Carnegie Stage 1a - DREM Database

IN VITRO STAGE 1a SPECIMENS (click here to view all figures)

In the primordial embryo the plasma membranes of the fertilizing gametes become confluent (Fig. 4). Non-fertilizing sperm may be present in the zona pellucida or in the subzonal space. The cytoplasm of the embryo protrudes where the fertilizing sperm enters. The protrusion is referred to as the fertilization cone (Fig. 3). It is devoid of microvilli and has numerous microfilaments forming a conspicuous band beneath the cell membrane. It later retracts supposedly drawing the sperm deep into the cytoplasm.

Once inside the cytoplasm the original envelope of the sperm vesiculates and is gradually dismantled. The head of the fertilizing sperm becomes decondensed and swells. Pronuclei are not yet present but a wave of granularity moves throughout the cytoplasm in 2 to 10 circular rotations. Some of the rotations are clockwise and some are counter clockwise. The movement lasts for twenty or more minutes and is referred to as the cortical reaction. The reaction involves the exocytosis of the contents of cortical granules. The membrane of the granules fuses with the overlying cell membrane then burst open releasing their contents into the subzonal space by a process similar to cell secretion (Figs. 13-15). Cortical granule release occurs three hours after sperm / oocyte fusion. Once the cortical granule contents are released into the subzonal space they interact with the inner part of the zona pellucida thus preventing further entrance of sperm into the oocyte and inhibiting polyspermia. The interaction is called the zonal reaction. (Fig. 16)

The second miotic division is completed at this time with the release of the second polar body into the subzonal space. Since the cytoplasm contains the genetic material from both the mother and father it is now technically an embryo (Figs. 1, 2, 3f, 7-12).

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