HumanHumans are the only living species in the Homo genus...
fertilization is the union of a humanoid
eggAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
and
spermA spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...
, usually occurring in the
ampulla of the uterine tubeThe ampulla is the second portion of the uterine tube. It is an intermediate dilated portion, which curves over the ovary.It is the most common site of human fertilization.The word ampulla is from Latin for flask....
. The result of this union is the production of a zygote, or fertilized egg, initiating prenatal development. Scientists discovered the dynamics of human fertilization in the nineteenth century.
HumanHumans are the only living species in the Homo genus...
fertilization is the union of a humanoid
eggAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
and
spermA spermatozoon is a motile sperm cell, or moving form of the haploid cell that is the male gamete. A spermatozoon joins an ovum to form a zygote...
, usually occurring in the
ampulla of the uterine tubeThe ampulla is the second portion of the uterine tube. It is an intermediate dilated portion, which curves over the ovary.It is the most common site of human fertilization.The word ampulla is from Latin for flask....
. The result of this union is the production of a zygote, or fertilized egg, initiating prenatal development. Scientists discovered the dynamics of human fertilization in the nineteenth century.
The process of fertilization involves a sperm fusing with an ovum—usually following
ejaculationEjaculation is the ejecting of semen from the male reproductory tract, and is usually accompanied by orgasm. It is usually the final stage and natural objective of male sexual stimulation, and an essential component of natural conception. In rare cases ejaculation occurs because of prostatic disease...
during copulation. It is possible, but less probable, for fertilization to occur without copulation,
artificial inseminationArtificial insemination, or AI, is the process by which sperm is placed into the reproductive tract of a female for the purpose of impregnating the female by using means other than sexual intercourse or natural insemination...
, or
In vitro fertilization. Upon encountering the ovum, the acrosome of the sperm produces enzymes which allow it to burrow through the outer jelly coat of the egg. The sperm plasma then fuses with the egg's plasma membrane, the sperm head disconnects from its flagellum and the egg travels down the Fallopian tube to reach the uterus.
In vitro fertilization (IVF) is a process by which egg cells are fertilized by sperm outside the womb,
in vitroIn vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
.
Corona radiata
The egg and the sperm bind through the
corona radiataThe corona radiata surround an ovum or unfertilized egg cell, and consist of two or three strata of follicular cells. They are attached to the outer protective layer of the ovum, the zona pellucida, and their main purpose in many animals is to supply vital proteins to the cell...
, a layer of follicle cells on the outside of the secondary
oocyteAn oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...
. Fertilization occurs when the nuclei of a sperm and an egg fuse. The successful fusion of gametes form a new organism.
Cone of attraction and Vitelline membrane
Where the spermatozoon is about to pierce, the yolk (ooplasm) is drawn out into a conical elevation, termed the cone of attraction. Once the spermatozoon has entered, the peripheral portion of the yolk changes into a membrane, the vitelline membrane, which prevents the passage of additional spermatozoa.
Acrosome reaction
The acrosome reaction must occur to mobilise enzymes within the head of the spermatozoon to degrade the zona pellucida.
example: hyaluronidase.
Zona pellucida
After binding to the corona radiata the sperm reaches the
zona pellucidaThe zona pellucida is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the latter, external but of essential importance to it...
, which is an extra-cellular matrix of glycoproteins. A special complementary molecule on the surface of the sperm head binds to a ZP2 glycoprotein in the zona pellucida. This binding triggers the acrosome to burst, releasing enzymes that help the sperm get through the zona pellucida.
Some sperm cells consume their
acrosomeThe acrosome is an organelle that develops over the anterior half of the head in the spermatozoa of many animals. It is a cap-like structure derived from the Golgi apparatus. Acrosome formation is completed during testicular maturation. In Eutherian mammals the acrosome contains digestive enzymes...
prematurely on the surface of the egg cell, facilitating the penetration by other sperm cells. As a population, sperm cells have on average 50% genome similarity so the premature acrosomal reactions aid fertilization by a member of the same cohort. It may be regarded as a mechanism of
kin selectionKin selection refers to apparent strategies in evolution that favor the reproductive success of an organism's relatives, even at a cost to the organism's own survival and reproduction. Charles Darwin was the first to discuss the concept of group/kin selection...
