Microspore

Microscopic photo of spores (in red) of Selaginella. The large three spores at the top are megaspores whereas the numerous smaller red spores at the bottom are microspores.

Microspores are land plant spores that develop into male gametophytes, whereas megaspores develop into female gametophytes. The male gametophyte gives rise to sperm cells, which are used for fertilization of an egg cell to form a zygote. Microspores are structures that are part of the alternation of generations in many seedless vascular cryptogams, all gymnosperms and all angiosperms. Plants with heterosporous life cycles using microspores and megaspores arose independently in several plant groups during the Devonian period. Microspores are haploid, and are produced from diploid microsporocytes by meiosis.

Embryogenesis 

This process is the most effective way of yielding haploid and double haploid plants through the use of male sex hormones. In the anther, after a microspore undergoes microsporogenesis, it can deviate towards embryogenesis and become star-like microspores. The microspore can then go one of four ways: Become an embryogenic microspore, undergo callogenesis to organogenesis (haploid/double haploid plant), become a pollen-like structure or die.

Morphology

The microspore consists of three different walled layers. The outer layer is called the perispore, the next is the exospore, and the inner layer is the endospore. The perispore is the thickest of the three layers while the exospore and endospore are relatively equal in width.

Seedless vascular plants

Microsporophylls bear microsporangia containing many microsporocytes that undergo meiosis, producing tiny microspores. Each microspore may become a male gametophyte consisting of a somewhat spherical antheridium within the microspore wall. Either 128 or 256 sperm cells with flagella are produced in each antheridium. In the ferns, the only heterosporous plants are aquatic or semi-aquatic, including the genera Marsilea,Regnellidium, Pilularia, Salvinia, and Azolla. This condition is also known in the lycopod genus Selaginella and in the quillwort genus Isoëtes.

Types of seedless vascular plants:

• Ferns
• Spikemosses
• Quillworts

Gymnosperms

In seed plants the microspores are the first cells to form into the male gametophyte and give rise to the pollen grains. The megaspores, in turn, are formed within the developing seed. Pollen cones usually develop toward the tips of the lower branches in clusters up to 50 or more. Microsporangia develop in pairs toward the bases of the scales. Each of the microsporocytes in the microsporangia undergoes meiosis, producing four haploid microspores. These develop into pollen grains; each grain consists of four cells and a pair of external air sacs. The air sacs give the pollen grains added buoyancy that could help with wind dispersal.

Types of Gymnosperms:

• Conifers
  • Pines
• Ginkgos
• Cycads
• Gnetophytes

Angiosperms

As the anther of a flowering plant develops, four patches of tissue differentiate from the main mass of cells. These patches of tissue contain many diploid microsporocyte cells, each of which undergoes meiosis producing a quartet of microspores. Four chambers (pollen sacs) lined with nutritive tapetal cells are visible by the time the microspores are produced. After meiosis, the haploid microspores undergo several changes:

1. The microspore divides by mitosis producing two cells. The first of the cells (the generative cell) is small and is formed inside the second larger cell (the tube cell).
2. The members of each part of the microspores separate from each other.
3. A double-layered wall then develops around each microspore.

These steps occur at relatively the same time and when complete, the microspores have become pollen grains.

See also

• Microsporangium
• Spore
• Megaspore