Video Transcript
In this video, we will learn how, where, and when fertilization in humans can occur. We will learn the events that follow fertilization in embryonic development to produce a new human being and how in some cases fertilization can result in the production of identical and nonidentical twins.
Humans reproduce sexually, which means that a diploid zygote is formed when the fusion of the male and female gametes occurs in a process called fertilization. The male gamete is called a sperm cell, and the female gamete is called an egg cell, or sometimes an ovum. Both of these gametes are described as haploid cells, which is sometimes represented as 𝑛, as they contain half the genetic information of a normal body cell. In humans, this means each gamete contains just 23 chromosomes instead of the usual 46 that are found in most cells. This is because when the gametes nuclei fuse together in fertilization, it forms a zygote with a full set of 23 pairs of chromosomes for 46 in total.
A diploid cell like the zygote is often represented as two 𝑛. Though in this diagram, the gametes look fairly similar in size, the sperm cells are actually the smallest cells in the male body, while the egg cell is often regarded as the largest cell in the female body. Each sperm cell is composed of a head, a neck, a midpiece, and tail. The head of the sperm cell contains its haploid nucleus, which has 23 chromosomes including one sex chromosome, which in sperm cells is either an X chromosome or a Y chromosome. The head of the sperm cell is covered with a cap-like structure called an acrosome. The acrosome contains enzymes that help the sperm to penetrate the egg cell.
The plasma membrane of the egg cell is surrounded by two layers. The inner layer is a clear, jelly-like coat called the zona pellucida, which is made primarily of glycoproteins. The corona radiata surrounds the zona pellucida and consists of multiple layers of follicle cells. The egg cell also contains 23 chromosomes including one sex chromosome, which is always an X chromosome. Because of this, when fertilization occurs, it’s the sex chromosome in the sperm cell that determines the biological sex of the zygote.
If a sperm cell with an X sex chromosome fertilizes the egg cell, the zygote will have two X chromosomes and be biologically female. Or if a sperm cell with a Y sex chromosome fertilizes the egg cell, the zygote will have one X chromosome and one Y chromosome and be biologically male. After the zygote has been produced, it divides to form an embryo full of diploid cells that will eventually develop into a fetus.
Let’s take a brief look at the main structures in the female reproductive system. The ovaries play a crucial function and reproduction, as they release the egg cells. They also behave as endocrine glands secreting the hormones estrogen and progesterone. Each of the two ovaries are connected to a fallopian tube, which is sometimes called an oviduct. The fallopian tubes connect each ovary to the uterus. In humans, the fallopian tubes are lined with tiny hair like structures called cilia that can waft the egg cell along moving it towards the uterus.
The uterus is a hollow organ with a lining called the endometrium into which an embryo might implant if fertilization is successful. There, the embryo is supported throughout its growth and development into a fetus. The walls of the uterus are made of smooth muscle, which contract strongly during childbirth. The uterus opens into the vagina through a tissue called the cervix. The wall of the vagina has folds that allow it to be stretched and expand during childbirth. The vagina is kept moist by secretions from glands in the cervix in either side of the vaginal opening.
Generally, the female reproductive system undergoes changes through periodic, approximately monthly, cycles called the menstrual cycle, which are regulated by changes in the concentration of certain hormones. For example, the concentration of estrogen and progesterone fluctuate throughout the menstrual cycle, resulting in changes in the thickness of the endometrium. Let’s take a look at this on a graph.
While the menstrual cycle is typically around 28 days in duration, longer or shorter cycles are also normal. Initially, levels of estrogen and progesterone are low, and the endometrium layer breaks down. The endometrium then starts to build up again from around day seven. This prepares the endometrium for the potential implantation of an embryo. In the middle of the menstrual cycle, around day 14, one of the ovaries usually releases a single egg cell in a process called ovulation. When the ovary releases the egg cell, it travels along the fallopian tube to a region called the ampulla, aided by the cilia in the fallopian tube’s lining.
But how does the sperm cell reach the egg cell for fertilization? Let’s take a look at the stages that usually occur to result in this phenomenon. Firstly, copulation occurs, which is also called sexual intercourse, coitus, or simply sex, during which the male inserts his penis into the female’s vagina. This often results in a process called ejaculation, during which hundreds of millions of sperm cells are released from the male’s penis and into the vagina.
On our diagram, let’s represent the sperm cells as one blue dot. From the vagina, the sperm can swim through the cervix and into the uterus. Only around 200 of the sperm cells that enter the vagina will make it to the fallopian tube. The egg cell releases chemicals that attract the sperm cell to it while it’s in the ampulla. If the sperm cells make it to the ampulla and an egg cell is also there, which has been released recently in ovulation, fertilization may occur.
