CD9 is a member of the tetraspanin superfamily proteins and is the only protein on the mouse oocyte which is known to be indispensable in sperm–egg fusion. Here, using indirect immunofluorescence we show that CD9 appears on the oolemma during the early stages of the growth of the oocyte, when it measures 13–22 μm in diameter. When the oocyte reaches a diameter of 17–22 μm, the density of CD9 in its oolemma is similar to the density of this protein in the cell membrane of the fully grown secondary oocyte. The appearance of CD9 in growing oocytes correlates with the previously reported time of the acquisition of fusibility between the spermatozoon and the egg. Accordingly we propose that during oogenesis the development of the ability of the oolemma to fuse with sperm may be regulated by synthesis of CD9 by the oocyte.

Tetraspanins, also called tetraspans or the transmembrane 4 superfamily (TM4SF), are cell-surface proteins that span the membrane four times and are found on many different cell types in many organisms. They display numerous properties that indicate their physiological importance in cell adhesion, motility, activation and proliferation, as well as their contribution to pathological conditions such as metastasis or viral infection. A major characteristic of tetraspanins is their ability to form cell-surface complexes with other molecules participating in cell adhesion, either to the extracellular matrix (ECM) or to other cells, and with molecules required for signalling. It is not yet known how the structure of the complexes might affect the functions of other molecules or what basic biochemical mechanisms allow their formation and regulation. Nevertheless, an intriguing association between tetraspanin expression and metastatic potential indicates that these molecules may provide novel insights into tumour progression.

L6, IL-TMP, and TM4SF5 are cell surface proteins predicted to have four transmembrane domains. Previous sequence analysis led to their assignment as members of the tetraspanin superfamily. In this paper, we identify a new sequence (L6D) that is strikingly similar to L6, IL-TMP, and TM4SF5. Analyses of these four sequences indicate that they are not significantly related to genuine tetraspanins, but instead constitute their own L6 superfamily.