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Microsporangia are sporangia that produce microspores and give rise to male gametes. Microsporangia occur in all plants that have heterosporic life cycles, such as spike mosses. In gymnosperms and angiosperm anthers, the microsporangia produce the microsporocytes, the microspore mother cells, which then produce four microspores through the process of meiosis. In the microsporocyte of Arabidopsis thaliana, meiosis depends on the expression of genes that facilitate DNA repair and homologous recombination. The microspores divide by mitosis to produce pollen grains.
In angiosperms, a very young anther (the part of the stamen that contains the pollen) consists of actively dividing meristematic cells surrounded by a layer of epidermis. It then becomes two-lobed. Each anther lobe develops two pollen sacs. Then, a two-lobed anther develops four pollen sacs that situate at four corners of the anther. Development of pollen sacs begins with the differentiation of archesporial cells in the hypodermal region below epidermis at four corners of the young anther. The archesporial cells divide by periclinal division to give a subepidermal primary parietal layer and a primary sporogenous layer. The cells of the primary parietal layer divide by successive periclinal and anticlinal divisions to form concentric layers of pollen sac wall. The wall layers from periphery to center consist of:
- A single layer of epidermis between, which becomes stretched and shrivels off at maturity
- A single layer of endothecium. The cells of endothecium have fibrous thickenings.
- One to three middle layers. Cells of these layers generally disintegrate in the mature anther
- A single layer of tapetum. The tapetal cells may be uni-, bi- or multinucleate and possess dense cytoplasm. The cells of the primary sporogenous layer divide further and give rise to diploid sporogenous tissue.