Internal and external fertilization
Depending on the species, spermatozoa can fertilize ova externally
or internally. In external fertilization, the spermatozoa fertilize the
ova directly, outside of the female's sexual organs. Female fish, for
example, spawn ova into their aquatic environment, where they are
fertilized by the semen of the male fish.
During internal fertilization, however, fertilization occurs inside
the female's sexual organs. Internal fertilization takes place after
insemination of a female by a male through copulation. In low
vertebrates (amphibians, reptiles, birds and monotreme mammals),
copulation is achieved through the physical mating of the cloaca of the
male and female. In marsupial and placental mammals, copulation occurs
through the vagina.
Composition of human semen
During the process of ejaculation, sperm passes through the
ejaculatory ducts and mixes with fluids from the seminal vesicles, the
prostate, and the bulbourethral glands to form the semen.
The seminal
vesicles produce a yellowish viscous fluid rich in fructose and other
substances that makes up about 70% of human semen.
The prostatic
secretion, influenced by dihydrotestosterone, is a whitish (sometimes
clear), thin fluid containing proteolytic enzymes, citric acid, acid
phosphatase and lipids.
Sertoli cells, which nurture and support developing spermatocytes,
secrete a fluid into seminiferous tubules that helps transport sperm to
the genital ducts. The ductuli efferentes possess cuboidal cells with
microvilli and lysosomal granules that modify the semen by reabsorbing
some fluid.
Once the semen enters the ductus epididymis the principle
cells, which contain pinocytotic vessels indicating fluid reabsorption,
secrete glycerophosphocholine which most likely inhibits premature
capacitation. The accessory genital ducts, the seminal vesicle,
prostate glands, and the bulbourethral glands, produce most of the
seminal fluid.
Seminal plasma of humans contains a complex range of organic and
inorganic constituents.
The seminal plasma provides a nutritive and protective medium for
the spermatozoa during their journey through the female reproductive
tract.
The normal environment of the vagina is a hostile one for sperm
cells, as it is very acidic (from the native microflora producing
lactic acid), viscous, and patrolled by immune cells.
The components in
the seminal plasma attempt to compensate for this hostile environment.
Basic amines such as putrescine, spermine, spermidine and cadaverine
are responsible for the smell and flavor of semen.
These alkaline bases
counteract the acidic environment of the vaginal canal, and protect DNA
inside the sperm from acidic denaturation.
The components and contributions of semen are as follows:
| Gland |
Approximate % |
Description |
| testes |
2-5% |
Approximately 200- to 500-million spermatozoa (also called
''sperm'' or ''spermatozoans''), produced in the testes, are released
per ejaculation. |
| seminal vesicle |
65-75% |
amino acids, citrate, enzymes, flavins, fructose (the main
energy source of sperm cells, which rely entirely on sugars from the
seminal plasma for energy), phosphorylcholine, prostaglandins (involved
in suppressing an immune response by the female against the foreign
semen), proteins, vitamin C |
| prostate |
25-30% |
acid phosphatase, citric acid, fibrinolysin, prostate
specific antigen, proteolytic enzymes, zinc (serves to help to
stabilize the DNA-containing chromatin in the sperm cells. A zinc
deficiency may result in lowered fertility because of increased sperm
fragility. Zinc deficiency can also adversely affect spermatogenesis.) |
| bulbourethral glands |
< 1% |
galactose, mucus (serve to increase the mobility of sperm
cells in the vagina and cervix by creating a less viscous channel for
the sperm cells to swim through, and preventing their diffusion out of
the semen. Contributes to the cohesive jelly-like texture of semen.),
pre-ejaculate, sialic acid |
A 1992 World Health Organization report described normal human semen
as having a volume of 2 ml or greater, pH of 7.2 to 8.0, sperm
concentration of 20x106 spermatozoa/ml or more, sperm count
of 40x106 spermatozoa per ejaculate or more, and motility of
50% or more with forward progression (categories a and b) of 25% or
more with rapid progression (category a) within 60 minutes of
ejaculation.
Further Reading
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"Semen"
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