Let’s find out what happens at a cellular level when the sperm cell meets the egg cell. You may recall that the acrosome contains enzymes. These enzymes are called hyaluronidases. You may also remember that the egg cell’s membrane has two layers surrounding it: the zona pellucida and the corona radiata. The corona radiata contains an intracellular matrix of proteins, carbohydrates, and a substance called hyaluronic acid. You might have noticed the similarity between hyaluronidase enzymes and hyaluronic acid. This is because hyaluronidases released from the acrosome digest hyaluronic acid.
Let’s look at a closer view of the egg cell to see this happening. The acrosome of the sperm cell releases these hyaluronidases, and they digest through the hyaluronic acid in the corona radiata, allowing the sperm cell to penetrate these outer layers. This allows molecules on the head of the sperm to bind to molecules on the surface of the zona pellucida, which triggers the acrosome reaction. This involves the acrosome releasing the enzyme acrosin, which is believed to digest a portion of the zona pellucida.
Eventually, the sperm cell will make its way through the whole zona pellucida. This allows the sperm cell to fuse with the egg cell’s plasma membrane. Then the sperm cell can inject its nucleus and its organelles into the egg cell cytoplasm. This penetration triggers the immature egg cell, which at ovulation was more precisely known as the secondary oocyte, to finally complete meiosis two, which halted before the secondary oocyte was released from the ovary during ovulation. This forms a mature egg cell, otherwise known as a mature ovum, with a haploid nucleus.
The haploid sperm nucleus can then fuse with a haploid egg cell nucleus. This forms a diploid zygote with a full set of 46 chromosomes completing the process of fertilization. After fertilization has occurred, the zona pellucida changes its structure to become impenetrable to other sperm. This prevents a condition called polyspermy, in which an egg cell is fertilized by more than one sperm cell. If polyspermy does occur, the zygote will have an abnormal number of chromosomes and probably would not survive.
Let’s take a look at the stages that follow fertilization and the development of this newly formed zygote. So far, we know that a secondary oocyte is released from an ovary in ovulation and that when the secondary oocyte is penetrated by a sperm cell, it forms a mature ovum. The completion of fertilization forms a diploid zygote. This zygote then continues its journey along the fallopian tube towards the uterus, pushed forward by the wafting cilia and muscular contractions.
Along the way, the cells of the zygote divide through mitosis. And each division is called a cleavage. This series of mitotic divisions first forms two cells, then four cells, and then each of these four cells divides by mitosis to form eight cells. At this point, the zygote becomes known as an embryo. And each cell within the embryo is known as a blastomere. Once the embryo contains eight to 32 tightly packed blastomeres, which takes about four days, it is called a morula. The morula continues to divide, and by day six, it transforms into a blastocyst as it moves into the uterus. At this stage, the blastocyst hatches out of the zona pellucida layer that was originally surrounding the egg cell.
As you can see, the blastocyst is surrounded by a layer of cells called the trophoblast. Within the trophoblast is a group of cells called the inner cell mass, as well as a fluid filled cavity called the blastocoel. The trophoblast attaches itself to the endometrium, and the endometrial tissue envelopes the blastocyst embedding it in the lining of the uterus. This process is called implantation, and it occurs between six to 12 days after fertilization has occurred.
After implantation, the inner cell mass can begin to differentiate, eventually developing into the cells that form the organs of the embryo and later on the fetus over this period of pregnancy, which is sometimes called the gestation period. This amazing process can form a fully fledged new human. Interestingly, in some cases, more than one embryo can implant in the uterus, leading to multiple births most commonly through the formation of twins.
The prefix mono- means one, and monozygotic twins are those who develop from one zygote. This single zygote splits into two separate embryos between eight to 12 days after fertilization. As both of these embryos come from the same egg and sperm, and therefore from the same zygote, they’re genetically identical to each other. For this reason, monozygotic twins are sometimes called identical twins. The twins share all the same genes and characteristics and are therefore always the same biological sex, as they both have their genotype decided by the sex chromosome of a single sperm cell.
The prefix di- means two, so dizygotic twins develop from two separate zygotes. Sometimes during ovulation, two ova are released from the ovaries instead of the usual one. If each ovum is separately fertilized by different sperm cell, two zygotes are formed, which develop into two genetically different embryos, as two genetically different ova were fertilized by two genetically different sperm cells. Therefore, dizygotic twins are sometimes known as nonidentical twins.
The word fraternal is used to describe brothers or siblings, implying fraternal twins, which is sometimes used as a synonym for dizygotic twins, have the same level of genetic similarity that normal siblings would. In fact depending on the sex chromosome that the sperm cell which fertilized each of the two ova carries, fraternal twins may even be different biological sexes.
