Figure 2. β-galactosidase expression pattern in UL1 during early embryogenesis. Successive panels are images of progressively older embryos obtained using a confocal microscope. Each panel is a summation of optical sections through a part (B,D,F,H,J,K,L,N) or through the whole (A,C,E,G,I,M) of an embryo. The left half of each panel shows the location of β-galactosidase and the position of the P- granules of the germ line cells (P4 in A-E, Z2 and Z3 in F-N). The right half of each panel shows the distribution of DNA in the same embryo. The stage of development of a stained embryo was determined by counting the number of nuclei present through the optical sections. Cells descended from a particular founder cell divide at the same time during early embryogenesis and this was used in cell identification. For example, in the 46-cell embryo of B the 4 descendants of the founder cell MS are dividing and the D founder cell is about to divide. The images are presented with anterior to the left but the direction of the dorsal-ventral axis varies between panels because the staining procedure meant that embryos could not be viewed in a particular orientation. (A) 43-cell embryo. One of the AB descendants has not yet divided in this round of AB lineage cell divisions. β-galactosidase can be detected in anterior AB descendants. (B) 46-cell embryo. Cells of the MS lineage are dividing and the D founder cell is about to divide. β-galactosidase is now at higher levels in 4 anterior AB descendants and detectable in additional AB descendants on the anterior-ventral and posterior- dorsal surfaces. (C) 51-cell embryo. The MS lineage and D founder cell have completed cell division. (D) 58-cell embryo. Cells of the AB lineage are in the process of dividing, some have just completed cell division, others are about to start. (E) 87-cell embryo. AB lineage cell divisions have finished and two patches of β-galactosidase containing cells are present, 20 cells anteriorly and 8 cells posteriorly. (F) 100-cell embryo. P4 has now divided to give Z2 and Z3. Da and Dp are about to divide and &
sup2;-galactosidase is detectable in these cells. (G) 103-cell embryo. Division in the D cell lineage (cells labelled D) is now complete and another round of cell division in the AB lineage has begun. (H) 185-cell embryo. β-galactosidase levels are now very high in the D lineage and beginning to drop in the AB lineage. The posterior group of AB cells expressing β-galactosidase have begun to move towards the anterior. (I) 189-cell embryo. Cell division is about to begin in both the D lineage and the AB lineage. (J) 220-cell embryo. β-galactosidase in the AB lineage has almost disappeared. The cell divisions in the D lineage are close to completion, generating 8 cells containing β-galactosidase. (K) 338-cell embryo. The round of cell division in the AB lineage is now complete. There are now 256 descendants of AB, none of which contain detectable levels of β-galactosidase. β-galactosidase is restricted to the 8 decendants of D. (L) 385-cell embryo. Another round of cell division in the D lineage has created 16 cells containing β-galactosidase, but at a lower level now. These cells have begun to move towards the anterior away from Z2 and Z3. (M) This embryo has at least 400 cells but elongation has not yet begun. β-galactosidase was not detected in any cells of embryos at this stage of development. (N) Elongation has begun and two cells, Z1 and Z4, begin to express β-galactosidase. The subcellular distribution of β-galactosidase was affected by cell division. In non- dividing cells the β-galactosidase was nuclear localized, possibly a consequence of the nuclear localization signal encoded in the vector part of pUL#24C7. In a cell beginning to divide the nuclear envelope breaks down and the β-galactosidase spreads throughout the cell, with the exception of the condensed chromosomes from which it appears to be excluded. (e.g. cells of the AB lineage in D and cells of the D lineage in F.) Immediately after cell division and reformation of the nuclear envelope the β-galactosidase remains throughout the cell with the concentration in the nucleus increasing as the level in the surrounding cytoplasm decreases. (e.g. cells of the AB lineage in E and cells of the D lineage in G.) Disappearance of β-galactosidase from the cytoplasm after cell division may depend on degradation of the enzyme or uptake into the nucleus.