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MILITA ROY BEJOY NARAYAN MAHAVIDYALAYA The six stages of vertebrate development Fertilization Cleavage Gastrulation Neurulation Neural crest formation Organogenesis Cleavage in a frog embryo Cross section of a frog blastula Gastrulation - The point in embryogenesis where the basic organization of the organism is established. A. Establishment of the three basic germ layers Ectoderm, Mesoderm, Endoderm B. Groups of cells that will give rise to specific organ systems and tissues are moved into the right position both externally and internally. C. Groups of cells that will influence each others differentiation are positioned so they can have an effect on each other. How is the characteristic body plan for any organism developed? Gastrulation: the first step in the process of body formation. It transforms a complex sphere into 3 basic germ layers from which all other tissues develop. Ectoderm is the outer layer = forms epidermis and nervous system. Mesoderm is in the middle and forms a many structures (i.e., heart, muscles). Endoderm is the inner layer and forms the ‘gut’ and related organs. The first change is to generate the rudiment of the digestive tract, hence the name gastrulation (gastric = stomach). 1. How does gastrulation occur ? 2. What are the molecular and cellular mechanisms that cause movement and rearrangement? Gastrulation is the first step of morphogenesis Morphogenesis is the process whereby individual cells undergo complex movements that generate the organ rudiments. Gastrulation generates the three basic germ layers from which organs arise. How do sheets of cells (epithelia) move during gastrulation? 5 methods. Invagination is the local inward movement of cells from a cavity Involution is similar, but more dramatic. It is an inward expansion of epithelial cells around an edge such as the blastpore. Convergent extension is elongation of an epithelium in one direction while it shortens in the other direction (stretching taffy). The cells can keep their relative positions and elongate or they can interdigitate interdigitate.. Epiboly is spreading movement of an epithelium to a deeper or thinner layer. Delamination is the splitting of one layer into 2 different layers. Different combinations of these basic movements yield a variety of changes that characterize gastrulation How do individual cells move during gastrulation? Seven basic types of cell movement lead to the changes in epithelial sheets that characterize gastrulation. 1. Migration is the movement of an individual cell over other cells or a substrate. 2. Intercalation is wedging of cells between their neighbors. Lateral intercalation involves lateral movements of cells in the same layer between one another = convergent extension. Radial intercalation involves wedging of 2 different layers. This process often leads to epiboly, the surface area of the epithelium increases while the thickness decreases. 3. Ingression is the movement of individual cells from an epithelium into an embryonic cavity. 4. Shape changes are coordinated changes in cell shape that cause an epithelium to invaginate, buckle or undergo convergent extension. 5. Cell division without growth increases the total number of ‘building blocks’ 6. Changes in adhesiveness: cells adhere to one another and to extracellular material via different types of specialized attachments. Loss of these attachments can facilitate cell movement. Gastrulation in Amphibians Amphibian gastrulation is much more complex than sea urchins, due in part to the large amount of yolk. There are also more layers in the blastula Different areas of the gastrula show distinct behavior: Animal cap: about 3 layers deep and derived from the animal hemisphere of the egg. It expands by epiboly until it covers about half of the surface of the gastrula. Non involuting marginal zone: a wide belt of cells 4 to 5 layers deep. It expands by convergent extension to cover the lower part of the gastrula Involuting marginal zone: involutes during gastrulation and comes to lie in the interior. The limit of involution reaches the edge of the blastopore by the end of gastrulation. The superficial layer of the involuting zone forms the roof of the archenteron (the lining of the future gut). The deep layer of the involuting zone forms the future trunk mesoderm (muscles of the body) Deep zone: a ring of cells that migrate towards the animal pole. Deep zone cells become head mesoderm and heart. Vegetal base: formed by the large yolky cells in the vegetal hemisphere. During gastrulation this base is tilted and displaced ventrally. It will form the floor of the archenteron (gut). Ectoderm = animal cap and non involuting marginal zone Mesoderm = deep zone and deep layer of involuting marginal zone Endoderm = vegetal cells and superficial layer of involuting marginal zone Gastrulation in the frog