Fertilization and early development
Fertilization is the fusion of one sperm with one egg. It rests on meiosis, which gives sperm and eggs half the chromosome set of the body's other cells, and on the development of each: sperm continuously from puberty, eggs one a month from a stock present at birth. A sperm must be changed in the female reproductive tract and pass the layers around the egg before it can fuse with it, and its fusion sets off a calcium signal that activates the egg. The zygote divides on its way down the fallopian tube and reaches the uterus about a week later. The environment around conception and in early life may shape reproductive health in adulthood through epigenetic processes.
Why fertilization matters to biology and medicine
Fertilization is the fusion of one sperm cell with one egg cell. How animals produce offspring of the same species as their parents but with characteristics of their own has been a biological phenomenon of great historical interest. One key to answering it has been uncovering the cellular and molecular events of fertilization. Understanding how sperm and eggs develop and interact has advanced the understanding of genetics, evolution, cell–cell fusion, cell division and cell signalling. It has also supported the cell theory, that all cells come from cells, which is a cornerstone of biology. Fertilization research has also advanced the technologies of contraception, such as the birth control pill, and of conception, such as in vitro fertilization, and these have affected human fecundity. So understanding human fertilization gives insight into both biological processes and clinical reproductive techniques.
Chromosome number, mitosis and meiosis
Most cells of the human body carry 46 chromosomes, organised into 23 pairs. A chromosome is a linear strand of DNA wound up with proteins. It is held in the cell nucleus, passed from one generation to the next, and carries the instructions for the development of an organism of a particular species. The two members of a pair contain genes for the same characteristics and are called homologous chromosomes. Cells with two sets of each chromosome are diploid. Most body cells with 46 chromosomes duplicate themselves by mitosis, in which one cell divides into two identical cells with the same chromosome number as the parent. So a cheek or liver cell with 46 chromosomes produces two cells with 46 chromosomes each.
Exceptions include mature egg and sperm cells, which have 23 chromosomes each. They are haploid, holding one member of each homologous pair, and only sperm and egg cells reach that state. The cells that produce them divide by meiosis, which occurs only during the development of sex cells. In meiosis a diploid cell replicates its DNA so that each chromosome has two chromatids, then completes two cell divisions to create haploid cells with 23 chromosomes each. The first division separates the homologous pairs, and the second separates the chromatids.
One sperm with 23 chromosomes fuses with one egg with 23 chromosomes to produce the zygote, a single diploid cell with 46. The zygote then divides mitotically into an embryo, a fetus and finally a baby, whose cells each carry 46 chromosomes until mature sperm and eggs are made again at puberty. So fertilization continues a species whose members have a characteristic chromosome number, and it also creates a new genetic mix, half the chromosomes coming from each parent. This can facilitate evolution by producing new phenotypes that may be better adapted to a changing environment.
Sperm development
In male humans meiosis occurs in the testes, inside the seminiferous tubules. These channels contain support cells and the cells that undergo meiosis to create haploid sperm. From puberty, hormones stimulate special diploid cells there to divide and produce cells that will undergo meiosis. Each meiotic event yields four haploid cells with 23 chromosomes each. Those cells then undergo changes of form that include ejecting a cytoplasmic droplet and growing a long flagellum, or tail. An acrosome, a membrane-bound sac of enzymes that are important in fertilization, develops in the cytoplasm of the sperm head. The sperm cells enter the central canal of the seminiferous tubules and are flushed to the epididymis, where they finish maturing, are stored, and gain the ability to swim.
One human testicle makes about 100 million sperm a day, and it takes about 70 days to make one mature sperm cell. Unused sperm are reabsorbed or excreted in the urine.
Egg development
Baby girls are born with millions of immature eggs, called oocytes, arrested at an early stage of meiosis in the ovaries. A male produces sex cells throughout his life after puberty, while a girl is born with all the sex cells she will ever produce. From puberty, hormones trigger one oocyte in one ovary each month to mature inside a follicle, a spherical cluster of cells in the ovary that surrounds and supports it. The process repeats monthly until menopause, by which time a woman will have ovulated 200 to 400 eggs. Which ovary supplies the egg in a given month appears to be random.
Both oocyte and follicle grow each month. The oocyte increases in diameter from about 10 µm to about 80–100 µm and stockpiles the messenger RNA, proteins and other materials that embryogenesis will need if fertilization occurs. It is surrounded by the zona pellucida, a transparent, elastic, noncellular layer of glycoproteins. The zona protects the oocyte, the egg and the early embryo, and it is important in facilitating successful fertilization. A rise in luteinizing hormone, secreted by the pituitary gland, triggers both ovulation and the first meiotic division. In ovulation a mature follicle ruptures and releases the oocyte from the ovary.
