ANAT 1502 Embryo Formation

ANAT 1502 Embryo Formation - Ch01.qxd 17/9/04 10:50 AM Page...

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The 1st week A fertilized ovum has a diploid number of chromosomes and once the second meiotic division has been completed, the stage of cleavage can begin. This consists of a series of rapid mitotic cell divisions in which the ovum divides over a period of about 3 days resulting in the so-called 16-cell-stage embryo (Figs 1.1, 1.2A). Each cell is known as a blastomere . After each cleavage division, whilst the number of cells increases, the size of each cell diminishes. The solid sphere of cells that forms is known as a morula , because it was thought to resemble a mulberry! Each of these new daughter cells is, at this stage, pluripotential. In other words, each daughter cell has the potential to differentiate into cells of any lineage. 1 Chapter 1 How does an embryo form? The 1st week 1 The 2nd week 2 Further development of the embryo 3 Folding of the embryo 7 By de±nition, an embryo comprises the tissues formed once mitosis of an ovum (a fertilized oocyte) begins, thus even at the two-cell stage it is an embryo. These few cells multiply in number over an 8-week period into a fetus, by which time it will consist of many millions of cells. The differentiation that establishes the organ systems takes place in the ±rst 8 weeks following fertilization (the embryonic period ). The period of time from the end of week 8 to full term (38 weeks) is a phase of growth and enlargement (the fetal period ). The crucial phase during which there is potential for malformation is in the ±rst 8 weeks, and this period is when the embryo is most vulnerable to environmental agents such as viruses and other teratogens. Table 1.1 summarizes the major events of the prenatal stages of development. The stages of the formation of an embryo are often described in relation to weeks of development. The stages leading up to fertilization (including gametogenesis and the histology of the uterus at the time of implantation), however, are beyond the scope of this book. Table 1.1 Stages of development before birth Time period Stage Main events 1st week Cleavage Fertilized ovum undergoes mitosis. Formation of morula; appearance of blastocyst; blastocyst implanted 2nd to 8th week Embryonic period Germ layers and placenta develop. Main body systems form 9th week to birth Fetal period Further growth and development of organs. Locomotor system becomes functional A B C D E F G H I Fig. 1.1 Stages of pre-embryonic development during the first week. ( A ) = ovulated oocyte; ( B ) = fertilization; ( C ) = stage of pronuclei formation; ( D ) = first cleavage spindle; ( E–G ) = cleavage of zygote; ( H ) = morula; ( I ) = blastocyst formation. Ch01.qxd 17/9/04 10:50 AM Page 1
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The morula soon shows signs of further differentiation. Cavities appear within the centre of the sphere of cells,
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This note was uploaded on 11/21/2011 for the course ANAT 1502 taught by Professor Johnh.romfh during the Spring '10 term at Life Chiropractic College West.

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ANAT 1502 Embryo Formation - Ch01.qxd 17/9/04 10:50 AM Page...

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