Abstract
Knowledge about the mechanism that establishes embryonic polarity is fundamental in understanding mammalian development. In re-addressing several controversial claims, we recently proposed a model in which mouse embryonic polarity is not specified until the blastocyst stage. Before fertilization, the fully differentiated oocyte has been characterized as “polarized,” and we indeed observed that the sperm preferentially enters the polar body half. Here we show that preferential sperm entry is not due to an intrinsic polarity of the oocyte, since fertilization takes place uniformly when the zona pellucida is removed. We suggest that the term “asymmetry” denotes morphological differences, whereas “polarity” in addition implies developmental consequences. Thus, the mouse oocyte can be considered “asymmetric” but “non-polarized.” The penetration through the zona pellucida is also random, and a significant proportion of sperm binds to the oocyte membrane at a point distant from the zona penetration site. Time-lapse recordings confirmed that sperm swim around the perivitelline space before fertilization. Experimental enlargement of the perivitelline space in the non-polar body half increased the regional probability of fertilization. Based on these experiments, we propose a model in which the space asymmetry exerted by the first polar body and the zona pellucida directs sperm entry preferentially to the polar body half, with no need for oocyte polarity.
Highlights
The notion of prepatterning in the mammalian egg has long been controversial [1,2]
The regulative capacity of the preimplantation embryos has suggested the absence of predetermination [3,4,5,6], studies in the last decade have claimed its presence in the mouse egg [7,8,9,10]
We recently showed that the first cleavage plane is not predetermined [11], and there is compelling evidence for the absence of embryonic polarity specification until the blastocyst stage [12,13,14,15,16]
Summary
Space Asymmetry Directs Preferential Sperm Entry in the Absence of Polarity in the Mouse Oocyte. The fully differentiated oocyte has been characterized as ‘‘polarized,’’ and we observed that the sperm preferentially enters the polar body half. We show that preferential sperm entry is not due to an intrinsic polarity of the oocyte, since fertilization takes place uniformly when the zona pellucida is removed. Experimental enlargement of the perivitelline space in the non-polar body half increased the regional probability of fertilization. Based on these experiments, we propose a model in which the space asymmetry exerted by the first polar body and the zona pellucida directs sperm entry preferentially to the polar body half, with no need for oocyte polarity
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