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.
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. The fourth right arch forms the right subclavian artery, while the fourth left arch forms the arch of the aorta. The proximal part of the sixth right arch persists as the proximal right pulmonary artery.
The sixth left arch gives off the left pulmonary artery. Approximately 30 posterolateral branches arise off the dorsal aortae and will form the intercostal arteries, the upper and lower extremity arteries, the lumbar arteries, and the lateral sacral arteries. The ventral branches consist of the vitelline and umbilical arteries. The venous system develops from the vitelline veins, umbillical veins, and the cardinal veins, all of which empty into the sinus venosus. Key Terms sinus venosus : A large quadrangular cavity that precedes the atrium on the venous side of the chordate heart.
In humans, it exists distinctly only in the embryonic heart, where it is found between the two venae cavae. They are ventral to the dorsal aorta. In an anastomosis by anterior cardinal veins, the left brachiocephalic vein is produced. Clinical Example 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. Chorionic Villi and Placental Development In the placenta, chorionic villi develop to maximize surface-area contact with the maternal blood for nutrient and gas exchange.
Learning Objectives Summarize the development of the chorionic villi and placenta. Key Takeaways Key Points Chorionic 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. The villi begin primary development in the fourth week, becoming fully vascularized between the fifth and sixth weeks. Placental development begins with implantation of the blastocyst; this leads to its differentiation into several layers that allow nutrient, gas, and waste exchange to the developing embryo and fetus —as well as forming a protective barrier.
Gallus gallus domestic chicken is a major model system in embryology. It was one of the first organisms used for developmental research in the nineteenth century because the egg could be opened and the development of the embryo inside could be seen without the use of a powerful microscope. In the s, gastrulation of the chick was not well understood or documented. There were two suggested explanations of chick gastrulation. The first suggested that the mesoderm formed from the epiblast , the early stage totipotent layer of cells, and the mesoderm then differentiated into the endoderm.
The other suggestion was that the epiblast and endoderm developed together first, followed by the mesoderm. Rauber emphasized that the mesoderm initiates the ectoderm and endoderm to differentiate and that the blastoderm was essentially the canvas for gastrulation. The improvement of microscopes, staining methods, and microtomes helped those documents provide detailed descriptions of embryonic stages of chick development.
With the help of researchers such as Rauber, Haeckel, Hamburguer, and Hamilton, people now understand that chick gastrulation begins approximately seven to eight hours after fertilization. In the chick epiblast , a totipotent primordial cell layer, cells begin to rearrange at the posterior end. Those cells migrate inward to form the primitive streak , a midline thickening of the epiblast.
During that time, the epiblast is separated from the hypoblast, a deeper layer of cells in the blastoderm , by the blastocoel , a fluid filled cavity. Future endoderm cells are the first cells to pass through the primitive streak. Those cells displace the hypoblast cells moving them towards the anterior pole of the embryo.
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