- There are two main events that follow implantation into the second week
- Placenta formation (6-12 days)
- 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:
- Cytotrophoblast:
- Basal layer of trophoblast which surrounds the blastocyst and produces more cells to migrate into the syncytium
- Syncytiotrophoblast:
- Cell membranes break down to form a syncytium
- Also release hCG (which maintains the endometrium)
- Cytotrophoblast:
- 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
- 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
- Epiblast
- There are therefore 2 cavities to consider:
- The amniotic cavity which is ringed by the epiblasts (including amnioblasts which make the fluid in the cavity)
- 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
