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Human Reproductive Cells

The creation of a brand new human involves many complicated processes. The main ‘building blocks’ are human reproductive cells, known as gametes. Female gametes are egg cells, and male gametes are sperm.

In mammals, a zygote (or fertilised cell) forms when an egg from the mother and a sperm from the father come together, allowing their genetic material to fuse. Once safely embedded in the mother’s uterine lining, the human zygote grows into a fully formed baby over the course of nine months.

Ovaries & egg cells

The ovaries are almond-shaped organs that form part of the female reproductive system. Each woman has two of them, located on either side of the uterus at the ends of the fallopian tubes.

The ovaries produce the female sex hormones progesterone and oestrogen. These are at the origin of the development of secondary sexual characteristics, such as enlarged breasts, widened hips, pubic and armpit hair. They determine the function of the female menstrual cycle, ovulation and menstruation, from puberty through menopause.

The ovaries contain many follicles, each of which holds an immature egg cell, or oocyte (a mature egg cell is called an ovum). Most cells aren’t visible to the naked eye. Human eggs cells are an exception, averaging 100 microns in diameter they are about as wide as a strand of hair.

Egg cells consist of:

  • the haploid nucleus, which contains one copy of each chromosome
  • mitochondria, which supply energy for cell replication and division, and contain mitochondrial DNA
  • cytoplasm, a thick solution mainly composed of water, salt, and proteins
  • the zona pellucida, a jelly-like barrier made of glycoproteins that surrounds the cell membrane to protect the contents of the cell
  • cortical granules that harden the outside of the egg cell once it has been fertilised


Egg cells are also formed of the corona radiata, the outermost layer of the egg cell, and the first polar body, a small haploid cell that is a byproduct of cell division.

The egg cell is designed to prevent polyspermy (an egg being fertilised by more than one sperm). A zygote is supposed to contain two copies of each chromosome—if there are more (as a result of multiple sperm fertilising the cell), the zygote is usually unviable.


In early foetal development, the ovaries in a female foetus contain around 6 million egg cells. At birth, that number has fallen to around 1 million. Entering puberty, the number has further decreased to 300,000–400,000. After puberty, the number decreases much more slowly—around 1000 per month.

As we age, our cells do the same, and the female egg cell is no exception. Although women are born with a large quantity of eggs, their number decreases over time. By the time she reaches 40 years of age, a woman will have only 3% of her original egg supply. The number can vary depending on her lifestyle—smoking, for example, accelerates egg cell loss.

The quality of egg cells also decreases with age. Just before ovulation, egg cells divide. Older egg cells are more prone to errors during this process, which decreases viability and increases the chance of developmental abnormalities.

There is a common misconception that certain hormonal processes (such as hormonal contraception or pregnancy) can interrupt the natural deterioration of egg cells. This is untrue. Hormones have a part to play in almost everything related to the human body, but this particular process is more dependent on mitochondrial deterioration.

Eggs can be donated. The potential donor will be carefully examined and, if healthy enough to go through the procedure, will be given drugs that stimulate follicular maturation. Eight to fourteen days later the mature eggs are removed from the donor's body laparoscopically and stored until needed.

In many countries, egg donation is the subject of ongoing discussions about donor health and the ethical aspects of donation, for example, whether it is acceptable to receive payment for an egg donation.

Ovulation

During the ovulation stage of the menstrual cycle, one of the ovaries brings a few follicles to maturity, and releases a mature egg cell, or ovum—the rest of the mature follicles are reabsorbed by the body. This happens regardless of whether the woman is sexually active. The egg is expelled towards the fallopian tube, which it then travels through, towards the uterus. This is where the female egg cell can be fertilized by a sperm (if unprotected intercourse has occurred).

Once ovulation has taken place, the released egg begins to deteriorate very quickly, and the so-called fertile window begins to close. If the egg is fertilised, it continues to travel through the fallopian tubes to the uterus, where it embeds itself in the uterine wall and proceeds to develop into an embryo. If the egg is not fertilized, it breaks down and is expelled from the body along with the uterine lining as the woman’s period.

Compared to eggs, sperm are more resistant and can survive in the female reproductive system for up to five days (but are unlikely to survive outside it). The survival of sperm depends on the right conditions, such as the consistency of cervical mucus. Timing intercourse relative to ovulation is a key step in planning a pregnancy.

Hormonal birth control works by suppressing ovulation. When ovulation doesn’t happen in a regular menstrual cycle, this is called an anovulatory cycle. Anovulatory cycles are common and most women are likely to experience them at some point in their lives.


You may not realize you haven’t ovulated if you aren’t tracking your menstrual cycle. Tracking your cycle allows you to notice patterns, which helps you realise when your body is behaving unusually. Being aware of what is (and isn’t) normal for your body can be vital when it comes to diagnosing certain conditions.

The formation of twins

Some women may release two eggs per cycle, this can lead to the conception of twins.

