Perivitelline space

Transcription

when ejaculation occurs during intercourse approximately 200 million sperm or spermatozoa are deposited into the vagina they swim through the cervix propelled by with whip-like motions of their tails or Flagela after which muscular contractions of the uterus direct them to the uterine tubes this process usually takes between 30 minutes and two hours only around 200 spermatozoa will reach the secondary oocyte in the uterine tube and of these only one will fertilize it fertilization cannot occur until two processes have taken place capacitation and the acrosomal reaction these can take several hours capacitation is not fully understood but secretions from the uterus wall and uterine tube destabilize the plasma membrane surrounding the head the spermatozoa or acrosome resulting in the membrane becoming more fluid which helps to prepare the spermatozoa for the events a fertilization the spermatozoa become hyperactive their flagella beat more frequently and their heads move laterally the capacitated spermatozoa moved through the corona radiata a dense layer of granulosa cells surrounding the oocyte and come into contact with the zona pellucida the zona pellucida expresses specific receptor proteins called ZP3 which bind to proteins expressed in the heads of the spermatozoa the binding ZP3 triggers the acrosome reaction during which the enzymatic contents of the acrosome are released these enzymes help to digest a path through the zona pellucida allowing the spermatozoa to enter the perivitelline space and reach the plasma membrane of the secondary oocyte with which it fuses to ensure that only one spermatozoa and penetrates the zona pellucida infuses with the oocyte membrane fusion of the spermatozoon and oocyte membranes activates a fast and a slow block to polyspermy during fast blocked polyspermy after fusion the oocyte membrane depolarizes preventing other spermatozoa from fusing with it slow block to polyspermy is also stimulated by this depolarization during slow block to polyspermy a wave of intracellular calcium is released causing small cortical granules beneath the oocyte membrane to release their contents renderings ZP3 inactive and making the zona pellucida impermeable upon the spermatozoon entering the oocyte undergoes meiosis II and further develops into the female pro nucleus during this time the sperm develops into the male pro nucleus and the two Pro nuclei fuse to form a single diploid nucleus or zygote