Embryonic alkaline phosphatase is expressed at M-phase in the spermatogenic lineage of the mouse

We have recently cloned and characterized a novel embryonic alkaline phosphatase (EAP) expressed at the two-cell to blastocyst stage of preimplantation development in the mouse. The isozyme is re-expressed in trace amounts in the thymus, intestine and testis during adult life. In the present report, we find that EAP transcripts can be detected, by RT-PCR analysis, in very low amounts in the testes of newborn mice, but at 24 days of age EAP mRNA levels reach the highest concentrations, remaining high at 40 and even 117 days of age. We produced a synthetic peptide and a corresponding rabbit anti-peptide antiserum (Rb-1434), which was characterized by enzyme antigen immunoassays and reactivity with Chinese hamster ovary cell transfectants, as reacting specifically with EAP. The Rb-1434 antibody enabled us to examine immunohistochemically what cell types in the testis are responsible for the expression of EAP during different developmental stages. No positive cells were recognized in the testis of newborns (day 0) and 8-day-old mice. Positive cells were first observed at day 15 and, at 24 days of age, many positive M-phase cells, morphologically corresponding to spermatocytes in mid to late prophase of meiotic division I, were strongly positive for EAP expression. Positive M-phase cells were also observed at 40 days and 151 days of age. Transgenic mice expressing the human GCAP isozyme in a tissue-specific manner in the testis, showed equivalent stages of M-phase figures when stained immunohistochemically with a specific rabbit polyclonal antiserum. Our results demonstrate that EAP is expressed in M-phase spermatogenic cells during postnatal development, suggesting a role for this AP isozyme during meiosis.