Implantation 3

Question 1

Describe the developmental progress of the placenta by day 7.5 to 8 after fertilization.

Answer 1

• Embryoblast normally at the invasion site with differentiation of the underlying trophoblast into:
  
  • cytotrophoblast
    
    • rapidly proliferating inner layer of mononuclear cells
  • outer nultinucleated zone without cell membranes (true syncytium) - syncytiotrophoblast
• Cytotrophoblast contributes the cells to the syncytiotrophoblast by fusion
• Inner cell mass differentiates into two layers
  
  • hypoblast
  • epiblast
    
    • small cavity appears destined to form the amniotic cavity
    • cells adjacent to the cytotrophoblast are called amnioblasts
    • will give rise to the amnion
• Endometrial stroma (decidua) adjacent to the implantation is highly vascular and edematous

Question 2

What happens during day 9 of placental development?

Answer 2

• Lacunar stage
  
  • blastocyst is more deeply imbedded into the endometrium
  • penetration defect at the uterine surface covered by a fibrin clot
  • vacuoles begin to form within the invading syncytium
    
    • these fuse to form large lacunae (no blood yet)

Question 3

What happens during day 11-12 of placental development?

Answer 3

• Blastocyst completely embedded in the endometrial stroma
• Re-epithelialization of the uterine surface
• Embryonic pole continues to be the site of syncytiotophoblast invasion and lacunae formation
• Opposite (abembryonic) pole, trophoblast consists almost entirely of cytotrophoblast
• Syncytiotrophoblasts begin to invade maternal vessels, fillin glacunae with blood
  
  • Establishment of the uteroplacental circulation
• A new population of cells arise
  
  • extraembryonic mesoderm
    
    • from between the cytotrophoblast and outer surface of the exocoelomic cavity (derived from yolk sac cells)
    • cells destined to become chorion
    • large spaces form in the extraembryonic mesoderm and become confluent to form the chorionic cavity

Question 4

What happens by day 13 of placental development?

Answer 4

• Cytotrophoblast cells proliferate at the invasion front
  
  • penetrate into the syncytiotrophoblast forming cell columns
    
    • referred to as primary villi

Question 5

What has happened by the beginning o the third week in early placental development?

Answer 5

• Primary villi consist of a cytotrophoblast core covered by syncytiotrophoblast
• Extraembryonic mesoderm cells begin to invade the cytotrophoblast core growing towardss the decidua
  
  • This is now referred to as a secondary villous

Question 6

What has happened by the end of the third week in the development of the placenta?

Answer 6

• Mesoderm derived cells begin to differentiate into blood cells and small vessels
• Villi now referred to as teriary or definitive villi
• Capillaries in the villi will connect to the developing vessels in the chorionic plate and connecting stalk, which will connect to the intraembryonic blood vessels
  
  • connects fetal and placental circulations in preparation for the beginning of a continuous fetal circulation when the fetal heart begins to beat in the fourth week of development
• Cytotrophoblasts continue to invade the syncytiotrophoblast and penetrate to the maternal endometrium
  
  • forms outer cytotrophoblast shell
    
    • direct contact with decidua
    • will become extravillous trophoblasts at the basal plate

Question 7

What has happened by the second month of the development of the placenta?

Answer 7

• Secondary and tertiary villi have formed from the stem villi
• Surface of the villi in direct contact with the maternal blood of the intervillous space now covered by syncytiotrophoblasts
• Erosion of the maternal spiral arteries occurs

Question 8

What happens to the villi at the embryonic pol during placental development? What about the abembryonic pole villi?

Answer 8

• Embryonic pole
  
  • continually expand and give rise to the chorion frondosum (bushy chorion)
• Abembryonic pole
  
  • villi degenerate and form sthe chorion laeve (smooth chorion)
  • will come in contact with the uterine wall and fuse, obliterating the uterine cavity

Question 9

What has happened by the third month to placental development?

Answer 9

• Only chorion frondosum and decidualis basalis will participate in gas and nutrient exchange
  
  • these make up the definitive placenta
• Amnion overgrows the mbryo to completely surround it
  
  • eventually fuses with the chorion
  • obliterates the chorionic cavity and forms the amniochorionic membranes (ruptures during labor)

Question 10

What are the specialized cell types that the cytotrphoblast can differentiate into?

Answer 10

• Extravillous trophoblast
  
  • endovascular trophoblast - invades and remodel maternal vessels
  • trophoblast giant cells - can invade into myometrium and others
• Syncytiotrophoblast

Question 11

What has happened by the 4th and 5th month to placental development?

Answer 11

• Several decidual septae form that project into the intervillous space
  
  • Separate the maternal surface into 15-20 cotyledons
• Placenta continues to enlarge, covering about 15-30% of the internal surface of the uterus

Question 12

What happens to the capillaries in the terminal villi as maturation of the fetus proceeds?

Answer 12

• More closely associated with the syncytiotrophoblast layer
  
  • increases diffusion efficacy (decreases diffusion distance
• Terminal villi get smaller and more numerous
• Syncytial knots (Tenney-Parker change) increase as gestation proceeds
• Calcifications also increase as term approaches