question: How does the sperm know that it is supposed to look for the egg and to find it?
comment: As ridiculous as it sounds, a sperm finds an egg because it can smell it. It moves in reaction to specific substances being released by an egg that contact its olfactory receptors, and this automatic action guides it to the source of where those chemicals are coming from.
passages: Section::::Sperm guidance in mammals.:III. Rheotaxis.
   Section::::Sperm guidance in mammals.:All mechanisms combined.
   Section::::Sperm guidance in non-mammalian species.:Molecular mechanism.
   Sperm guidance
  
  Sperm guidance is the process by which sperm cells (spermatozoa) are directed to the oocyte (egg) for the aim of fertilization. In the case of marine invertebrates the guidance is done by chemotaxis. In the case of mammals, it appears to be done by chemotaxis, thermotaxis and rheotaxis.
  
  Section::::Background.
   Section::::Sperm guidance in mammals.:I. Chemotaxis.:Molecular mechanism.
   Section::::Sperm guidance in non-mammalian species.
  
  Sperm guidance in non-mammalian species is performed by chemotaxis. The oocyte secretes a chemoattractant, which, as it diffuses away, forms a concentration gradient: a high concentration close to the egg, and a gradually lower concentration as the distance from the oocyte increases. Spermatozoa can sense this chemoattractant and orient their swimming direction up the concentration gradient towards the oocyte. Sperm chemotaxis was demonstrated in a large number of non-mammalian species, from marine invertebrates to frogs.
  
  Section::::Sperm guidance in non-mammalian species.:Chemoattractants.
   Section::::Sperm guidance in mammals.
  
  Three different guidance mechanisms have been proposed to occur in the mammalian oviduct: thermotaxis, rheotaxis, and chemotaxis. Indeed, due to obvious restrictions, all these mechanisms were demonstrated "in vitro" only. However, the discoveries of proper stimuli in the female – an ovulation-dependent temperature gradient in the oviduct, post-coitus oviductal fluid flow in female mice, and sperm chemoattractants secreted from the oocyte and its surrounding cumulus cells, respectively – strongly suggest the mutual occurrence of these mechanisms "in vivo".
  
  Section::::Sperm guidance in mammals.:I. Chemotaxis.
   DNA damage in sperm cells may be detected by using Raman spectroscopy. It is not specific enough to detect individual traits, however. The sperm cells having least DNA damage may subsequently be injected into the egg cell by intracytoplasmic sperm injection (ICSI). Many other methods for sperm sorting have been proposed or are currently tested.
   Section::::Sperm guidance in mammals.:II. Thermotaxis.:Behavioral mechanism of mammalian sperm thermotaxis.
   Section::::Sperm guidance in mammals.:II. Thermotaxis.:Establishing sperm thermotaxis as an active process.
   Figure 2. A simplified scheme describing the suggested sequence of active sperm guidance mechanisms in mammals. In addition, two passive processes, sperm rheotaxis and contractions of the oviduct, may assist sperm movement towards the fertilization site.
   Section::::Sperm guidance in mammals.:II. Thermotaxis.:Temperature sensing.
  
  The response of spermatozoa to temporal temperature changes even when the temperature is kept constant spatially suggests that, as in the case of human sperm chemotaxis, sperm thermotaxis involves temporal gradient sensing. In other words, spermatozoa apparently compare the temperature (or a temperature-dependent function) between consecutive time points. This, however, does not exclude the occurrence of spatial temperature sensing in addition to temporal sensing.
   Like in any other highly essential system in biology, mammalian sperm guidance is expected to involve redundancy. Indeed, at least three guidance mechanisms are likely to act in the female genital tract, two active mechanisms — chemotaxis and thermotaxis, and a passive mechanism — rheotaxis. When one of these mechanisms is not functional for any reason, guidance is not expected to be lost and the cells should still be able to navigate to the oocyte. This resembles guidance of migrating birds, where the birds’ navigation is unaffected when one of the guidance mechanisms is not functional. 
   Human sperm and eggs also lack molecules for the immune recognition of "self". These immune markers are also known as major histocompatibility complex (MHC) antigens or more specifically in humans as human leukocyte antigens (HLA)). Therefore, a major question is how are human gametes recognized by immune effector cells. Specifically, their lack of MHC recognition markers should trigger the immune system, resulting in lysis of both sperm and eggs by leukocytes known as natural killer, or NK cells. These cells target and kill other cells lacking such MHC markers, a concept known as "missing self". One distinct possibility is that sperm and eggs are recognized via oligosaccharides expressed on their surfaces. For example, human gametes are coated with carbohydrate sequences that have been implicated in the suppression of NK cell mediated responses.
   When progesterone is the dominant hormone, as it would be in the second half of a normal cycle, the fern pattern is no longer discernible, and the pattern is completely absent by the 22nd day of a woman's cycle. The disappearance of the fern pattern after the 22nd day suggests ovulation, and its persistence throughout the menstrual cycle suggests an-ovulation (infertility).
   One of the processes used is the 'swim up' method, whereby a sperm extender is added to the donor's freshly ejaculated sperm and the test-tube is left to settle. After about half-an-hour, the lighter sperm, containing the male chromosome pair (XY), will have swum to the top, leaving the heavier sperm, containing the female chromosome pair (XX), at the bottom, thus allowing selection and storage according to sex.
   It has become clear to reproductive urologists who regularly perform testis sperm retrieval in azoospermic men for IVF-ICSI that spermatogenesis varies geographically within the testis. This underscores the limitation and inadequacy of a single, localized testis biopsy or a single FNA specimen to accurately reflect the biology of the entire organ. In fact, the current clinical challenges are: (1) to determine which infertile men with azoospermia harbor sperm for IVF-ICSI and (2) to precisely locate the areas of sperm production within atrophic, nonobstructed testes. This clinical need led to the development of testis FNA “mapping” in male infertility by a team of University of California San Francisco physicians led by Dr. Paul J. Turek (Figure 1).
   Section::::Sperm guidance in mammals.:I. Chemotaxis.:Species specificity.
  
