Migration of Sperms
The migration of the sperms from the vault of the vagina to the outer part of the Fallopian tube, and hence to the ovum, is not satisfactorily understood.
Approximately 400 million sperms are present in each ejaculation and they are initially protected by the semi-gelatinous seminal fluid which liquefies after an interval of 15 or 20 minutes, and the sperms are then exposed to the vaginal acidity. Being very sensitive to acid, those which have not gained access to the cervical canal are rapidly rendered immobile or killed. Many lame, abnormal or damaged sperms are thus automatically destroyed, although they may occasionally fertilize an ovum, resulting in a ‘blighted ovum’. Those that reach the cervical canal are protected and nourished by its alkaline mucus. For a few days before ovulation the mucus in the cervical canal is especially prepared for sperms and becomes transparent and less viscous than during the remainder of the menstrual cycle.
If only 10 per cent of the sperms reach the cervical canal a total of approximately 40 million sperms will have gained this favourable haven. The journey of about 23 centimetres from the cervix, up the uterus and along the Fallopian tube takes approximately 45 minutes and between 1,000 and 2,000 sperms reach the outer portion of each Fallopian tube. The cells lining the Fallopian tube secrete an alkaline mucus which is rich in sugar and nourishes the sperms on their journey. They will survive in the outer part of the Fallopian tube for between 48 and 72 hours, lying in wait for the newly forming follicle to discharge its ovum. If ovulation has occurred within 18 hours before the sperms reach the outer end of the Fallopian tube then fertilization occurs immediately the sperms reaches the ovum.
While only one sperm is capable of actually fertilizing an ovum, it is probable that many sperms have to be present in order for fertilization to occur. The ovum is surrounded by a gelatinous material which can be liquefied by hyaluronidase, an enzyme carried by the sperms. No single sperm can carry sufficient hyaluronidase to liquefy enough of the gelatinous material to penetrate the ovum.
The hyaluronidase of several sperms is thought to be required for sufficient liquefaction to allow one sperm to penetrate the ovum.
The actual mechanism of penetration of the ovum by the sperm is not known. It is believed that the sperm makes head-on contact with the ovum and gradually penetrates the wall of the ovum by virtue of its hard swimming velocity. In the human the lashing tail does not enter the ovum but is left on the outer side of the capsule. It is also likely that only one sperm actually penetrates the ovum, unlike some animals where several sperms do so although only one is used for fertilization.
During the first few hours after penetration of the ovum by the sperm, the female and the male nuclei, which contain the chromosomes that will endow the offspring with its hereditary characteristics, enlarge and move towards the centre of the ovum where they fuse or coalesce to form a single nucleus. Within a short time the nucleus divides into two equal parts, each part containing an equal portion of maternal and paternal chromosomes. During the next 72 hours the whole fertilized cell divides into two and subsequently sub-divides to form 4, 8, 16, 32 and then 64 separate cells, when it is known as a morula. This process continues in the human for 266 days from conception, when the specialization of different tissues is complete and a 3 kg. baby has grown and is ready for delivery.
After four days a cystic space appears in the centre of the newly formed morula and it is then called a blastocyst. The cells around the outer surface of the blastocyst continue to multiply and by the seventh day have formed sprout-like projections called chorionic villi which will immediately embed the blastocyst in whatever tissue it finds itself. It has normally reached the cavity of the uterus seven days after fertilization (the 21st day of the menstrual cycle). The chorionic villi burrow into the superficial cells of the lining of the uterus which they digest and erode. The egg can then burrow deeper into the endometrium where it finds a new source of nourishment, for the chorionic villi can gently open the maternal blood vessels and extract therefrom the oxygen and nutrients that the developing pregnancy requires. Implantation is usually in the upper and posterior aspect of the body of the uterus. The blastocyst is not visible to the naked eye and a woman will not yet know that she is pregnant, since she has not missed her first menstrual period and even the earliest symptoms of pregnancy do not begin to manifest themselves for several days.