embryo readings - Part 1: Cell Fate Determination pg. 1...

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pg. 1 Part 1: Cell Fate Determination
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pg. 2 Abridged and modified (by A. Schivell) from: Carlson (1996) Foundations of Embryology, 6 th ed, Ch. 1. Restriction and Determination Within the zygote lies the capability to form an entire organism. In many vertebrates the individual cells resulting from the first few divisions after fertilization retain this capability. The embryological term for these cells is totipotent . As development continues, most cells gradually lose the ability to form all the types of cells that are found in the adult body. It is as if they were funneled into progressively narrower channels. The reduction of the developmental options permitted to a cell is called cell fate restriction . Little is known about the mechanisms that bring about restriction, and the sequence and time course of restriction vary considerably from one species to another. An example representing a general pattern of restriction during development may serve to clarify the concept (Fig. 1-20).
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pg. 3 Shortly after fertilization the zygote undergoes a series of cell divisions, called cleavage . Early in cleavage the cells commonly remain totipotent. The period of cleavage comes to an end when certain cells in the embryo undertake extensive migrations and rearrange themselves into three primary germ layers during a process known as gastrulation . Named on the basis of their relative positions, the outermost layer is the ectoderm , the innermost is the endoderm , and between the two is the mesoderm . By this time at least one stage of restriction has usually occurred, so that the cells of the three germ layers are now locked into separate developmental channels and are no longer freely interchangeable – they are known as pluripotent . For example, the potential options open to the cells of the ectoderm are many, such as epidermis (skin) and nervous tissue, but not muscle or blood. In the next major developmental event, part of the ectoderm becomes thickened and is henceforth committed to forming the brain, the spinal cord, and other associated structures. This stage of development is commonly called neurulation . The remainder of the ectodermal cells can no longer form these structures and have thus undergone another phase of restriction. Soon, as a result of tissue interactions with the newly forming brain, groups of ectodermal cells become committed to forming the lens and inner ear, whereas the remainder of the ectoderm ultimately loses this capacity. Subsequent developmental events see the ectoderm further subdivided into groups of cells destined to form cornea; hair, scales, or feathers; cutaneous glands; or simply epidermis. When restriction has proceeded to the point at which a group of cells becomes committed to a single developmental fate (e.g., the formation of cornea), we say that determination of these cells has taken place. Thus, determination represents the final step in the process of restriction. The mechanisms that bring about determination of various groups of cells are receiving
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This note was uploaded on 06/03/2010 for the course BIOL BIO 101 taught by Professor Drumheller during the Spring '10 term at University of Washington.

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embryo readings - Part 1: Cell Fate Determination pg. 1...

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