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The human egg may only be fertilized by one spermatozoon to prevent polyploidy. In most mammals, the primary block to polyspermy occurs at the zona pellucida (ZP). Little is known of the human ZP and the changes occurring following fertilization to prevent polyploidy. Using antibodies directed against synthetic peptides predicted from the human ZP2 and ZP3 cDNA, we identified ZP3 as a 53–60 kDa glycoprotein and ZP2 as a 90–110 kDa glycoprotein in prophase-I oocytes. Characterization of the ZP from metaphase II arrested eggs (inseminated–unfertilized and fertilized–uncleaved), shows no visible modification of ZP3, but demonstrates that ZP2 undergoes limited proteolysis in the amino terminal domain, to a 60–73 kDa species, denoted ZP2p, which remains linked to the proteolysed fragments by intramolecular disulphide bonds. A lack of ZP2 proteolytic activity in acrosomal supernatants is consistent with an oocyte origin for the protease. The ZP2-specific protease may be released during cortical granule exocytosis which occurs during meiotic maturation and following sperm–egg fusion as part of the block to polyspermy. Since mouse ZP2 acts as a secondary sperm receptor, it is possible that intact ZP2 binds a secondary egg binding protein, whereas cleaved ZP2 does not, suggesting a possible mechanism for the block to polyspermy.
glycosylation, human zona pellucida, proteolysis, ZP2, ZP3
The zona pellucida (ZP) is a transparent, porous, glycoprotein coat that surrounds mammalian eggs. The ZP is formed in the early stages of ovarian follicular development and plays an important role in fertilization and early development.
It contains the species-specific receptors for spermatozoa and can induce the acrosome reaction.
The ZP is involved in establishing the ZP block to polyspermy as well as serving to protect the cleaving embryos as they traverse the female reproductive tract (Wassarman, 1988; Snell and White, 1996).
Much of what is known about the ZP in mammalian fertilization has been obtained from studies on the mouse. The mouse ZP (mZP) is composed of three glycoproteins: ZP1, ZP2 and ZP3 (Bleil and Wassarman, 1980a). Fertilization begins when a capacitated mouse spermatozoon binds the ZP, activating the acrosome reaction.
ZP3 acts as the primary sperm receptor, mediating both initial binding of the spermatozoon to the egg and activation of the acrosome reaction (Bleil and Wassarman, 1980b, 1983).
Following induction of the acrosome reaction, ZP2 acts as the secondary sperm receptor, binding acrosome-reacted spermatozoa and facilitating penetration of the ZP for fusion with the egg plasma membrane (Bleil and Wassarman, 1986).
Following sperm–egg fusion, the egg releases its cortical granule contents into the ZP, modifying the zona to prevent any further sperm binding and penetration. Both ZP2 and ZP3 are modified by the zona reaction: ZP2 undergoes limited proteolysis (Bleil et al., 1981; Moller and Wassarman, 1989) and ZP3 loses both sperm receptor activity and ability to induce the acrosome reaction (Wassarman, 1988).
Human homologues of the mZP1, mZP2 and mZP3 cDNA have been isolated (hZP3, Chamberlain and Dean, 1990; hZP2, Liang and Dean, 1993; hZP1, Harris et al., 1994) and recombinant hZP3 expressed in chinese hamster ovary cells has been shown to activate the acrosome reaction (van Duin et al., 1994).
However, very little is known of the native human ZP (hZP) proteins.
Sodium dodecy sulphate–polyacrylamide gel electrophoresis (SDS–PAGE) analysis of solubilized ZP demonstrates three ZP proteins, 90–110 kDa (ZP1), 64–78 kDa (ZP2) and 57–73 kDa (ZP3), with the 90–110 kDa protein disappearing following fertilization (Shabinowitz and O'Rand, 1988a,b; Bercegeay et al., 1995).
