Fertilization:
Fertilization (also known as
conception or
syngamy) occurs when a spermatozoan and an ovum unite to form a diploid
zygote. It isn’t quite as simple a process as you might think.
The terms “egg” and “ovum” are often used as if they’re interchangeable, but they aren’t, really. An “egg” consists of an ovum
plus various supporting structures. In mammals, for instance, the ovum is surrounded by a clear layer of glycoproteins known as the
zona pellucida. Surrounding the zona pellucida is a layer of cells known as the
corona radiata. These structures that surround and support the ovum can make things difficult for spermatozoa, since a spermatozoan must penetrate them in order to fuse with the ovum.
That’s one of the reasons that many, many spermatozoa must be produced per ejaculation. First of all, since it lacks most of the organelles that a cell needs in order to survive for any length of time, a spermatozoan has a short life expectancy. Second, if fertilization is internal, the female’s reproductive tract (which is typically rather acidic) can be a rather hostile place from the perspective of spermatozoa. (By some estimates, more than half the spermatozoa deposited into a woman’s vagina when her partner ejaculates are killed within a minute.) Finally, it usually takes the combined actions of several spermatozoa to penetrate the corona radiata and zona pellucida.
If all goes well, however, a spermatozoan penetrates into the ovum and fertilization occurs.
Fertilization
Linear Mode
