Lecture - Fertilization

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Introduction

Image of early human zygote
Early Human Zygote

Development is 1 embryonic cell producing about 100,000,000,000,000 cells in the adult at any one time (over time with cell death and ongoing replacement this is substantially more).

This is where the first embryonic cell begins!

Fertilization is the fusion of haploid gametes, egg (oocyte) and sperm (spermatozoa), 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.


This lecture and the associated laboratory will cover gametogenesis and fertilization.

Some Recent Research - Podcast Biosights 18 March 2013 - Breaking egg symmetry

Objectives

Cell division - 3 types
  1. Broad understanding of reproductive cycles.
  2. Understand the key features of gametogenesis.
  3. Understand the differences in male and female gametogenesis.
  4. Brief understanding of the differences between mitosis and meiosis.
  5. Understanding of the events in fertilization.

Resources

Movies
Oocyte Meiosis 01 icon.jpg
 ‎‎Oocyte Meiosis
Page | Play
Rabbit-ovulation.jpg
 ‎‎Ovulation
Page | Play | Audio
Follicle 001 icon.jpg
 ‎‎Ovulation
Page | Play
Bovine uterine tube oocyte transport 1.jpg
 ‎‎Oocyte Transport
Page | Play
Spermatozoa animation icon.jpg
 ‎‎Spermatozoa
Page | Play
Spermatozoa motility icon 01.jpg
 ‎‎Spermatozoa Motility
Page | Play
Human fertilization 1 icon.jpg
 ‎‎Fertilisation to
4 Blastomere
Page | Play
Human fertilization 2 icon.jpg
 ‎‎Fertilization
Page | Play
Fertilization 002 icon.jpg
 ‎‎Fertilization
Page | Play
Pronuclear fusion 001 icon.jpg
 ‎‎Pronuclear Fusion
Page | Play
DNA bead-induced ectopic polar body-icon.jpg
 ‎‎Ectopic Polar Body
Page | Play
Mouse spermatozoa mito movie icon.jpg
 ‎‎Male Mitochondria
Page | Play
References
UNSW Embryology

Logo.png

Hill, M.A. (2020). UNSW Embryology (20th ed.) Retrieved March 28, 2024, from https://embryology.med.unsw.edu.au

Cell Division Links: meiosis | mitosis | Lecture - Cell Division and Fertilization | spermatozoa | oocyte | fertilization | zygote | Genetics
The Developing Human: Clinically oriented embryology

The Developing Human, 9th edn.jpg

Citation: The Developing Human: clinically oriented embryology 9th ed. Keith L. Moore, T.V.N. Persaud, Mark G. Torchia. Philadelphia, PA: Saunders, 2011.

The following chapter links only work with a UNSW connection.

Larsen's Human Embryology

Larsen's human embryology 4th edn.jpg

Citation: Larsen's human embryology 4th ed. Schoenwolf, Gary C; Larsen, William J, (William James). Philadelphia, PA : Elsevier/Churchill Livingstone, c2009.

The following chapter links only work with a UNSW connection.


Human Reproductive Cycle

Female Male
  • Menstrual Cycle a regular cycle of reproduction (28 days)
  • begins at puberty, release of 1 egg (oocyte) every cycle
  • Endocrine controlled (HPG axis) Hypothalamus - Pituitary - Gonad
  • continuous production of sperm (spermatozoa)
  • begins at puberty, release millions of spermatozoa
  • Endocrine controlled (HPG axis) Hypothalamus - Pituitary - Gonad

XXhpgaxis.jpg Menstrual cycle.png


Gametogenesis

Meiosis in the gonad (ovary or testis) produces the haploid gametes, oocyte and spermatozoa (egg and sperm).


Male

The testes have two functions.

  1. produce the male gametes or spermatozoa
  2. produce male sexual hormone, testosterone (internal and external genitalia, sex characteristics)

Human spermatozoa take about 48 days from entering meiosis until morphologically mature spermatozoa.