In this study we compare the ZP from prophase I (Pro I) oocytes and metaphase II (Met II) arrested human eggs which were inseminated–unfertilized and fertilized–uncleaved [from failed in-vitro fertilization (IVF) preparations]. ZP2- and ZP3-specific antibodies were used to examine the changes in the ZP which may be responsible for establishing the ZP block to polyspermy.
Materials and methods
Rabbit polyclonal antisera against ZP2 and ZP3 were obtained using synthetic peptides coupled to keyhole limpet haemocyanin (KLH) as immunogen. Coupling to KLH was carried out using m-maleimidobenzoic acid N-hydroxysuccinimide ester (Pierce Chemical Co.
, Rockford, USA) essentially according to manufacturer's instructions.
For the anti-ZP2 serum, synthetic peptides corresponding to amino acid residues 424–440 (CGTRYKFEDDKVVYENE) and 535–545 (NRDDPNIKLVLDDC) (Liang and Dean, 1993) were co-injected, and for the anti-ZP3 serum the synthetic peptide corresponding to residues 327–341 (CGTPSHSRRQPHVMS) (Chamberlain and Dean, 1990) were used. The ZP3 peptide has already been shown to generate antibodies capable of recognizing the native protein (Mahi-Brown and Moran, 1995). Specificity was demonstrated by immunoprecipitation analysis of heat-solubilized iodinated ZP.
Collection of oocytes
Human ZP were obtained from post-mortem-derived Pro I oocytes and Met II arrested eggs obtained from follicular aspirates from patients participating in the IVF–embryo transfer programme at Tygerberg Hospital, Cape Town as previously described (Franken et al., 1991a, 1996).
All prescribed legal and ethical procedures concerning the Human Tissue Act have been fulfilled throughout the study. Pools of Met II arrested eggs, induced by exogenous gonadotrophins, had been subjected to IVF by either regular insemination or intracytoplasmic sperm injection.
Pools of eggs which were inseminated but failed to fertilize or which fertilized but failed to undergo cleavage were used with the patients' consent and represent materials which would normally have been discarded. These eggs have been collectively termed `failed IVF' throughout.
Post-mortem-derived oocytes are termed Pro I oocytes throughout. All eggs were stored in 1.5 mol/l MgCl2, 0.1% polyvinylpyrrolidone, 40 mmol/l HEPES, pH 7.2 at 4°C for up to several weeks prior to use.
ZP retain their biological activity and function following storage in salt solution for at least 1 month (Franken et al., 1991b; Kruger et al., 1991).
Isolation and radiolabelling of ZP
Zonae were mechanically isolated from oocytes using glass micropipettes under a stereomicroscope (Franken et al., 1996). ZP were cleaned of cumulus–corona cells as described (Franken et al.
, 1996) and either heat-solubilized at 70°C for 90 min in distilled water adjusted to pH 9 with Na2CO3 (Dunbar et al., 1980) or acid-solubilized in 5 mmol/l NaH2PO4, pH 2.5 and then neutralized (Bleil and Wassarman, 1980; Franken et al., 1996). Solubilized ZP were iodinated using 0.
1 mCi Na125I (Amersham) and iodobeads (Pierce Chemical Co.) essentially according to manufacturer's instructions.
Immunoprecipitation and gel electrophoresis
Iodinated heat-solubilized ZP were diluted in 50 mmol/l HEPES, pH 7.4, 1% Triton X-100, 5 mmol/l EDTA, 0.1% bovine serum albumin and various protease inhibitors (2 mmol/l phenylmethylsulphonyl fluoride, 5 μg/ml leupeptin, 0.3 U/ml aprotinin).
For non-reducing SDS–PAGE analysis, 50 mM iodoacetamide was included. Immunoprecipitation was carried out essentially as described (Bauskin et al., 1991). Antisera were used at dilutions of 1:75 for anti-ZP3 serum and 1:100 for anti-ZP2 serum.