Let’s learn about how the embryo develops into a fetus after implantation has occurred. During the second week, after the embryo has been implanted at its blastocyst stage, the cells begin to differentiate into layers. Some of these layers form outside the embryo and are called extraembryonic membranes to make up a structure called the amniotic sac, which we can see at a later stage of pregnancy in this diagram. The prefix extra- means outside, which reminds us that these layers are found outside the developing embryo. These layers protect the embryo as it develops into a fetus during the gestation period.
You might notice from the diagram that there are two layers that form the amniotic sac. The inner layer of the amniotic sac is called the amnion. The cavity within the amnion becomes filled with amniotic fluid, eventually surrounding the entire embryo and behaving as a shock absorber. This protects the embryo from sudden temperature fluctuations, trauma, and sudden sharp movements that cause shock. Interestingly, nonidentical twins are likely to develop within two separate amniotic sacks.
The outermost extra embryonic membrane is called the chorion, surrounding both the amnion and the developing embryo, which is now called a fetus. The chorionic membrane forms finger-like projections called chorionic villi. Together, these villi and part of the mother’s endometrium forms an organ called the placenta, which we can see a region of in the diagram here. Up until about 10 weeks into pregnancy, the cells of the endometrium are responsible for nourishing the developing embryo. After this point, the placenta takes over this role.
The placenta is a temporary organ that nourishes the fetus during its development. It’s connected to the fetus through the umbilical cord. By looking more closely at the umbilical cord, we can see that it contains three blood vessels: two umbilical arteries and one large umbilical vein. The umbilical arteries transport deoxygenated blood from the fetus to the placenta. This deoxygenated blood contains carbon dioxide and other waste products that will enter the mother’s circulatory system so it can eventually leave her body. The umbilical vein, on the other hand, transports oxygenated blood from the placenta to the fetus.
As we can see in the diagram, the fetal blood vessels and the mother’s blood vessels never actually come into direct contact with each other. Instead, the mother’s blood pools into a region called the intervillous space in the placenta. There, the mother’s blood can exchange gases and nutrients with the fetal blood vessels by diffusion across the chorionic villi membranes, which provide a large surface area for the exchange of materials. It’s important to note that not only useful substances will diffuse from the mother’s blood into the fetus’s blood; toxic materials like alcohol, nicotine, and other harmful drugs can also cross the placenta and reach the developing fetus.
Aside from its role in nourishing the fetus, the placenta also secretes hormones that help to support and maintain the pregnancy. For example, the placenta secretes a hormone called human chorionic gonadotrophin, or HCG. HCG is partly responsible for maintaining the endometrial thickness throughout pregnancy. HCG also ensures that the corpus luteum, a temporary structure in the ovary, continues to secrete progesterone in early pregnancy. After the fourth month of pregnancy, the corpus luteum breaks down, and the placenta takes over the role of secreting progesterone for the remaining months of pregnancy.
Progesterone prevents the uterus from contracting too early and increases the blood flow to the uterus. The placenta also secretes estrogen, which increases vascularization, or the formation of blood vessels in the uterus, which helps support the developing fetus. Interestingly, identical twins usually share a single placenta, though two embryos are connected to the placenta through two separate umbilical cords. Nonidentical twins, however, usually have two separate placentas and two umbilical cords.
Let’s outline the different stages of fetal development during pregnancy. In humans, the gestation period is usually around nine months, which is about 40 weeks or 280 days long. Pregnancy is usually divided into three separate trimesters, during which the fetus gradually grows, and its organs and organ systems develop. The first trimester is the period from week one to week 12 of pregnancy. The fetus’s heart begins to develop as early as week three of pregnancy, and the heartbeats can be detected through ultrasound from around week five. The nervous system begins to develop, especially the spinal cord, as do the limbs, hands, eyes, and sex organs.
In a male fetus, the testes start to develop around week five to six, while in the female fetus the ovaries develop it around week 12. The skeletal system also begins to develop during the first trimester, and this process can continue well into adulthood up to the age of about 25.
The second trimester spans from week 13 to week 26 of pregnancy. The sense organs of the fetus begin to develop, and the fetus begins to experience some sensory stimulation. The nervous system becomes more advanced, and the first signs of movement are usually detected during this stage.
The third and final trimester from week 27 until birth is the longest one. During this trimester, extensive brain development occurs, and all the internal organ systems, such as the respiratory system that includes the lungs, become fully developed. At the end of this approximately nine-month gestation period, the fetus is ready to be born.
Let’s summarize what we’ve learned about fertilization and pregnancy by reviewing the key points. Male and female gametes fuse in fertilization to form a zygote and then an embryo. The embryo then implants into the uterus, where it develops into a fetus, which is nourished by a structure called the placenta. The organs and organ systems of the fetus can then develop over the nine months of pregnancy until birth.
