Implantation and placenta formation

• Implantation


  The process by which the blastocyst gets attached, embedded and included within the uterine wall
• Implantation begins


  6 days through the end of the 1st week after fertilization
• Implantation


  1. The blastocyst attaches to the endometrial epithelium, usually adjacent to the embryonic pole

  2. By the end of 7th day, the blastocyst gets implanted in the superficial compact layer of endometrium and derives its nourishment from the eroded endometrium
• Trophoblast


  • Proliferates rapidly and differentiates into two layers: inner cellular cytotrophoblast, and outer mass of syncytiotrophoblast (multinucleated protoplasm with no cell boundaries)
  • Finger like processes of syncytiotrophoblast extend through the endometrium and invade the endometrial connective tissue
• Implantation cont'd


  1. The blastocyst gradually embeds deeper in the endometrium

  2. By 10th day it is completely buried within the 'Functional layer' (stratum compactum + stratum spongiosum) of the endometrium

  3. The defect in the endometrial epithelium is filled by closing plug (day 10)

  4. The defect gradually disappears as the endometrial epithelium is repaired (day 12 & 13) by the proliferation of the surrounding cells

  5. By the end of the 2nd week the blastocyst is completely embedded, and the surface defect in the mucosa has healed
• Uteroplacental circulation


  • Ensues earlier than now, as the embryo derived its nutrients from the primitive yolk sac and uterine glandular secretions
• Implantation cont'd


  Small cavities appear in syncytiotrophoblast, and get filled with maternal blood, establishing primitive uteroplacental circulation
• 2nd week


  • Trophoblast differentiates to Cytotrophoblast and Syncytiotrophoblast
  • Embryoblast differentiates to Epiblast and Hypoblast
  • Extraembryonic mesoderm differentiates to Somatopleuric and Splanchnopleuric
  • Blastocystic cavity differentiates to amniotic and yolk sac cavities
  • Major changes occur due to increase in nutritional and surface area as well as other factors between the fetal and maternal components to facilitate exchange
• Placental development


  1. Week 2: primary villi formation - 1st stage of chorionic villi development, trophoblastic shell cells form fingerlike extension into maternal decidual

  2. Week 3: secondary villi formation- 2nd stage of chorionic villi development, extraembryonic mesoderm grows into villi, covers the entire surface of the chorionic sac

  3. Week 4: tertiary villi formation- 3rd stage of chorionic villi development, mesenchyme differentiates to blood vessel and cells fuse with placental vessels
• Placenta at birth


  • Embryo/Maternal organ which is villous chorion and decidual basalis respectively, continuous with amniotic and chorionic sacs
  • Discoid, 20cm across, 3cm thick, weighing 500-600gm (1/6-1/7 of fetal birth weight)
  • Has varying shapes: accessory, bidiscoid, diffuse, horse-shoe etc.
  • On the maternal side cotyledons forms cobblestone appearance where it is covered with decidual basalis
• Umbilical cord at birth

  • On the fetal surface, has umbilical cord attachment 1-2cm across, 30-90cm long, covered with amniotic membrane and attached to chorionic plate
  • Shiny, grey, soft, coiled, compressible on the fetal surface of the placenta
  • Contains 1 vein, 2 arteries, Wharton's jelly
  • Extremely short - 5cm, Excessively long - 150cm
• Placental classification


  • Based on histology, structural organisation and layers between fetomaternal circulation
  • Hemochorial - chorion in direct contact with maternal blood (e.g. Human)
  • Endotheliochorial - maternal endometrial blood vessels are bare to endothelium in contact with chorion (e.g. cats, dogs)
  • Epitheliochorial - maternal epithelium of the uterus in contact with primitive chorion (e.g. pigs, cows)
• Types of placenta


  • Discoid - human, mice, rat, rabbit and monkey
  • Zonary - dog, cat, bear, seal
  • Cotyledonary - cow, deer, goat, giraffe
  • Diffuse - horse, pig, camel, lemur, whale, kangaroo
• Placenta Types in Human (discoid)

  • Circummarginata/circumvillate - double fold of chorion & amnion whitish ring with fibrin in between
  • Succenturiate lobe placenta - has accessory cotyledon
  • Bipartite placenta - has two separate lobes
  • Placenta fenestra - has avascular window in succenturiate variant
  • Placenta membranacea - vascularised decidual capsularis
  • Placenta praevia - located in lower uterine segment
• Placental blood vessels


  • Arteries are paired, carry deoxygenated blood and waste product away from the placenta and to the placenta villi
  • Veins initially paired then singled, at the end of the embryonic period, carry oxygenated blood to the embryo (sinus venosus)
• Placental nutrition


  • Histiotrophic nutrition - early placental development and initial transfer of nutrients from mother to fetus
  • Hemotrophic nutrition - later blood borne nutrition
• Placental barrier


  • A pseudo barrier, selectively permeable membrane. The maternal blood in the intervillus space is separated from the fetal blood by chorionic derivative
  • In utero fetomaternal blood does not mix, these are the layers that separate the mother and the fetus blood: Cytotrophoblast, Syncytiotrophoblast, Villus core connective tissue, Fetal capillary endothelium
• By the 4th month placenta has 2 components: fetal portion (chorion frondosum) and maternal portion (decidual basalis)
• Most maternal hormones do not cross the placenta, e.g. thyroxine, progestins, synthetic estrogens
• Despite the acclaimed protective function of placenta barrier, some viruses traverse placental barrier with ease e.g. rubella, cytomegalovirus, coxsackie, variola, varicella, measles, poliomyelitis virus which may result in variable extent of birth defect or even death
• Drugs and their metabolites traverse the barrier readily to cause serious congenital malformations. Habituation has been linked to maternal cocaine or heroine use
• Functions of placenta


  • Hormone production
  • Nutrients and electrolyte exchange
  • Gaseous exchange
  • Transmission of immunoglobulins (N.B. Rh incompatibility, isoimmunisation, erythroblastosis/hydrops foetalis)
• In the 4-5th months

  1. Decidua forms several septa (decidual septa) which project into intervillous spaces, but do not reach the chorionic plate

  2. These septa have core of maternal tissue but their surfaces are covered by layers of syncytial cells, so that syncytial layer separates maternal blood in intervillous lakes from the fetal tissue of the villi, this organisation divides the placenta into cotyledons
• Continuous fetal and uterine growth enlarges the placenta in a parallel fashion with the former structures throughout the pregnancy. It covers 15-30% of the maternal surface of the uterus
• Increase in the thickness of the placenta results from arborisation of existing villi
• Attachment of the umbilical cord is usually eccentric but occasionally marginal and rarely velamentous
• The intervillous space of a mature placenta contains about 150ml of blood, being replenished 3-4 times per minute
• The surface area of chorionic villi is 4-14m2, the microvilli increases the surface area of the intervillous space to increase the exchange rate between maternal and fetal circulation
• Normal Implantation Sites


  • The implantation site determines placenta localization
  • Normally it occurs in the upper part of the body of uterus, more often on the posterior wall
• Abnormal Implantation Sites


  • Implantation in the lower segment leads to placenta praevia
  • Extrauterine: Fallopian tube, Ovary, Abdominal cavity
• Clinical correlates (abnormalities)


  • Placenta praevia, abruptio, increta, accreta, percreta, vasa praevia, hydatidiform mole, chorionic intervillousitis
• Placenta pathologies


  • Chorionic villitis, meconium myonecrosis, thrombophilia, neuroblastoma
• Ectopic sites of implantation


  • Ovary, Cervix