Immunoprecipitated proteins were analysed on polyacrylamide gels in the presence of SDS either with reduction (Laemmli, 1970) or without (Olson et al., 1988). Experiments intended for parallel analysis by both non-reducing and reducing SDS–PAGE were treated as previously described (Bauskin et al., 1991).
Both reducing and non-reducing gels were 7.5%. Sequential immunoprecipitation analysis was carried out as described (Bauskin et al., 1991).
For endoglycosidase analysis, either heat solubilized iodinated ZP or immunoprecipitated proteins eluted from antibody coated protein A Sepharose beads were treated with N-glycosidase F, endoglycosidase H, O-glycosidase and neuraminidase (Boerhinger Mannheim, Mannheim, Germany) essentially according to manufacturer's instructions.
Preparation of acrosomal extract
Semen was provided by normal donors and motile spermatozoa were selected by the swim-up procedure as previously described (Franken et al., 1994). Acrosomal exocytosis of washed spermatozoa (300×106) was induced by A23187 ionophore (Sigma, St Louis, MO, USA) and Ca2+ as described (Cross et al., 1986).
Confirmation of acrosomal exocytosis was obtained by staining with fluorescein isothiocyanate-conjugated Pisum sativum agglutinin (Sigma) before and after incubation with ionophore and calcium, (Cross et al., 1986). Acrosomal supernatant was collected after centifugation of spermatozoa (400 g, 5 min) for incubation with Pro I ZP.
Pro I ZP were incubated with acrosomal supernatant for 2 h at 37°C prior to solubilization and immunoprecipitation analysis with anti-ZP2-specific serum.
Characterization of the ZP proteins from immature prophase I oocytes
Reducing SDS–PAGE analysis of iodinated heat-solubilized ZP isolated from Pro I oocytes demonstrated two major bands at 95–110 and 57–73 kDa (Figure 1A).
Immunoprecipitation analysis with anti-ZP3 serum raised against a synthetic ZP3 peptide (residues 327–341; Chamberlain and Dean, 1990) demonstrates ZP3 as a 53–60 kDa component of the lower major band (lane 4).
Antiserum raised against two ZP2 synthetic peptides predicted from the ZP2 cDNA sequence (residues 424–440 and 535–545; Liang and Dean, 1993) precipitates the upper major 105–110 kDa band (lane 2). There also appears to be a minor reactivity of the serum with a 60–73 kDa component of the lower major band (lane 2).
As solubilization by heating may not completely solubilize the ZP and result in the formation of supramolecular complexes, sequential immunoprecipitation analysis was performed.
Iodinated heat-solubilized ZP was immunoprecipitated with anti-ZP2 serum, the precipitated proteins eluted with reducing SDS–PAGE sample buffer, then reprecipitated with anti-ZP2 serum. Both the major upper 105–110 kDa band as well as the minor 60–73 kDa component reprecipitated (lane 3), suggesting that there are two forms of ZP2 which may arise from post-translational modification (see below).
More Than a Simple Lock and Key Mechanism: Unraveling the Intricacies of Sperm-Zona Pellucida Binding
Open access peer-reviewed chapter
By Kate A. Redgrove, R. John Aitken and Brett Nixon
Submitted: November 27th 2011Reviewed: June 6th 2012Published: September 19th 2012
Mammalian fertilization involves a concerted interplay between the male and female gametes that ultimately results in the creation of new life. However, despite the fundamental importance of gamete interaction, the precise molecular mechanisms that underpin and regulate this complex event remain to be fully elucidated.
Such knowledge is crucial in our attempts to resolve the global problems of population control and infertility. The current world population has surpassed 7 billion people, and continues to grow at a rate of approximately 200 000 each day (UN, 2009).
Alarmingly, the majority of this population growth is occurring in developing nations, and is driven in part by an unmet need for effective and accessible contraceptive technologies. Indeed, a recent study by the Global Health Council revealed that of the 205 million pregnancies recorded worldwide each year, 60-80 million of these are deemed to be unplanned or unwanted (Guttmacher, 2007).
