The movements and fusion of the pronuclei at fertilization of the sea urchin Lytechinus variegatus: Time‐lapse video microscopy

Abstract

The penetration of the sperm into the egg, and the movements of the male and female pronuclei were followed from sperm attachment through pronuclear fusion, using time‐lapse video microscopy of gametes and zygotes of the sea urchin Lytechinus variegatus (23° C). The pronuclei move in four stages: I. Sperm Entry Phase, following sperm‐egg fusion and a rapid radiating surface contraction (5.9 ± 1.3 μm/second) when egg microvilli engulf the sperm head, midpiece, and tail to form the fertilization cone and the sperm tail beats in the egg cytoplasm; II. Formation of the Sperm Aster, which pushes the male pronucleus centripetally at a rate of 4.9 ± 1.7 μm/minute starting 4.4 ± 0.5 minutes after sperm‐egg fusion, as the male pronucleus undergoes chromatin decondensation; III. Movement of the Female Pronucleus, the greatest and fastest of the pronuclear motions at a rate of 14.6 ± 3.5 μm/minute at 6.8 ± 1.2 minute after sperm‐egg fusion, which establishes the contact between the pronuclei; and IV. Centration of the Pronuclei to the egg center at a rate of 2.6 ± 0.9 μm/minute by 14.1 ± 2.6 minutes after sperm‐egg fusion. Pronuclear fusion typically occurs after stage IV and proceeds rapidly starting 14.7 ± 3.6 minutes after sperm‐egg fusion with the male pronucleus coalescing into the female pronucleus at a rate of 14.2 ± 2.6 μm/minute.