Microgametogenesis

Introduction

Microgametogenesis is the process by which a microgametophyte develops from a pollen grain to a three-celled stage of development in plants. It happens in flowering plants with a microspore mother cell inside the plant’s anther.

When the microgametophyte first emerges from the pollen grain, four sets of fertile cells known as sporogenous cells are visible. These cells are surrounded by a sterile cell wall called the tapetum, which supplies food to the cell and eventually becomes the pollen grain’s cell wall. These sporogenous cell groups eventually mature into diploid microspore mother cells. These microspore mother cells, also known as microsporocytes, go through meiosis and divide into four microspore haploid cells.

These new microspore cells then undergo mitosis, resulting in the formation of a tube cell and a generative cell. The generative cell then goes through mitosis a second time to form two male gametes, also known as sperm.

Microgametogenesis 

Microgametogenesis refers to the events that occur during the progression of unicellular microspores into mature microgametophytes containing gametes. This phase starts with the microspore expanding, which is commonly associated with the formation of a single large vacuole. The microspore nucleus is displaced to an eccentric position against the microspore wall as a result of vacuolation. 

In this position, the nucleus goes through pollen mitosis I, which results in the formation of two unequal cells, a large vegetative cell and a small generative cell, each with a haploid nucleus. The generative cell then separates from the pollen grain wall and is engulfed by the vegetative cell, resulting in a unique ‘cell within a cell’ structure.

Gametogenesis: Formation of Male Gametes

In angiosperms, gametogenesis leads to the formation of male gametes in two stages:

• Microsporogenesis

• Microgametogenesis

The plant’s androecium is made up of stamen, filament, and anthers. The anthers contain the pollen grains that carry the male gametes. The anther is anatomically divided into lobes, which are further divided into chambers known as microsporangia.

Pollens can be found in these microsporangia. Pollen sacs are another name for microsporangia. The pollen sac is said to develop from a parent cell known as the archesporial cell. This archesporial cell is divided into two layers: outer and inner. The sporogenous tissue or cell that forms the pollen mother cell is formed by the inner layer.

This sporogenous tissue is surrounded by the tapetum layer, which feeds the pollen or microspore mother cell found in the sporogenous tissue. The mother cell of a microspore is diploid and divides meiotically to produce haploid microspores or pollen. This is a description of the process of microsporogenesis.

The pollen grain is a haploid cell with two layers: an outer exine layer derived from the tapetum and an inner layer called intine. Except for a small area where the pollen tube emerges after pollination, exine is present throughout the pollen grain. The germ pore is the name given to this small part.

The pollen grain is divided into two halves: a small generative nucleus and a larger vegetative nucleus. The generative cell produces two male nuclei, whereas the vegetative cell produces the pollen tube. This explains the microgametogenesis process.

Gametogenesis: Formation of Female Gametes

Female gametogenesis, like male gametogenesis, occurs in two stages in angiosperms:

• Megasporogenesis

• Megagametogenesis

In females, the term ‘mega-‘ is used instead of ‘micro-‘ because female gametes are said to be larger than male gametes.

The ovules are found in multiple lobes of the ovary. A cell within the ovule develops into a megaspore mother cell. The mother cell of a megaspore is a diploid. This megaspore mother cell goes through meiosis to produce four haploid megaspores. Three of the four megaspores degenerate, leaving only one megaspore in each ovule. This is referred to as megasporogenesis.

This megaspore nucleus is now dividing mitotically to form eight nuclei. Six of the eight nuclei migrate to opposing poles (3 each), while the remaining two nuclei remain in the centre. Polar nuclei are the nuclei that remain in the centre. The secondary nucleus is formed when these polar nuclei fuse. A megaspore develops into an embryo sac. Megagametogenesis refers to the entire process.

Conclusion

Microgametogenesis is the process by which a microgametophyte develops from a pollen grain to a three-celled stage of development in plants. These cells are surrounded by a sterile cell wall called the tapetum, which supplies food to the cell and eventually becomes the pollen grain’s cell wall. The microspore nucleus is displaced to an eccentric position against the microspore wall as a result of vacuolation. In this position, the nucleus goes through pollen mitosis, which results in the formation of two unequal cells, a large vegetative cell and a small generative cell, each with a haploid nucleus. The generative cell then separates from the pollen grain wall and is engulfed by the vegetative cell, resulting in a unique cell within a cell structure.