Ovum and sperms: (In vitro)

From this photograph, it should be clear that the heads of human sperm are less than 1/20 the diameter of human eggs.

Arrows point to sperm heads

Fertilization

Stage 1 of fertilization:The acrosome reaction Occurs when sperms come into contact مع with the يتصل

corona radiata of the ovum البويضة

Passage مرورof sperm throughخالل the corona radiata depends on على :enzyme action يعتمد hyaluronidase released from sperm acrosome Tubal mucosal enzymes

Flagella action also aidsيساعد corona radiata penetrationاختراق

Stage 2 of fertilization:Penetration اختراقof the zona pellucida around the ovum: Acrosomal enzymes: esterases, acrosin, and

neuraminidase cause lysis تحللof the zona pellucida

Once ان sperm penetrates zona pellucida, theبمجردzona reaction occurs: This reaction makes the zona pellucida impermeable غير

.to other sperms منفذة When more than one sperm manages to enter the ovum

(dispermy = 2; triploidy = 3), the fetus nearly always aborts يجهض.

Stages 3 & 4 of fertilization:

Head and tail of a sperm enter the cytoplasm of the ovum

Stage 5 of fertilization:Formation of male and female pronuclei:Chromosomal material of the sperm

decondensates and enlargesChromosomal material of the ovum

decondensates following the completion of meiosis

The male and female pronuclei are indistinguishable from one another.

The second polar body can be seen (arrow).

The plasma membranes of the two pronuclei are dissolving and one diploid nucleus will remain.

Fusion of the pronuclei:(in vitro)

Stage 6 of fertilization:

Membranes of the pronuclei break down, chromosomes condense and arrange themselves for mitotic cell division (cleavage) التفلجThe first cleavage follows shortly, leaving 2

cells, each with 46 chromosomes.

The results of fertilization:Restores the normal diploid number of chromosomes (46).Results in variation of human species as maternal and paternal chromosomes intermingle.Determines the sex of the embryo. The sex chromosome (Y or X) carried by the successful

sperm determines embryonic sex.

Twins: still 1 sperm per eggMonozygotic (monoovular):A fertilized, single egg splits into two developing

zygotes at a very early stage. Identical twins; same sex.

Dizygotic (polyovular):Result from the fertilization by two sperm of two

separate ova that have reached maturation at the same time.

Not identical twins; can be different sexes Incidence increases with age of the mother

Triploidy (in vitro)

There are 3 pronuclei within this one zygote. In the laboratory, such embryos are discarded. In vivo, such embryos almost always abort spontaneously.