A zygote, also known as a fertilized ovum or fertilized egg, is the union of a sperm cell and an egg cell. The zygote begins as a single cell but divides rapidly in the days following fertilization. The zygote’s single cell contains all of the 46 necessary chromosomes, getting 23 from the sperm and 23 from the egg. The zygote phase is brief, lasting only about four days. Around the fifth day, the mass of cells becomes known as a blastocyst. The embryo develops from the blastocyst. In order for reproduction to take place, a single sperm cell must penetrate the outer surface of an egg in a process known as fertilization. During a healthy reproductive cycle, a single egg cell is released from the follicle into the fallopian tube at ovulation. If sperm are present, thousands will attempt to penetrate this single egg cell. Once a single sperm has broken through the outer surface, a zygote is formed. Chemical changes in the surface of the egg prevent other sperm from entering. Medically assisted fertilization is also possible and becoming increasingly common. Intrauterine insemination (IUI) and in vitro fertilization (IVF) are two frequently used assisted reproductive techniques. During IUI, semen is inserted into the uterus using a catheter with fertilization taking place inside the body. With IVF, eggs are removed from the ovaries and fertilized in a lab. The blastocyst is then implanted in the uterus. Zygotes divide through a process known as mitosis, in which each cell doubles (one cell becomes two, two becomes four, and so on). This two-week stage is known as the germinal period of development and covers the time of fertilization (also called conception) to the implantation of the blastocyst in the uterus. The sperm cell contains paternal genetic information while the egg cell contains maternal genetic information. Because each cell contains half of the genetic material, each cell is known as a haploid cell. When these two haploid cells join, they form a single diploid cell that contains all necessary chromosomes. The zygote then travels down the fallopian tube to the uterus. As it travels, its cells rapidly divide and it becomes a blastocyst. Once in the uterus, the blastocyst must implant in the lining in order to obtain the nourishment it needs to grow and survive. The embryonic period of development lasts from two weeks after conception through the eighth week, during which time the organism is known as an embryo. At the ninth week post-conception, the fetal period begins. From this point until birth, the organism is known as a fetus.
Conception occurs when an egg is fertilized, but pregnancy does not actually begin until a blastocyst implants into the uterus. It’s not usually possible to know whether fertilization has occurred at this early stage, considered week 3 of pregnancy. Symptoms and pregnancy hormone levels are usually not notable until week 4 or 5. Identical twins are monozygotic. With monozygotic twins, one egg is fertilized and one zygote is formed, but at the blastocyst phase, it splits to form two embryos. Monozygotic twins share the same genetic material. Fraternal twins, on the other hand, are dizygotic, which means that two eggs are fertilized resulting in two zygotes. Those two zygotes go on to develop into two embryos. Unlike monozygotic twins, dizygotic twins do not share identical genes. Not all zygotes make it to the next stage of prenatal development. Researchers estimate that 30% to 70% of all naturally occurring conceptions fail either before or at the time of implantation. Researchers suspect these losses are connected to abnormalities. In cases of recurrent miscarriage, a parental chromosomal anomaly is often to blame. In the case of these very early miscarriages, also known as chemical pregnancies, a person may not be aware that fertilization had occurred because they may experience bleeding similar to and around the time of their expected menstrual period. With the advent of early result home pregnancy tests, however, more people are able to detect chemical pregnancies as early as four or five days before their expected menstrual cycle. IUI and IVF can fail as well. Success rates for IUI range from 7% to 20%. Studies have linked poor semen parameters with IUI failure. Poor quality eggs and hormone deficiencies are other known reasons for IUI failure. IVF success rates vary by age, with the greatest chance of success in parents who are younger than 35. At the younger end of the spectrum, the chance of success with IVF is around 54% but those numbers diminish over time, with success rates at only 4% by age 43. Things that can influence IVF success or failure include a parent’s age, prior pregnancies and losses, the viability of the eggs, and the underlying cause of infertility.