These concerning statistics highlight the inadequacies of our current armory of contraceptives and demonstrate the need for the development of novel methods for fertility control.
By virtue of its specificity and its ability to be suppressed in both males and females, sperm interaction with the outer vestments of the oocyte, a structure known as the zona pellucida (ZP), represents an attractive target for the development of novel contraceptives. However, the realization of such technologies is predicated on a thorough understanding of the molecular mechanisms that underpin this intricate binding event.
Such knowledge will also contribute to the development of novel diagnostic and therapeutic strategies for the paradoxical increase in male infertility that is being experienced by Western countries.
Indeed, male infertility has become a distressingly common condition affecting at least 1 in 20 men of reproductive age (McLachlan and de Kretser, 2001).
In a vast majority (>80%) of infertile patients sufficient numbers of spermatozoa are produced to achieve fertilization, however the functionality of these cells has become compromised, making defective sperm function the largest single defined cause of human infertility (Hull, et al., 1985, Ombelet, et al., 1997).
Biologically, a major cause of impaired sperm function is a failure of these cells to recognize the surface of the egg. Defective sperm- zona pellucida interactions is thus a major cause of fertilization failure in vitro and bioassays of sperm- zona pellucida interaction are able to predict male infertility in vivo with great accuracy (Arslan, et al., 2006).
In this review we explore our current understanding of the mechanisms that are responsible for sperm- zona pellucida interactions. Consideration is given to well-established paradigms of receptor-ligand binding with an emphasis on the emerging evidence for models involving the participation of multimeric receptor complexes and the maturation events that promote their assembly.
The zona pellucida (ZP) is a porous extracellular matrix that surrounds the oocyte (Dunbar, et al., 1994, Wassarman and Litscher, 2008).
In the most widely accepted models of gamete interaction, the zona pellucida plays a critical role in tethering spermatozoa, and inducing the release of their acrosomal contents (Bleil and Wassarman, 1983).
Binding to the zona pellucida is a highly selective and carefully regulated process that serves as an inter-species barrier to fertilization by preventing adherence of non-homologous sperm to eggs (Hardy and Garbers, 1994).
Although all mammalian eggs are enclosed in a zona pellucida matrix, it’s thickness (~1-25μm) and protein content (~1-10ng) varies considerably for eggs derived from different species (Wassarman, 1988). In mice, the zona pellucida comprises three major sulfated glycoproteins designated ZP1 (200kDa), ZP2 (120kDa) and ZP3 (83kDa).
Current evidence suggests that these proteins assemble into a non-covalently linked structure comprising ZP2-ZP3 dimers that polymerize into filaments and are cross-linked by ZP1 (Greve and Wassarman, 1985, Wassarman and Mortillo, 1991).
In addition to orthologues of the three mouse zona pellucida proteins [hZP1 (100kDa), hZP2 (75kDa) and hZP3 (55kDa)], the human zona pellucida comprises a fourth glycoprotein, hZP4 (65kDa) (Bauskin, et al., 1999, Lefievre, et al., 2004), which is thought to be dysfunctional in the mouse (Lefievre, et al., 2004).
The biological significance of the increased complexity in the zona pellucida of humans awaits further investigation. Given that the mouse remains the most widely studied model for understanding sperm- zona pellucida interaction, this species will serve as the focus for the following discussion.
Sperm- zona pellucida interaction encompasses a complex sequence of events that relies on each gamete having achieved an appropriate level of maturity.
Spermatozoa that approach the oocyte have undergone a behavioral and functional reprogramming event within the female reproductive tract, termed capacitation (see section 188.8.131.52), which ultimately endows the cells with the competence for fertilization.
