Carnegie Stage 1b - DREM Database

IN VITRO STAGE 1b SPECMENS (click here to view all figures)

The unique feature of pronuclear embryos is the presence of pronuclei which appear within 11 hours of in vitro insemination. The pronuclei each measure approximately 30 µm in diameter before fixation. The paternal (male) and maternal (female) pronuclei usually form simultaneously (Figs. 11, 12). The paternal pronucleus is usually larger and forms near the site where the sperm entered (fertilization cone); the maternal pronucleus forms near the site where the second polar body is extruded.

After their formation the pronuclei are located a distance from each other but subsequently each moves toward the center of the cell (Figs. 2, 3). By 15 hours after in vitro insemination they lie close to one another (Fig. 6). They remain closely associated until the onset of syngamy (See below). As they approach one another, adjacent areas of each pronuclear envelope appear to flatten out. At the same time nucleoli move from random locations within each pronucleus to line up in the flattened pronuclear envelope regions (Figs. 6, 7). Nucleoli vary in size and number from as few as one to as many as nine. The envelope of the maternal pronucleus often begins to break down before the paternal one. The envelopes of the pronuclei do not fuse or interlock.

A straight line can be drawn from the polar bodies through the cytoplasm that divides the cell into equal parts. This line is called the polar axis (or anterior-posterior polar axis) and roughly indicates the location of the first cleavage plane. The pronuclei rotate into the polar axis and align along it (Figs. 4, 5). This is considered the correct position for syngamy and the first cleavage.

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