Sperm

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Overview

The term sperm is derived from the word spermos (meaning "seed") and refers to the male reproductive cells. Sperm cells are the smaller gametes involved in fertilization. In these types of sexual reproduction, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell. A uniflagellar sperm cell that is motile is also referred to as spermatozoon, whereas a non-motile sperm cell is referred to as spermatium. Sperm cells cannot divide and have a limited life span, but they can fuse with egg cells during fertilization to form a totipotent zygote with the potential to develop into a new organism.

The spermatozoa of animals are produced through spermatogenesis inside the male gonads (testicles) through meiosis. Sperm cells in algal and many plant gametophytes are produced in male gametangia (antheridia) through mitosis. In flowering plants, sperm nuclei are produced inside pollen.

Motile sperm cells

Motile sperm cells typically move via flagella and require water in order to swim toward the egg for fertilization. The uniflagellated sperm cells (with one flagellum) produced in most animals are referred to as spermatozoa, and are known to vary in size.

In nematodes, the sperm cells crawl, rather than swim, towards the egg cell.

Non-motile sperm cells

Non-motile sperm cells called spermatia lack flagella and therefore cannot swim. They are often confused with conidia. Conidia are spores that germinate independently of fertilization, whereas spermatia are gametes that cannot give rise to a new organism by themselves, but instead are required for fertilization. Spermatia are produced in a spermatangium.

Because spermatia cannot swim, they depend on their environment to carry them to the egg cell. Some red algae produce non-motile spermatia that are spread by water currents after their release. The spermatia of rust fungi are covered with a sticky substance. They are produced in flask-shaped structures containing nectar, which attract flies that transfer the spermatia to nearby hyphae for fertilization in a mechanism similar to insect pollination in flowering plants.

Sperm nuclei

In many land plants, including most gymnosperms and all angiosperms, the male gametophytes (pollen grains) are the primary mode of dispersal, for example via wind or insect pollination, eliminating the need for water to bridge the gap between male and female. Each pollen grain contains a spermatogenous (generative) cell. Once the pollen lands on the stigma of a receptive flower, it germinates and starts growing a pollen tube through the carpel. Before the tube reaches the ovule, the nucleus of the generative cell in the pollen grain divides and gives rise to two sperm nuclei which are then discharged through the tube into the ovule for fertilization.

In some protists, fertilization also involves sperm nuclei, rather than cells, migrating toward the egg cell through a fertilization tube. Oomycetes form sperm nuclei in a syncytical antheridium surrounding the egg cells. The sperm nuclei reach the eggs through fertilization tubes, similar to the pollen tube mechanism in plants.

See also

Bibliography

  • P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): Biology of Plants, 7th Edition, W.H. Freeman and Company Publishers, New York, ISBN 0-7167-1007-2

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