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Fertilisation and Embryo Development
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University of Edinburgh Lecture notes for Reproductive Biology 3 Lecture "Fertilisation and Embryo Development"
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Fertilisation and Embryo Development• It is normal to have 20 million-300 million sperm per ml of semen
o 100-1000 of these sperm reach the ampulla
• The vagina is acidic
o Sperm are lost here by leakage or destruction by leukocytes
o Cervix- filters poor quality sperm
▪ Not all species have cervixes
o Uterine Tubal Junction (UTJ)- used in sperm selection
▪ Different mechanisms between species
o Isthmus – stores sperm
▪ Preserves motility and viability until ovulation
▪ Sperm that reach this point are protected
o Ampulla- holds oocytes
▪ Site of fertilisation
o Infundibulum- the opening of the oviduct
▪ Oocyte picked up by fimbriae and swept into here
o
• When sperm reach the UTJ they undergo a series of selective events
o Knockout mice used to identify 13 genes that are important for sperm getting
through the UTJ
▪ 11 of genes interact with the gene Adam3
▪ Adam3 has to be located in the right area of the sperm
• Adam3 believed to be involved with sperm binding to the lining of
the UTJ
• The isthmus has a thicker wall
o Has a thinner lumen
o Contains cilia
o Sperm are shielded from leukocytes in the isthmus
o Sperm can bind to the isthmus for up to 24 hours to rest
▪ Sperm are released from walls in waves
• Believed to be related to ovulation
• This limits the number of sperm around the egg at one time
o Decreases the chance of polyspermy
, o
• Sperm’s journey to the ampulla
o Sperm move due to thermotaxis
▪ Sperm swim towards the warmer environment of the ampulla
▪ Human sperm can detect temperature changes as small as 0.006
degrees C
o Sperm also move due to chemotaxis
▪ Sperm swim towards progesterone released from cumulus cells around
oocyte
o Sperm also move by rheotaxis
▪ Rheotaxis is the movement of sperm against a current
▪ Current is created by tubal fluid stretching from the ampulla to the
isthmus
▪ Sperm must become hyperactivated (capacitated) to do this
o
o Capacitation- occurs in the isthmus
▪ Gives the sperm the capacity to fertilise egg
▪ Membrane of the sperm head changes to become more fluid
• Cholesterol, phospholipids and glycoproteins on the surface are
lost
▪ Capacitation can only occur when the sperm is in the oviduct because it
makes it more unstable
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