Notwithstanding recent evidence to the balance of evidence favors a model for sperm- zona pellucida interaction that involves three distinct stages: the first comprises primary binding of acrosome-intact spermatozoa to the zona pellucida, this is then followed by secondary binding of acrosome-reacted spermatozoa to the zona pellucida, and finally penetration of the acrosome-reacted sperm through the zona pellucida and into the perivitelline space (Florman and Storey, 1982, Inoue and Wolf, 1975, Saling, et al., 1979, Swenson and Dunbar, 1982).
Oocyte and developing zona pellucida in the ovary
(Latin, zona pellucida = transparent zone) The zona pellucida (ZP) is a specialized extracellular matrix surrounding the developing oocyte (egg, ovum) within each follicle within the ovary. This thick matrix is thought to be formed by secretions from the oocyte and the follicle granulosa cells and in human oocytes consists of four types of zona pellucida glycoproteins ZP1, ZP2, ZP3 and ZP4 which have different roles in fertilization. Note that mice also have 4 ZP genes, but multiple stop and missense codons in ZP4 means that only three are produced. Polymers of ZP2 and ZP3 organized into extended filaments that are cross-linked by ZP1 homodimers.
Note that depending upon species and type of study, the zona pellucida can also be called the: oolemma, egg coat or vitelline membrane.
The zona pellucid has many different roles including in oocyte development, protection during growth and transport, fertilization, spermatozoa binding, preventing polyspermy, blastocyst development, and preventing premature implantation (ectopic pregnancy).
|Human oocyte contained inside zona pellucida.||Early zygote inside zona pellucida|
In human development, during the first week of development following fertilization the zona pellucida remains surrounding the blastocyst from which it ‚hatches‘ to commence implantation.
Some Recent Findings
Human zygote inside the zone pellucida Mouse germinal vesicle
Эмбрион человека, окруженный Zona pellucida.
лат. Zona pellucida (ранее также zona striata, в русском языке соответствует термин «блестящая оболочка», используемый редко) — гликопротеиновая оболочка вокруг плазматической мембраны яйцеклетки млекопитающих животных (в том числе человека).
Zona pellucida представляет собой прозрачную эластичную гликопротеиновую оболочку, окружающую яйцеклетку. Её толщина у человека составляет 5-10 мкм. Через неё свободно проникает вода и растворенные в ней вещества.
В индивидуальном развитии яйцеклетки zona pellucida образуется на стадии роста, когда она развивается в составе первичного фолликула. Белки zona pellucida синтезирует яйцеклетка.
Вопреки мнению, распространенному в середине 20-го века, фолликулярные клетки в образовании zona pellucida не участвуют.
В строении zona pellucida принимают участие четыре гликопротеина ZP1, ZP2, ZP3, ZP4. Два гликопротеина (ZP2 и ZP3), связываясь попеременно, образуют нити, которые соединены друг с другом «перемычками» из ZP1 и ZP4. Сахаридные части гликопротеинов ZP2 и ZP3 являются лигандами для связывания сперматозоидами, сахаридные части гликопротеинов видоспецифичны.
Часто в литературе указывают, что в строении zona pellucida участвуют только три белка ZP1, ZP2, ZP3. Это связано с тем, что изначально строение zona pellucida было изучено на лабораторной мыши, у которой ген ZP4 мутантен, белок ZP4 не производится; у прочих млекопитающих, включая человека, в строении zona pellucida участвуют четыре белка.
Zona pellucida выполняет две функции:
- Препятствует проникновению более одного сперматозоида в яйцеклетку (так называемый «блок полиспермии»). После проникновения сперматозоида в яйцеклетке запускается каскад биохимических реакций, приводящий к опорожнению кортикальных везикул — мембранных пузырьков, содержащих литические ферменты. Содержимое кортикальных везикул выбрасывается наружу (в перивителлиновое пространство) и модифицирует zona pellucida таким образом, что последующие сперматозоиды уже не способны проникнуть через неё. Модификация касается отщепления сахаристых остатков от гликопротеинов ZP2 и ZP3, также от ZP2 отщепляется полипептидная часть. Именно сахаристые остатки гликопротеинов ZP2 и ZP3 являются лигандами для рецепторов на головке сперматозоидов, ответственных за связывание с Zona pellucida и запуск акросомной реакции. Проникновение одного сперматозоида в яйцеклетку — необходимое условие для нормального развития эмбриона.
