Background: The primary cilium is a microtubule-based organelle that extends transiently from the apical cell surface to act as a sensory antenna. Initially viewed as a cellular appendage of obscure significance, the primary cilium is now acknowledged as a key coordinator of signaling pathways during development and in tissue homeostasis. Objectives: The aim of this review was to present the structure and function of this overlooked organelle,with an emphasis on its epididymal context and contribution to male infertility issues. Materials and Methods: A systematic review has been performed in order to include main references relevant to the aforementioned topic. Results: Increasing evidence demonstrates that primary cilia dysfunctions are associated with impaired male reproductive system development and male infertility issues. Discussion: While a large amount of data exists regarding the role of primary cilia in most organs and tissues, few studies investigated the contribution of these organelles to male reproductive tract development and homeostasis. Conclusion: Functional studies of primary cilia constitute an emergent and exciting new area in reproductive biology research.

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Histone H5, an early marker of the avian erythroid lineage, is expressed at low levels in early erythroid precursors and at higher levels in more mature cells. We show that the increase in H5 expression is due to transcriptional activation of the H5 gene following differentiation of precursor CFU(E). We have found and characterized two upstream enhancers, El (between -2233 and -1878 from the site of transcription initiation, +1) and E3 (between -1321 and -1163), and confirmed the presence of a downstream enhancer (C. D.Trainor, S. J. Stamler, and J. D. Engel, Nature [London] 328:827-830, 1987) E7 (between +846 and + 1181) which are responsible for the increase in H5 gene transcription. The enhancers had a weak effect in nondifferentiated CFU(E) but a strong effect when the cells were induced to differentiate. Cooperation among the three enhancers, however, was not required for HS gene activity in the differentiated cells. The enhancers contain binding sites for several ubiquitous and erythroid cell-specific nuclear proteins, including GATA-1, as demonstrated with GATA-1-specific antibodies. Although the GATA sites were required for enhancer function, the concentration of GATA-1, GATA-2, and GATA-3 decreased during cell differentiation, and overexpression of these factors had little effect on H5 transcription. Hence, the differentiation-specific effect of the enhancers is not mediated by changes in relative levels of the GATA factors. Functional analysis of the H5 promoter indicated that the requirement of several elements, including a GC box necessary for transcription enhancement, did not change during the early stages of CFU(E) differentiation. However, the UPE, a positive element in proliferating CFU(E) recognized by the transcription factor H4TF2, was dispensable in the differentiated cells. These results suggest that as the cells enter the final stages of differentiation, there is a reprogramming of the regulatory factors that control H5 transcription and that the enhancers rescue and increase the activity of the promoter.Vertebrate erythropoiesis is characterized by the expression of sets of tissue-specific genes which are temporally regulated during maturation of the committed precursors. The chicken histone H5 gene is an interesting marker by which to study how these processes are regulated, because it responds both to early and late differentiation cues during erythropoiesis. Contrary to late erythroid markers, including the adult globin chains and cell surface proteins (2), H5 is already present in the early precursor CFU(E) (3, 45), and its expression increases further with the onset of the expression of these late genes (1). Transcription run-on analysis of erythroid cells at different stages of maturation have also indicated that the activity of the H5 gene is lower in proliferating erythroid precursors than in more mature cells (1). Thus, H5 expression during maturation is predominantly regulated at the transcription level.In an attempt to understand the molecular mechanisms accounting for the t...

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