In many cultures, marriage — along with birth and death — is considered the most pivotal life event. For pioneering developmental biologist Lewis Wolpert, however, these life events are overrated. According to Wolpert, "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Gastrulation is a major biological event that occurs early in the embryonic stage of human development. Figure \(\PageIndex{1}\): Wedding couple in Kandy Sri Lanka
After a blastocyst implants in the uterus around the end of the first week after fertilization, its internal cell mass, which was called the embryoblast, is now known as the embryo. The embryonic stage lasts through the eighth week following fertilization, after which the embryo is called a fetus. The embryonic stage is short, lasting only about seven weeks in total, but developments that occur during this stage bring about enormous changes in the embryo. During the embryonic stage, the embryo becomes not only bigger but also much more complex. Figure \(\PageIndex{2}\) shows an eight to nine week old embryo. The embryo's finger, toes, head, eyes, and other structures are visible. It is no exaggeration to say that the embryonic stage lays the necessary groundwork for all of the remaining stages of life. Figure \(\PageIndex{2}\): An eight to nine-week-old embryo
Starting in the second week after fertilization, the embryo starts to develop distinct cell layers, form the nervous system, make blood cells, and form many organs. By the end of the embryonic stage, most organs have started to form, although they will continue to develop and grow in the next stage (that of the fetus). As the embryo undergoes all of these changes, its cells continuously undergo mitosis, allowing the embryo to grow in size, as well as complexity. Figure \(\PageIndex{3}\): Blastula and Gastrula. The blastula is composed of one layer with a Blastocoel inside. Some cells of the outer layer fold into the Blastocoel to create a Blastopore. This invagination also gives rise to three germ layers. This diagram is color-coded. Ectoderm, blue. Endoderm, green. Blastocoel (the yolk sack), yellow. Archenteron (the gut), purple.
Late in the second week after fertilization, gastrulation occurs when a blastula, made up of one layer, folds inward and enlarges to create a gastrula. A gastrula has 3 germ layers--the ectoderm, the mesoderm, and the endoderm. Some of the ectoderm cells from the blastula collapse inward and form the endoderm. The final phase of gastrulation is the formation of the primitive gut that will eventually develop into the gastrointestinal tract. A tiny hole, called a blastopore, develops in one side of the embryo. The blastopore deepens and becomes the anus. The blastopore continues to tunnel through the embryo to the other side, where it forms an opening that will become the mouth. Whether this blastospore develops into a mouth or an anus determines whether the organism is a protostome or a deuterostome. With a functioning digestive tube, gastrulation is now complete. Each of the three germ layers of the embryo will eventually give rise to different cells, tissues, and organs that make up the entire organism, which is illustrated in Figure \(\PageIndex{4}\). For example, the inner layer (the endoderm) will eventually form cells of many internal glands and organs, including the lungs, intestines, thyroid, pancreas, and bladder. The middle layer (the mesoderm) will form cells of the heart, blood, bones, muscles, and kidneys. The outer layer (the ectoderm) will form cells of the epidermis, nervous system, eyes, inner ears, and many connective tissues.
Figure \(\PageIndex{4}\): Neural precursor cells fold and elongate to form the neural tube. Mesoderm cells condense to form a rod which will send out signals to redirect the ectoderm cells above. This fold along the neural tube sets up the vertebrate central nervous system. Following gastrulation, the next major development in the embryo is neurulation, which occurs during weeks three and four after fertilization. This is a process in which the embryo develops structures that will eventually become the nervous system. Neurulation is illustrated in Figure \(\PageIndex{4}\). It begins when a structure of differentiated cells called a neural plate forms from the ectoderm. The neural plate then starts to fold inward until its borders converge. The convergence of the neural plate borders also results in the formation of a neural tube. Most of the neural tube will eventually become the spinal cord. The neural tube also develops a bulge at one end, which will later become the brain.
In addition to neurulation, gastrulation is followed by organogenesis, when organs develop within the newly formed germ layers. Most organs start to develop during the third to eighth weeks following fertilization. They will continue to develop and grow during the following fetal period. The heart is the first functional organ to develop in the embryo. The primitive blood vessels start to develop in the mesoderm during the third week after fertilization. A couple of days later, the heart starts to form in the mesoderm when two endocardial tubes grow. The tubes migrate toward each other and fuse to form a single primitive heart tube. By about day 21 or 22, the tubular heart starts to beat and pump blood, even as it continues to develop. By day 23, the primitive heart has formed five distinct regions. These regions will develop into the chambers of the heart and the septa (walls) that separate them by the end of the eighth week after fertilization.
