Fertilization

Introduction

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

| Morula | Blastocyst | original page

Some Recent Findings

Gamete recognition in mice depends on the cleavage status of an egg's zona pellucida protein"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	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

• 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)

• 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

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

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

• 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

Fertilization- Oocyte

• Membrane Depolarization - caused by sperm membrane fusion, 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)

Sperm-Oocye Interaction

• 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

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

• Sperm contributes - centriole which organizes mitotic spindle
• Oocyte contributes - mitochondria (maternally inherited)

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.

Protein Expression Classified by Molecular Functions

MII oocyte	Zygote

External Links

Original Embryology Links - Week 1 Notes | Week 1 - In Vivo Fertilization | Week 1 - In Vitro Fertilization | Movie - Fertilization | Animation - Fertilization | Week 1 - Abnormalities | Embryology for K12 Students - Week 1

References

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

• IJDB Vol. 52 Nos. 5/6 (2008) Fertilization

Search Pubmed

April 2010

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

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

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Cite this page: Hill, M.A. (2024, May 3) 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