and endocrinology
Placental Transport General aspects aspects of
placental transport are similar to that seen in
other species. However, immunoglobulins are not
transported across the placenta from mother to
foetus, and therefore, barring foetal infections,
the young ruminant is born without circulating
antibodies. Hence importance of colostrum to
newborn cattle,sheep. Placental
Endocrinology The major hormones of ruminant
placentae are progesterone and other progestins,
estrogens and placental lactogen. The sheep
placenta produces enough progesterone that by
roughly day 70 the corpora lutea can be removed
and pregnancy will not be interrupted. In
contrast, luteal progesterone is required
throughout gestation in cattle and goats because
their placentae secrete much smaller quantities
of progesterone. In reality, a large amount of
progesterone is synthesized by the goat placenta,
but most is converted to a biologically inactive
pregnane before secretion.
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Is placenta protective?
Chlamydia psittaci
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Is placenta immune?