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. Defining the Embryonic StageAfter 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 embryoEmbryonic DevelopmentStarting 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. GastrulationLate in the second week after fertilization,gastrulationoccurs 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. Table \(\PageIndex{1}\): The germ layers and what they give rise to
NeurulationFigure \(\PageIndex{4}\): 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. OrganogenesisIn 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. Other Developments in the EmbryoSeveral other major developments that occur during the embryonic stage are summarized chronologically below, starting with the fifth week after fertilization. Week FiveBy 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:
Week SixBy 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:
Week SevenBy week seven, the embryo measures about 13 mm (0.51 in.) in length. During this week, some of the developments that take place include:
Week EightBy 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:
Genetic and Environmental Risks to Embryonic DevelopmentThe 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:
Extraembryonic StructuresSeveral 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. PlacentaThe 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. Chorion, Yolk Sac, and AmnionBesides 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 \(\PageIndex{5}\).
Feature: My Human BodyAssume 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. Review
Explore Morebio.libretexts.org/link?17805#Explore_More Learn more about spina bifida here: Attributions
During which stage of prenatal development do the heart spine and brain emerge?Week 6: The neural tube closes
Just four weeks after conception, the neural tube along your baby's back is closing. The baby's brain and spinal cord will develop from the neural tube. The heart and other organs also are starting to form. Structures necessary to the formation of the eyes and ears develop.
At what stage of fetal development does the heart begin to develop?The development of the heart begins as early as the third week of gestation with the 4-chamber fetal heart formed by gestational week 7.
What are the 3 phases of prenatal development?Normal prenatal development lasts about 38 weeks and is divided into three stages: germinal, embryonic, and fetal.
What happens during the germinal stage?The germinal stage of pregnancy starts at the time of conception, when the sperm and the egg combine to form a zygote. During the germinal stage, the zygote begins to divide in order to implant into the uterine wall. Once implantation is complete, the embryonic stage begins.
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