Both meiotic divisions in the female are unequal. In the first, one daughter cell retains most of the cytoplasm, and this large cell is what is then often called the egg. The other retains very little and is called the first polar body, which may not go on to divide again. After ovulation the egg and the follicle cells that accompany it are captured by a fallopian tube, and the egg moves slowly down the tube toward the uterus. The tube transports the egg, and then the developing embryo if fertilization occurs, to the uterus. If sperm are present, fertilization can take place in the tube. The interaction of a sperm with the egg is what stimulates the second meiotic division and completes egg meiosis. That division is unequal too. So each meiotic event in the female produces one large egg with 23 chromosomes and two or three small polar bodies, which later degenerate. This lets the egg keep enough of the nutrients, proteins and other materials that embryogenesis requires.
A mammalian egg that has completed the first meiotic division can be about 100 µm in diameter. Its first polar body lies next to it, under the zona pellucida.
From sperm deposition to sperm–egg fusion
After sperm are deposited in the vagina, they must travel past the cervix, through the uterus and into the one fallopian tube that holds an egg. The journey is often depicted as a race won by the sperm that reaches the egg first, but that depiction is not necessarily true. A sperm that reaches the egg too quickly may not be capable of fertilising it. To fertilise an egg, a sperm must first spend long enough in the female reproductive tract to undergo capacitation, a series of molecular changes that include modifications to the components of the sperm plasma membrane. The changes are not completely understood, but research suggests that they change sperm motility and components of the plasma membrane. A sperm that reaches an egg without having completed capacitation cannot fertilise it.
About 200 million sperm are typically deposited in one ejaculation, but only about 200 reach the egg in the fallopian tube. Sperm usually reach the egg between 30 minutes and 6 days after they are deposited. Sperm that arrive in as little as 30 minutes are believed to be helped by contractions of the uterus, and they are likely incapable of fertilization, because not enough time may have passed for capacitation to complete.
When an egg is ovulated, some cells from the follicle that surrounded the developing oocyte are ovulated with it, and in vivo these small follicle cells surround the egg. A capacitated sperm that reaches the egg must work through that layer, helped by a membrane-bound enzyme on its surface. It must then penetrate the zona pellucida. Binding to the zona triggers the acrosome reaction, in which rising calcium in the sperm cytoplasm causes exocytosis of the acrosomal vesicle from the sperm head. The vesicle releases digestive enzymes onto the zona, and they cut a hole through it for the sperm to swim through. The sperm plasma membrane then binds the egg plasma membrane and the two fuse. The sperm nucleus, with its 23 chromosomes, then enters the egg cytoplasm.
The interactions between the two membranes during binding and fusion are not completely understood, but they trigger the egg to release enzymes that modify the zona pellucida so that no further sperm can penetrate it. That modification is the block to polyspermy, the fertilization of an egg by more than one sperm. Polyspermy produces an embryo with an abnormal chromosome number, as two sperm give 69 chromosomes rather than 46, and it usually leads to embryonic death. Binding and fusion also trigger the egg to complete its second meiotic division, creating an egg nucleus with 23 chromosomes. Twins are not produced by two sperm fertilizing one egg. Identical twins arise when an early embryo separates into two clusters of cells that each develop into a fetus, and nonidentical twins arise when a woman ovulates two eggs that are each fertilized by one sperm.
Egg activation by calcium
By the early twentieth century, researchers had realised that a sperm not only delivers a haploid nucleus to the egg but also somehow signals, or activates, the egg to prepare for embryogenesis. In mammals the activation events include the polyspermy block and the completion of meiosis. The signal was not discovered until 1977, when two research groups studying sea urchin and fish eggs independently found that a large increase in calcium occurs in the egg cytoplasm at fertilization. That calcium rise has since been found at fertilization in eggs from all animal and plant species examined, among them maize, jellyfish, bivalves such as clams, annelids such as segmented marine worms, sea urchins, starfish, sea squirts, frogs, mice and humans.
If the rise is blocked, further development is inhibited, because the egg does not re-enter the cell cycle. In mammals, inhibiting the rise prevents the egg from creating the polyspermy block, finishing meiosis and beginning embryogenesis. In mammalian eggs the sperm triggers calcium to be released into the cytoplasm from the egg's endoplasmic reticulum, an organelle important in protein synthesis and in calcium storage.
Calcium indicators and microscopy show that in the eggs of vertebrates such as fish, frogs and mice the rise begins at the point of sperm–egg interaction and spreads across the egg as a wave. In the mammalian egg, cytoplasmic calcium then falls until the next wave. Some vertebrate eggs, such as those of fish and frogs, undergo only one wave. Mammalian eggs, including those of mice and humans, undergo multiple waves, or oscillations, that continue for several hours after fertilization. In humans the oscillations can occur about every 10 to 30 minutes. In mammals multiple waves are required to induce the egg to create the polyspermy block and to finish meiosis. In mouse eggs, creating the block requires four to eight oscillations and completing meiosis requires 16 to 24. The signalling molecules the sperm uses to initiate the rise are not completely understood.
