A sperm cell, also known as a spermatozoon, has one function – to fertilize an egg and pass on the male genome. It is the smallest cell in the body and contains dense DNA that produces energy to drive movement through the female reproductive tract.
Sperm cells are produced in the testes by a complex and precisely orchestrated process called spermatogenesis. This Collection will explore the biological mechanisms that balance sperm production and quality with reproductive efficiency.
What is a spermatid?
A spermatid is an immature, male sexual cell that can fertilize a female egg cell and cause conception. A spermatid forms from primary germ cells, or spermatogonia, in the testis during a process called spermatogenesis. Spermatogonia are diploid cells that undergo a series of cell divisions, or meiosis, to form four haploid spermatids from each primary spermatocyte.
A series of specialized processes then transforms spermatids into mature, motile sperm cells that can penetrate and fertilize an egg cell in the vagina or in the epididymis of the penis. Sperm cells develop tails and other specializations to help them move through a woman’s reproductive tract. A sperm cell is a haploid male gamete, meaning it contains only half the genetic material that is found in a woman’s egg cell.
The development of sperm cells begins when undifferentiated primary germ cells, or spermatogonia, form in the testis. Each spermatocyte goes through two quick successive meiotic reductive divisions to produce haploid spermatids. The spermatids then go through a process called cytodifferentiation, which takes place over 2-3 weeks in mice and rats, to become streamlined sperm cells capable of fertilizing an egg. These sperm cells are then released from the seminiferous tubules and transported to the epididymis or the vagina. The sperm cells are arranged in a pattern called the spermatogenic wave.
What is a sperm cell?
A sperm cell is a small, specialized cell that can fertilize a female egg. It develops from a primary germ cell (spermatogonia) during a process called spermiogenesis, which occurs in the testicular seminiferous tubules. As it develops, the sperm cell becomes more and more specialized for its specific role of fertilizing an egg. In the final stage of spermiogenesis, the spermatid cells become mature sperm cells, or spermatozoa.
Sperm cells have a head and tail region separated by a plasma membrane. The head contains a condensed haploid nucleus, and the tail has a long flagellum that propels the sperm to an egg. The sperm’s structure is adapted for its function, with tightly packed mitochondria, which provide the energy needed to move through seminal fluid, and a specialized cap-like structure, called the acrosome, which is capable of softening the egg’s outer shell and allowing penetration.
As a sperm cell moves, it creates an electrical current that causes the flagellum to rotate in a wave-like motion, which helps the sperm to swim toward the egg and penetrate its shell. The sperm cell also produces enzymes that can destroy obstacles in its path and help it get through the egg’s protective outer coating. The sperm cell also produces a long, thin neck piece that connects the head and tail. The neck is soft and flexible, which allows the sperm to curve and turn easily as it searches for and enters the egg.
How is a sperm cell formed?
Sperm cells are created in the testes in a process called spermatogenesis. This process starts when diploid germ cells (spermatogonia) present at the base of the epithelial lining of a long and coiled tubular structure called a seminiferous tubule begin to divide. These divisions give rise to haploid spermatids, which undergo a process called meiosis that reduces their number of chromosomes by half. The spermatids then mature and construct a tail, called the flagellum, to propel them through seminal fluid to an egg.
In a sperm cell, the nucleus contains 23 chromosomes, and its DNA is tightly packed with simple proteins called protamines. The sperm’s head is shaped like a teardrop and has a cap called the acrosome, which covers its nucleus. The neck of a sperm is soft and slender, and it contains mitochondria that generate the energy to power its movements. At the end of a sperm’s tail is a whip-like structure that contains protein fibers that contract to propel it through a female reproductive tract.
Because a sperm cell has only half the genetic material of a complete cell, it is considered a semi-living organism, and can metabolize sugars for energy, grow, move, and reproduce. It can also fuse with a fertilized egg to form a new zygote. However, a sperm cell cannot continue to divide on its own and has a limited lifespan. It can only survive for a short time after it has fused with an egg cell and is released into a fluid called semen.
How is a sperm cell fertilized?
Of the hundreds of millions of sperm cells deposited in the vagina during coitus, only a few hundred actually reach the egg to fertilize it. Those that do have a perilous journey ahead of them. First they have to survive the acidity of the vagina and mucus in the cervix, then they must make it past the layer of follicle cells surrounding the ovulated egg known as the zona pellucida.
In a process called capacitation, or priming, fluids in the female reproductive tract prepare sperm for this final stage of spermiogenesis by improving their motility and altering the structure of the membrane surrounding the acrosome. The acrosome contains the digestive enzymes that will allow sperm to penetrate the oocyte once it contacts it.
As sperm approach the egg, they bind to the corona radiata and the zona pellucida, triggering the acrosomal reaction. Digestive enzymes in the acrosome degrade the outer layers of these cells until one sperm is able to tunnel through to the egg’s plasma membrane and release its haploid genetic material into the egg.
During this frantic battle, the surface of the egg changes to prevent other sperm from penetrating it, and once the lucky sperm cell succeeds, it immediately undergoes a chemical reaction that ensures no other sperm can follow suit. The sperm and egg then fuse to form a zygote, or fertilized egg.