Fertilization in Human

The union of a male gamete with the egg cell to form a zygote is called fertilization.

The fusion of the male gamete (sperm) and female gamete (ovum) to form a zygote is called fertilization.

or

The formation of a zygote by the union of a sperm and an ovum is called fertilization.

Fertilization (also called 'syngamy') is defined as the process of fusion of a haploid male gamete (sperm, n) with a haploid female gamete (ovum, n) to form a diploid zygote (2n). In humans and other mammals, fertilization is internal.

Site of Fertilization:

Fertilization takes place at the ampullary-isthmic junction of the fallopian tube (uterine tube) – the junction between the ampulla and isthmus regions, roughly midway between the ovary and uterus.

During copulation (coitus), 2–4 ml of semen containing 200–400 million spermatozoa is deposited in the vagina (insemination). The sperm must traverse a long, hostile path to reach the ovum.

Path taken by the sperm and ovum for fertilisation

• Ovum is released from a ruptured Graafian follicle; this is called ovulation.
• Ovulation occurs on approximately Day 14 of the menstrual cycle, triggered by the LH surge.
• The ovum at ovulation is actually a secondary oocyte arrested at metaphase II, not a fully mature egg.
• It is swept into the fallopian tube by the fimbriae and transported to the ampullary-isthmic junction.
• The ovum measures approximately 100 μm in diameter and is surrounded by the zona pellucida and corona radiata.
• The ovum can be fertilized only within 12–24 hours after ovulation. After this window, it degenerates.
• The ovum is classified as 'alecithal' – almost devoid of yolk – as seen in mammals.

Capacitation is the process by which sperm acquire the ability (capacity) to fertilize the ovum after spending time in the female genital tract. Sperm do not have this ability immediately after ejaculation.

• ~50% of sperm are demobilized/broken/destroyed in the vagina before capacitation.
• Secretions of the female genital tract remove coating substances (decapacitation factors) from the sperm's surface.
• Ca²⁺ (calcium ions) enter the sperm → tail begins rapid whiplash movements.
• The acrosomal membrane thins, exposing lytic enzymes for the acrosomal reaction.
• Sperm speed increases as the tail changes from gentle to vigorous movement.
• Duration: approximately 5–6 hours in the female genital tract.
• After capacitation, sperm can reach the ampulla within 5 minutes, aided by tubal contractions.

This reaction serves as a species-specificity (compatibility) check – ensuring that fertilization occurs only between sperm and ovum of the same species.

• The reaction between fertilizin and antifertilizin causes agglutination of sperm to the egg membrane; only a few sperm can react with the egg surface.
• Since the reaction is species-specific, cross-species fertilisation is prevented.
• Fertilizin also activates sperm and initiates the subsequent acrosomal reaction.
• This reaction is called the compatibility reaction.

When the capacitated sperm head contacts the zona pellucida of the ovum at the animal pole, the acrosome (cap-like structure on sperm head) ruptures and releases lytic enzymes. This is called the acrosomal reaction.

Ovum surrounded by few sperms

Sequence of Penetration:

• Sperm migrates through the coat of follicle cells (corona radiata) and binds to ZP3 receptor proteins of the zona pellucida.
• Acrosomal enzymes dissolve the zona pellucida - sperm head pushes through.
• A small fertilization cone/cone of reception forms on the egg membrane at the site of contact.
• Sperm head makes contact with egg plasma membrane → membranes of both cells dissolve at the contact point.
• The sperm nucleus and proximal centriole enter the egg cytoplasm. The tail and mitochondria remain outside.

Only ONE sperm must fertilise the ovum. The body uses two sequential blocks to prevent the entry of additional sperm:

The entry of sperm into the secondary oocyte activates the ovum, which was arrested at Metaphase II of meiosis.

Karyogamy (also called Amphimixis) is the fusion of the male pronucleus (from sperm head) and the female pronucleus (from ovum) to form the diploid zygote (2n). This is the final event of fertilisation.

• Fertilization restores the diploid chromosome number (46 chromosomes) in the zygote.
• It completes meiosis II in the secondary oocyte and forms the mature ovum.
• The sperm provides the centriole required for the first cell division of the zygote.
• Fusion of male and female gametes creates genetic variation in the offspring.
• Fertilization determines the sex of the child and initiates embryo development.

• Fertilization in humans is the fusion of male and female gametes (sperm and ovum) and is an internal process occurring in the oviduct, leading to the formation of a diploid zygote.
• During copulation, millions of sperms are released into the vagina, but only one sperm reaches the ovum due to selection and protective barriers around the egg.
• Before fertilization, sperm undergo capacitation and move towards the ovum; interaction between sperm and egg involves recognition molecules like fertilizin (ovum) and antifertilizin (sperm).
• The sperm’s acrosome releases enzymes that help digest the egg coverings (corona radiata and zona pellucida), allowing sperm entry through the membrane.
• After entry, the sperm nucleus and the ovum nucleus fuse (syngamy), forming a zygote (2n); this process restores the diploid chromosome number (46 in humans).
• Fertilization triggers completion of meiosis II in the oocyte, formation of the second polar body, and formation of the female pronucleus and male pronucleus.
• In humans, sperm carry 22+X or 22+Y chromosomes, while ova carry 22+X only, determining the genetic sex of the offspring.