The zona pellucida surrounds all mammalian oocytes and plays a vital role at fertilization and in early development. The genes that code for two of the mouse zona proteins (ZP2 and ZP3) represent a developmentally regulated set of genes whose expression serves as markers of mouse oocyte growth and differentiation. We previously characterized the single-copy Zp-3 gene and showed that its expression is oocyte specific and restricted to a narrow window of oocyte development. We now define the Zp-2 gene transcript and show that it is coordinately expressed with Zp-3 only during the 2-week growth phase of oogenesis that occurs prior to ovulation. Like Zp-3, the expression of Zp-2 is restricted to oocytes, and, although not detectable in resting oocytes, both ZP2 and ZP3 transcripts accumulate to become very abundant messengers in 50-,um-diameter oocytes. Ovulated eggs contain ZP2 and ZP3 transcripts which are 200 nucleotides shorter than those found in growing oocytes and have an abundance of less than 5% of the peak levels. In an attempt to understand the molecular details associated with the developmentally regulated, tissue-specific gene expression of the zona genes, the Zp-2 genetic locus has been characterized and its 5' flanking sequences have been compared with those of Zp-3. Both genes contain three short (8-to 12-base-pair) DNA sequences of 80 to 88% identity located within 250 base pairs of their transcription start sites.At birth the mouse ovary contains 10,000 to 15,000 primordial oocytes (29), the vast majority of which are in the prophase of the first meiotic division. During the reproductive life of the female, cohorts of these resting oocytes (10 to 15 p.m in diameter) enter into a 2-week growth phase which culminates in meiotic maturation and subsequent ovulation (3, 11). Virtually nothing is known about the signals that induce this growth and differentiation, but one potential marker of these phenomena is the expression of the zona pellucida genes.The zona pellucida surrounds all mammalian oocytes; in the mouse it is composed of three sulfated glycoproteins, ZP1, ZP2, and ZP3 (7,45). The mouse zona proteins are coordinately synthesized in the growing oocyte (8,45) and are secreted to form a filamentous zona matrix (39) in which ZP2 and ZP3 complex into copolymers cross-linked by ZP1 (24). At fertilization, sperm initially bind to ZP3 via 0-linked oligosaccharide side chains (9, 10, 21). Following the induction of the sperm acrosome reaction on the surface of the zona, ZP2 acts as a secondary sperm receptor that is necessary for the maintenance of sperm binding to the egg (6). ZP2 is proteolytically cleaved after fertilization (5, 36), and this modification, along with presumed changes in ZP3, are postulated to play an important role in the postfertilization block to polyspermy.The genes that code for the mouse zona pellucida represent a remarkable set of developmentally regulated, oocytespecific genes which specifiy products vital to fertilization and early mammalian development. We have previously ...