Fertilization is the fusion of haploid gametes, egg and sperm, to form the diploid zygote. Note though there can be subtle differences in the fertilization process which occurs naturally within the body or through reproductive technologies outside the body, the overall product in both cases is a diplod zygote. In fertilization research, after humans the mouse is the most studied species followed by domestic and farm animals. The process of fertilization involves components of, and signaling between, both sperm (spermatozoa) and egg (oocyte). In addition to in vivo fertilization there are many new in vitro technologies related to human infertility (Assisted Reproductive Technologies]] and animal production somatic cell nuclear transfer (SCNT) to generate a zygote.
- Fertilization Links: Oocyte | Spermatozoa | Fertilization | Assisted Reproductive Technology | In Vitro Fertilization | Lecture - Fertilization | Lecture - Week 1 and 2 | Lecture - Genital Development| Menstrual Cycle | Morula | Blastocyst
Some Recent Findings
- Understand the mechanisms of gamete formation.
- Understand the mechanisms of cell division.
- Describe the differences between mitosis and meiosis.
- Understand the mechanisms of fertilization, both in vivo and in vitro.
- Describe the cleavage of the zygote.
- Have a preliminary understanding of the role and process in male sex determination and X inactivation.
- Understand the abnormalities that occur during this period of development.
Prior to the fertilization process commencing both the gametes oocyte (egg) and spermatozoa (sperm) require completion of a number of biological processes.
- Oocyte Meiosis - completes Meiosis 1 and commences Meiosis 2 (arrests at Metaphase II).
- Spermatozoa Capacitation - following release (ejaculation) and mixing with other glandular secretions, activates motility and acrosome preparation.
- Migration - both Oocyte and Spermatozoa.
- oocyte ovulation and release with associated cells, from ovary into fimbria then into uterine tube (oviduct, uterine horn, fallopian tube) and epithelial cilia mediated movement.
- spermatozoa ejaculation, deposited in vagina, movement of tail to "swim" in uterine secretions through cervix, uterine body and into uterine tube, have approximately 24-72h to fertilize oocyte.
- Process of oogonia mature into oocytes (ova, ovum, egg)
- all oogonia form primary oocytes before birth, therefore a maturation of preexisting cells in the female gonad, ovary
- humans usually only 1 ovum released every menstrual cycle (IVF- superovulation)
- oocyte and its surrounding cells = follicle
- primary -> secondary -> ovulation releases
Ovary- Histology - whole transverse section (cortex, medulla)
- Primary Oocyte - arrested at early Meiosis 1
- diploid: 22 chromosome pairs + 1 pair X chromosomes (46, XX)
- autosomes and sex chromosome
- Oogenesis- pre-antral then antral follicle (Graafian follicle is mature antral follicle released)
- Secondary oocyte
- 1 Day before ovulation completes (stim by LH) Meiosis 1
- haploid: 22 chromosomes + 1 X chromosome (23, X)
- nondisjunction- abnormal chromosome segregation
- begins Meiosis 2 and arrests at metaphase
- note no interphase replication of DNA, only fertilization will complete Meiosis 2
Ovulation (HPG Axis)
- Hypothalmus releases gonadotropin releasing hormone (GRH, luteinizing hormone–releasing hormone, LHRH) -> Pituitary releases follicle stimulating hormone (FSH) and lutenizing hormone (LH) -> ovary follicle development and ovulation.
- release of the secondary oocyte and formation of corpus luteum
- secondary oocyte encased in zona pellucida and corona radiata
- Ovulation associated with follicle rupture and ampulla movement.
- glycoprotein shell ZP1, ZP2, ZP3
- mechanical protection of egg
- involved in the fertilization process
- sperm binding
- adhesion of sperm to egg
- acrosome reaction
- releases enzymes to locally breakdown
- block of polyspermy
- altered to prevent more than 1 sperm penetrating
- may also have a role in development of the blastocyst
- granulosa cells and extracellular matrix
- protective and nutritional role for cells during transport
- cells are also lost during transport along oviduct
Gamete formation- Spermatogenesis
- process of spermatagonia mature into spermatazoa (sperm)
- continuously throughout life occurs in the seminiferous tubules in the male gonad- testis (plural testes)
- at puberty spermatagonia activate and proliferate (mitosis)
- primary spermatocyte -> secondary spermatocyte-> spermatid->sperm
- Seminiferous Tubule is site of maturation involving meiosis and spermiogenesis
- Spermatogenesis- Meiosis
- meiosis is reductive cell division
- 1 spermatagonia (diploid) 46, XY (also written 44+XY) = 4 sperm (haploid); 23, X 23, X 23, Y 23, Y
- morphological (shape) change from round spermatids to elongated sperm
- loose cytoplasm
- Transform golgi apparatus into acrosome (in head)
- Organize microtubules for motility (in tail, flagellum)
- Segregate mitochondria for energy (in tail)
- Links: Spermatozoa Development
- Human Ejaculate
- By volume <10 % sperm and accessory glands contribute majority of volume (60 % seminal vesicle, 10 % bulbourethral, 30 % prostate)
- 3.5 ml, 200-600 million sperm.
