Week 2: Placenta & Bilaminar Disc Formation

  • There are two main events that follow implantation into the second week
    1. Placenta formation (6-12 days)
    2. Bilaminar disc formation
  • This is the week of twos
    • 2 trophoblast layers – cytotrophoblast and syncytiotrophoblast
    • 2 embryoblast layers – epiblast and hypoblast
    • 2 cavities – amnion and yolk sac
    • 2 extraembryonic mesoderm layers

Placenta development

  • The trophoblast (which surrounds the embryo/blastocyst) differentiates into two forms:
    1. Cytotrophoblast:
      • Basal layer of trophoblast which surrounds the blastocyst and produces more cells to migrate into the syncytium
    2. Syncytiotrophoblast:
      • Cell membranes break down to form a syncytium
      • Also release hCG (which maintains the endometrium)

  • The syncytiotrophoblast invades into the endometrium to develop the placenta
    • These do not cause an immune response in the mother
    • Forms large vacuoles within it called lacunae
    • As it disrupts maternal sinusoids (capillaries), the lacunae fill with maternal blood
  • The syncytiotrophoblast then develops primary villi that project into the lacunae
    • These have a cytotrophoblastic core with a syncytiotrophoblast outer
    • Nutrients diffuse from lacunar blood  into the syncytiotrophoblast (there is no direct contact between embryo and intravascular blood)
      • This is called histiotrophic nutrition – i.e. circulation which relies on diffusion across the interface
      • This is facilitated by the decidua reaction – endometrial cells become larger, oedematous, and fill with glycogen/lipids
    • Thus, the primitive uteroplacental circulation begins
  • To understand this , look at the image below from Langman’s which shows the lacunae and primary villi
Bilaminar disc and 1º uteroplacental circulation
  • NB: As the blastocyst fully embeds within the endometrium by day 12, it becomes encapsulated by syncytium
    • Furthermore, the uterus wall closes behind it
    • Can lead to some bleeding confused with menstruation at around 10 days

Bilaminar disc formation (Day 8-9)

  • This occurs in week 2 at around the same time as trophoblast differentiation
    • Overlaps with placenta formation (see above)
    • This process establishes the primary body map
  • In this process, the inner cell mass (embryoblast) develops into the bilaminar embryonic disc by differentiation into 2 layers:
    • Epiblast
      • Dorsal layer adjacent to the amnion
      • Forms all germ cells
    • Hypoblast
      • Ventral towards the blastocele cavity
      • This migrates to line the inside of the trophoblast/blastocele cavity to form the exocoelomic membrane which lines the primitive yolk sac/exocoelomic cavity
        • This later forms the extraembryonic mesoderm
  • There are therefore 2 cavities to consider:
    1. The amniotic cavity which is ringed by the epiblasts (including amnioblasts which make the fluid in the cavity)
    2. The primitive yolk sac which is next to the hypoblast

The extraembryonic mesoderm

  • At Day 12, the extra-embryonic mesoderm forms
    • This is loose connective tissue that comes from the exocoelomic membrane cells
    • Separates the trophoblast from the embryoblast (dorsally)
    • There are 2 separate layers
      • Somatopleuric mesoderm: lines the cytotrophoblast/yolk sac/amnion
      • Splanchnopleuric mesoderm: lines the embryo
  • Day 13 – the Secondary Yolk Sac forms
    • Develops as the hypoblast again produces cells that migrate along the extracoelomic membrane
    • Parts of the yolk sac are pinched off and develop exocoelomic cysts
    • This forms a new, smaller cavity – the secondary yolk sac
  • Cavities also develop within the extraembryonic mesoderm to form the extraembryonic coelom (aka the chorionic cavity)
    • Further separates embryoblast and trophoblast
    • The connecting stalk remains to keep the trophoblast and embryoblast connected and becomes umbilical cord
Langmans- Secondary yolk sac and chorionic cavity formation.