Immunolocalization of Cystathionine β-Synthase, Cystathionine γ-Lyase, Heme Oxygenase-2 and Nitric Oxide Synthase in the Human Fetal Retina in Different Gestational Trimesters

The endogenous gaseous transmitters (GTs) —nitrogen oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H2S)—make up a special neuromodulatory system which mediates the development, maturation and plastic modification of nervous centers. We addressed immunolocalization of the key enzymes of GT synthesis, cystathionine β-synthase (CBS), cystathionine γ-lyase (CSE), heme oxygenase-2 (HO-2) and constitutive NO synthase (nNOS), at different ontogenetic stages of the human fetal retina. CBS, CSE and HO-2 were found to be expressed in photoreceptor, bipolar and amacrine neurons, the number of which increases in the first and decreases in the third trimesters of gestation. The number of nNOS-immunopositive amacrine and ganglion neurons demonstrates inverse dynamics with maximal values in the third trimester. Uneven distribution patterns of the marker enzymes are discussed in the light of the modulatory function of GTs in neurogenesis of the human retina and their involvement in cytoprotective mechanisms.