Spermatozoa development: primordial germ cell - spermatogonia - primary spermatocyte - secondary spermatocytes - spermatid - spermatozoa

Sertoli cells (support cells) Interstitial cells or Leydig cells (produce hormone)

Seminiferous tubule cartoon.jpg

Female

The ovaries have two functions.

  1. produce the female gametes or oocytes
  2. produce female hormones, estrogen and progesterone (secondary sex characteristics, menstrual cycle)

In an adult human female the development of a primordial follicle containing an oocyte to a preovulatory follicle takes in excess of 120 days.

Human ovary follicle development.jpg

Human ovary follicle development

Ovarian Follicle Stages: primordial follicle - primary follicle - secondary follicle - tertiary follicle - preovulatory follicle


Follicle cells (support cells) Theca cells (produce hormone)


Links: Spermatozoa Development | Oocyte Development | MBoC - Figure 20-18. Influence of Sry on gonad development | Endocrinology - Comparative anatomy of male and female reproductive tracts

Fertilization

Early zygote showing polar bodies
  • Oogenesis - 1 gamete produced/meiosis + 3 polar bodies, meiosis is slow, 1 egg produced and released at ovulation
  • Spermatogenesis - 4 gametes produced/meiosis, meiosis is fast, 200-600 million sperm released at ejaculation


Ovulation icon.jpg Fertilization 001 icon.jpg

Fertilization Site

  • Fertilization usually occurs in first 1/3 of uterine tube (oviduct, Fallopian tube)
  • Fertilization can also occur outside uterine tube associated with Assisted Reproductive Technologies (IVF, GIFT, ZIFT...) and ectopic pregnancy
  • The majority of fertilized eggs do not go on to form an embryo

Fertilization - Spermatozoa

Fertilization - Oocyte

Embryo mitosis icon.jpg Week1 001 icon.jpg








Cell Division

Historic drawing of mitosis

Features Two Mechanical Processes

  • Mitosis - microtubule based segregation of chromosomes and formation of 2 nuclei
  • Cytokinesis - microfilament based splitting of the cell cytoplasmic contents as a whole into 2 daughter cells

Features Two Types

  • Mitosis - occurs in all cells, producing genetically identical progeny.
  • Meiosis - occurs only in germ cells (sperm=spermatozoa and egg=oocyte), producing genetically different progeny.
    • progeny = daughter cells, offspring

Cell Changes

Nucleus

  • Chromosome condensation
  • Nuclear envelope breakdown

Cytoplasm

  • Cytoskeleton reorganization
  • Spindle formation (microtubule, MT) Contractile ring (microfilament, MF)
  • Organelle redistribution

Mitosis

Mitosis Movie[1] See also MCB Movie - The stages of mitosis and cytokinesis in an animal cell
  • Based on light microscopy of living cells light and electron microscopy of fixed and stained cells
  • 5 Phases - prophase, prometaphase, metaphase, anaphase, and telophase
  • Cytokinesis 6th stage overlaps the end of mitosis

Note that DNA duplication has occurred earlier in the S Phase of the cell cycle.

Mitosis 01 icon.jpg
 ‎‎Mitosis
Page | Play

1. Prophase

  • Chromosome DNA has been earlier duplicated (S Phase)
  • Chromosomes begin condensing
  • Chromosome pairs (chromatids) held together at centromere
  • Microtubules disassemble
  • Mitotic spindle begins to form
  • Prophase ends when nuclear envelope breaks down

2. Prometaphase

  • Microtubules now enter nuclear region
  • Nuclear envelope forms vesicles around mitotic spindle
  • Kinetochores form on centromere attach to some MTs of spindle
  • Prometaphase ends when chromosomes move to metaphase plate

MCB Movie - Centromeric attachment of microtubules

3. Metaphase

Mitosis - Metaphase.jpg

Metaphase fluorescent image of Mitotic spindle and Chromosomes
  • Kinetochore MTs align chromosomes in one midpoint plane
  • Metaphase ends when sister kinetochores separate

