To the stages occuring within the life cycle of plants, animals and man, one can distinguish a distinct sequence including the adult individual stage, meiosis and gamete formation, fusion of male and female gamete or fertilization, formation of a zygote and subsequencely young organism, and then again adult individual. During this cycle an alternation of generation occurs, from diploid (indicated as "2n"; diploid derived from the Greek word diplous = double) to haploid (1n; from haplous = single) to diploid again.
All land plants, and some algae, have life cycles in which a diploid multicellular sporophyte (the generation that has a double set of chromosomes) alternates with a haploid gametophyte generation. The sporophyte produces spores (hence the name), by meiosis. These meiospores develop into a gametophyte. Both the meiospores and the resulting gametophyte are haploid, meaning they only have one set of homologous chromosomes. The mature gametophyte produces male or female gametes (or both) by mitosis. The male sex cell is usually small and it moves toward the immobile, larger female sex cell. The fusion of male and female gametes (fertilization) produces a diploid zygote which develops into a new sporophyte.
Life cycle in flowering plant and man
Courtesy of Dr. J. Derksen; drawings Lidwien van der Horst
Formation of reproductive cells in higher plants, animals and man (under construction)
The male and female reproductive cells (gametes = sex cells) develop from the meiocytes through post-meiotic differentiation in both animals and plants. The male sex cells are called sperm cells (from the Greek sperma = seed), the female are named egg cell or ovum (the latin word for egg).
Reproductive cells in plants or in animals or man
In plants
Megagametophyt and embryo sac in tobacco. Developement of pollen in tobacco and pollen tube in pine (Photographs P. de Groot, K. Weterings and A. Jansen
In animals and man
Spermatides and sperm cells of a grass hopper. Mature, unfertilized egg cell of the sea urchin. Sperm cell of man labeled with a fluorescent marker for reseach purposes
In animals (under construction)
There are mostly two genders of each animal species, which differ in their genital organs, but also in their morphological characteristics:
In male animals (also in man) meiosis occurs in the testis. A sperm mothercell produces four sperm cells.
In female animals (also in woman) meiosis occurs in the ovaria. During the meiotic process asymmetrical divisions occur so that all cytoplasm is gathered in only one of the meiotic products . This large cell becomes an egg cell, whereas the other cells produced by meiosis disappear.
In plants
Both lower and higher plants show meiosis during their life cycle in special organs. In gymnosperms meiosis occurs in cones and in angiosperms in the flowers that contain anthers (the male reproductive organs) and pistils (the female organs).
Pollen development in the anthers
A. Cross-section through an anther of Lilie (Lilium)sp.) with on the left and the right side two loculi each. In the loculi sporemothercells (SMCs) can be seen from which the four spores develop through meiosis I and II. Inbetween the loculi of each pair a thin layer of cells (arrow) is visible along which the loculus can burst open atmaturity and release the pollen grains. In the middle the cross-sectioned filament (Fi) to which the anther is attached is indicated. In the upper part the vascular bundle (v) of the loculus can be distinguished.
B. Loculus. The lumen contains developing pollen. On the inner wall (w) of the loculus a layer constitued of block-shaped single cells is present, the tapetum (t). The tapetum feeds the developing spore and -later- pollen.
C. Tetrad stage during pollen development. After the two meiotic divisions the four daughter cells are still interconnected and form a tetrad. They are still surrounded by the wall (arrow) of the original cell, the microspore mothercell (MMC).
D. Mitotic division in the spore leading to the formation of a microgametophyte or pollen. Only the metaphase is shown here. The chromosomes lay in the equatorial plane of the cell.
E. Nearly ripe pollen grain: visible are a vegetative cell with nucleus (VN), which later will form the pollen tube, and a generative cell with its own nucleus (GN), which later will divide into two sperm cells.
F. Ripe pollen grain in which the texture of the outer cell wall, the exine, can be recognized. The grainy dark purple structure in the middle of the pollen grain is the vegetative nucleus.
G. Diagram in 3 parts: Ripe pollen grain consisting of the vegetative cell (VC) and therein the smaller generative cell (GC). After landing on the stigma (St) the pollen grain germinates and forms a pollen tube. In the pollen tube the generative cell divides into two sperm cells (SC). The pollen tube grows to the embryo sac (ES) and delivers the two sperm cells that are involved in double fertilization.
At the male side, in the loculi of the anthers, a microspore mothercell produces four microspores that are initially joined in a tetrad (see photograph here below). From each -haploid- microspore a microsporophyt develops by mitosis. In angiosperms the microsporophyte correspond to the pollen grains. In each pollen grain goes to the first mitotic division to form a vegetative and a generative cell. The generative cell leads generates through a second mitosis two gametes (sperm cells). More images and information on pollen development can be found elsewehre on this site.
In the pistil, meiosis occurs in the ovaria. Also here asymmetrical divisions during meiosis leads to accumulation of the cytoplasm in only one of the meiotic products: the macrosporemothercell (also called megasporemothercell). (The other cells disappear.) The large cell forms an embryo sac, the actual macrogametophyt. Within the ovules three further mitotic divisions usually occur, giving in many species rise to three antipodes, two polar nuclei, one egg cells and two synergids.
