HR (Human Reproduction) Blastocyst Development

HR (Human Reproduction) Lecture Notes: Blastocyst
Development
Single cell to blastocyst
● Single cell to morula
○ 8 cell, same overall size but higher nuclear:cytoplasmic ratio
◆ All are totipotent so can remove some for testing
◆ Maternal RNA and protein used till 4-8 cell stage when

genome is turned on
◆ Maternal RNA can stay till blastocyst stage
○ Zona pellucida remains (holds cells together and prevents early

implantation (ectopic pregnancy, needs to be terminated))
● Compaction
○ More expression of adhesion molecules
○ Polarised cells -> asymmetric division -> inner cells different from

outer cells
● Cavitation
○ Tight junctions, pumping ions, blastocoel
○ Trophectoderm (Cdx2, Gata3, Eomes) -> placental trophoblast;

ICM (Oct4, Nanog, Gata6) -> embryo proper + placental
mesoderm)
○ Cdx2
◆ More cell contact (ICM) -> activates HIPPO -> inhibits

(phosphorylates) Yap transcription factor -> no Cdx2
◆ So less cell contact (trophectoderm) -> more Cdx2


Nutrition, embryo uses diffusion and active transport
● Some fallopian cells (non-ciliated) secrete nutrients
○ Low pO2 which good for stem cells (organogenesis is most

sensitive to teratogenesis)
● Then endometrial glands secrete nutrients

Implantation (uterus)
● Need progesterone and oestrogen
○ Uterus is usually non-receptive to embryo (to stop infection)
○ Only receptive for day or two, mid progesterone phase
● Repulsive window
○ More progesterone receptors (progesterone promotes endometrial

development)
○ Negative charge on surface membrane (repels blastocyst)
○ Thick with mucin glycoprotein coat (adhesion proteins aren’t

revealed)
○ Long microvilli (no surface to attach to)
● Receptive window
○ Less progesterone receptors
○ Loss of negative charge