4. Anaphase

  • Separation of sister Kinetochores
  • shortening of Kinetochore microtubules pulls chromosome to spindle pole
  • Anaphase ends as nuclear envelope (membrane) begins to reform

5. Telophase

  • Chromosomes arrive at spindle poles
  • Kinetochore MTs lost
  • Condensed chromosomes begin expanding
    • Continues through cytokinesis

Cytokinesis

  • Division of cytoplasmic contents
  • Contractile ring forms at midpoint under membrane
  • Microfilament ring Contracts forming cleavage furrow
  • Eventually fully divides cytoplasm

Cell Organelles

  • Mitochondria - Divide independently of cell mitosis, distributed into daughter cells
  • Peroxisomes - localise at spindle poles
  • Endoplasmic Reticulum - associated with the nuclear envelope vesicles.
  • Golgi Apparatus- Golgi stack undergoes a continuous fragmentation process, fragments are distributed into daughter cells, then reassembled into new Golgi stacks

Meiosis

Mitosis and meiosis.jpg

Comparison of Mitosis and Meiosis

Meiosis Germ cell division (haploid)

  • Reductive division generates the haploid gametes (egg, sperm)
  • Each genetically distinct from the parent
  • Genetic recombination (prophase 1)
    • Exchanges portions of chromosomes maternal/paternal homologous pairs
  • Independent assortment of paternal chromosomes (meiosis 1)

Homologous chromosomes pairing unique to meiosis

  • Each chromosome duplicated and exists as attached sister chromatids before pairing occurs
  • Genetic Recombination shown by chromosomes part red and part black
    • chromosome pairing in meiosis involves crossing-over between homologous chromosomes

Meiosis I and II

  • Meiosis I - separates the pairs of homologous chromosomes, reduces the cell from diploid to haploid.
  • Meiosis II - separates each chromosome into two chromatids (chromosome behavior in meiosis II is like that of mitosis).

Figure 14.32. Comparison of meiosis and mitosis

Prophase I - The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Prophase I is divided into five phases:

  1. Leptotene - chromosomes start to condense.
  2. Zygotene - homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes consisting of four chromatids (tetrads).
  3. Pachytene - crossing over between pairs of homologous chromosomes to form chiasmata (form between two nonsister chromatids at points where they have crossed over)
  4. Diplotene - homologous chromosomes begin to separate but remain attached by chiasmata.
  5. Diakinesis - homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.

Prometaphase I - Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores.

Metaphase I - Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate. (This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 223, which is over 8 million.)

Anaphase I - The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell.

Telophase I - The chromosomes become diffuse and the nuclear membrane reforms.

Cytokinesis I - Cellular cytoplasmic division to form two new cells, followed by Meiosis II.

Prophase II - Chromosomes begin to condense, nuclear membrane breaks down and spindle forms.

Metaphase II - Spindle fibres attach to chromosomes, chromosomes align in cell centre.

Anaphase II - Chromosomes separate and move to the opposite poles of the cell.

Telophase II - Chromosomes reach spindle pole ends and the nuclear membrane reforms.

Cytokinesis - Cellular cytoplasmic division to form new cells.

Comparison of Meiosis/Mitosis

McGraw-Hill Animation comparing Mitosis and Meiosis

  • After DNA replication 2 nuclear (and cell) divisions required to produce haploid gametes
  • Each diploid cell in meiosis produces 4 haploid cells (sperm) 1 haploid cell (egg)
  • Each diploid cell mitosis produces 2 diploid cells

Meiosis Differences

Male - Spermatogenesis

Male gametogenesis

MBoC - Figure 20-27. The stages of spermatogenesis

  • Meiosis initiated continuously in a mitotically dividing stem cell population
  • 4 gametes produced / meiosis
  • Meiosis completed in days or weeks
  • Meiosis and differentiation proceed continuously without cell cycle arrest
  • Differentiation of gamete occurs while haploid after meiosis ends
  • Sex chromosomes excluded from recombination and transcription during first meiotic prophase

