Conceptual Overview

The male reproductive structures are involved in the elaboration of pollen grains, which give rise to male gametes, i.e. sperm cells, or simply sperms. Pollen grains are formed inside the anthers, which are specialized parts of the stamens.

The process of pollen grain formation includes microsporogenesis and microgametogenesis. During microsporogenesis, microspore mother cells (or meiocytes) are formed in the sporogenous tissue of the anther. These cells undergo meiosis to produce tetrads of haploid uninucleate microspores. Microgametogenesis comprises the subsequent mitoses and development of microspores into microgametophyte. The microgametophytes in turn produce male gametes (or sperm cells). During microgametogeneis, microspores divide mitotically once (or less frequently twice) and become the male gametophytes, or microgametophyte (pollen grains). If a single division occurs, the mature pollen grain consists of a large vegetative cell (future pollen tube) and a small generative cell (sperm mother cell). In plants with such bi-cellular pollen grains, the generative cell divides to form two sperm cells in the pollen tube after germination of the grain. In plants with tri-cellular pollen grains, the generative cell divides and produces two sperms during maturation of a grain, but before pollen tube emergence.

The shape of pollen grains varies greatly. The pollen grain may be spherical, ellipsoidal, thread-like, etc. The size of pollen grains also varies, from a few micrometers to as much as a quarter of a millimeter.

Characteristic of pollen grains are the successive changes in the structure and composition of their cell walls during development. First, the amorphous polysaccharide callose is deposited in meiocytes to the inside of very thin cellulosic cell walls, and is also laid down after meiosis around each microspore of the tetrad. During callosic wall formation, all of the plasmodesmatal connections are lost and the cells become isolated from the maternal sporophyte. Thus, the development switches from sporophytic to the gametophytic phase. While still within the tetrad and encapsulated by callose, the exine begins to be deposited. The exine is composed of sporopollenin, a complex organic hydrophobic biopolymer that is highly resistant to degradation. Upon the establishment of a first layer of sporopollenin that starts to form the young exine, dissolution of the callosic wall begins. The rate of deposition of sporopollenin increases after the liberation of microspores from the tetrad. The microspores grow in circumference and, finally, an inner cellulosic cell wall, the intine, is deposited. The intine is similar in composition to the primary wall of typical plant cells.

Various substances may accumulate on the exine surface. In entomophylous (insect-pollinated) plants, pollen grains are covered with oily, sticky and colored materials that facilitate the attraction of insects, and the adhesion and subsequent transfer of pollen. In anemophilous (wind-pollinated) plants the thickness, surface sculpturing, and stickiness of the exine are all generally reduced.

The surface of the exine is ornamented with a complex pattern of spines, net-like ridges and/or other projections. These ornamentations vary greatly but, at the same time, the pattern is constant in plant taxa of various ranks (taxon-specific) and may be used for taxonomic purposes. From the analysis of pollen it is possible to determine which plants have grown in a certain geographical region and during a certain geological time.

Exine deposition is absent over the apertures (pore sites through which pollen tubes emerge during germination). The apertures may be circular or furrow-like, and the number of them varies from one in monocotyledons to three or more in dicotyledons.

Pollen is a source of allergenic proteins that elicit the formation of a specific class of antibodies, immunoglobulin E, in sensitized humans causing "hay fever." These proteins are synthesized in the vegetative cell.

Starch grains and lipid droplets may accumulate in the vegetative cell before full maturity of the pollen grain. At the final stage of maturation, desiccation and transition to a somewhat dormant state occurs in pollen grains. Pollen apertures are the gates through which water evaporates in the desiccation process.

The anther, a site of pollen grain development, is a terminal portion of a stamen. The anther is supported on a stalk, or filament. The anther usually has two longitudinal lobes that are united by a band of parenchyma cells called the connective, which is a continuation of the filament. Each anther lobe contains two longitudinal pollen sacs (or locules) within which the pollen grains are produced. A single vascular band extends from the filament into the center of the connective and continues into the anther.

In its earliest stage, the anther consists of a uniform mass of meristematic cells. Soon in this homogeneous meristem, four separate groups of cells, the sporogenous tissue, become discernible. The sporogenous tissue is composed of numerous microspore mother cells. Thus, unlike the nucellus (megasporangium) where a single megaspore mother cell and then single megagametophyte (embryo sac) are formed, sporogenous tissue is composed of many microspore mother cells, and then numerous microgametophytes (pollen grains) are developed within a pollen sac (the microsporangium).

