What is the largest cell in the human body?

The largest cell in the human body is the egg (female gamete). Unlike almost all other cells in the body, the egg is spherical and can be seen with the naked eye, measuring about 0.12 mm (120 μm) in diameter. This is huge compared to other cells. For example, a typical red blood cell is about 7.5 μm in diameter, meaning the egg is 20 times the diameter of a blood cell.⁽¹⁾.

Furthermore, if we think in real size, the egg occupies so much volume because it has abundant cytoplasm, where molecules and components are stored that help initiate the development of the fetus after fertilization. Therefore, it is not big “because it is big”, but because it plays a key biological role.

As a curious fact, the egg cell exceeds in diameter and volume any other cell in the human body, including neurons and muscle cells, which although they can be very long, have a much smaller cell body.

What is the second largest cell in the human body?

The second largest cell in volume is adipocytes (fat cells). Human adipocytes reach typical diameters of about 40 μm and can measure up to 200 μm when expanded by lipid accumulation, but their volume is still up to ten times smaller than that of the egg.⁽²⁾.

The human egg has a spherical shape and is immobile, so it does not actively “swim” like the sperm. Its diameter (approx. 0.1–0.15 mm) is equivalent to the thickness of a hair, which explains why it can be distinguished without a microscope under suitable conditions. If compared to other cells, the difference in size is gigantic. An egg is like an onion with many layers of protection, while a sperm is just a small tip. In fact, some calculate that if the sperm were a 2 cm marble, the egg would be a ball about 63 cm.

As a specialized gamete, it is surrounded by several protective layers that also perform recognition and support functions. The parts of this cell are:

• YesExternal surface: we find the corona radiata (layer of follicular cells that accompany it from the ovary) and the zona pellucida (a resistant glycoprotein matrix). These layers act as a selective barrier that helps only the “correct” sperm adhere, activate chemical signals, and pass through. Furthermore, once fertilization occurs, changes are triggered that reduce the entry of other sperm, avoiding polyspermy.
• Interior: there is abundant cytoplasm, full of organelles and nutritional reserves. The cytoplasm of the egg contains essential structures (nucleus, mitochondria, ribosomes, etc.). In fact, its nutritional substance is called yolk or yolk, and it provides the nutrients that the future embryo will need in its first divisions. In the center of the egg is its haploid nucleus, with the 23 female chromosomes. This nucleus is relatively large and contains all the maternal genetic information.

The main function of the egg is reproductive. It is a haploid gamete, that is, it contains half of the chromosomes (23) necessary to form a new being.

When fertilized by a sperm (another haploid gamete), its nuclei unite and restore the diploid number (46 chromosomes) in the zygote. This zygote is the first cell of the embryo and, through successive cell divisions and differentiation processes, will give rise to the baby.

In other words, the egg provides the new organism:

• 50% of your DNA (the entire maternal genome).
• Mitochondria and other maternal organelles.
• Initial cellular environment, including cytoplasm, proteins, RNA and energy reserves.

This is crucial because the sperm mainly delivers genetic material, while the egg also provides the initial cellular environment: cytoplasm, proteins, RNA and reserves that allow the embryo to begin functioning before implanting. In fact, all the mitochondria of the embryo come from those of the egg, so that only the mother transmits mitochondria, providing the initial energy for development and marking maternal mitochondrial inheritance in the offspring.

Resources of the egg and its role in the embryo

The ovule also contains:

• messenger RNA.
• Proteins.
• Energy reserves.

These components support the first cell divisions of the embryo. It can be considered a “seed” loaded with resources: when it fuses with the sperm, it begins a complex process (embryogenesis) that, ideally, will culminate in a fetus and then in a healthy baby.

Without a healthy and nourished egg, life could not begin. In addition, the characteristics of the egg determine future traits of the child, such as:

• Height.
• Eye color.
• genetic predispositions.

That is, the egg is essential for fertilization and to transmit maternal inheritance. Its large size allows it to contain everything necessary (genetic material, nutrients and organelles) that drives the first stages of the development of the new being.

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