- Capacitation is the removal of glycoprotein coat and seminal proteins and alteration of sperm mitochondria.
- Ovulation-inducing factor identified in several species.
Marcelo H Ratto, Yvonne A Leduc, Ximena P Valderrama, Karin E van Straaten, Louis T J Delbaere, Roger A Pierson, Gregg P Adams The nerve of ovulation-inducing factor in semen. Proc. Natl. Acad. Sci. U.S.A.: 2012, 109(37);15042-7 PMID:22908303
- Infertility can be due to Oligospermia, Azoospermia, Immotile Cilia Syndrome
- Oligospermia (Low Sperm Count) - less than 20 million sperm after 72 hour abstinence from sex
- Azoospermia (Absent Sperm) - blockage of duct network
- Immotile Cilia Syndrome - lack of sperm motility
Clinical guidelines have typically identified the "fertile window" between days 10 and 17 within the typical 28 day menstrual cycle.
- fertile window occurred during a broad range of days in the menstrual cycle.
- between days 6 and 21 women had at minimum a 10% probability of being in their fertile window.
- women cannot predict a sporadic late ovulation; 4 - 6% of women whose cycles had not yet resumed were potentially fertile in the fifth week of their cycle.
- only about 30% of women is the fertile window entirely within the days of the menstrual cycle identified by clinical guidelines (between days 10 and 17)
- most women reach their fertile window earlier and others much later.
- women should be advised that the timing of their fertile window can be highly unpredictable, even if their cycles are usually regular.
- Fertilization usually occurs in first 1/3 of oviduct
- Fertilization can also occur outside oviduct, associated with In Vitro Fertilization (IVF, GIFT, ZIFT...) and ectopic pregnancy
- The majority of fertilized eggs do not go on to form an embryo
Fertilization - Spermatozoa
- Sperm Binding - zona pellucida protein ZP3 acts as receptor for sperm
- Acrosome Reaction - exyocytosis of acrosome contents (Calcium mediated) MBoC - Figure 20-31. The acrosome reaction that occurs when a mammalian sperm fertilizes an egg
- enzymes to digest the zona pellucida
- exposes sperm surface proteins to bind ZP2
- Membrane Fusion - between sperm and egg, allows sperm nuclei passage into egg cytoplasm
- Membrane Depolarization - in non-mammalian species, caused by sperm membrane fusion, acts as a primary block to polyspermy.
- Cortical Reaction - IP3 pathway elevates intracellular Calcium, exocytosis of cortical granules MBoC - Figure 20-32. How the cortical reaction in a mouse egg is thought to prevent additional sperm from entering the egg
- enzyme alters ZP3 so it will no longer bind sperm plasma membrane
- Meiosis 2 - completion of 2nd meiotic division
- forms second polar body (a third polar body may be formed by meiotic division of the first polar body)
- Contact between sperm and oocyte egg coat (zona pellucida [ZP]) glycoproteins triggers increases in intracellular calcium ion (iCa2+) concentration in sperm
- CATSPER channels on the distal portion of sperm (the principal piece) are required for the ZP-induced iCa2+ increases
- iCa2+ increase starts from the sperm tail and propagates toward the head
- Store depletion-activated Ca(2+) entry is thought to mediate the sustained phase
Formation of the Zygote
- Pronuclei - Male and Female haploid nuclei approach each other and nuclear membranes break down
- chromosomal pairing, DNA replicates, first mitotic division
- Spermatozoa contributes - centriole which organizes mitotic spindle
- Oocyte contributes - mitochondria (maternally inherited)
- Mitochondria of the spermatozoa are specifically destroyed in early development by proteolysis (mouse 4 to 8 cell transition).
- Metaphase II oocytes in rats have an average mitochondrial DNA (mtDNA) copy number of 147,600 (+/-3000) that only increases at the 8-cell stage.
- based upon whether an X or Y carrying sperm has fertilized the egg, should be 1.0 sex ratio.
- actually 1.05, 105 males for every 100 females, some studies show more males 2+ days after ovulation.
- cell totipotent (equivalent to a stem cell, can form any tissue of the body)
Men - Y Chromosome
- Y Chromosome carries Sry gene, protein product activates pathway for male gonad (covered in genital development)
Women - X Chromosome
- Gene dosage, one X chromosome in each female embryo cell has to be inactivated
- process is apparently random and therefore 50% of cells have father's X, 50% have mother's X
- Note that because men only have 1 X chromosome, if abnormal, this leads to X-linked diseases more common in male that female where bothe X's need to be abnormal.