Female - Oogenesis

Female gametogenesis

The Cell - Figure 14.37. Meiosis of vertebrate oocytes

  • Meiosis initiated once in a finite population of cells
  • 1 gamete produced / meiosis
  • Completion of meiosis delayed for months or years
  • Meiosis arrested at 1st meiotic prophase and reinitiated in a smaller population of cells
  • Differentiation of gamete occurs while diploid in first meiotic prophase
  • All chromosomes exhibit equivalent transcription and recombination during meiotic prophase

Polar Bodies

Early zygote showing polar bodies
  • In female gametogenesis only a single (1) haploid egg is produced from meiosis. In male gametogenesis four (4) haploid sperm are produced from meiosis. So what happens to all the extra DNA in producing this single egg?
    • In Meiosis 1 the "extra" DNA is excluded to the periphery as a 1st polar body, which encloses the extra DNA.
    • In Meiosis 2 the "extra" DNA is once again excluded as a 2nd polar body. The first polar body may also under go meiosis 2 producing a 3rd polar body.
  • These polar bodies are not gametes.
  • Polar bodies appear to have no other function other than to dispose of the extra DNA in oogenesis.
    • Recent research in mice suggest that the position of oocyte polar body may influence fertilization site.

Fertilization

Mark Hill.jpg Gamete formation, menstrual cycle and fertilization will be covered in detail in this week's Laboratory and next week's lecture. The notes below are included for information purposes, we will not complete all content shown below within the lecture.

Fertilization is the complete process resulting in the fusion of haploid gametes, egg and sperm, to form the diploid zygote. The recent development of aided fertilization is described as in vitro fertilization (in vitro = "in glass", outside the body, IVF). Clinically, all these aided fertilization techniques are grouped as Assisted Reproductive Technologies or ART.

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 24-48h to fertilize oocyte.

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

Oogenesis

Oocyte Development (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 and 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

Spermatozoa Development (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

Fertilization- Oocyte

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.

Abnormalities

Trisomy21arrow.gifTrisomy21female.jpgTrisomy21male.jpg

  • The most common chromosome abnormality is aneuploidy, the gain or loss of whole chromosomes.
  • Caused by meiotic nondisjunction, the failure of chromosomes to correctly separate homologues during meiosis I or sister chromatids during meiosis II.
  • Down Syndrome - caused by an extra copy of chromosome 21. Abnormal Development - Trisomy 21 (Down Syndrome)
  • Chromosomal translocations occur when there is an inappropriate exchange of chromosomal material. Philadelphia chromosome
  • Philadelphia chromosome - piece of Chr9 exchanged with Chr22 Generates truncated abl, overstimulates cell production, leads to chronic myelogenous leukemia

UNSW Embryology Links

Cell Division Links: meiosis | mitosis | Lecture - Cell Division and Fertilization | spermatozoa | oocyte | fertilization | zygote | Genetics

References

  1. <pubmed>12105179</pubmed>

Online Textbooks

Search

Reviews

<pubmed>18163464</pubmed> <pubmed>17189856</pubmed>


Terms

  • diploid - (Greek, di = double + ploion = vessel) Having two sets of chromosomes, the normal state for all cells other than the gametes.
  • haploid - (Greek, haploos = single) Having a single set of chromosomes as in mature germ/sex cells (oocyte, spermatozoa) following reductive cell division by meiosis. Normally cells are diploid, containing 2 sets of chromosomes.
  • meiosis - The cell division that occurs only in production of germ cells where there is a reduction in the number of chromosomes (diploid to haploid) which is the basis of sexual reproduction. All other non-germ cells in the body divide by mitosis.
  • mitosis - The normal division of all cells, except germ cells, where chromosome number is maintained (diploid). In germ cell division (oocyte, spermatozoa) meiosis is a modified form of this division resulting in reduction in genetic content (haploid). Mitosis, division of the nucleus, is followed by cytokinesis the division of the cell cytoplasm and the cytoplasmic contents. cytokinesis overlaps with telophase.

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Cite this page: Hill, M.A. (2024, March 28) Embryology Lecture - Fertilization. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Lecture_-_Fertilization

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