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Sperm guidance in mammals — an unpaved road to the egg

Abstract

Contrary to the prevalent view, there seems to be no competition in the mammalian female genital tract among large numbers of sperm cells that are racing towards the egg. Instead, small numbers of the ejaculated sperm cells enter the Fallopian tube, and these few must be guided to make the remaining long, obstructed way to the egg. Here, we review the mechanisms by which mammalian sperm cells are guided to the egg.

Key Points

  • Mammalian spermatozoa employ at least two guidance mechanisms: chemotaxis and thermotaxis. These mechanisms are restricted to capacitated spermatozoa only, constituting 10% of the sperm population in humans.

  • The capacitated, chemotactic, thermotactic state is temporary and its timing in different mammalian species seems to be programmed according to the time at which an ovulated egg is available in the female genital tract.

  • Sperm chemoattractants are secreted from both the egg and its surrounding cumulus cells. One chemoattractant that is secreted from the cumulus cells is progesterone, which is active in the pM concentration range.

  • One of the chemotaxis receptors on human spermatozoa is an olfactory receptor, OR17-4. Its agonist, the floral scent bourgeonal, acts as a sperm chemoattractant in vitro.

  • Progesterone and bourgeonal each cause a transient rise in intracellular Ca2+ concentrations, which results in a behavioural response. The signalling cascade that mediates these responses is not known for either of these chemoattractants.

  • At ovulation, the temperature at the sperm's storage site decreases by almost 1°C in rabbits, resulting in an ovulation-dependent temperature gradient that can be exploited for sperm thermotaxis from the storage to the fertilization site.

  • Sperm guidance to the egg in the mammalian female genital tract seems to be a multistep process, involving long-range thermotaxis and at least two, possibly more, steps of chemotaxis.

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The authors declare no competing financial interests.

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Acknowledgements

M.E. is an incumbent of Jack and Simon Djanogly Professorial Chair in Biochemistry. L.C.G. is a member of the research staff of the National Council of Research (CONICET, Argentina).

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Correspondence to Michael Eisenbach.

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Glossary

Capacitation

A ripening process that spermatozoa must undergo in order to penetrate the female's egg and fertilize it.

Acrosome reaction

The release of proteolytic enzymes from the top part of the sperm's head, known as the acrosome, which enables sperm penetration through the egg coat.

Chemotaxis

The movement of cells in the direction of a chemoattractant gradient.

Chemoattractant

A factor (a peptide or any other chemical) that attracts specific cells by chemotaxis.

Thermotaxis

The movement of cells that is directed according to a temperature gradient.

Follicular fluid

A fluid consisting of sex steroid hormones, plasma proteins, mucopolysaccharides and electrolytes that surrounds the ovum in the vesicular ovarian follicle (Graafian follicle).

Oviduct

A tube between the ovary and the uterus, through which the egg is transported from the former to the latter and in which fertilization occurs. It consists of two parts: the isthmus — a narrow part that is closer to the uterus — and the ampulla — a wider part that is closer to the ovary.

Cumulus cells

The cells that form dense layers surrounding a mature egg.

Axoneme

An axial filament complex at the centre of the sperm tail.

Hydrozoa

A class of radially symmetrical marine or freshwater invertebrates of the phylum Cnidaria, with one end of the body bearing the mouth and tentacles. This class includes polyps and medusa.

Hydromedusa

A hydrozoan in the medusoid stage of its life cycle.

Ascidian

A marine invertebrate animal that has a transparent sac-shaped body with openings through which water passes; also known as sea squirt.

Chemokinesis

The speed enhancement of actively moving cells in response to a stimulus.

Photorelease

The rapid release of a compound from its caged (protected) analogue by a short pulse of light.

Hyperactivation

A motility pattern that is characterized by increased velocity, decreased linearity, increased amplitude of lateral head displacement, and flagellar whiplash movement.

Granulosa cells

The cells that form layers surrounding the oocyte within the follicle.

Olfactory receptor

An integral membrane protein that is associated with a G-protein and is involved in effecting the sense of smell.

Spermatid

An immature gamete that develops into a spermatozoon.

Nuclear progesterone receptor

A progesterone-inducible transcription factor that is located intracellularly.

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Figure 1: The mammalian female genital tract and possible guidance mechanisms.
Figure 2: Two types of sperm response to chemoattractants.
Figure 3: Models for the molecular mechanisms of sperm chemotaxis in marine species and mammals.