- Удерживает клетки раннего эмбриона вместе. Бластомеры (клетки эмбриона на этапе дробления) у млекопитающих не образуют клеточных контактов и не способны к самостоятельному формированию единого зародыша. In vitro при искусственном удалении zona pellucida бластомеры разъединяются. Клеточные контакты между бластомерами начинают формироваться после 3-го деления дробления и становятся устойчивыми к моменту образования трофэктодермы. Перед имплантацией эмбриону необходимо освободиться от Zona pellucida, так как блестящая оболочка препятствует контакту трофэктодермы и маточного эпителия. Преждевременное освобождение от zona pellucida может приводить к образованию однояйцевых близнецов.
Выход эмбриона из zona pellucida
После достижения стадии бластоцисты эмбриону требуется выйти из блестящей оболочки, чтобы приступить к имплантации. Процесс выхода из zona pellucida называется хетчинг (англ. hatching — вылупление). Эмбрион человека совершает хетчинг на 5-7 день развития, эмбрион мыши на 4-5 день развития. Механизм выхода сложен.
Разрыв оболочки обусловлен двумя факторами: механическим воздействием эмбриона (бластоциста наполняет свою полость водой и увеличивается в размере, давя на стенки zona pellucida изнутри), химическим воздействием эмбриона (трофэктодерма эмбриона выделяет протеолитический фермент «стрипсин», который растворяет оболочку).
Эмбрион (бластоциста) покидает zona pellucida, выходя через образовавшуюся щель с помощью «амебоидных» движений.
Источник — https://ru.wikipedia.org/w/index.php?title=Zona_pellucida&oldid=98062428
Egg coat or pellucid zone) is a glycoprotein layer surrounding the plasma membrane of mammalian oocytes.Zona pellucidaHuman ovum: The zona pellucida is seen as a thick clear girdle surrounded by the cells of the corona radiata.IdentifiersMeSHD015044FMA18674Anatomical terminology[edit on Wikidata]
The zona pellucida (plural zonae pellucidae, also egg coat or pellucid zone) is a glycoprotein layer surrounding the plasma membrane of mammalian oocytes. It is a vital constitutive part of the oocyte. The zona pellucida first appears in unilaminar primary oocytes. It is secreted by both the oocyte and the ovarian follicles. The zona pellucida is surrounded by the corona radiata. The corona is composed of cells that care for the egg when it is emitted from the ovary.
This structure binds spermatozoa, and is required to initiate the acrosome reaction. In the mouse (the best characterised mammalian system), the zona glycoprotein, ZP3, is responsible for sperm binding, adhering to proteins on the sperm plasma membrane.
ZP3 is then involved in the induction of the acrosome reaction, whereby a spermatozoon releases the contents of the acrosomal vesicle.
The exact characterisation of what occurs in other species has become more complicated as further zona proteins have been identified.
In humans, five days after the fertilization, the blastocyst performs zona hatching; the zona pellucida degenerates and decomposes, to be replaced by the underlying layer of trophoblastic cells.The zona pellucida is essential for oocyte growth and fertilization.
The zona pellucida is a translucent matrix of glycoproteins that surrounds the mammalian oocyte, and its formation is critical to successful fertilization. In non-mammals it is called the vitelline membrane or vitelline envelope.
The thick membrane of the zona pellucida functions to only allow species-specific fertilization; to prevent polyspermy, and enable the acrosome reaction for the successful adhesion and penetration of the sperm cell. The major glycoproteins of the egg coat responsible, are known as sperm-binding proteins.
