During this week, the following developments take place:. By week six after fertilization, the embryo measures about 8 mm 0. During the sixth week, some of the developments that occur include:. By week seven, the embryo measures about 13 mm 0.
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. During this week, some of the developments that occur include:. The embryonic stage is a critical period of development. 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.
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. 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.
Besides the placenta, the chorion , yolk sac, and amnion also form around or near the developing embryo in the uterus. 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. A 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.
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.
Defining the Embryonic Stage 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. Embryonic Development 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.
Gastrulation 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.
Digestive, endocrine, and adrenal cortex glands. 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. Organogenesis In addition to neurulation, gastrulation is followed by organogenesis , when organs develop within the newly formed germ layers. Other Developments in the Embryo Several other major developments that occur during the embryonic stage are summarized chronologically below, starting with the fifth week after fertilization.
Week Five By week five after fertilization, the embryo measures about 4 mm 0. During this week, the following developments take place: Grooves called pharyngeal arches form. These will develop into the face and neck. The inner ears begin to form. Arm buds are visible. The liver, pancreas, spleen, and gallbladder start to form. Week Six By week six after fertilization, the embryo measures about 8 mm 0.
During the sixth week, some of the developments that occur include: The eyes and nose start to develop. Leg buds form and the hands form as flat paddles at the ends of the arms. The precursors of the kidneys begin to form. The stomach starts to develop. Week Seven By week seven, the embryo measures about 13 mm 0. During this week, some of the developments that take place include: The lungs begin to form. This video of a living Xenopus frog embryo shows both gastrulation and neurulation.
You should recognize the beginning of the film from our discussion of gastrulation. The open neural plate on the dorsal side has formed by the time the blastopore closes. The closure of the neural plate into a tube is accompanied by elongation of the embryo.
Animal development: Organogenesis. Organogeneis is the period of animal development during which the embryo is becoming a fully functional organism capable of independent survivial.
Organogenesis is the process by which specific organs and structures are formed , and involves both cell movements and cell differentiation. Organogenesis requires interactions between different tissues. These are often reciprocal interactions between epithelial sheets and mesenchymal cells. The study of organogenesis is important not only because of its relevance to understanding fundamental mechanisms of animal development, but also because it may lead to medical applications , such as the repair and replacement of tissues affected by genetic disorders, disease or injury.
The metanephros is the permanent kidney found mammals and in birds and reptiles , and forms at the region between the mesonephros and the cloaca below. Balinsky's figure of mesonephric and pronephric anatomy from Peter Vize. The development of the adult kidney metanephros provides a good example of reciprocal epithelial-mesenchyme interactions. Mature metanephric kidneys form from reciprocal inductions between the metanephric mesenchyme and the epithelial ureteric buds.
The metanephric mesenchyme forms the nephrons, which are the functional units of the kidneys, and the epithelial ureteric buds form the collecting ducts and ureter. Metanephric kidney development is a multistep process. Mesenchyme cells induces the ureteric bud to elongate and branch.
The ureteric bud induces mesenchyme to aggregate transition from mesenchyme to epithelium. Each aggregate forms a nephron: first a comma shape is observed, and then the S-shaped tubule, which connects to the branched ureteric bud. What is the experimental evidence for reciprocal induction? The metanephric mesenchyme doesn't condense into epithelial cells if cultured in isolation, but does if it is cultured with ureteric bud tissue.
The ureteric bud doesn't branch if cultured in isolation, but does in combination with mesenchymal cells. Similar experiments using a filter to separate the tissues showed that these inductions only work if cell processes can extend through the filter and directly contact the responding cells.
Vertebrate limbs develop from limb buds. The vertebrate limb bud consists of a core of l oose mesenchymal mesoderm covered by an epithelial ectodermal layer.
Cells within the progress zone rapidly divide, and differentiation only occurs once cells have left the progress zone. Because of this process, differentiation proceeds distally as the limb extends that is, the proximal end of the limb develops before the distal end. The apical ectodermal ridge at tip of limb bud induces the formation of the progress zone. Pattern formation organizes cell types into their proper locations based on positional information.
Anterior-posterior patterning is regulated by the zone of polarizing activity, or ZPA. The current model is that proximal-distal pattern formation is regulated by the amount of time a cell spends in the progress zone. Dorsal-ventral patterning is controlled by the overlying ectoderm. What makes forelimbs and hindlimbs different from one another? Pattern formation is regulated by the same signals in both limbs, although these signals are interpreted differently.
In this case, the right aorta will have to arch across from the esophagus, causing difficulty breathing or swallowing. In the placenta, chorionic villi develop to maximize surface-area contact with the maternal blood for nutrient and gas exchange.
