The early embryo 2016 Paul Billiet ODWS The

The early embryo © 2016 Paul Billiet ODWS

The zygote 2 polar bodies Zona pellucida 2 pronuclei © 2016 Paul Billiet ODWS Human zygote

The 4 -cell embryo Follicular cells (corona radiata) l l Rapid cell division but little growth (cleavage) The ball of cells remains about the same size as the oocyte. Sperms © 2016 Paul Billiet ODWS Zona pelucida

The 8 -cell embryo l l Zona pelucida: a shell of glycoprotein It protects the developing embryo. © 2016 Paul Billiet ODWS Zona pellucida

The morula Zona pelucida Supernumary sperms © 2016 Paul Billiet ODWS

The blastocyst l l 5 -6 days after fertilisation Zona pellucida lost. © 2016 Paul Billiet ODWS Inner cell mass (becomes the embryo) Human blastocyst

Implantation The blastocyst arrives in the uterus About 1 week after fertilisation. © 2016 Paul Billiet ODWS

Implantation © 2016 Paul Billiet ODWS

Implantation l l l Maternal blood vessels start to break down Pools of blood form Less barriers to exchange. © 2016 Paul Billiet ODWS

The beginning of the placenta Placenta Amniotic sac Embryo © 2016 Paul Billiet ODWS

Implantation © 2016 Paul Billiet ODWS

Implantation Embryo Implantation Filmed for the First Time (2 min 51) http: //www. scientificamerican. com/video/embryoimplantation-filmed-for-the-firsttime/? &WT. mc_id=SA_BS_20140314 © 2016 Paul Billiet ODWS

The placenta l l l Provides an exchange system for the foetus Large surface area (villi and microvilli) Reduced barriers to exchange (pools of maternal blood) Gas exchange Nutrients Excretory products. © 2016 Paul Billiet ODWS

The placenta l l It is also an endocrine organ Secreting progesterone and oestrogens Maintaining pregnancy As the corpus luteum degenerates (at 2 months), the placenta takes over. © 2016 Paul Billiet ODWS

Umbilical cord l l l In mammals the yolk sac is very small The umbilical cord connects the embryo to the placenta Foetal blood vessels irrigate the placenta © 2016 Paul Billiet ODWS

Amniotic sac at 5 weeks The amniotic sac grows around the embryo © 2016 Paul Billiet ODWS

The foetus l l At 8 weeks the embryo is complete A period of growth (the foetus) begins © 2016 Paul Billiet ODWS