Several other major developments that occur during the embryonic stage are summarized chronologically below, starting with the fifth week after fertilization.
By week five after fertilization, the embryo measures about 4 mm (0.16 in.) in length and has begun to curve into a C shape. During this week, the following developments take place:
By week six after fertilization, the embryo measures about 8 mm (0.31 in.) in length. During the sixth week, some of the developments that occur include:
By week seven, the embryo measures about 13 mm (0.51 in.) in length. During this week, some of the developments that take place include:
By week eight — which is the final week of the embryonic stage — the embryo measures about 20 mm (0.79 in.) in length. During this week, some of the developments that occur include:
The embryonic stage is a critical period of development. Events that occur in the embryo lay the foundation for virtually all of the body’s different cells, tissues, organs, and organ systems. Genetic defects or harmful environmental exposures during this stage are likely to have devastating effects on the developing organism. They may cause the embryo to die and be spontaneously aborted (also called a miscarriage). If the embryo survives and goes on to develop and grow as a fetus, it is likely to have birth defects. Environmental exposures are known to have adverse effects on the embryo include:
Several structures form simultaneously with the embryo. These structures help the embryo grow and develop. These extraembryonic structures include the placenta, chorion, yolk sac, and amnion.
The placenta is a temporary organ that provides a connection between a developing embryo (and later the fetus) and the mother. It serves as a conduit from the maternal organism to the offspring for the transfer of nutrients, oxygen, antibodies, hormones, and other needed substances. It also passes waste products (such as urea and carbon dioxide) from the offspring to the mother’s blood for excretion from the body of the mother. Figure \(\PageIndex{5}\): The placenta is a lifeline that develops between the embryo and mother. It allows the transfer of substances between them. The amniotic cavity is surrounded by a membrane called the amnion, which forms as a sac around the developing embryo. The yolk sac nourishes the early embryo, and the chorion develops into the fetal portion of the placenta.The placenta starts to develop after the blastocyst has implanted in the uterine lining. The placenta consists of both maternal and fetal tissues. The maternal portion of the placenta develops from the endometrial tissues lining the uterus. The fetal portion develops from the trophoblast, which forms a fetal membrane called the chorion (described below). Finger-like villi from the chorion penetrate the endometrium. The villi begin to branch and develop blood vessels from the embryo. As shown in Figure \(\PageIndex{5}\), maternal blood flows into the spaces between the chorionic villi, allowing the exchange of substances between the fetal blood and the maternal blood without the two sources of blood actually intermixing. The embryo is joined to the fetal portion of the placenta by a narrow connecting stalk. This stalk develops into the umbilical cord, which contains two arteries and a vein. Blood from the fetus enters the placenta through the umbilical arteries, exchanges gases, and other substances with the mother’s blood, and travels back to the fetus through the umbilical vein.
Besides the placenta, the chorion, yolk sac, and amnion also form around or near the developing embryo in the uterus. Their early development in the bilaminar embryonic disc is illustrated in Figure \(\PageIndex{5}\).
Assume that you’ve been trying to conceive for many months and that you have just found out that you’re finally pregnant. You may be tempted to celebrate the good news with a champagne toast, but it’s not worth the risk. Alcohol can cross the placenta and enter the embryo’s (or fetus’s) blood. In essence, when a pregnant woman drinks alcohol, so does her unborn child. Alcohol in the embryo (or fetus) may cause many abnormalities in growth and development. Figure \(\PageIndex{6}\): Fetal Alcohol syndrome facial recognitionA child exposed to alcohol in utero may be born with a fetal alcohol spectrum disorder (FASD), the most severe of which is fetal alcohol syndrome (FAS). Signs and symptoms of FAS may include abnormal craniofacial appearance (Figure \(\PageIndex{6}\)), short height, low body weight, cognitive deficits, and behavioral problems, among others. The risk of FASDs and their severity if they occur depend on the amount and frequency of alcohol consumption, and also on the age of the embryo or fetus when the alcohol is consumed. Generally, greater consumption earlier in pregnancy is more detrimental. However, there is no known amount, frequency, or time at which drinking is known to be safe during pregnancy. The good news is that FASDs are completely preventable by abstaining from alcohol during pregnancy and while trying to conceive.
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