Embryology - 1 Flashcards
How long can sperm and oocyte survive?
Sperm - 5 days
Oocyte - 24 hours after ovulation
Describe the journey of the egg during fertilisation
Egg leaves ovary and enters fallopian tube
Sperm enters egg and unites with nucleus
Fertilised egg divides - cell numbers increase and blastocyst is formed
Cells attach to uterus
Describe the process of fertilisation
Sperm makes contact with egg
Acrosome reacts with zona pellucida - release hydrolytic enzymes
Acrosome reacts with perivitelline space
Plasma membranes of sperm and egg fuse
Sperm nucleus enters egg
What happens when sperm nucleus enters egg?
Cortical granules fuse with egg plasma membrane which render the vitelline layer impermeable to sperm
Describe early embryo development
Day 1 - formation of pronuclei
2 - first cell division
3 - cell division continues
4 - formation of morula (ball of cells)
5 - blastocyst forms
What does the cells in the blastocyst organise into?
2 layers - trophoblast (outer) where placenta develops and inner cell mass where embryo develops into foetus
Which layer of the blastocyst invades the maternal endometrium for implantation?
Trophoblast
What are the 4 steps of implantation?
Hatching
Apposition
Adhesion
Invasion
Describe the hatching stage of implantation
Trophectoderm cells produce protease to dissolve the zona in preparation for implantation
Describe the apposition stage of implantation
First connection between blastocyst and endometrium, apposes to microvilli like structure called pinopodes expressed on receptive endometrium
Describe adhesion stage of implantation
Trophoblast of the blastocyst adheres to the epithelial layer of maternal endometrium, embryonic tissue starts to actively secrete hCG
Describe the invasion stage of implantation
Trophoblast proliferation, differentiation, crossing of epithelial membrane and invasion of endometrial stroma to form placenta
Uterine spiral arteries remodelled by invasive trophoblast
What happens if there is inability to hatch?
Can result in infertility and premature hatching can result in abnormal implantation in the uterine tube - ectopic can result
What is the stages of apposition and adherence mediated by?
Growth factors and cytokines
Hormones
Free oxygen radicals
What is decidualisation?
Pre-requisite for trophoblast invasion
Involves transformation of stromal cells of maternal endometrium every month under progesterone
What are the changes which happen during decidualisation?
Stromal cell differentiation converted to decidual
Angiogenesis - new blood vessel formation
Increased macrophages, lymphocytes and decidual leukocytes for maternal immune tolerance
What do the changes of decidualisation transform the endometrium into?
Vascular receptive tissue for blastocyst invasion
What are the stages of placental formation?
Blastocyst comes into contact with endometrium
Trophoblast differentiation and villi formation
Placenta is fully derived from embryo
What forms primary chorionic villi in placentation?
Invading trophoblast
What makes primary chorionic villi become secondary?
With infiltration of extra embryonic mesoderm
When do secondary chorionic villi become tertiary?
When capillaries form
What does invasion of cytotrophoblast do in placentation?
Remodels spiral arteries of endometrium to increase blood flow
What is the mature placenta formed of?
Chorionic villi
Intervillous space
Placental septae
Cotyledons
What are the functions of the placenta?
Acts as an immunological barrier
Gas exchange
Nutrient exchange
Waste excretion
Endocrine functions
