Microsporangia

A microsporangium (plural microsporangia) is a sporangium that produces spores that give rise to male gametophytes. Microsporangia are notable in spikemosses, and a minority of ferns. In gymnosperms and angiosperms (flowering plants), the microsporangia produce the microsporocyte, also known as the microspore mother cell, which then creates four microspores through meiosis. The microspores divide through mitosis to create pollen grains. The term is only applicable to plants that exhibit heterospory and is not used for bryophytes.

A very young anther consists of actively dividing meristematic cells surrounded by a layer of epidermis. It then becomes two-lobed. Each anther lobe develops two pollen sacs. Thus, a two-lobed anther develops four pollen sacs situated 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 centre consist of:

...
Wikipedia