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K.V. Allebrandt, P. Layer; Acetylcholinesterase Expression During Differentiation and Death of Photoreceptors in the Chick Pineal Gland . Invest. Ophthalmol. Vis. Sci. 2006;47(13):4180.
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The avian pineal represents a transitional type between the photosensory organ of lower vertebrates and the endocrine gland of mammals, presenting neuroendocrine photoreceptor cells (PRCs). Pineal PRC peculiarly display acetylcholinesterase activity, although the pineal organ does not receive significant cholinergic input. Here, we investigated the regulated–expression of AChE in developing PRC.
AChE activity and the expression of the photopigment pinopsin were detected by histochemistry and immunohistochemistry, respectively. Genomic DNA fragmentation was revealed by TUNEL assay.
The onset of PRC differentiation was detected by E12. By this stage, AChE positive–cells start to populate the luminal surface of follicles, where PRC are located. AChE activity increases at the luminal surface of follicles throughout development, accompanying PRC differentiation. By E15, three PRC morphologies were identified under microscope, one of which has not been reported for post–hatching chick pineals. This embryonic type of PRC seems to display lamellar complexes. Indeed, a sensory regression of the avian pineal organ occurs from embryonic to post–hatching periods. Follicles are invaded by connective tissue initiating the loss of follicular structures, and a functional transition from a photosensory organ to a more endocrine gland takes place. A massive apoptotic activity in follicles was confirmed by TUNEL assay, indicating that PRC are dying. The degeneration of pineal PRC correlates with an increase of AChE–expression at the luminal surface of follicles.
Interestingly, in vitro studies have shown that AChE is essential for assembly of the apoptosome (Park et al., 2004), implicated in intrinsic apoptotic mechanisms. Here, we have shown that AChE activity accompanies PRC differentiation until death, being most concentrated in apoptotic areas of the pineal epithelium. Therefore, we suggest that AChE participates in the natural apoptotic process occurring during development. Reference:
Park, S.E.; Kim, N.D. and Yoo, Y.H. (2004) Acetylcholinesterase plays a pivotal role in apoptosome formation. Cancer Res, 64, 2652–5. Erratum in: (2004) Cancer Res, 64, 9230.
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