.
Recent studies have shown that the egg is not passive during this process.
Cortical reaction
Once the sperm cells find their way past the zona pellucida, the
cortical reactionThe cortical reaction occurs when a sperm cell unites with the egg's plasma membrane, modification of the zona pellucida that blocks polyspermy; enzymes released by cortical granules digest sperm receptor proteins ZP2 and ZP3 so that they can no longer bind sperm, in mammals.The cortical reaction...
occurs: cortical granules inside the secondary oocyte fuse with the plasma membrane of the cell, causing enzymes inside these granules to be expelled by exocytosis to the zona pellucida. This in turn causes the glyco-proteins in the zona pellucida to cross-link with each other—that is, the enzymes cause the ZP2 to hydrolyse into ZP2f—making the whole matrix hard and impermeable to sperm. This prevents fertilization of an egg by more than one sperm.
Fusion
After the sperm enters the cytoplasm of the oocyte, the cortical reaction takes place, preventing other sperm from fertilizing the same egg. The oocyte now undergoes its second meiotic division producing the haploid ovum and releasing a polar body. The sperm nucleus then fuses with the ovum, enabling fusion of their genetic material.
Transformations
In preparation for the fusion of their genetic material both the oocyte and the sperm undergo transformations as a reaction to the fusion of cell membranes.
The
oocyteAn oocyte, ovocyte, or rarely ocyte, is a female gametocyte or germ cell involved in reproduction. In other words, it is an immature ovum, or egg cell. An oocyte is produced in the ovary during female gametogenesis. The female germ cells produce a primordial germ cell which undergoes a mitotic...
completes its
second meiotic divisionMeiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....
. This results in a mature
ovumAn ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
. The nucleus of the oocyte is called a
pronucleusA pronucleus is the nucleus of a sperm or an egg cell during the process of fertilization, after the sperm enters the ovum, but before they fuse. Sperm and egg cells are haploid, meaning they carry half the number of chromosomes...
in this process, to distinguish it from the nuclei that are the result of fertilization.
The sperm's tail and mitochondria degenerate with the formation of the male
pronucleusA pronucleus is the nucleus of a sperm or an egg cell during the process of fertilization, after the sperm enters the ovum, but before they fuse. Sperm and egg cells are haploid, meaning they carry half the number of chromosomes...
. This is why all mitochondria in humans are of maternal origin.
Replication
The pronuclei migrate toward the center of the oocyte, rapidly replicating their
DNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
as they do so to prepare the embryo for its first mitotic division.
Mitosis
The male and female pronuclei don't fuse, although their genetic material do. Instead, their membranes dissolve, leaving no barriers between the male and female
chromosomeA chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...
s. During this dissolution, a
mitotic spindleIn cell biology, the spindle fibers are the structure that separates the chromosomes into the daughter cells during cell division. It is part of the cytoskeleton in eukaryotic cells...
forms between them. The spindle captures the chromosomes before they disperse in the egg cytoplasm. Upon subsequently undergoing mitosis (which includes pulling of chromatids towards centrioles in anaphase) the cell gathers genetic material from the male and female together. Thus, the first mitosis of the union of sperm and oocyte is the actual fusion of their chromosomes.
Each of the two daughter cells resulting from that mitosis has one replica of each chromatid that was replicated in the previous stage. Thus, they are genetically identical.
Diseases
Various disorders can arise from defects in the fertilization process.
- Polyspermy
In biology, polyspermy describes an egg that has been fertilized by more than one sperm. Diploid organisms normally contain two copies of each chromosome, one from each parent. The cell resulting from polyspermy, on the other hand, contains three or more copies of each chromosome -- one from the...
results from multiple sperm fertilizing an egg.
However, some researchers have found that in rare pairs of fraternal twins, their origin might have been from the fertilization of one egg cell from the mother and two sperm cells from the father. This possibility has been investigated by computer simulations of the fertilization process.