Spindle membranes in mitosis and meiosis of the heteropteran insect Dysdercus intermedius. A study of the interrelationship of spindle architecture and the kinetic organization of chromosomes.

The fate of cytoplasmic membranes has been traced from spermatogonial interphase to the telophase of the second meiotic division with the aid of osmium ferrocyanide staining. At prophase, the nuclear membrane is indented to a funnel shape due to microtubules which radiate from the poles. These indentations open at prometaphase admitting microtubules to the nuclear interior. Since the nuclear envelope widens at prometaphase with only few lateral defects, organelles are excluded and the spindle is essentially intranuclear. Membranes surround the spindle until late anaphase. There are no cytoplasmic vesicles within the spindle, but there is an abundance of vesicles around the spindle poles following the radiating microtubules. At telophase, small vesicles around the chromosomes contribute to the new nuclear envelope while others fuse into large blisters which disappear gradually. A new vesicle system is formed in the daughter cells. At meiosis, the nuclear membrane is not indented by the polar microtubules which follow the contour of the nucleus together with vesicles. At late diakinesis, the bivalents are individually ensheathed by vesicles and lamellae which separate them from the remainder of the nucleus. In building up the metaphase configuration, the chromosomal sheaths become interconnected with the X1/X2-pseudotetrad in the center. Lamellae from these sheaths extend to the poles following the kinetochore microtubule bundles. In contrast to mitosis, the spindle body is pervaded by a structural framework of membranes with mitochondria in between. Essentially the same spindle architecture exists in the second meiotic division with the exception of the tandemly joined X1 and X2 chromosomes which are ensheathed by endoplasmic reticulum but lie outside the autosome group as they form spindle connections to one pole only. Since the chromosomes form a holocentric group at mitosis but behave as individual telocentrics at meiosis, their ensheathment by membranes during meiosis is thought to be essential for their orientation and orderly disjunction in the absence of true kinetochore plates.