Cyclic AMP- responsive enhancer- binding protein (CREB) expressed in germ cells of the testis. 
Shown is a photomicrograph of a cross-section of two seminiferous tubules at different develop- 
mental stages, VII (left) and V ( right). The intense orange-brown structures are the nuclei of the 
round haploid spermatids stained with an antiserum to CREB. The relative acellular region is the 
interstitial space between the two tubules lined with the blue nuclei of the spermatogonia and 
spermatocytes, the cellular precursors of the spermatids. The round spermatids will differentiate 
into maturing spermatozoa, which are the linear blue structures adjacent to the spermatids (left ) 
or juxtaposed between columns of spermatids ( right). CREB is believed to activate the transcrip- 
tion of genes involved in spermatogenesis, under the regulation of cAMP-dependent protein ki- 
nase. Magnification, X 1,500. 
Data of Gerard Waeber, Heather Hermann, and foel F. Habener. 
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