Sperm

The term sperm is derived from the Greek word (σπέρμα) sperma (meaning "seed") and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell. The human sperm cell is haploid, so that its 23 chromosomes can join the 23 chromosomes of the female egg to form a diploid cell. A uniflagellar sperm cell that is motile is referred to as a spermatozoon, whereas a non-motile sperm cell is referred to as a spermatium. Sperm cells cannot divide and have a limited life span, but after fusion with egg cells during fertilization, a totipotent zygote is formed with the potential to develop into a new organism. Mammalian sperm cells can live for up to 3 days inside the female reproductive system. The spermatozoa of animals are produced through spermatogenesis inside the male gonads (testicles) via meiotic division. Sperm cells in algal and many plant gametophytes are produced in male gametangia (antheridia) via mitotic division. In flowering plants, sperm nuclei are produced inside pollen.

Contents

Anatomy

The sperm cell consists of a head, a midpiece and a tail. The head contains the nucleus with densely coiled chromatin fibres, surrounded anteriorly by an acrosome, which contains enzymes used for penetrating the female egg. The midpiece has a central filamentous core with many mitochondria spiralled around it, used for ATP production for the journey through the female cervix, uterus and uterine tubes. The tail or "flagellum" executes the lashing movements that propel the spermatocyte.

Motile sperm cells

Motile sperm cells of algae and seedless plants.[1]

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.

Motile sperm are also produced by many protists and the gametophytes of bryophytes, ferns and some gymnosperms such as cycads and ginkgo. The sperm cells are the only flagellated cells in the life cycle of these plants. In many ferns and lycophytes, they are multi-flagellated (carrying more than one flagellum).[1]

In nematodes, the sperm cells are amoeboid and crawl, rather than swim, towards the egg cell.[2]

In order to move towards the ova the sperm swim through the womans vaginal juices coating the walls of the womb and cervix.

Non-motile sperm cells

Non-motile sperm cells called spermatia lack flagella and therefore cannot swim. Spermatia are produced in a spermatangium.[1]

Because spermatia cannot swim,they depend on their environment to carry them to the egg cell. Some red algae, such as Polysiphonia, produce non-motile spermatia that are spread by water currents after their release.[1] 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.[3]

Fungal spermatia (also called pycnidiospores) may be confused with conidia. Conidia are spores that germinate independently of fertilization, whereas spermatia are gametes that are required for fertilization. In some fungi, such as Neurospora crassa, spermatia are identical with microconidia as they can perform both functions of fertilization as well as giving rise to new organisms without fertilization.[4]

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.[1]

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.[1]

Sperm quality

Main article: Semen quality

A study by the University of California has suggested that folate may protect sperm cells from aneuploidy.[1]

Market

Further information: Sperm donation

On the global market, Denmark has a well developed system of sperm export. This success mainly comes from the reputation of Danish sperm donors for being of high quality[5] and, in contrast with the law in the other Nordic countries, gives donors the choice of being either anonymous or non-anonymous to the receiving couple.[5] Furthermore, Nordic sperm donors tend to be tall and highly educated[6] and have altruistic motives for their donations[6], partly due to the relatively low monetary compensation in Nordic countries. More than 50 countries worldwide are importers of Danish sperm, including Paraguay, Canada, Kenya, and Hong Kong.[5] However, the Food and Drug Administration (FDA) of the US has banned import of any sperm, motivated by a risk of mad cow disease, although such a risk is insignificant, since artificial insemination is very different from the route of transmission of mad cow disease.[7] The prevalence of mad cow disease is one in a million, probably less for donors. If prevalence was the case, the infectious proteins would then have to cross the blood-testis barrier and the sperm would have to be swallowed orally to make transmission possible.[7]

See also

References

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Raven, Peter H.; Ray F. Evert, Susan E. Eichhorn (2005). Biology of Plants, 7th Edition. New York: W.H. Freeman and Company Publishers. ISBN 0-7167-1007-2. 
  2. ↑ Bottino D, Mogilner A, Roberts T, Stewart M, Oster G (2002). "How nematode sperm crawl". J. Cell. Sci. 115 (Pt 2): 367–84. PMID 11839788. 
  3. ↑ Sumbali, Geeta (2005). The Fungi. Alpha Science Int'l Ltd.. ISBN 1842651536. 
  4. ↑ Maheshwari R (1999). "Microconidia of Neurospora crassa". Fungal Genet. Biol. 26 (1): 1–18. doi:10.1006/fgbi.1998.1103. PMID 10072316. 
  5. ↑ 5.0 5.1 5.2 Assisted Reproduction in the Nordic Countries ncbio.org
  6. ↑ 6.0 6.1 FDA Rules Block Import of Prized Danish Sperm Posted Aug 13, 08 7:37 AM CDT in World, Science & Health
  7. ↑ 7.0 7.1 The God of Sperm By Steven Kotler
Preceded by
None		 Stages of human development
Sperm		 Succeeded by
Zygote