Pathways of pollen development
Pollen is produced within the anthers (microsporangia or pollen sacs) of the flower. During its development from an undifferentiated mound of cells (anther primordium) the anther forms two general groups of cells. The reproductive or sporogenous cells give rise to the microspores and are formed from cells located centrally within the developing anther. The non-reproductive cells form discrete anther tissues layers and include the epidermal, cortical and tapetal cell layers surrounding the sporogenous cells. The tapetum which is the innermost layer of the pollen sac plays a dominant role particularly during the microspore stage. For example, many male sterile mutations affect tapetal cell functions and development is often arrested during the microspore stage.
Microsporogenesis and microgametogenesis
Two distinct and successive developmental phases, microsporogenesis and microgametogenesis, lead to the production of the mature microgametophytes.
Microsporogenesis comprises the events which lead to the formation of the haploid unicellular microspores. During microsporogenesis the diploid sporogenous cells differentiate as microsporocytes (pollen mother cells or meiocytes) which divide by meiosis to form four haploid microspores. Each diploid meiocyte gives rise to a tetrad of four haploid microspores and microsporogenesis is complete with the formation of distinct single-celled haploid microspores.
Microgametogenesis comprises events which lead to the progressive development of the unicellular microspores into mature microgametophytes containing the gametes. This phase begins with the expansion of the microspore which is commonly associated with the formation of a single large vacuole. Vacuolation is accompanied by the displacement of the microspore nucleus to an eccentric position against the microspore wall. In this position the nucleus undergoes first pollen mitosis (pollen mitosis I) which results in the formation of two unequal cells, a large vegetative cell and a small generative cell each containing a haploid nucleus. The generative cell subsequently detaches from the pollen grain wall and is engulfed by the vegetative cell forming a unique 'cell within a cell' structure. The engulfed generative cell divides once more by mitosis (pollen mitosis II) to form the two sperm cells completely enclosed within the vegetative cell cytoplasm either before pollen is shed (tricellular pollen) or within the pollen tube (bicellular pollen).