Chorionic villi sprout from the chorion after their rapid proliferation in order to give a maximum area of contact with the maternal blood. These villi invade and destroy the uterine decidua while at the same time they absorb nutritive materials from it to support the growth of the embryo. Chorionic artery : An image showing the chorionic villi and the maternal vessels.
During the primary stage the end of fourth week , the chorionic villi are small, nonvascular, and contain only the trophoblast. During the secondary stage the fifth week , the villi increase in size and ramify, while the mesoderm grows into them; at this point the villi contain trophoblast and mesoderm. During the tertiary stage fifth to sixth week , the branches of the umbilical vessels grow into the mesoderm; in this way, the chorionic villi are vascularized.
At this point, the villi contain trophoblast, mesoderm, and blood vessels. Embryonic blood is carried to the villi by the branches of the umbilical arteries. After circulating through the capillaries of the villi, it is returned to the embryo by the umbilical veins. Chorionic villi are vital in pregnancy from a histomorphologic perspective and are, by definition, products of conception. The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium.
The placenta functions as a fetomaternal organ with two components: the fetal placenta chorion frondosum , which develops from the same blastocyst that forms the fetus; and the maternal placenta decidua basalis , which develops from the maternal uterine tissue. The outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This layer is divided into two further layers: the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer.
The latter is a multinucleated, continuous cell layer that covers the surface of the placenta. It forms as a result of the differentiation and fusion of the underlying cytotrophoblast cells, a process that continues throughout placental development. The syncytiotrophoblast otherwise known as syncytium thereby contributes to the barrier function of the placenta.
Placenta : Image illustrating the placenta and chorionic villi. The umbilical cord is seen connected to the fetus and the placenta. Privacy Policy. Skip to main content. Human Development and Pregnancy. Search for:. Third Week of Development. Gastrulation During gastrulation, the embryo develops three germ layers endoderm, mesoderm, and ectoderm that differentiate into distinct tissues.
Learning Objectives Describe gastrulation and germ-layer formation. Key Takeaways Key Points Gastrulation takes place after cleavage and the formation of the blastula.
Formation of the primitive streak is the beginning of gastrulation. It is followed by organogenesis—when individual organs develop within the newly-formed germ layers. The ectoderm layer will give rise to neural tissue, as well as the epidermis.
The mesoderm develops into somites that differentiate into skeletal and muscle tissues, the notochord, blood vessels, dermis, and connective tissues. The endoderm gives rise to the epithelium of the digestive and respiratory systems and the organs associated with the digestive system, such as the liver and pancreas. Key Terms somite : One of the paired masses of mesoderm, distributed along the sides of the neural tube, that will eventually become dermis, skeletal muscle, or vertebrae.
Neurulation Following gastrulation, the neurulation process develops the neural tube in the ectoderm, above the notochord of the mesoderm. Learning Objectives Outline the process of neurulation. Key Takeaways Key Points The notochord stimulates neurulation in the ectoderm after its development.
The neuronal cells running along the back of the embryo form the neural plate, which folds outward to become a groove. During primary neurulation, the folds of the groove fuse to form the neural tube. The anterior portion of the tube forms the basal plate, the posterior portion forms the alar plate, and the center forms the neural canal.
The ends of the neural tube close at the conclusion of the fourth week of gestation. Key Terms basal plate : In the developing nervous system, this is the region of the neural tube ventral to the sulcus limitans. It extends from the rostral mesencephalon to the end of the spinal cord and contains primarily motor neurons. The caudal part later becomes the sensory axon part of the spinal cord.
Clinical Example Spina bifida is a developmental congenital disorder caused by the incomplete closing of the neural tube during neurulation. Somite Development Somites develop from the paraxial mesoderm and participate in the facilitation of multiple developmental processes.
Learning Objectives Describe the functions of somites. Key Takeaways Key Points The paraxial mesoderm is distinct from the mesoderm found more internally in the embryo. Alongside the neural tube, the mesoderm develops distinct paired structures called somites that develop into dermis, skeletal muscle, and vertebrae. Each somite has four compartments: the sclerotome, myotome, dermatome, and the syndetome. Each becomes a specific tissue during development. Key Terms neural crest cells : A transient, multipotent, migratory cell population that gives rise to a diverse cell lineage including melanocytes, craniofacial cartilage, bone, smooth muscle, peripheral and enteric neurons, and glia.
Development of the Cardiovascular System The circulatory system develops initially via vasculogenesis, with the arterial and venous systems developing from distinct embryonic areas. Learning Objectives Outline the development of the cardiovascular system. Key Takeaways Key Points The aortic arches are a series of six, paired, embryological vascular structures that give rise to several major arteries.
The first and second arches disappear early. The third arch becomes the carotid artery.
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