Chapter 4

The process whereby a spermatozoon and an oocyte fuse to form a single‐celled zygote is termed fertilisation. Following penetration of the vitelline membrane by the spermatozoon, the activated oocyte completes meiosis and extrudes the second polar body. The chromosomes contained in the haploid male pronucleus align with their corresponding chromosomes in the female pronucleus. The paternal and maternal chromosomes condense, become attached to mitotic spindles and align themselves centrally. The first mitotic, or cleavage, division follows. The integration of the paternal and maternal genetic material, which occurs during these processes, is referred to as syngamy. As a consequence of fertilisation, the diploid number of chromosomes is restored, the sex of the individual is determined (by the spermatozoon) and biological variation results from the integration of paternal and maternal hereditary characteristics.

Many aquatic animals release oocytes and spermatozoa into the water and fertilisation takes place in this aqueous environment. The fact that gametes are released at approximately the same time and in close proximity to each other, usually as a consequence of courtship, contributes to the likelihood of fertilisation taking place. In addition, mutual chemical attraction between male and female gametes increases the probability of fertilisation. This selective attraction is considered to be important in the attachment of the spermatozoon to the oocyte and in the inhibition of cross‐fertilisation between unrelated species. Relative to mammalian species, aquatic animals and amphibians produce large numbers of zygotes; however, the parental energy invested per zygote is low. In contrast, birds and mammals produce far fewer zygotes; thus, the parental energy invested per zygote is much greater. This investment can take many forms and in mammalian species it encompasses the parental investment provided both during gestation and postnatally.

In mammals, oocytes are retained within the female reproductive tract and are fertilised by spermatozoa deposited there. This type of fertilisation is referred to as internal fertilisation. Factors which increase the probability of fertilisation taking place are the high numbers of spermatozoa released at copulation and the relatively large size of the oocyte. Despite the fact that millions or even billions of spermatozoa are deposited in the female tract, only hundreds of spermatozoa reach the site of fertilisation. Involvement of more than one spermatozoon in fertilisation (polyspermy) is an abnormal occurrence in mammals and invariably leads to early embryonic death. Accordingly, the female tract controls the transport of spermatozoa so that the number reaching the site of fertilisation is sufficient to fertilise oocytes released from the ovary without the likelihood of polyspermy.

Depending on the species, spermatozoa may be deposited in the vagina, the cervix or the uterus at coitus (Table 4.1). From this location they are transported to the uterine tube which is arbitrarily divided on a functional basis into three regions, infundibulum, ampulla and isthmus. The infundibulum, the region closest to the ovary, is funnel‐shaped and its free edge possesses regular processes known as fimbriae which play an important role in the capture of the oocyte(s) at ovulation. The infundibulum is continuous with a tubular portion which is divided into two regions of comparable length. The proximal region where fertilisation takes place is the ampulla and the narrower terminal segment, which opens into the uterus, is the isthmus. Despite past uncertainty about the rate of transportation of spermatozoa within the female reproductive tract, it is now generally accepted that transportation occurs in two phases, a rapid phase and a slow phase. The rapid phase is associated with muscular contraction of the tract following coitus, with spermatozoa present in the ampulla of the uterine tube within 5 to 15 minutes after mating.

Table 4.1 Volume of ejaculate, number of spermatozoa per ml, and site of deposition of spermatozoa in the female reproductive tract of domestic animals.

Species Approximate volume of ejaculate (ml) Number of spermatozoa per ml (× 106) Site of deposition of spermatozoa in the female reproductive tract
Cats 0.5 60 Vagina
Cattle 4.0 800 to 1500 Vagina
Dogs 10 250 Uterus
Horses 70 150 to 300 Uterus
Pigs 250 200 to 300 Cervix / uterus
2000 to 3000
15 to 20

In the slow sustained phase of transport, which continues for some hours, the spermatozoa move from the vagina or uterus to the isthmus, an important reservoir for spermatozoa in the reproductive tracts of cattle, sheep and pigs. On reaching the isthmus, a proportion of viable spermatozoa bind to the mucosal epithelium resulting in suppression of their motility. At ovulation, some bound spermatozoa are gradually released from the epithelium by unspecified factors. These released spermatozoa exhibit vigorous motility and are hyperactive. This enhanced activity may assist in the transfer of spermatozoa to the ampulla and penetration of the layers surrounding the oocyte. An interval of 6 to 12 hours post insemination is required in cattle and sheep for adequate numbers of spermatozoa to reach the isthmus to increase the likelihood of fertilisation. Spermatozoa may remain viable in the isthmus for up to 20 hours in cattle and sheep and up to 36 hours in pigs. There is general agreement that transportation of spermatozoa within the uterus relies principally on the muscular activity of the female reproductive tract. Contractions of the uterus, which increase during oestrus, are augmented by copulation due to release of oxytocin from the neurohypophysis. Prostaglandins present in seminal plasma may also promote uterine contractions.


Before spermatozoa can fertilise oocytes, they must first undergo biochemical and physiological modifications within the female reproductive tract. This process, referred to as capacitation, involves the removal of cholesterol and many glycoproteins from the surface of the spermatozoon, resulting in increased fluidity of the cell membrane. Capacitation, which commences in the uterus, is completed in the isthmus.

Cellular events in the process of fertilisation

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Sep 27, 2017 | Posted by in GENERAL | Comments Off on Fertilisation
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