  Species specificity was not detected in experiments that compared the chemotactic responsiveness of human and rabbit spermatozoa to follicular fluids or egg-conditioned media obtained from human, bovine, and rabbit. The subsequent findings that cumulus cells of both human and rabbit (and, probably, of other mammals as well) secrete the chemoattractant progesterone is sufficient to account for the lack of specificity in the chemotactic response of mammalian spermatozoa.
  
  Section::::Sperm guidance in mammals.:I. Chemotaxis.:Behavioral mechanism.
   In contrast, in natural fertilization sperm compete and when the first sperm penetrates the oolemma, the oolemma hardens to block the entry of any other sperm. Concern has been raised that in ICSI this sperm selection process is bypassed and the sperm is selected by the embryologist without any specific testing. However, in mid-2006 the FDA cleared a device that allows embryologists to select mature sperm for ICSI based on sperm binding to hyaluronan, the main constituent of the gel layer (cumulus oophorus) surrounding the oocyte. The device provides microscopic droplets of hyaluronan hydrogel attached to the culture dish. The embryologist places the prepared sperm on the microdot, selects and captures sperm that bind to the dot. Basic research on the maturation of sperm shows that hyaluronan-binding sperm are more mature and show fewer DNA strand breaks and significantly lower levels of aneuploidy than the sperm population from which they were selected. A brand name for one such sperm selection device is PICSI. A recent clinical trial showed a sharp reduction in miscarriage with embryos derived from PICSI sperm selection.
   Detectives Benson and Amaro (Mariska Hargitay and Danny Pino) investigate the disappearance of a girl, Cate (Shannon Maree Walsh), who was researching children born through sperm donation. They learn that Cate was conceived with donor sperm, and that she had tracked down the donor. She was going to meet the man against her mother's wishes.
   Here, too, there is no single rule. In some species (for example, in hydroids like "Campanularia" or tunicate like "Ciona"), the swimming direction of the spermatozoa changes abruptly towards the chemoattractant source. In others (for example, in sea urchin, hydromedusa, fern, or fish such as Japanese bitterlings), the approach to the chemoattractant source is indirect and the movement is by repetitive loops of small radii. In some species (for example, herring or the ascidian Ciona) activation of motility precedes chemotaxis. In chemotaxis, cells may either sense a temporal gradient of the chemoattractant, comparing the occupancy of its receptors at different time points (as do bacteria), or they may detect a spatial gradient, comparing the occupancy of receptors at different locations along the cell (as do leukocytes). In the best-studied species, sea urchin, the spermatozoa sense a temporal gradient and respond to it with a transient increase in flagellar asymmetry. The outcome is a turn in the swimming path, followed by a period of straight swimming, leading to the observed epicycloid-like movements directed towards the chemoattractant source.
   Microsatellite markers have been demonstrated to nullify these problems in "P. annularis". They are single-locus, codominant markers that vary dramatically for number of simple sequence repeats. They can be readily amplified using the polymerase chain reaction, and the samples of DNA can be minuscule. This allows scientists to access the genotypes of very young embryos and even sperm from spermathecae. Through the genotypes of stored sperm, alleles of dead fathers are recovered. They can then combine these data with the maternal genotype to improve the accuracy of the maternity assignment.
  
  Section::::Taxonomy and phylogeny.
   Oocyte activation
  
  Oocyte (or ovum/egg) activation is a series of processes that occur in the oocyte during fertilization.
  
  Sperm entry causes calcium release into the oocyte. In mammals, this has been proposed to be caused by the introduction of phospholipase C isoform zeta (PLCζ) from the sperm cytoplasm, although this remains to be established definitively. Activation of the ovum includes the following events:
  
  BULLET::::- Cortical reaction to block against other sperm cells
  
  BULLET::::- Activation of egg metabolism
  
  BULLET::::- Reactivation of meiosis
  
  BULLET::::- DNA synthesis
  
  Section::::Sperm trigger of egg activation.
   Section::::Other techniques.
  
  Ovulation predictor kits (OPKs) can detect imminent ovulation from the concentration of lutenizing hormone (LH) in a woman's urine. A positive OPK is usually followed by ovulation within 12–36 hours.
  
  Saliva microscopes, when correctly used, can detect ferning structures in the saliva that precede ovulation. Ferning is usually detected beginning three days before ovulation, and continuing until ovulation has occurred. During this window, ferning structures occur in cervical mucus as well as saliva.
   It should be noted that chemotaxis is a short-range guidance mechanism. As such, it can guide spermatozoa for short distances only, estimated at the order of millimeters.
  
  Section::::Sperm guidance in mammals.:I. Chemotaxis.:Chemoattractants.
  