An observation on the early fertilization of Squaliobarbus curriculus by scanning electron microscope

The mature eggs,spermatozoa of Squaliobarbus curriculus and the early fertilization have been observed under scanning electron microscope(SEM).The spermatozoon consists of two parts:the head and the tail.The total length of the spermatozoon is 19.6 to 22.4?μm.Diameter of the spermatozoon head is 1.65?μm,and the length of the tail is 17.8 to 20.6?μm.The mature egg of Squaliobarbus curriculus is elliptic and has a single micropyle on the animal pole.The micropyle is funneled with an annular fertilized aperture on the bottom.The diameter of the micropyle is 12.46 to 14.10?μm;the thickness of egg membrane is 2.78?μm.The egg membrane is watched smooth under the lower magnification of SEM;under the high magnification of SEM,there are no fossae and ridges found but many pore canals distributing on the egg membrane homogeneously.The diameter of the pore canals is 0.14?μm.Around the smooth fertilized aperture,there are some radiating bulking.The inner diameter of the fertilized aperture is 2.71μm,and the outer is 5.21?μm.The process of spermatozoa penetrating eggs is observed at 27?℃ and described as follows:2 to 4 second after spermatozoa penetrating eggs,the spermatozoa arrive the certain area of egg near the micropyle,the head of the spermatozoon is opposite to the micropyle.From 8 to 18 second,there are many spermatozoa around the micropyle,one of them begin entering the fertilized aperture and left the tail out of it.During this period,there isn't any fiberized embolus blocking the micropyle.Then,along with the cortical reaction caused by fertilization,the micropyle is gradually blocked by many cortical granules.At this time,the process of spermatozoon penetrating egg is finished.From 20 to 28 second,the micropyle is blocked by fiberized embolus;the fertilized aperture is closed too.From 30 to 60 second,the fiberized embolus in the miropyle is becoming more and more,the spermatozoa outside the micropyle can not enter the egg,and loose the capability of voluntary movement.In 40 second,the disorganized spermatozoa can be seen under the SEM;in 55 second,the micropyle is blocked with some large embolus,the spermatozoa are disorganized completely.Above all,these conclusions are drawn:Mechanism of Squaliobarbus curriculus 's fertilization follows monospermy,Only one spermatozoon can get through the micropyle canal of one egg and enter it,while the others will be disorganized automatically.In 18 second after a spermatozoon goes into an egg,the embolus will be formed to prevent the other sperms getting into the egg,which insures the mechanism of monospermy.