In this article we will discuss about:- 1. Morphology of Orbicules 2. Development of Orbicules 3. Functions.
Ubisch bodies were first discovered by Rosanoff (1865) in close association with the tapetum. Kosmath (1927) suggested that these bodies should be called “Ubisch granules” while Erdtman (1961) proposed the term “orbicules”.
Morphology of Orbicules:
Orbicules are small and generally spherical structures (2-5 pm) found in the anthers of many genera of angiosperms, both monocotyledons, e.g. Triticum aestivum and dicotyledons, e.g. Betula pendulaand many gymnosperms, e.g. Ephedra foliata.
They are often loosely attached to the extinct spores/pollen exine and also occur as free bodies in the palynological macerations The shape may however, vary in different species from spheroidal with spinulose surface, rod like, doughnut shape, oval or rounded triangular or plate like with undulating or jagged margin, etc. They frequently fuse into large compound aggregates as, in Euphorbia caputmedusae.
They generally have a central core and thick wall with micro-channels. It has been seen that when the anthers of Betula pendula were freeze in liquid nitrogen, fractured and examined with SEM, orbicules were connected together by filamentous structures. In addition they were also connected to the wall of the microspores.
Development of Orbicules:
The first sign of Ubisch body formation is the appearance of a number of medium electron dense bodies surrounded by a limiting membrane, in the tapetal cells. These are referred as pro-Ubisch bodies and in many respects resemble the sphaerosomes.
As anther development proceeds, the pro-Ubisch bodies increase in number. At the time of late tetrad formation, the pro-Ubisch bodies are surrounded by a zone of ribosomes which radiate from the bodies like the spokes of a wheel.
As recognizable exinous elements of the pollen grain wall are evident, profiles of endoplasmic reticulum come in close association with the pro-Ubisch bodies. There appears to be an open continuity between the lumen of the pro-Ubisch bodies and the cisternae of the endoplasmic reticulum. Thus both ribosomes and endoplasmic reticulum are involved in the synthesis and metabolism of sporopollenin precursors.
The pro-Ubisch bodies are extruded from the tapetal cytoplasm into the anther locules where they rapidly become invested with a layer of electron dense material, which has been chemically tested to be sporopollenin, which initially is thought to be in a low state of polymerization.
Due to increased polymerization of the sporopollenin the Ubisch bodies become increasingly more electron dense. The density and apparent rate of sporopollenin deposition continues parallel with the deposition of exinous material on the pollen grains. At the time of final dissolution of the tapetum, the Ubisch bodies and remaining pro-Ubisch bodies become intermingled with the pollen grains in the anther sac.
Functions of Orbicules:
1. A transport mechanism for the conveyance of sporopollenin between the tapetum and the developing microspores taking part in sporoderm formation.
2. Provide a non-wettable layer lining the anther sac from which the pollen grains are readily detached or they may be associated with pollen dispersal.
3. Temporary packaging of sensitive material for transport through the locular sap. The locular fluid probably contains exo-cellular enzymes against which the sporopollenin wall of the orbicules forms an effective barrier.
4. Orbicules have no specific function; they are a by product of tapetal cell metabolism.
5. Orbicules might actively participate in lysis and degradation of tapetal cells. In Lilium henryi\he orbicules get connected with the tapetal plasmalemma and then become lytically active.
6. Prevent osmosis and collapse of the developing microspores.