- Male Gamete Development
The angiosperm reproductive cycle begins with the process of microsporogenesis, or microspore formation. The stamen consists of a filament and the anther, also known as the microsporangium. In most of the cases, the anther consists of four pollen sacs, or locules.
Within each locule, the archesporial cell develops through mitosis and extends as a row of cells throughout the entire length of the young anther. These mitotic cell divisions generate the anther wall, which is made up of several cell layers, the outermost of which transforms itself into the epidermis. The layer of cells below the epidermis is known as the endothecium.
During anther development, the endothecial cells acquire thickenings whose function is related to anther opening and pollen release. The innermost layer of the anther wall is the tapetum, whose primary function correlates with the nourishment of the young pollen and the deposition of the exine, a coating of the pollen grain.
As development proceeds, the sporogenous cells located below the tapetum transform into microsporocytes. These microsporocytes will undergo meiosis, and tetrads (units of four) of microspores will form.
Shortly after their formation, the tetrads separate into individual microspores. Each microspore is haploid, and often it will enlarge and separate from the tetrad, becoming sculptured by the deposition of sporopollenin and other substances that will turn into the ornamented surface of the pollen grain.
The second phase of pollen development is known as microgametogenesis.
The single nucleus of each microspore divides once by meiosis and one of the two daughter nuclei becomes a generative nucleus and accumulates little cytoplasm, while the other becomes a tube nucleus. The generative nucleus undergoes a single mitotic division and forms two male gametes or sperm cells. These form the pollen grain or a mature microgametophyte. In some Angiosperms, this division occurs after pollination in the pollen tube. When the anther opens, the mature male gametophytes or pollen grains will be disseminated and ready for germination.
- Female Gamete Development
The ovule (female sex organ) consists of two opposite ends: the micropyle, where the integuments come together, and a more distant end, where the ovular tissue is more massive. This part is also known as the chalaza, and it lies directly opposed to the micropyle.
The mature ovule is composed of three layers: the outer integument; the inner integument; and, underneath the integuments, the nucellus. During ovular development, one cell lying below the nucellar epidermis changes into a primary archesporial; this will divide to form the primary parietal cell and primary sporogenous cell.
The primary sporogenous cell functions as the megaspore mother cell, which divides meiotically, originating four haploid megaspores. In the majority of angiosperms, three of the megaspores will degenerate, and only the chalazal one will develop into the megagametophyte (embryo sac).
After the completion of the embryo sac stage, a series of cellular events occurs, ending with the formation of the mature embryo sac, ready for fertilization by the male gametes. The chalazal megaspore enlarges and undergoes three mitoses, giving rise to eight haploid cells. The mature megagametophyte consists of two groups of four cells located at both ends of the embryo sac.
The result is three antipodals at the chalazal end: the egg apparatus (consisting of the egg and two synergids at the micropylar end) and the polar nuclei. These two cells, present at both ends, usually fuse before pollination, and during fertilization they form the primary endosperm nucleus.