Find numerous photographs of all stages of the microsporogenesis and microgametogenesis (from meiosis tot pollen formation), and of the megasporogenesis and megagametogenesis(from meiosis to egg and fertilization or syngamy) in Lilium on the very complete webpages at the site of the International Association of Sexual Plant Reproduction Research (IASPRR). See also instructive and complete article by Ross E. Koning about "Pollen and Embryo Sac". Plant Physiology Website. 1994. (Copyright author: Ross Koning; download pdf version, with permission; 370 KB).
Double fertilization in flowering plants (under construction)
Fertilization in higher plants and in man
Double fertilization in flowering plants
Pollen grain = Po consisting of a Vegetative cell = VG and a Generative cell = GC, St = Stigma, St = Style, Pt = pollen tube, SC1 en SC2 = Sperm cell 1 and 2, O = ovule containing an embryo sac = ES (PRE = Before and POST = just after fertilization), E = egg cell, Sy = synergides, CC = Central cell, A = Antipodal cells, Zy = zygote, f CC = fertilized Central Cell (forms the endosperm)
Double fertilization in flowering plants (see page on the Anatomy of the flower) requires the and features around When pollen grains reach a receptive stigma of the same species (Pollination), in principle each vegetative cell of the pollen grain can develop a tube. The much smaller generative cell, or her mitotic products, the two -haploid- sperm cells, are enclosed inside the vegetative compartment and migrate forward along with the growing tube tip. The pollen tube penetrates the stigma and continues to grow through the style to an ovule inside the ovarium. Each ovule contains one embryo sac (ES),in fact a small female gametophyte, bearing a.o. one egg cell, one central cell, and mostof the time synergis and antipodal cells. When the pollen tube reaches the embryo sac it bursts open and releases the two sperm cells, the actual sex cells. Then double fertilization occurs. One sperm cell fertilizes the egg cell so that a diploid zygote arises, from which an embryo develops. The other sperm cell fertilizes the large central cell in the middle of the embryo sac and give rises through series of divisions to the endosperm, which function as a storage organ of nutrient for the seed. (Sometimes the storage food is transfered from the endosperm to the cotyledons). The ripe dry ovule with the mature embryo and the endosperm is the seed. Fruits are the ripened ovary with inside one (e.g. wheat) or several (e.g. tomato) seed(s).
Pollen tube growth through the pistil in tomato
Pollen tubes in a pollinated pistil of tomato (Solanum Lycopersicum cv money maker). The course of pollen tubes could be visualized by treatment with the fluorescing marker aniline blue that specifically binds to callose, a special cell wall component which is abundant in pollen tubes. Research project of Kimberly Koens, Bart Schimmel and Maaike de Jong.
A. Pollen tubes growing through the stigma (St) and the style (Sty). B. Detail of pollen tubes (arrows) in the style. The bright spots are callose plugs which separate the living and growing part of the pollen tube in the tip and the remaining degenerating part. C. Fluorescing pollen tubes can be seen in the lower part of the style (Sty)and the underlying ovarium (Ova) containing ovules (Ovu). D. Detail of three ovules (Ovu). The two upper ovules have been penetrated by pollen tubes (arrow) growing toward the embryo sac (ES, corresponding with the dark ovale shape on this photograph). In each pollen tube two sperm cells are present which are involved in double fertilization
Embryosac and mature embryo in plants
A. Transverse section through an ovule of Lily (Lilium sp.).
In the center an embryo sac (ES) can be seen surrounded by its two protective layers the integumenten (I1 and I2). Together they form the ovule. Ovules lay in the lumen of the ovarium. Bw: Inner wall of the ovarium. On one side the integuments come together and a small channel remains open, the micropyle (M),throguh which the pollen tube can penetrate in order to release the sperm cells. Inside the embryo sac the synergiden (Sy) that help conducting the sperm nuclei to the egg cell (E) a bit furtherup is located close the micropyle, whereas the large central cell (CC) is in the center. The embryo develops from the fertilized egg cell, the endosperm (Es) with a nutritional function from the fertilized central cell and the integuments become part of the seed coat.
B. Cross section through a cereal fruit, here: Wheat (Triticum aestivum).
According to botanical definitions, wheat grains are single seed fruits. The seed coat, which arises from the integuments, grows together with the ovary wall (pericarp). Such fruit is called a caryopsis and it occurrs nearly exclusively in grasses. The fruit incorporates the endosperm (Es) and the embryo (Em) a complete plantlet with rootlet (= radicula R) and the first leaves (leaf shoot or plumula, Pl). The one cotyledon is transformed into a shield-shaped organ, called scutellum (Sc). The scutellum is appressed against the endosperm and absorbs storage material from the endosperm to transfer it to the growing embryo.
Screenshots from films on pollen tube growth and fertilization
Extract from the film by Dr. Olga Erdelska and Dr. Hans-Henning Heunert on double fertilization in plants (in snowdrop; Galanthus nivalis): embryo sac - sperm cells - fertilization of the egg cell - fertilization of the central cell - endosperm formation. Unfortunately the Institut für den Wissenschaftlichen Film which showed and sold these movies has been closed.
From a film by dr. I. Lichtscheidl cs on pollen tube growth and fertilization in plants. Source: site of the department for Cell Physiology and Scientific Film of the university of Vienna)
Webpages and photographs:Jan Derksen, Kimberly Koens, Bart Schimmel, Maaike de Jong and Elisabeth Pierson
Drawings: Lidwien van der Horst
Web development: Remco Aalbers
