Microsporogenesis and Microgametogenesis

The unicellular pollen grain represents the microspore of seed plants, the multicellular pollen grain the male gametophytic generation of seed plants and is source and transport unit for the male gametes (or their progenitor cell). The development of a pollen grain includes (micro)sporogenesis [1-4] and (micro)gametogenesis [5-9]. Microsporogenesis starts with the differentiation of microspore mother cells (MMC) resp. pollen mother cells (PMC) [1]. These diploid cells become enclosed by a thick callose wall and undergo meiosis, forming a tetrad of four haploid microspores, each encased in a second callose wall insulating them from each other and from the surrounding diploid tapetal cells [2].

Cytokinesis following meiotic nuclear divisions is accompanied by the formation of cleavage planes determined by the configuration and orientation of the meiotic spindle axes. In the case of successive cytokinesis, planes are formed after the first and second meiotic divisions leading to the formation of various tetrad types (see "Pollen Morphology"). During simultaneous cytokinesis the cleavage planes are formed concurrently after the second meiotic division; in this case microspores are arranged in a tetrahedral tetrad.

Pollen wall formation starts when the microspores are still arranged in tetrads and encapsuled by callose [3]. The first step consists of the deposition of the primexine, a fibrillar polysaccharide material, on the surface of the microspores. The primexine forms a template where sporopollenin precursors and finally sporopollenin are subsequently deposited, building the final pollen wall. Apertures are developed where the endoplasmic reticulum has prevented the deposition of primexine.

During pollen formation and maturation the tapetum plays an important role, usually

Tetrad Formation

Microsporogenesis tetrads

Scrophularia nodosa Scrophulariaceae tetrad tetrahedral iodid

Spiraea sp. Rosaceae tetrad tetrahedral PA+TCH+SP

Orobanche hederae Orobanchaceae tetrad planar KMnO,

Microsporogenesis tetrads

Scrophularia nodosa Scrophulariaceae tetrad tetrahedral iodid

Spiraea sp. Rosaceae tetrad tetrahedral PA+TCH+SP

Orobanche hederae Orobanchaceae tetrad planar KMnO, forming a single layer of cells circumscribing the loculus. Tapetal cells are specialized and have a short lifespan;they finally lose their cellular organization and are reabsorbed. Two types of tapetum are known: the secretory (or glandular or parietal) and the amoeboid (or periplasmodial). In the secretory type (e.g., in Apiaceae) the tapetal cells remain stationary until they finish their physiological functions. In the amoeboid tapetum type (e.g., in Araceae) cells lose their individuality in an early developmental stage by degeneration of the cell walls. The protoplasts then fuse and intrude into the locule where they enclose the pollen grains.

The tapetum plays an important role during several stages of pollen development. Its main function is the nourishment of the microspores but it also synthesizes enzymes (e.g., callase), exine precursors, pollen coatings, forms Ubisch bodies and viscin threads (both equivalents to the ektexine). The most striking material produced by the tapetum is pollenkitt (and tryphine in Brassicaceae, elastoviscin in Orchidaceae), a sticky, heterogeneous material composed of neutral lipids, flavonoids, carotenoids, proteins and polysaccharides. Pollenkitt serves numerous functions: for example, keeping pollen grains together during transport; protecting pollen from water loss, ultraviolet radiation, hydrolysis and exocellular enzymes; maintaining sporophytic proteins inside exine cavities.

Microgametogenesis in angiosperms includes first and second pollen mitosis, leading to the formation of the male gametes (sperm cells). Gametogenesis starts with formation of a central vacuole within the uninucleate microspore, pushing the nucleus towards the wall [5]. As long as the nucleus is in a central position within the cytoplasm, the cell is called a microspore [4]. With the dislocation of the microspore nucleus the cell becomes the young pollen grain.

The first pollen mitosis is followed by an asymmetric cell division, leading to the formation of a smaller generative cell and a larger vegetative cell [6]. When the generative cell is formed it is pressed against the pollen wall; it later separates and is then located within the cytoplasm of the vegetative cell [7]. After detachment, the generative cell, which is sparse in organelles, becomes modified in shape from spherical to spindle-shaped (the shape of the generative nucleus changes correspondingly). During the second pollen mitosis, which is followed by a symmetric cell division, the generative cell is divided into two sperm cells, the final stage of gametophytic development [8]. In about 25 % of flowering plants investigated, the pollen grains are three-celled at the time of anthesis [8];in 75% of flowering plants, pollen grains are shed from the anther at a two-celled stage. In the latter case the second pollen mitosis takes place in the pollen tube, after germination of the pollen grain onto a stigma or a corresponding structure [9].

Microgametogenesis in gymnosperms includes several mitotic divisions. Normally, pollen grains of gymnosperms are multi-celled at anthesis, and comprise prothallial cell(s), a large tube cell and a small anth-eridial cell. The tube cell becomes a pollen tube; the antheridial cell undergoes division into the stalk cell and the spermatogenous cell, the latter finally dividing into the male gametes (sperm cells or spermatozoids).

Pollen Development Stages
Pollen development in angiosperms

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  • maeve
    How microsporogenesis is different from microgametogenesis?
    2 years ago

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