Fertilization

From Embryology

Introduction

Early zygote.jpg

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 and egg.

The first polar body deforms the mammalian egg away from its encapsulating zona pellucida


Fertilization Links: fertilization | oocyte | spermatozoa | meiosis | | ovary | testis | menstrual cycle | zona pellucida | zygote | granulosa cell Lecture - Fertilization | 2016 Lecture | mitosis | Lecture - Week 1 and 2 | hydatidiform mole | Assisted Reproductive Technology | | morula | blastocyst | Lecture - Genital Development | Category:Fertilization
Historic Embryology - Fertilization 
1910 Fertilization | 1919 Human Ovum | 1921 The Ovum | 1927 First polar body | 1929 Oocyte Size | 1943 Fertilization | 1944 In vitro fertilization | 1948 In vitro fertilization


| Morula | Blastocyst | original page


Human-spermatozoa EM01.jpg

Some Recent Findings

  • CD9 tetraspanin generates fusion competent sites on the egg membrane for mammalian fertilization[1] "CD9 tetraspanin is the only egg membrane protein known to be essential for fertilization. To investigate its role, we have measured, on a unique acrosome reacted sperm brought in contact with an egg, the adhesion probability and strength with a sensitivity of a single molecule attachment. Probing the binding events at different locations of wild-type egg we described different modes of interaction. Here, we show that more gamete adhesion events occur on Cd9 null eggs but that the strongest interaction mode disappears. We propose that sperm-egg fusion is a direct consequence of CD9 controlled sperm-egg adhesion properties. CD9 generates adhesion sites responsible for the strongest of the observed gamete interaction. These strong adhesion sites impose, during the whole interaction lifetime, a tight proximity of the gamete membranes, which is a requirement for fusion to take place. The CD9-induced adhesion sites would be the actual location where fusion occurs."
  • Gamete recognition in mice depends on the cleavage status of an egg's zona pellucida protein[2]"sperm-egg recognition depends on the cleavage status of ZP2 and that binding at the surface of the zona is not sufficient to induce sperm acrosome exocytosis."

Objectives

  • 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.

Movies

Fertilization 002 icon.jpg Fertilization 001 icon.jpg Pronuclear fusion 001 icon.jpg
Fertilization Fertilization in Mouse Pronuclear Fusion
Flash | Quicktime Flash | Quicktime Flash | Quicktime

Fertilization Preparation

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.

Endocrinology - Diagram of the comparative anatomy of the male and female reproductive tracts

Oogenesis

Histology of the Ovary
Preantral Follicle
Antral Follicle and 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

Oocytenumber.jpg

  • 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)

Menstrual Cycle

  • 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)

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.

Zona Pellucida

MBoC - Figure 20-21. The zona pellucida

  • 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


Links: Zona Pellucida

Corona Radiata

  • 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

Spermiogenesis

  • 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)

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
  • 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

Fertility Window

Probability of women with regular or irregular cycles being in their fertile window

Clinical guidelines have typically identified the "fertile window" between days 10 and 17 within the typical 28 day menstrual cycle.

Data from a large USA NIEHS - Early Pregnancy Study (1982-86) identified the timing of the “fertile window” within a range of different menstrual cycles.[3]

  • 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 Site

  • 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

Fertilization- Oocyte

Sperm-Oocye Interaction

  • Contact between sperm and oocyte egg coat (zona pellucida [ZP]) glycoproteins triggers increases in intracellular calcium ion (iCa2+) concentration in sperm[4]
  • 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

Early human zygote showing Pronuclei
  • 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

  • Mitochondria of the spermatozoa are specifically destroyed in early development by proteolysis (mouse 4 to 8 cell transition).[5]
  • 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.[6]

Sex Determination

  • 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.[7]

Protein Expression Classified by Molecular Functions

MII oocyte
Zygote
Mouse- MII oocyte protein expression.jpg Mouse- zygote protein expression.jpg

References

  1. 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
  2. <pubmed>20616279</pubmed>
  3. <pubmed>11082086</pubmed>| PMC27529 | BMJ
  4. <pubmed>19211808</pubmed>
  5. <pubmed>11041517</pubmed>
  6. <pubmed>17307910</pubmed>
  7. <pubmed>20876089</pubmed>| PNAS

Textbooks

  • 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

Search NCBI Bookshelf fertilization | fertilisation

Reviews

Articles

<pubmed>9557845</pubmed>| Hum Reprod.

Search Pubmed

April 2010

  • fertilization - All (51803) Review (5928) Free Full Text (11715)


Search Pubmed Now: fertilization | fertilisation | zona pellucida | zygote

External Links


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Cite this page: Hill, M.A. (2024, March 29) Embryology Fertilization. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Fertilization

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© Dr Mark Hill 2024, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G