The fourth left arch constitutes the arch of the aorta between the origin of the left carotid artery and the termination of the ductus arteriosus. The fifth arch disappears on both sides. The proximal part of the sixth right arch persists as the proximal part of the right pulmonary artery, while the distal section degenerates. The sixth left arch gives off the left pulmonary artery and forms the ductus arteriosus.
This duct remains during fetal life, but closes within the first few days after birth due to increased O 2 concentration. This causes the production of bradykinin which causes the ductus to constrict, occluding all flow. Within one to three months, the ductus is obliterated and becomes the ligamentum arteriosum. The dorsal aortae are initially bilateral and then fuse to form the definitive dorsal aorta.
Approximately 30 posterolateral branches arise off the aorta and will form the intercostal arteries, upper and lower extremity arteries, lumbar arteries, and the lateral sacral arteries. The lateral branches of the aorta form the definitive renal, suprarenal, and gonadal arteries. Finally, the ventral branches of the aorta consist of the vitelline arteries and umbilical arteries. The vitelline arteries form the celiac, and superior and inferior mesenteric arteries of the gastrointestinal tract.
After birth, the umbilical arteries will form the internal iliac arteries. The human venous system develops mainly from the vitelline, umbilical, and cardinal veins, all of which empty into the sinus venosus. The venous system arises during the fourth to eighth weeks of human development. Most defects of the great arteries arise as a result of the persistence of aortic arches that normally should regress or due to the regression of arches that normally should not.
A double aortic arch occurs with the development of an abnormal right aortic arch, in addition to the left aortic arch, forming a vascular ring around the trachea and esophagus, which usually causes difficulty breathing and swallowing. Occasionally, the entire right dorsal aorta abnormally persists and the left dorsal aorta regresses. 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 gastrulation occurs after the cleavage. After the formation of the gastrula, the embryo enters the gastrula and begins the organogenesis process. The newly formed cells of the three germ layers will combine and develop into organs. The cells of each germ layer can develop into specific organs and tissues.
The ectoderm develops into the epidermis, neural crest, and tissue that later develops into the nervous system. Mesoderm cells are located between ectodermal cells and endoderm cells and can develop into somite, muscles, and cartilage belonging to ribs and vertebrae. In addition, the mesoderm can also develop into the dermis, spinal cord, blood vessels and blood, bone, and connective tissue.
Endoderm cells develop into the epithelium of the digestive system and respiratory system, such as the liver and pancreas. After the endocytic formation process, the cells of the body are organized into a group surrounded by cells associated with the epithelium. The molecular mechanisms of gastrulation and the time required are different in different organisms. However, gastrulation between different organisms still has something in common, such as: first, the topology of the embryo changes, from a single connected surface like a spherical surface to a non-single connected surface like a ring surface.
Second, embryonic cells will differentiate into ectodermal cells, mesoderm cells, and endoderm cells some lower organisms without mesodermal cells. Third, endoderm cells will have digestive function. In addition, although the specific patterns of animal gastrulation are very different, in general, during the process of gastrulation, the movement of cells can be summarized into five types: invagination, involution, ingression, delamination, and epiboly.
In the preparation stage of gastrulation, the embryo forms a proximal-distal axis and an anterior-posterior axis, and asymmetry also occurs with the occurrence of these two axes. The formation of the embryonic egg cylinder marks the formation of the far and near axis: the proximal extraembryonic tissue forms the placenta-like tissue.
Signal transduction pathways mediated by signaling molecules such as bone molding protein BMP , fibroblast growth factor FGF , Nodal, and Wnt are involved in this process. The visceral endoderm surrounds the epidermis. The distal visceral endoderm DVE migrates to the anterior part of the embryo, forming the anterior visceral endoderm AVE , breaking the symmetry before and after. The above process is regulated by the Nodal signaling pathway. The original strip is produced in the initial stage of gastrulation.
The original strip is located at the junction between the posterior extraembryonic tissue and the epidermis and the region where the internal migration occurs. The formation of the original streak is closely related to the Nodal signaling pathway in cells in the coriolis region and the BMP4 signaling pathway activated by extra-embryonic tissues. Cer1 and Lefty1 can limit the formation of primitive streaks to specific regions by antagonizing the Nodal signaling pathway.
The original strip area after formation will continue to grow distally. Many scientists are trying to compare the embryonic experiments with in vitro experiments to further study the process of gastrulation. This video show the surface of a Xenopus embryo surface during gastrulation. Early on, the dorsal lip of the blastopore forms due to the contraction of bottle cells see below.
The blastopore continues to develop from the early "frown" until it can be observed as a complete circular ring of involuting cells. Convergent extension closes the blastopore at the yolk plug and elongates the embryo along the anterior--posterior axis. The posterior end of the embryo is pointed at you. How does the the blastopore lip form? A small group of cells change shape, narrowing at the exterior edge of the blastula. This change in cell shape, called apical constriction, creates a local invagination, which pushes more interior cells upwards and begins to roll a sheet of cells towards the interior.
The constricted cells are called bottle cells , due to their shape like an upside down bottle in these images. During gastrulation in birds and mammals, epiblast cells converge at the midline and ingress at the primitive streak. Ingression of these cells results in formation of the mesoderm and replacement of some of the hypoblast cells to produce the definitive endoderm.
As gastrulation proceeds, the primitive groove extends anteriorly. A cross-section through the embryo allows us to observe the three germ layers that form during gastrulation: ectoderm , mesoderm , and endoderm.
Show below are images of human embryos during gastrulation,13 - 19 days post ovulation. Notice the primitive streak, which is analogous to the blastopore of Xenopus. Neurulation in vertebrates results in the formation of the neural tube , which gives rise to both the spinal cord and the brain.
Neural crest cells are also created during neurulation. Neural crest cells migrate away from the neural tube and give rise to a variety of cell types, including pigment cells and neurons.
Neurulation begins with the formation of a neural plate , a thickening of the ectoderm caused when cuboidal epithelial cells become columnar. Changes in cell shape and cell adhesion cause the edges of the plate fold and rise, meeting in the midline to form a tube. The cells at the tips of the neural folds come to lie between the neural tube and the overlying epidermis.
These cells become the neural crest cells. Both epidermis and neural plate are capable of giving rise to neural crest cells. What regulates the proper location and formation of the neural tube? The notochord is necessary in order to induce neural plate formation. Below are scanning electron micrographs of a chick embryo during neurulation.
During neurulation, somites form in pairs flanking the neural tube. Somites are blocks of cells that form a segmental pattern in the vertebrate embryo. Somites produce cells that become vertebrae, ribs, muscles, and skin. The region where neural tube closure begins varies between different classes of vertebrates. In amphibians such as Xenopus , the neural tube closes almost simultaneously along its entire length.
In birds , the neural tube closes in the anterior to posterior direction, as Hensen's node regresses. Mammalian neurulation is similar to that of birds, however the bulky anterior neural plate seems to resist closure - the middle of the tube closes first, followed by both ends. Watch this animation of mammalian neurulation!
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