Structure and morphogenesis of the secretory placental cells in Lilium

Abstract: Cells of the ovarian transmitting tissue of Lilium form a secretory epithelium on the placenta. These placental cells have been studied by electron microscopy and confocal scanning microscopy. They follow a basipetal order of maturity. Placental cells have a characteristic papillose shape and are typified as transfer cells with a prominent secretion zone. After initiation at anthesis day, the secretion zone continues to grow by addition of vesicles from the dictyosomes to reach maturity four days after anthesis. The epithelial cells are rich in organelles characteristic of secretory cells. Secretion commences before anthesis and the cuticle is sloughed off. The outermost layer of the domed cell wall is rugged and the surface shows signs of corrosion. The original central vacuole of the immature cells is replaced by a system of small vacuoles which are supplemented by autophagic vacuoles derived from the ER. The ER system undergoes a transition from agranular to granular form. Starch grains in the plastids are digested during early maturation but new ones accumulate six days after anthesis. Microtubules are organised as cortical and central arrays, which are distinct from one another. The cortical array undergoes a unique transition in the upper part of the cell which develops wall thickenings. The time and deposition of the cell wall closely coincides with the realignment of the microtubules. We propose that microtubules in the cortical array act as templates for the deposition of the cellulose microfibrils in the secondary cell wall and hence influence the pattern of wall thickenings. Microtubules comprising the central array make their appearance one day before anthesis, in the upper half of the cell, and grows into an intricate network extending throughout the cell. This array may be involved in trafficking of vesicles and organelles in these transfer cells. A unique network of membranes originating from the inner nuclear envelope is seen in the nucleus and has been termed the nuclear reticulum. I deduce these membranes to be an extension of the endomembrane system into the nucleus with a luminal continuity between the ER and the nuclear reticulum through the perinuclear space. These membranes sequester calcium and may be channels for communication between the cytoplasm and the nucleoplasm.