  • Paternal, or identical twins are created when the fertilized egg splits in half. Identical twins grow up in the same amniotic sac (also known as the bag of waters) and, unlike fraternal twins, are genetically identical. Most identical twins come by chance.
  • Fraternal twins occur when two separate eggs are fertilized by two separate sperm. Fraternal twins do not share the exact same genes — they are no more alike to each other than they are to their siblings from different pregnancies. Fraternal twins tend to run in some families.
  • Multiple births can be fraternal, identical, or a combination. Multiples associated with fertility treatments are mainly fraternal.

For scientists and biomedical researchers, twins provide a valuable opportunity to differentiate the influence of genes from the influence of the environment—nature vs. nurture. Because identical twins come from a single fertilized egg that has divided in two, they share the same genetic code. Any difference between them (e.g. a twin with younger-looking skin) should be due to environmental factors (for example, less time in the sun).

Additionally, by comparing the experiences of identical twins to those of fraternal twins, we can quantify how much our genes affect our lives.

Testicles & sperm

The sperm is the male reproductive cell, or gamete.


The term ‘gamete’ [derived from the Ancient Greek word for marriage] implies that the cell is half of a whole—it contains half of the genetic material necessary to form a new human life. Typically, male and and female gametes each contain 23 chromosomes. When a sperm merges with an egg cell it forms a zygote containing 46 chromosomes, which then develops into an embryo.

The production of seminal fluid takes place in the testicles. They also produce testosterone, which is a sex hormone responsible for male secondary sex characteristics, including facial and chest hair, a masculine pelvic build (lack of rounded hips), a muscular upper body build, and the ability to generate muscle mass at a faster rate than a woman.

Spermatogenesis is the process of creating sperm. This process begins in the seminiferous tubules of the testes. These tubules produce sperm called spermatocytes. Spermatocytes go through several cycles of division to transform cells into spermatids. Spermatids are young sperm that grow and mature until they become sperm (a process that takes approximately 64 days).


Sperm have three identifiable parts:

  • The head of the sperm contains the nucleus that holds its DNA. Covering the head of sperm is a cap called an acrosome. The acrosome contains enzymes that help sperm break through the cell membrane of an egg.
  • The midsection contains energy producing mitochondria. These specialized structures provide energy the sperm uses to move.
  • The tail (also known as the flagellum) propels the sperm forward on its way from the vagina to the fallopian tube. A sperm can travel about 30 inches (76cm) per hour.

[quote] Unlike women, men aren’t born with reproductive cells. Instead, they start producing sperm at the onset of puberty—from around 12 years old onwards, men produce millions of new sperm every day. On average, a male produces approximately 73 million sperm cells per milliliter of ejaculate.

Two main factors that can affect male fertility are sperm count and sperm motility.

Sperm count refers to the average number of sperm present in one sample of semen. Doctors can assess sperm count during routine sperm analyses.

Experts consider a healthy sperm count to be about 15 million per milliliter, or at least 39 million per ejaculate. Doctors consider a sperm count under 15 million per ml to be low and likely to impede fertility. Testosterone levels have a significant impact on sperm number and quality. Certain medical conditions—including inherited genetic disorders, infections, and tumors—can also impact sperm count.

Some lifestyle choices and natural remedies can help support the hormones that control sperm production, which may aid the healthy development of sperm and improve sperm count.

Sperm motility is the ability of sperm to move efficiently. Sperm need to move through the woman’s reproductive tract to reach and fertilize her egg. Poor sperm motility can be a cause of male infertility.

There are two kinds of sperm motility:

  • Progressive motility refers to sperm that are swimming in a mostly straight line or large circles—behaviour that helps it progress towards the egg.
  • Non-progressive motility refers to sperm that do not travel in straight lines or that swim in very tight circles—behaviour that does not help it progress.

For the sperm to get through the cervical mucus to fertilize a woman’s egg, they need to have a progressive motility of at least 25 micrometers/second. Poor sperm motility or asthenozoospermia is diagnosed when less than 32 percent of the sperm are able to move efficiently.

Scientists are still researching the mechanisms that allow sperm to reach and fertilise an egg cell. The hormone progesterone is somehow involved. Scientists now think that sperm may be attracted to concentrations of progesterone, which is released by the egg and found in higher levels closest to it.

Sperm banking

It is possible to collect sperm for artificial insemination through either intrauterine insemination (IUI) or in vitro fertilization (IVF).

IUI involves placing the sperm directly into the female uterus to aid in fertilization; IVF involves introducing the sperm into the egg in a laboratory and later transferring viable embryos to the uterus.

To collect a semen sample, the male ejaculates into a sterile container. A lab technician will then take the sperm sample for immediate use with IUI or IVF, or freeze it for storage.

Sperm can also be donated. Donated sperm is rigorously tested for infectious diseases and genetic conditions.

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