Motility is a characteristic function of the male gamete, which allows
spermatozoa to actively reach and penetrate the female gamete in organisms with
internal and external fertilization. Sperm motility is acquired under the control of many
extrinsic and intrinsic factors and is based on the specialized structure of the sperm
flagellum. An overview of how the sperm flagellum is organized and works to support
cell motility is presented, with special focus on the molecular mechanisms and factors
involved in the development and maintenance of sperm motility. Data obtained both in
organisms with external fertilization, such as sea urchins, ascidians or fishes as well as
those relying on internal fertilization, such as mammals, are critically analyzed.
In most animal species, sperm motility is dependent on a long appendage called
flagellum. Flagella are essential organelles found in most eukaryotic cells: their basic
structure is the axoneme, built of a scaffold of microtubules and responsible for
movement generation in an autonomous manner provided energy in the form of ATP is
present. Beating of flagella allows movement, using thrust on the milieu surrounding
sperm cells and is responsible of the translational drive of spermatozoa either in the
fluid or by contact with structures cells or tissues. The present paper aims to describe:
1. The biochemical and structural elements of the “9+2” flagellar
structure, so called axoneme, a complicated arrangement of at least
250 different protein subunits which sustains motility.
2. The mechanisms of wave generation and propagation along the
axoneme of flagella, stating that in paradigms of wave propagation, a
clear distinction is made between the dynein dependent microtubule
sliding (oscillatory motor) and the bending mechanism (including
regulator of wave propagation). The waves propagation is supported
by a bending/relaxing cyclic mechanism which propagates in register,
but frame-shifted with the powering action of the dynein-ATPase
motors all along the axoneme. While knowledge has been largely
accumulated on the motor components, little is known about the
elements regulating the bending processes.
3. Guidance of spermatozoa is closely dependent on flagellar behavior.
An overview of the various ways by which a spermatozoon can orient
itself or be oriented to the corresponding egg in order to improve
fertilization success is presented. As specific guidance mechanisms
occur in response to chemicals such as Ca2+ ions controlling
asymmetry of flagella beating special emphasis is devoted to such
regulatory aspects. A discussion is also devoted to the way a cell
elaborates its own flagellum and details possible hypothesis able to
explain the origin of the axoneme, recognized as an ancestral structure
with a high degree of conservation, as well as phylological aspects of
this unique mechanical device used in an extremely large variety of
situations to insure efficient cell movements of the male gametes, but
also many unicells, such as the green algae Chlamydomonas, used as
models to better understand the properties of this ubiquitous organelle
due to its high degree of structure and function conservation all along
evolutionary tree.
Keywords: Axoneme, Calcium ions, Dynein, Flagella, Membrane, Microtubules,
Molecular motors, Motility, Motility activation, Regulation, Phosphorylation,
Osmolarity, Energy, Viscosity.
