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Spermatozoa of the Shrew, Suncus murinus, Undergo the Acrosome Reaction and Then Selectively Kill Cells in Penetrating the Cumulus Oophorus¹

Takayuki Mri^a

^a Laboratory of Zoology, Graduate School of Agriculture, Kyushu University, Fukuoka 812-8581, Japan ^b Department of Obstetrics and Gynecology, Weill Medical College of Cornell University, New York, New York 10021

ABSTRACT

In the musk shrew, Suncus murinus (and other shrews), the cumulus oophorus is ovulated as a discrete, compact, matrix-free ball of cells linked by specialized junctions. In examining how they penetrate the cumulus, Suncus spermatozoa were observed to first bind consistently by the ventral face over the acrosomal region to the exposed smooth surface of a peripheral cumulus cell. This was apparently followed by point fusions between the plasma and outer acrosomal membranes. Thereafter, spermatozoa without acrosomes were observed within cumulus cells that displayed signs of necrosis, as did some radially neighboring cumulus cells linked by zona adherens and gap junctions. Eventually, penetration of spermatozoa as far as the perizonal space around the zona pellucida left linear tracks of locally necrotic cells flanked by normal cumulus cells. Based on these and previous observations, we conclude that the acrosome reaction in Suncus is always induced by cumulus cells, and that reacted spermatozoa penetrate the cumulus by selective invasion and killing of cumulus cells along a linear track. Loss of the acrosome also exposes an apical body/perforatorium that is covered with barbs that appear to assist reacted fertilizing spermatozoa in binding to the zona pellucida. Because fertilized eggs displayed no other spermatozoa within or bound to the zona, an efficient block to polyspermy must prevent such binding of additional spermatozoa.

FOOTNOTES

First decision: 30 January 2001.

¹ This work was supported by a grant from the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists.

² Correspondence: Takayuki Mri, Laboratory of Zoology, Graduate School of Agriculture, Kyushu University, Higashiku Hakozaki 6-10-1, Fukuoka 812-8581, Japan. FAX: 092 642 2804; tmohri{at}agr.kyushu-u.ac.jp

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