Fertilization Protein Changes
A recent study in mice has shown that after fertilization the maternal proteins present in the original oocyte are quickly degraded by the zygote stage. MII oocytes have 185,643 different peptides while zygotes contain only 85,369 peptides.
Protein Expression Classified by Molecular Functions
The abnormalities listed below relate to genetic abnormalities occurring during fertilisation. There are of course many additional genetic abnormalities inherited and introduced by the recombined maternal and paternal genomes, other than trisomy 21 these will not be covered here.
Complete Hydatidiform Mole
- Chromosomal genetic material from the oocyte (ovum, egg) is lost, by an unknown process.
- Fertilization then occurs with one or two spermatozoa and an androgenic (from the male only) conceptus (fertilized oocyte) is formed.
- The embryo (fetus, baby) does not develop at all but the placenta does grow.
Partial Hydatidiform Mole
Three sets of chromosomes instead of the usual two and this is called triploidy.
- chromosomal (genetic) material from the oocyte (ovum, egg) is retained and the egg is fertilized by one or two spermatozoa.
- with partial mole there are maternal chromosomes and there is a fetus.
- the three sets of chromosomes means the fetus is always grossly abnormal and will not survive.
(Greek, parthenos = virgin, genesis = birth) The development of an unfertilized oocyte (no spermatozoa). Other than mammals, many different species (plants, insects, reptiles) can develop from unfertilized eggs. An embryo so formed without sperm contribution. Blocking of parthenogenesis in mammals appears to be related to genomic imprinting. Abnormal parthenogenic processes can occur in mammals, and more recently a parthenogenic mouse has been made in the laboratory.
Trisomy 21 (Down's or Down syndrome) is caused by nondisjunction of chromosome 21 in a parent who is chromosomally normal and is one of the most common chromosomal aneuploidy abnormalities in liveborn children. The frequency of trisomy 21 in the population is approximately 1 in 650 to 1,000 live births, in Australia between 1991-97 there were 2,358 Trisomy 21 infants. There are other less frequently occurring trisomies (18, 13 and X).
- Links: Trisomy 21
- ↑ Yasuhiro Yamauchi, Jeffrey A Shaman, W Steven Ward Non-genetic contributions of the sperm nucleus to embryonic development. Asian J. Androl.: 2011, 13(1);31-5 PMID:20953203
- ↑ Jégou A, Ziyyat A, Barraud-Lange V, Perez E, Wolf JP, Pincet F, Gourier C. CD9 tetraspanin generates fusion competent sites on the egg membrane for mammalian fertilization Proc Natl Acad Sci U S A. 2011 Jun 20. [Epub ahead of print] PMID: 21690351
- ↑ Gagandeep Gahlay, Lyn Gauthier, Boris Baibakov, Olga Epifano, Jurrien Dean Gamete recognition in mice depends on the cleavage status of an egg's zona pellucida protein. Science: 2010, 329(5988);216-9 PMID:20616279
- ↑ Paul M Wassarman Zona pellucida glycoproteins. J. Biol. Chem.: 2008, 283(36);24285-9 PMID:18539589 | J Biol Chem.
- ↑ A J Wilcox, D Dunson, D D Baird The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ: 2000, 321(7271);1259-62 PMID:11082086 | PMC27529 | BMJ
- ↑ Jingsheng Xia, Dejian Ren Egg coat proteins activate calcium entry into mouse sperm via CATSPER channels. Biol. Reprod.: 2009, 80(6);1092-8 PMID:19211808
- ↑ J M Cummins Fertilization and elimination of the paternal mitochondrial genome. Hum. Reprod.: 2000, 15 Suppl 2();92-101 PMID:11041517
- ↑ Yuichi Kameyama, France Filion, Jae Gyu Yoo, Lawrence C Smith Characterization of mitochondrial replication and transcription control during rat early development in vivo and in vitro. Reproduction: 2007, 133(2);423-32 PMID:17307910
- ↑ Shufang Wang, Zhaohui Kou, Zhiyi Jing, Yu Zhang, Xinzheng Guo, Mengqiu Dong, Ian Wilmut, Shaorong Gao Proteome of mouse oocytes at different developmental stages. Proc. Natl. Acad. Sci. U.S.A.: 2010, 107(41);17639-44 PMID:20876089 | PNAS
- Human Embryology (2nd ed.) Larson Ch1 p1-32
- The Developing Human: Clinically Oriented Embryology (6th ed.) Moore and Persaud
- Before we Are Born (5th ed.) Moore and Persaud Ch 2 p14-33
- Essentials of Human Embryology Larson Ch1 p1-16
- Human Embryology Fitzgerald and Fitzgerald Ch2 p8-14
- fertilization - All (51803) Review (5928) Free Full Text (11715)
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Cite this page:
Hill, M.A. (2013) Fertilization. Retrieved June 20, 2013, from http://php.med.unsw.edu.au/embryology/index.php?title=Fertilization
- Dr Mark Hill 2013, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G