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Medicine LibreTexts

26.4B: Sperm

Sperm are the male “seeds,” germ cells, or gametes.

 

LEARNING OBJECTIVE

 

Describe the anatomy and function of sperm

 

KEY TAKEAWAYS

Key Points

 

  • Sperm fertilize the oocyte, donate the paternal chromatin, and provide the centrosome that maintains the zygote’s microtubule system.
  • Sperm have three parts: a head, which holds the chromatin, a midpiece filled with mitochondria to provide energy, and a flagellum or tail to move the sperm from the vagina to the oocyte.
  • Sperm with one tail, such as human sperm, are referred to as spermatozoa.
  • Sperm quality and quantity decrease with age.

 

Key Terms

 

  • anisogamy: The form of sexual reproduction that involves the union or fusion of two gametes that differ in size and/or form.
  • spermatozoa: A motile sperm cell or moving form of the haploid cell that is the male gamete.
  • acrosome: A caplike structure over the anterior half of the sperm’s head.
  • ATP: An acronym for adenosine triphosphate, which transports chemical energy within cells for metabolism.
  • oogamy: A form of anisogamy (heterogamy) in which the female gamete (oocyte) is significantly larger than the male gamete (sperm) and is non-motile. The male gametes are highly motile and compete for the fertilization of the immotile oocyte.

 

EXAMPLE

 

The term sperm is derived from the Greek word for seed and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there are marked differences in the size of the gametes, with the smaller termed the “male” or sperm cells. Sperm cells cannot divide and have a limited lifespan. After fusion with egg cells during fertilization, a new organism forms, beginning as a totipotent zygote. 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. During fertilization, the sperm provides the following three essential parts to the oocyte:

  1. A signalling or activating factor that causes the metabolically dormant oocyte to activate
  2. The haploid paternal genome
  3. The centrosome, which is responsible for maintaining the microtubule system

Sperm Anatomy

 

Closeup of Mammalian Fertilization

 

Closeup of Mammalian Fertilization: Micrograph of a sperm poised to enter an ovum

Sperm develop in the testes and consist of a head, a midpiece, and a tail. The head contains the nucleus with densely coiled chromatin fibers, surrounded anteriorly by an acrosome that contains enzymes for penetrating the female egg. The midpiece has a central filamentous core with many mitochondria spiraled around it.

Sperm Physiology and Function

In animals, most of the energy (ATP) for sperm motility is derived from the metabolism of fructose carried in the seminal fluid. This takes place in the mitochondria located in the sperm’s midpiece. This energy is used for the journey through the female cervix, uterus, and uterine tubes.

Motile sperm cells typically move via flagella and require a water medium in order to swim toward the egg for fertilization.These cells cannot swim backwards due to the nature of their propulsion. The uniflagellated sperm cells (with one flagellum) of animals are referred to as spermatozoa.

This sperm diagram indicates the acrosome, plasma membrane, nucleus, centriole, mitochondria, terminal disc, axial filament, head, midpiece, tail, endpiece, periacrosomal space, cell membrane, acrosome, nuclear vacuoles, nucleus, nuclear envelope, subacrosomal space, outer acrosome membrane, ecuatorial segment, postacrosomal region, postacrosomal sheet, posterior ring, connecting piece, redundant nuclear envelope, mitochondrial sheath, outer dense fibers, central pair, and axoneme.

 

Human Sperm: Detailed and labeled diagram of a human spermatozoa

 

Fertility Factors

Sperm quantity and quality are the main parameters in semen quality, a measure of the ability of semen to accomplish fertilization. The genetic quality of sperm, as well as its volume and motility, all typically decrease with age.