404 
Monitoring Stem Cell Research 
Fig 1: Reprogramming in normal development and nuclear 
cloning. 
a. The genome of primordial germ cells (PGCs) is 
hypomethylated (“reset”, white boxes). Reprogramming and 
establishment of parent specific epigenetic marks occurs over the 
course of gametogenesis so that the genome of sperm and egg are 
competent to express the genes that need to be activated in early 
embryonic (box with wavy lines) and later (hatched box) 
development. During cleavage and early postimplantation 
development “embryonic” genes, such as Oct 3/4, become activated 
(black box) and are repressed at later stages (stippled boxes) when 
tissue specific genes (hatched boxes) are activated in adult tissues 
(A, B, C). Epigenetic reprogramming of imprinted and non-imprinted 
genes occurs during gametogenesis in contrast to X inactivation and 
the readjustment of telomere length which take place 
postzygotically. 
Fis lA Reprogramming in Nmtal Development 
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PRE -PUBLICATION VERSION 
