Introduction to Structure of Mammalian Sperm

Sperm are the male gametes that carry the haploid paternal genome to the egg for fertilization. Sperm were discovered in the 1670s, but their role in fertilization was not discovered until the mid-1800s by Karl Ernst von Baer, who showed the union of sperm and egg in sea urchins and tunicates. Structure of mammalian sperm and its function are adapted to the environmental conditions and reproductive strategies of different species. Sperm consist of a haploid nucleus, a propulsion system, and a sac of enzymes that enable the nucleus to enter the egg. Sperm undergo extensive modification and maturation during their development and capacitation.

Sperm nucleus

• The sperm nucleus contains the haploid paternal DNA that is tightly compressed and streamlined for efficient transport.
• The sperm nucleus is derived from the primary spermatocyte that undergoes two meiotic divisions to produce four haploid spermatids.
• The spermatids then undergo spermiogenesis, a process of differentiation that involves chromatin condensation, nuclear elongation, and cytoplasmic reduction.
• The sperm nucleus is located at the front or to the side of the sperm head.

Acrosome

• The acrosome is a large vesicle that contains enzymes that digest proteins and complex sugars.
• The acrosome is derived from the Golgi apparatus of the spermatid during spermiogenesis.
• The acrosome lies on top or to the side of the sperm nucleus and forms a cap-like structure.
• The acrosome plays a crucial role in fertilization by digesting a path through the outer coverings of the egg and facilitating sperm-egg recognition and binding.
• In many species, a region of globular actin proteins lies between the sperm nucleus and the acrosomal vesicle. These proteins are used to extend a fingerlike acrosomal process from the sperm during the early stages of fertilization.

Flagellum

• The flagellum is a long, slender, whip-like appendage that propels the sperm through fluid media.
• The flagellum is composed of three main parts: the axoneme, the midpiece, and the principal piece.
• The axoneme is the core structure of the flagellum that consists of two central microtubules surrounded by a row of nine doublet microtubules. These microtubules are made exclusively of tubulin, a dimeric protein.
• The midpiece is the region of the flagellum that connects the axoneme to the base of the sperm nucleus. It contains rings of mitochondria that provide ATP for flagellar movement and sperm metabolism.
• The principal piece is the longest part of the flagellum that extends from the midpiece to the end tip. It contains a layer of dense fibers that stiffens the sperm tail and prevents excessive bending.

Dynein

• Dynein is a protein attached to the microtubules of the axoneme that provides the force for sperm propulsion.
• Dynein is an ATPase—an enzyme that hydrolyzes ATP, converting the released chemical energy into mechanical energy that moves the sperm.
• Dynein allows the active sliding of the outer doublet microtubules, causing the flagellum to bend and generate waves of motion.

Capacitation

• Capacitation is the final stage of sperm maturation that occurs in mammals after the sperm has been inside the female reproductive tract for a certain period of time.
• Capacitation involves biochemical and physiological changes in the sperm that enable them to bind to and fertilize an egg.
• Capacitation includes changes in membrane fluidity, ion permeability, membrane potential, intracellular pH, cAMP levels, protein phosphorylation, motility patterns, and acrosomal responsiveness.

Summary

• Sperm are specialized cells that carry the paternal genome to the egg for fertilization.
• Sperm have a streamlined and compact structure that consists of a haploid nucleus, an acrosome, and a flagellum.
• Sperm are propelled by dynein-mediated sliding of microtubules in their flagellum.
• Sperm undergo capacitation in mammals to acquire fertilization competence.