Sperm
Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction. Sperm cells contribute approximately half of the nuclear genetic information to the diploid offspring. Animals produce motile sperm with a tail known as a flagellum, which are known as spermatozoa, while some red algae and fungi produce non-motile sperm cells, known as spermatia. Flowering plants contain non-motile sperm inside pollen, while some more basal plants like ferns and some gymnosperms have motile sperm.
Sperm cells form during the process known as spermatogenesis, which in amniotes takes place in the seminiferous tubules of the testicles. This process involves the production of several successive sperm cell precursors, starting with spermatogonia, which differentiate into spermatocytes. The spermatocytes then undergo meiosis, reducing their chromosome number by half, which produces spermatids. The spermatids then mature and, in animals, construct a tail, or flagellum, which gives rise to the mature, motile sperm cell. This whole process occurs constantly and takes around 3 months from start to finish.
Sperm cells cannot divide and have a limited lifespan, but after fusion with egg cells during fertilization, a new organism begins developing, starting 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 with 46 paired chromosomes. In mammals, sperm is stored in the epididymis and released through the penis in semen during ejaculation.
The word sperm is derived from the Greek word σπέρμα, sperma, meaning "seed".
Evolution
It is generally accepted that isogamy is the ancestor to sperm and eggs. Because there are no fossil records of the evolution of sperm and eggs from isogamy, there is a strong emphasis on mathematical models to understand the evolution of sperm.A widespread hypothesis states that sperm evolved rapidly, but there is no direct evidence that sperm evolved at a fast rate or before other male characteristics.
Sperm in humans
Function
The main sperm function is to reach the ovum and fuse with it to deliver two sub-cellular structures: the male pronucleus that contains the genetic material and the centrioles that are structures that help organize the microtubule cytoskeleton.The nuclear DNA in sperm cells is haploid, that is, they contribute only one copy of each paternal chromosome pair. Mitochondria in human sperm contain no or very little DNA because mtDNA is degraded while sperm cells are maturing, hence they typically do not contribute any genetic material to their offspring.
In mammals, sperm cells normally come in two types, "female" and "male", named for the resulting sex of the fertilized zygote each produces after fusing with the ovum. Sperm cells that produce female offspring carry an X-chromosome, while sperm cells that produce male offspring carry a Y-chromosome. Errors of meiosis may lead to the formation of sperm containing different arrangements of sex chromosomes, either altogether missing, or in multiples, such as "XX", "XY", etc... some of the conditions known as Disorders of Sex Development are the result of fertilization by such defective sperm.
The sperm cell of Homo sapiens is the small reproductive cell produced by males, and can only survive in warm environments; upon leaving the body, it starts to degrade, thereby decreasing the total sperm quality.
Semen has an alkaline nature and the spermatozoa do not reach full motility until they reach the vagina, where the alkaline pH is neutralized by acidic vaginal fluids. This gradual process takes 20–30 minutes. During this period, fibrinogen from the seminal vesicles forms a clot, securing and protecting the sperm. Just as they become hypermotile, fibrinolysin from the prostate gland dissolves the clot, allowing the sperm to progress optimally.
DNA damage and repair
DNA damages present in spermatozoa in the period after meiosis but before fertilization may be repaired in the fertilized egg, but if not repaired, can have serious deleterious effects on fertility and the developing embryo. Human spermatozoa are particularly vulnerable to free radical attack and the generation of oxidative DNA damage.Exposure of males to certain lifestyle, environmental or occupational hazards may increase the risk of aneuploid spermatozoa. In particular, risk of aneuploidy is increased by tobacco smoking, and occupational exposure to benzene, insecticides, and perfluorinated compounds. Increased aneuploidy of spermatozoa often occurs in association with increased DNA damage. DNA fragmentation and increased in situ DNA susceptibility to denaturation, the features similar to these seen during apoptosis of somatic cells, characterize abnormal spermatozoa in cases of male infertility.
Although DNA repair has long been considered impossible in human spermatozoa due to the high level of DNA compaction in these cells, human spermatozoa possess a truncated base excision repair pathway that is mediated by 8-oxoguanine DNA glycosylase 1. Thus mature spermatozoa appear to have a limited capacity to mount a DNA repair response to oxidative stress.
Avoidance of immune system response
molecules on the surface of ejaculated sperm cells are recognized by all human female immune systems, and interpreted as a signal that the cell should not be rejected. The female immune system might otherwise attack sperm in the reproductive tract. The specific glycoproteins coating sperm cells are also utilized by some cancerous and bacterial cells, some parasitic worms, and HIV-infected white blood cells, thereby avoiding an immune response from the host organism.The blood-testis barrier, maintained by the tight junctions between the Sertoli cells of the seminiferous tubules, prevents communication between the forming spermatozoa in the testis and the blood vessels within the interstitial space. This prevents them from eliciting an immune response. The blood-testis barrier is also important in preventing toxic substances from disrupting spermatogenesis.
