DSE-1BT: Developmental aspects of embryo

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Compiled and circulated by Dr. Parimal Dua, Assistant Professor, Dept. of Physiology, Narajole Raj college

Page | 1 DSE-1BT: Developmental aspects of embryo

DSE-1BT: Developmental aspects of embryo

Organogenesis:

 Organogenesis in human development

Organogenesis is the process by which three germ layers turn into the internal organs of animals. The outer layer is called ectoderm; the middle layer, the mesoderm; and the inner layer, the endoderm.

Organogenesis is the process by which the three germ tissue layers of the embryo, which are the ectoderm, endoderm, and mesoderm, develop into the internal organs of the organism. Organs form from the germ layers through the differentiation: the process by which a less-specialized cell becomes a more- specialized cell type. During organogenesis, the three germ layers of the embryo differentiate and further specialize to form the various organs of the body.

 Ectoderm will produce the epidermis (skin) and nervous system of the

adult.

 Mesoderm will produce many internal organs of the adult such as the muscles, spine and circulatory system.

 Endoderm will produce the digestive system and other internal organs of the adult.

 Development of eye as an example of reciprocal and repeated inductive events

Another feature of induction is the reciprocal nature of many inductive interactions. Once the lens has formed, it can then induce other tissues. One of these responding tissues is the optic vesicle itself. Now the inducer becomes the induced. Under the influence of factors secreted by the lens, the optic vesicle becomes the optic cup, and the wall of the optic cup

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differentiates into two layers, the pigmented retina and the neural retina. Such interactions are called reciprocal inductions.

At the same time, the lens is also inducing the ectoderm above it to become the cornea. Like the lens-forming ectoderm, the cornea-forming ectoderm has achieved a particular competence to respond to inductive signals, in this case the signals from the lens. Under the influence of the lens, the corneal ectodermal cells become columnar and secrete multiple layers of collagen.

Mesenchymal cells from the neural crest use this collagen matrix to enter the area and secrete a set of proteins (including the enzyme hyaluronidase) that further differentiate the cornea. A third signal, the hormone thyroxine, dehydrates the tissue and makes it transparent. Thus, there are sequential inductive events, and multiple causes for each induction.

Figure: Schematic diagram of the induction of the mouse lens. (A) At embryonic day 9, the optic vesicle

extends toward the surface ectoderm from the forebrain. The lens placode (the prospective lens) appears as a local thickening of the surface ectoderm near the optic vesicle. (B) By the middle of day 9, the lens placode has enlarged and the optic vesicle has formed an optic cup. (C) By the middle of day 10, the central portion of the lens-forming ectoderm invaginates, while the two layers of the retina become distinguished. (D) By the middle of day 11, the lens vesicle has formed, and by day 13 (E), the lens consists of anterior cuboidal epithelial cells and elongating posterior fiber cells. The cornea develops in front of the lens.

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(F) Summary of some of the inductive interactions during eye development.