THE NUCLEAR RETICULUM IN PLACENTAL CELLS OF LILIUM-REGALE IS A PART OF THE ENDOMEMBRANE SYSTEM

Placental cells line the ovarian transmitting tract in Lilium regale a nd produce exudates for secretion. Sections through the highly lobed n uclei of these cells reveal the presence of membrane profiles which fo rm vesicles with varying dimensions in cross section. Computer reconst ruction of the nucleus reveals that the vesicle profiles form a comple x reticulum of tubular cisternae, which spans the whole nucleus, enclo sing a maze of continuous lumen space. Connections between the vesicle s and the inner nuclear envelope are visible at various points along t he nuclear envelope. This complex network of tubules which constitutes the reticulum arises from the inner nuclear membrane. The nuclear ret iculum dramatically increases the inner-envelope surface area, compris ing 82% of the total membrane perimeter of inner nuclear envelope and nuclear reticulum. The inner nuclear envelope invaginates into the nuc leus forming the nuclear reticulum and the outer nuclear envelope evag inates into the endoplasmic reticulum (ER). indicating that there is a continuity between the lumens of the nuclear reticulum and the ER. Th e nuclear reticulum is labelled with zinc iodide-osmium tetroxide, a s taining pattern identical to that seen in the EP. Positive reaction to the zinc iodide-osmium tetroxide indicates that the nuclear reticulum is a site for Ca2+ deposition. The nuclear reticulum forms an extensi on of the endomembrane system which reaches deep into the nucleoplasm. The lumenal continuity of this system means that there is a channel f or communication from the cytoplasm into the nucleoplasm, and that thi s channel sequesters calcium.