Embryonic Stem Cells and Embryology

What are embryonic stem cells?  derived from embryos that develop from eggs that have been fertilized in vitro  not derived from eggs fertilized in a woman's body  typically four or five days old and are a hollow microscopic ball of cells called the blastocyst.

 do not have a specific gene code to be an exact cell  have all of the genes that the other cells do, but their genes are activated for a primary purpose of mitosis without a specialized tissue function  have not undergone differentiation  daughter cells derived from these stem cells are capable of differentiating into almost any kind of cell

 in the laboratory, stem cells can be manipulated to take on characteristics of specific cells, such as heart cells, nerve cells or even pancreatic cells ( a process called directed differentiation).  in the laboratory, stem cells can be manipulated to take on characteristics of specific cells, such as heart cells, nerve cells or even pancreatic cells ( a process called directed differentiation).  during this process, scientists activate certain genes, and deactivate other genes.  during this process, scientists activate certain genes, and deactivate other genes.  as a result, the newly coded cells develop specific structures to perform certain functions.

Adult Stem Cells  Stem cells can also come from any body cell  Do not have to be embryonic  Cells are already differentiated

Embryonic Development  Cleavage: division of cells in the early embryo  Zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote.  The different cells derived from cleavage are called blastomeres and form a compact mass called the morula blastomeresmorulablastomeresmorula  Cleavage ends with the formation of the blastula (hollow ball of single layer of cells) blastula

Early Embryological Development Week 1  Following fertilization, the major events of the first week of development are cleavage, formation of the blastocyst, and the beginning of the implantation process. fertilizationcleavageblastocystimplantationfertilizationcleavageblastocystimplantation  Blastula is formed

Week 2  By the end of the second week, the blastocyst has completed implantation

Week 3  Gastrulation occurs (bilaminar embryo  trilaminar embryo) - one side of blastula pushes inward - three cell lineages that will eventually form every system ectoderm, mesoderm, and endoderm  This is the time of early tissue and organ differentiation of the nervous and cardiovascular systems, as well as the formation of future body cavities.

Embryonic Differentiation  Ectoderm nervous system, skin nervous system, skin  Mesoderm muscles, circulatory, skeletal, excretory, and reproductive systems muscles, circulatory, skeletal, excretory, and reproductive systems  Endoderm linings of digestive and respiratory tracts, liver, pancreas linings of digestive and respiratory tracts, liver, pancreas

Week 4  In week 4, the embryo undergoes major morphological changes as it changes from a trilaminar disk-shaped embryo to a cylindrical embryo.  This is also an important week in terms of determining placement of future organs.  Following median and horizontal folding, many organs and body cavities will begin to form or will be repositioned.