Anatomy
The mammalian sperm cell can be divided in 2 parts connected by a neck:- Head: contains the nucleus with densely coiled chromatin fibers, surrounded anteriorly by a thin, flattened sac called the acrosome, formed by modification of the Golgi body, which contains enzymes such as spermlysin used for penetrating the female egg. It also contains vacuoles. As the spermatozoon approaches the ovum, it undergoes the acrosome reaction in which the membrane surrounding the acrosome fuses with the plasma membrane of the sperm's head, exposing the contents of the acrosome. The head of a human sperm is disc shaped, and approximately.
- Tail: also called the flagellum, is the longest part, at approximately. It is capable of wave-like motion that propels sperm for swimming and aids in the penetration of the egg. The flagellum propels the sperm cell at about. The tail was formerly thought to move symmetrically in a helical shape.
- Neck: also called connecting piece contains one typical centriole and one atypical centriole such as the proximal centriole-like. The proximal centriole is retained in the mature spermatozoon; the distal centriole disappears after axoneme assembly. The proximal centriole enters into the ovum, which has no centriole, and starts the first cleavage division of the zygote thus formed. The distal centriole gives rise to the axial filament which forms the tail and has a arrangement. A transitory membrane called the Manchette lies in the midpiece.
- Midpiece: It has 10–14 spirals of mitochondria surrounding the axial filament in the cytoplasm. It provides motility, and hence is called the powerhouse of the sperm. It also has a ring centriole that form a diffusion barrier between the midpiece and the principal piece and serve as a stabilizing structure for tail rigidity.
During fertilization, the sperm provides three essential parts to the oocyte: a signalling or oocyte-activating factor, which causes the metabolically dormant oocyte to activate; the haploid paternal genome; the centriole, which is responsible for forming the centrosome and microtubule system. It may also contribute with paternal messenger RNA, also contributing to embryonic development.
The spermatozoon is characterized by a minimum of cytoplasm and the most densely packed DNA known in eukaryotes. Compared to mitotic chromosomes in somatic cells, sperm DNA is at least sixfold more highly condensed.
The human spermatozoon contains at least 7500 different proteins.
Human sperm genetics has been associated with human evolution, per a 2020 study.
In humans, recombination rates differ between maternal and paternal DNA:
- Maternal DNA: Recombines approximately 42 times on average.
- Paternal DNA: Recombines approximately 27 times on average.
Sperm size
Sperm activation
Approaching the egg cell is a rather complex, multistep process of chemotaxis guided by different chemical substances/stimuli on individual levels of phylogeny. One of the most significant, common signaling characters of the event is that a prototype of professional chemotaxis receptors, formyl peptide receptor as well as the activator ability of its ligand formyl Met-Leu-Phe have been demonstrated in the surface membrane even in the case of human sperms.Mammalian sperm cells become even more active when they approach an egg cell in a process called sperm activation. Sperm activation has been shown to be caused by calcium ionophores in vitro, progesterone released by nearby cumulus cells and binding to ZP3 of the zona pellucida. The cumulus cells are embedded in a gel-like substance made primarily of hyaluronic acid, and developed in the ovary with the egg and support it as it grows.
The initial change is called "hyperactivation", which causes a change in spermatozoa motility. They swim faster and their tail movements become more forceful and erratic.
A recent discovery links hyperactivation to a sudden influx of calcium ion into the tails. The whip-like tail of the sperm is studded with ion channels formed by proteins called CatSper. These channels are selective, allowing only calcium ions to pass. The opening of CatSper channels is responsible for the influx of calcium. The sudden rise in calcium levels causes the flagellum to form deeper bends, propelling the sperm more forcefully through the viscous environment. Sperm hyperactivity is necessary for breaking through two physical barriers that protect the egg from fertilization.
The second process in sperm activation is the acrosome reaction. This involves releasing the contents of the acrosome, which disperse, and the exposure of enzymes attached to the inner acrosomal membrane of the sperm. This occurs after the sperm first meets the egg. This lock-and-key type mechanism is species-specific and prevents the sperm and egg of different species from fusing. There is some evidence that this binding is what triggers the acrosome to release the enzymes that allow the sperm to fuse with the egg.
ZP3, one of the proteins that make up the zona pellucida, then binds to a partner molecule on the sperm. Enzymes on the inner acrosomal membrane digest the zona pellucida. After the sperm penetrates the zona pellucida, part of the sperm's cell membrane then fuses with the egg cell's membrane, and the contents of the head diffuse into the egg.
Upon penetration, the oocyte is said to have become activated. It undergoes its secondary meiotic division, and the two haploid nuclei fuse to form a zygote. In order to prevent polyspermy and minimise the possibility of producing a triploid zygote, several changes to the egg's zona pellucida renders them impenetrable shortly after the first sperm enters the egg.