In a human egg injected with aequorin, a calcium indicator that emits light on binding calcium, the first calcium rise occurred about 16 minutes after sperm were added, and multiple further rises followed. Each spike corresponded to one wave crossing the egg.
The first week of development
The sperm and egg nuclei, each haploid, migrate toward each other through the egg cytoplasm over about 12 hours. When they meet, the nuclear membranes break down and the 46 chromosomes line up in single file in the centre of the zygote for the first mitotic division of embryogenesis. That division produces two daughter cells with 46 chromosomes each. The early divisions take place in the fallopian tube while the embryo continues toward the uterus. The zona pellucida protects the developing embryo and helps prevent it from embedding in the tube. An embryo embedded there is an ectopic pregnancy, which usually results in pregnancy loss.
About a week after fertilization the multicellular embryo reaches the uterus at the blastocyst stage. A blastocyst consists of an outer layer of cells called the trophoblast, a fluid-filled cavity, and a cluster of cells to one side called the inner cell mass. The trophoblast forms the embryonic portion of the placenta, and the inner cell mass develops into the embryo proper. The blastocyst releases enzymes that create a hole in the zona pellucida, which lets it hatch from the zona and embed within the uterine lining. The rest of development takes place in the uterus until birth. In humans, embryo names the organism from fertilization through the eighth week after it, and fetus names it from the ninth week until birth.
The completion of meiosis, the divisions in the fallopian tube and the embryo's embedding in the uterine lining trigger the hormonal changes that maintain the pregnancy and prevent oocyte maturation and ovulation during gestation.
Open questions
As of 2012, many questions about sperm and egg development and about fertilization remained unanswered. They included how a cell is signalled to undergo meiosis rather than mitosis, which modifications a sperm undergoes during capacitation are essential, which key molecules are involved in sperm–egg binding and fusion, and how a sperm triggers the calcium rise in a mammalian egg.
The early-life environment and reproductive health
Epigenetics regulates gene expression through biochemical mechanisms that alter the proteins that structure chromatin, without changing the genetic code. It is essential for setting cell diversity and for adaptation to different environmental conditions. It can be induced by endogenous and external signals. It modulates gene expression in four main ways: chemical modification of cytosine in DNA, post-translational modification of chromatin structure through histone proteins, genomic imprinting, and modulation by non-coding RNA.
Environmental conditions can shape epigenetics during critical developmental periods: preconception, prenatal and early postnatal life, together called the first 1,000 days. This plasticity lets the developing organism acclimatise to the intrauterine environment, which reflects external conditions. Studies in humans and animals underpin the Developmental Origins of Health and Disease (DOHaD) hypothesis, that the early-life environment profoundly shapes the future health of the developing organism. In the two decades to 2024, preclinical and clinical studies consolidated the hypothesis and its impact on reproductive development and function in adulthood. Evidence shows that the hypothesis applies to both metabolic and adult fertility conditions.
Fertility in humans depends on the interaction of the hypothalamic–pituitary–gonadal (HPG) axis, a neuroendocrine system whose aspects develop and are fine-tuned at different developmental stages, especially the embryological period. Gametogenesis and the development of the axis occur during gastrulation. So environmental effects during the first 1,000 days can explain numerous neuroendocrine deregulations in adult reproductive life.
Gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus are the central mechanism leading the HPG axis. They regulate the axis at different developmental stages and have a fundamental role in controlling puberty onset and reproductive function. In the two decades to 2024, kisspeptins emerged as the master regulators of GnRH neurons, and Kiss1 neurons, also in the hypothalamus, encode them. Preclinical studies show epigenetic regulation acting on these neuronal populations. Adverse external conditions can affect aspects of adult reproduction such as puberty maturation, brain sex differentiation and gametogenesis.
Clinical results have shown that maternal malnutrition during pregnancy and breastfeeding is a crucial factor in the offspring's reproductive health, affecting pubertal maturation, gonadotropic function and gamete quality. The intrauterine environment can modulate transgenerational effects through epigenetic mechanisms acting on the precursors of oocytes and sperm. The parents' environment around conception has also been shown to play a fundamental role in determining the reproductive phenotype of their offspring. Malnutrition and stress in either parent before conception can change the oocyte and sperm and so modify the heritable epigenetic pattern, which may raise susceptibility to disease in adulthood. In humans the environment before and after conception is hard to address separately. In animals, reciprocal embryo transfer has separated the effect of diet before pregnancy from that of diet during it on the offspring's metabolic features. Longitudinal studies of births to patients who received donated embryos could be of scientific interest.
The mechanism of intergenerational transmission of non-genetic traits is not yet fully elucidated, but recent evidence suggests that epigenetic processes may be the primary agents. The mechanistic basis of epigenetic phenomena is hard to establish for two main reasons: the time between a stressor and its physiological result, and the tissue-dependent nature of the regulation. More epidemiological studies in fertile and infertile human populations could clarify the regulation. Epigenetics may be the clue to diagnosing infertility and to possible future neuroendocrine therapies.