How does Development take place in Animals

The developmental pattern of a fertilized egg into an embryo is almost identical in all animal forms at least in the initial stages. Following fertilization, the zygote undergoes a series of divisions that leads from a single cell to a collection of cells in the form of a hollow sphere, known as blastula. The cells increase in number very rapidly at the same time the size of the cells decreases. During this period some of the blastula cells begin to differentiate into endoderm, mesoderms and ectoderm.

The endoderm generally gives rise to epithelial lining of the gut, the mesoderm forms the muscles, internal skeleton; and the ectoderm develops into nerves and the outer covering of the animal. During this stage there is the rearrangement in the position of the cellular layers by a process known as gastrulation. After gastrulation the endoderm becomes the innermost layer of cells and the mesoderm surrounds the endoderm and the outermost layer is the ectoderm. Endoderm can further specialize into liver, pancreas, lung or many other cell types, but cannot reverse course and become ectoderm or mesoderm. This stage of embryo is known as gastrula. There is no significant growth in size between zygote and gastrula. All these stages are so important that all vertebrates, in spite of their great anatomical and physiological differences, follow this developmental pattern.

Once the gastrulation has taken place with the rearrangement of different layers, the cell differentiation starts rapidly. The three layers of cells differentiate and develop into various organs needed to make a functional individual. Just three weeks after fertilization, human embryos will develop a heart and by the eighth week of development the head is completely identifiable in an embryo with a 2.5 cm length. Almost all types of tissues and organs start developing by this time. By the twelfth week of development it develops external recognizable parts such as sex organs, fingers, nails, and toes. A gut also develops from the endoderm during this period.

How does an animal develop from a single cell? The answer to this fundamental question of embryology and developmental biology is based on asymmetry in the egg cell and instructions in the DNA of the developing animal. Structures form in the developing embryo under the guidance of the DNA instructions that are the same in each cell, and external cues that let the cell know where it is and what type of cell it should become. Signals include information from neighbor-neighbor contact and from gradients of protein or small-molecule morphogens.

Tags : Bio Technology, Bio Genetics, Animal Development

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