Studies on the Endoplasmic Reticulum

Abstract

Cells of onion and garlic root tips were examined under the electron and phase contrast microscopes after fixation in KMnO4. Special attention was focused on the distribution and behavior of the endoplasmic reticulum (ER) during the several phases of mitosis.Slender profiles, recognized as sections through thin lamellar units of the ER (most prominent in KMnOa-fixed material), are distributed more or less uniformly in the cytoplasm of interphase cells and show occasional continuity with the nuclear envelope. In late prophase the nuclear envelope breaks down and its remnants plus cytoplasmic elements of the ER, which are morphologically identical, surround the spindle in a zone from which mitochondria, etc., are excluded. During metaphase these ER elements persist and concentrate as two separate systems in the polar caps or zones of the spindle. At about this same time they begin to proliferate and to invade the ends of the spindle. The invading lamellar units form drape-like partitions between the anaphase chromosomes. In late anaphase, their advancing margins reach the middle zone of the spindle and begin to fray out. Finally, in telophase, while elements of the ER in the poles of the spindle coalesce around the chromosomes to form the new envelope, the advancing edges of those in the middle zone reticulate at the level of the equator to form a close lattice of tubular elements. Within this, which is identified as the phragmoplast, the earliest signs of the cell plate appear in the form of small vesicles. These subsequently grow and fuse to complete the separation of the two protoplasts. Other morphological units apparently participating in mitosis are described.Speculation is provided on the equal division or not of the nuclear envelope and the contribution the envelope fragments make to the ER of the new cell.

Lamellae in the Spindle of Mitotic Cells of Walker 256 Carcinoma

“…Except for their double nature the spindle lamellae are similar to parts of the E R described by Porter and Machado (13) in plant cells, and these authors, whom I quoted earlier, suggested that this might be the case. The observations show that in Walker tumor cells in anaphase the spindle lamellae come into relation with the surface of the chromosomes and appear to be involved in the formation of the nuclear membrane.…”

“…Chentsov (5) has independently and very briefly referred to some equivalent "five-layered" structures in cells of an induced rhabdomyoblastoma. Recently, the finding of the "spindle filaments" in animal cells was briefly reviewed by Porter and Machado (13) who compared the cytoplasmic structures of mitotic animal and plant cells. A component of the spindle in the latter was found to be certain membranes of the endoplasmic reticulum (ER) which were suspended, like drapes, among the other parts of the spindle.…”

Mitotic Cell Studies Based on <i>in vivo</i> Observations

“…Cell plate formation Porter and Machado (1960) studied the mitosis of onion root tip cells under the electron microscope. They found vesicle-like particles of various sizes ("Phragmosomes") in the equatorial region of dividing cells in telophase.…”

“…Neither phragmosomes nor any kind of cytoplasmic elements visible in phragmoplast (Courtesy of Dr. Sato). 4, schematic diagram of phragmoplast and cell plate in onion root tip cell at mid-telophase treated with KMnO4 drawn after the electron micro graph of Porter and Machado (1960). Polysaccharide-micelles and fibrillar texture of phrag moplast destroyed by fixation.…”

“…In their report, Porter and Machado (1960) have briefly dealt with the con tributions by Inoue (1953) 3. Spindle unit fibrils Even in animal cell mitosis, Wilson (1928) pointed out the presence of the anastral type of mitosis, viz., spindles arising wholly from the nucleus , the latter being drawn out as a whole to form the spindle, without visible participation of the cytoplasm.…”