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  Organogenesis and Formation of Germ Layers, Biology tutorial

Orgauogenesis in animal:

At some point after different genn layers are stated, organogenesis starts. First stage in vertebrates is known as neurulation, where neural plate folds forming neural tube. Other common organs or structures that arise at this time comprise heart and somites, but from now on embryogenesis follows no common pattern among different taxa of animal kingdom. In most animals organogenesis along with moiphogenesis will result in the larva. The hatching of the larva, which must then undergo metamorphosis, marks the end of embryonic development.

Product of Three Germ Layer:

The proceeding graph below represents the products produced by the three germ layers. Endoderm produces tissue inside lungs, thyroid, and pancreas. Mesoderm helps in production of cardiac muscle, smooth muscle, skeletal muscle, tissues inside kidneys, and red blood cells. Ectoderm generates tissues inside epidermis and helps in formation of neurons inside brain, and melanocytes.

Formation of the germ layers:

Embryonic disk becomes oval and then pear-shaped, wider end being directed forward. Near the narrow, posterior end, the opaque streak, known as primitive streak, makes its appearance and extends along middle of the disk for approx one-half of its length; at anterior end of the streak there is knob-like thickening termed Hensen's knot. The shallow groove, primitive groove, seems on surface of streak, and anterior end of groove communicates through aperture, the blastopore, with the yolk sac. Primitive streak is generated by the thickening of axial part of ectoderm, cells of which multiply, grow downward, and blend with those of subjacent entoderm. From sides of the primitive streak a third layer of cells, the mesoderm, extends lateralward between ectoderm and entoderm; caudal end of the primitive streak forms the cloacal membrane. Blastoderm now comprises of three layers, named from without inward: ectoderm, mesoderm, and entoderm; each has distinctive features and gives rise to certain tissues of body. For several mammals, it is at time during formation of germ layers that implantation of embryo in the uterus of mother takes place.

Formation of early nervous system - neural groove, tube and notochord:

In front of primitive streak, two longitudinal ridges, caused by the folding up of ectoderm, make their appearance, one on either side of middle line formed by streak. These are named neural folds; they begin some little distance behind anterior end of embryonic disk, where they are continuous with each other, and from there slowly extend backward, one on either side of anterior end of primitive streak. Between the folds is the shallow median groove, neural groove. Groove slowly deepens as neural folds become elevated, and eventually folds meet and unite in middle line and convert groove in closed tube, the neural tube or canal, ectodermal wall of which forms the rudiment of nervous system. After coalescence of neural folds over anterior end of primitive streak, the blastopore no longer opens on surface but in the closed canal of neural tube, and therefore transitory communication, neurenteric canal, is established between neural tube and primitive digestive tube. Coalescence of neural folds happens first in region of the hind brain, and from there extends forward and backward; toward the end of the third week, the front opening (anterior neuropore) of tube lastly closes at anterior end of the future brain, and creates a recess that is in contact, for a time, with overlying ectoderm; the hinder part of neural groove presents for the time a rhomboidal shape, and to this expanded segment the term sinus rhomboidalis has been applied. Previous to neural groove is closed, ridge of ectodermal cells appears along the major margin of each neural fold; this is termed neural crest or ganglion ridge, and from it the spinal and cranial nerve ganglia and ganglia of sympathetic nervous system are developed.

Formation of early septum:

The extension of mesoderm occurs throughout the whole of embryonic and extraembryonic areas of ovum, except in certain regions. One of these is observed immediately in front of neural tube. Mesoderm extends forward in form of two crescentic masses that meet in middle line so as to enclose behind them an area that is devoid of mesoderm. Over this area, ectoderm and entoderm come in direct contact with each other and comprise thin membrane, buccopharyngeal membrane that forms septum between primitive mouth and pharynx.

Early formation of heart and other primitive structures:

In front of buccopharyngeal area, where lateral crescents of mesoderm fuse in the middle line, pericardium is later developed, and this area is thus designated pericardial area. The second region where mesoderm is absent, at least for a time, is that immediately in front of pericardial area. This is termed the proamniotic area, and is the region where the proamnion is developed; in humans, though, a proamnion is actually never formed. The third region is at hind end of embryo where ectoderm and entoderm come in apposition and form Cloacal membrane.

Somitogenesis:

Somitogenesis is the procedure by which somites are generated. These segmented tissue blocks distinguish in skeletal muscle, vertebrae, and dermis of all vertebrates. Somitogenesis starts with formation of somitomeres (whorls of concentric mesoderm) marking future somites in presomitic mesoderm (unsegmented paraxial). Presomitic mesoderm gives rise to successive pairs of somites, identical in appearance and which distinguish in same cell types but structures formed by the cells differ depending on anteroposterior (like thoracic vertebrates have ribs, lumbar vertebrates do not). Somites have exclusive positional values along axis and it is thought that these are stated by Hox (homeotic) genes.

Toward the end of second week after fertilization, transverse segmentation of paraxial mesoderm starts, and it is converted in series of well-defined, more or less cubical masses, also called as primitive segments that occupy complete length of trunk on either side of middle line from occipital region of the head. Every segment has a central cavity (called as myocoel) that, though, is soon filled with angular and spindle-shaped cells. Primitive segments lie immediately under ectoderm on lateral aspect of neural tube and notochord, and are joined to lateral mesoderm by intermediate cell mass. Those of the trunk may be arranged in the given groups, that is: cervical 8, thoracic 12, lumbar 5, sacral 5, and coccygeal from 5 to 8. Those of occipital region of the head are generally described as being four in number. In mammals, primitive segments of head can be identified only in occipital region, but the study of lower vertebrates leads to idea that they are present also in anterior part of the head and that, in total nine segments are signified in cephalic region.

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