The sporogenous tissue is delineated from the anther wall by a single-layer of cells designated as the tapetum. The cells of the tapetum enlarge and develop a complex ultrastructure, which indicates that they become very active metabolically. At the time of meiotic division of microsporocytes, the nuclei of the tapetal cells also divide, but mitotically. However, mitosis is not followed by cytokinesis, and tapetal cells subsequently become bi-nucleate. Two types of tapetum may be distinguished according to subsequent development of the cells û a secretory tapetum or a periplasmodial tapetum. The cells of the secretory tapetum remain intact and persist in situ, whereas in the periplasmodial tapetum, the cell walls break down and the protoplasts intrude into the pollen sac eventually forming a coenocytic (multinucleate) plasmodium around the developing pollen grains. The tapetum is involved in the nourishment of the meiocytes and pollen grains, and in the synthesis and deposition of sporopollenin and other wall materials onto the surface of the developing exine. The tapetal cells also synthesize and secrete callase, an enzyme responsible for the dissolution of callose around the microspore tetrads. Before an anther matures, the tapetum degenerates and its remains are deposited on the pollen grain surface.

At the final stage of pollen grain maturation, and immediately before anthesis (the time of maturation of the male and female organs of the flower), the filament undergoes rapid elongation resulting in the disruption of the tracheary elements in the vascular bundle and, therefore, the cessation of the water supply to the anther. The process of anther and pollen desiccation commences. It terminates in anther dehiscence and the subsequent dispersal of pollen. Just before anther maturation, conspicuous secondary wall thickenings are deposited on the anticlinal and inner cell walls of the sub-epidermal cell layer, the endothecium (or fibrous layer, or contractile layer) of the anthers. The thickenings of the endothecial cells cause tangential shrinkage during anther desiccation, leading to the rupture and outward bending of the anther wall. In other words, it brings about the dehiscence of the anther. Each of the two anther lobes dehisces by a longitudinal slit called a stomium. The endothecial cells do not develop secondary thickenings in the region of the future stomium, thereby providing for the specific location and orientation of the slit for dehiscence.

When a pollen grain is released from the anther, it exists as an extremely reduced haploid male plant (male gametophyte) until it is carried to the stigma either by wind, insects or directly by contact of the anther with the stigma. The interaction between pollen and the pistil starts with the adhesion of pollen grains to the stigmatic surface. This is facilitated by the sticky nature of the pollen surface and an exudate on the stigmatic surface. Adhesion of pollen grains is soon followed by the uptake of water through the apertures in the exine. This imbibition of water is accompanied by a release of proteins from the pollen wall. These proteins participate in recognition reactions between the pollen and the stigma. In incompatible combinations pollen germination is suppressed, or newly emerged pollen tubes fail to penetrate the stigma.

Rehydration results in the mobilization of food reserves, the resumption of metabolic activity and the subsequent germination of pollen grains. Germination occurs by the emergence of a pollen tube from one of the apertures. The walls of the growing pollen tubes (i.e., the walls of the vegetative cell) are cellulosic and both similar to, and continuous with, the intine of the pollen grain. After recognition and acceptance, the pollen tubes (carriers of the sperm cells) grow through the stigma to the stylar transmitting tissue, and then down to the ovary. When the tubes reach the ovary, they grow along the surface of the placenta towards the ovules.

As in other freely growing cells such as root hairs or fungal hyphae, the growth of the pollen tubes is restricted to the extreme tip. This apical tip growth occurs very rapidly and may reach a rate of 1 cm per hour in some plants, such as corn pollen tubes. However, the actual growth zone is only a few micrometers long. This zone is filled with secretory Golgi vesicles that export the cell polysaccharides and membranes, which supply the growing wall and plasmalemma with new materials. The rate of the vesicle production has been estimated to be as high as more than 5,000 per minute. The subapical zone is rich in dictyosomes, the producers of secretory vesicles, as well as mitochondria and endoplasmic reticulum. The vegetative nucleus and the generative cell, (or sperm cells) are located in an adjacent vacuolated zone.

After reaching the ovule, the pollen tube enters the embryo sac through the micropyle. The obturator, or papillate placental cells that are formed in some plants, provide a well-defined pathway that guides the pollen tube to the tip of the ovule. It is generally assumed that the directed growth of the pollen tube occurs due to chemotropically active substances produced and secreted by the synergid cells. After its arrival to the embryo sac, the pollen tube enters one of the synergids through its intracellular filiform apparatus. When the pollen tube is within the synergid cytoplasm, it bursts and the vegetative nucleus and both sperm cells are discharged into the synergid. Thereafter, double fertilization occurs.