The four major sperm-binding proteins, or sperm-receptors, are ZP1, ZP2, ZP3, and ZP4. They bind to capacitated spermatozoa and induce the acrosome reaction. Successful fertilization depends on the ability of sperm to penetrate the extracellular matrix of the zona pellucida that surrounds the egg.In the mouse:
- ZP3 allows species-specific sperm binding
- ZP2 mediates subsequent sperm binding
- ZP1 cross-links ZP2 and ZP3.
Data with native human protein are not currently available.
Main article: Immunocontraception
ZP module-containing glycoproteins ZP1, ZP2, ZP3 and ZP4 are targets for immunocontraception in mammals.
In non-mammals, the zona pellucida is called the vitelline membrane or envelope, and the vitelline envelope in insects, and plays an important role in preventing cross-breeding of different species, especially in species such as fish that fertilize outside of the body.
The zona pellucida is commonly used to control wildlife population problems by immunocontraception.
When the zona pellucida of one animal species is injected into the bloodstream of another, it results in sterility of the second species due to immune response.
This effect can be temporary or permanent, depending on the method used. In New Jersey, immunocontraception using porcine zona pellucida has been trialled for the control of deer.
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- Bagnell C (2005). ‚Animal Reproduction‘. Rutgers University Department of Animal Sciences.[verification needed]
- Jovine, Luca; Darie, Costel C.; Litscher, Eveline S.; Wassarman, Paul M. (2005). ‚Zona Pellucida Domain Proteins‘. Annual Review of Biochemistry. 74: 83–114. doi:10.1146/annurev.biochem.74.082803.133039. PMID 15952882.
- Monné, Magnus; Han, Ling; Jovine, Luca (2006). ‚Tracking Down the ZP Domain: From the Mammalian Zona Pellucida to the Molluscan Vitelline Envelope‘. Seminars in Reproductive Medicine. 24 (4): 204–16. doi:10.1055/s-2006-948550. PMID 16944418.
- Wassarman, P. M. (2008). ‚Zona Pellucida Glycoproteins‘. Journal of Biological Chemistry. 283 (36): 24285–9. doi:10.1074/jbc.R800027200. PMC 2528931. PMID 18539589.
- Wassarman, Paul M.; Litscher, Eveline S. (2008). ‚Mammalian fertilization:the eggs multifunctional zona pellucida‘. The International Journal of Developmental Biology. 52 (5–6): 665–76. doi:10.1387/ijdb.072524pw. PMID 18649280.
- Monné, Magnus; Han, Ling; Schwend, Thomas; Burendahl, Sofia; Jovine, Luca (2008). ‚Crystal structure of the ZP-N domain of ZP3 reveals the core fold of animal egg coats‘. Nature. 456 (7222): 653–7. Bibcode:2008Natur.456..653M. doi:10.1038/nature07599. PMID 19052627.
- Han, Ling; Monné, Magnus; Okumura, Hiroki; Schwend, Thomas; Cherry, Amy L.; Flot, David; Matsuda, Tsukasa; Jovine, Luca (2010). ‚Insights into Egg Coat Assembly and Egg-Sperm Interaction from the X-Ray Structure of Full-Length ZP3‘. Cell. 143 (3): 404–15. doi:10.1016/j.cell.2010.09.041. PMID 20970175.
- Histology image: 18404loa – Histology Learning System at Boston University – ‚Female Reproductive System: ovary, cumulus oophorus ‚
- Histology image: 14805loa – Histology Learning System at Boston University – ‚Female Reproductive System: ovary, multilaminar primary follicle‘
- Anatomy photo: Reproductive/mammal/ovary2/ovary7 – Comparative Organology at University of California, Davis – ‚Mammal, canine ovary (LM, High)‘
- Image at um.edu.mt
- Image at um.edu.mt
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