Lecture - Fertilization
This lecture will introduce the key concepts of biology cell division and cellular sexual development. Cell division and fertilisation will also be explored further in the Wednesday laboratory.
|Quicktime | Flash|
|File:Pronuclear_fusion 001 icon.jpg</wikiflv>||This animation shows the maternal (egg) and paternal (sperm) gamete pronuclei coming together following fertilization to form the first diploid cell, the zygote.
- Identify the key features of cell division.
- Identify the stages and differences between mitosis and meiosis.
- Understand the differences in male and female gamete meiosis.
- Understanding of gamete fusion during fertilization.
The Developing Human: Clinically oriented embryology
|Citation: The developing human : clinically oriented embryology 8th ed. Moore, Keith L; Persaud, T V N; Torchia, Mark G Philadelphia, PA : Saunders/Elsevier, c2008.|
Larsen's human embryology
|Citation: Larsen's human embryology 4th ed. Schoenwolf, Gary C; Larsen, William J, (William James). Philadelphia, PA : Elsevier/Churchill Livingstone, c2009.|
|Hill, M.A. (2011) UNSW Embryology (11th ed.). Sydney:UNSW.|
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).
- Cell Division Milestones, Recent Nobel Prizes
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
- Chromosome condensation
- Nuclear envelope breakdown
- Cytoskeleton reorganization
- Spindle formation (microtubule, MT) Contractile ring (microfilament, MF)
- Organelle redistribution
|Quicktime | Flash|
Mitosis Movie 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.
- 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
- 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
- Kinetochore MTs align chromosomes in one midpoint plane
- Metaphase ends when sister kinetochores separate
- Separation of sister Kinetochores
- shortening of Kinetochore microtubules pulls chromosome to spindle pole
- Anaphase ends as nuclear envelope (membrane) begins to reform
- Chromosomes arrive at spindle poles
- Kinetochore MTs lost
- Condensed chromosomes begin expanding
- Continues through cytokinesis
- Division of cytoplasmic contents
- Contractile ring forms at midpoint under membrane
- Microfilament ring Contracts forming cleavage furrow
- Eventually fully divides cytoplasm
- 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 Germ cell division (haploid)
- Reductive division
- Generates haploid gametes (egg, sperm)
- Each genetically distinct from 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).
- The homologous chromosomes pair and exchange DNA to form recombinant chromosomes.
- Prophase I is divided into five phases:
- Leptotene - chromosomes start to condense.
- Zygotene - homologous chromosomes become closely associated (synapsis) to form pairs of chromosomes consisting of four chromatids (tetrads).
- Pachytene - crossing over between pairs of homologous chromosomes to form chiasmata (form between two nonsister chromatids at points where they have crossed over)
- Diplotene - homologous chromosomes begin to separate but remain attached by chiasmata.
- Diakinesis - homologous chromosomes continue to separate, and chiasmata move to the ends of the chromosomes.
- Spindle apparatus formed, and chromosomes attached to spindle fibres by kinetochores.
- 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.)
The homologous chromosomes in each bivalent are separated and move to the opposite poles of the cell.
The chromosomes become diffuse and the nuclear membrane reforms.
- Cellular cytoplasmic division to form two new cells, followed by Meiosis II.
- Chromosomes begin to condense, nuclear membrane breaks down and spindle forms.
- Spindle fibres attach to chromosomes, chromosomes align in cell centre.
- Chromosomes separate and move to the opposite poles of the cell.
- Chromosomes reach spindle pole ends and the nuclear membrane reforms.
Cellular cytoplasmic division to form new cells.
Comparison of Meiosis/Mitosis
- 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
Female - Oogenesis
- 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
Male - 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
- 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 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.
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.
- 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)
- 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 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 - 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)
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
- Sperm contributes - centriole which organizes mitotic spindle
- Oocyte contributes - mitochondria (maternally inherited)
- 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.
- 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|
- The Developing Human: Clinically Oriented Embryology (8th Edition) by Keith L. Moore and T.V.N Persaud - Chapter 2
- Larsen’s Human Embryology by GC. Schoenwolf, SB. Bleyl, PR. Brauer and PH. Francis-West - Chapter 1
- Developmental Biology by Gilbert, Scott F. Sunderland (MA): Sinauer Associates, Inc.; c2000Figure 2.9. Summary of meiosis | fusion of egg and sperm plasma membranes
- Molecular Biology of the Cell 4th ed. Alberts, Bruce; Johnson, Alexander; Lewis, Julian; Raff, Martin; Roberts, Keith; Walter, Peter New York and London: Garland Science; c2002 - IV. Internal Organization of the Cell Chapter 17. The Cell Cycle and Programmed Cell Death Programmed Cell Death | An Overview of the Cell Cycle | Figure 17-1. The cell cycle | Fertilization
- Molecular Cell Biology by Lodish, Harvey; Berk, Arnold; Zipursky, S. Lawrence; Matsudaira, Paul; Baltimore, David; Darnell, James E. New York: W. H. Freeman & Co.; c1999 Chapter 13. Regulation of the Eukaryotic Cell Cycle Regulation of the Eukaryotic Cell Cycle | Overview of the Cell Cycle and Its Control | Figure 13-2. Current model for regulation of the eukaryotic cell cycle | Movies Proposed alternative mechanisms for chromosome congression. | Centromeric attachment of microtubules. | The stages of mitosis and cytokinesis in an animal cell.
- The Cell - A Molecular Approach by Cooper, Geoffrey M. Sunderland (MA): Sinauer Associates, Inc.; c2000- IV. Cell Regulation Chapter 14. The Cell Cycle The Eukaryotic Cell Cycle | Figure 14.1. Phases of the cell cycle | Figure 14.32. Comparison of meiosis and mitosis | Figure 14.37. Meiosis of vertebrate oocytes
- HSTAT - In Vitro Fertilization As A Medical Treatment For Male or Female Infertility
- Cell cycle studies based upon quantitative image analysis. Stacey DW, Hitomi M. Cytometry A. 2008 Apr;73(4):270-8. Review. PMID: 18163464
- Analysis of cell cycle phases and progression in cultured mammalian cells. Schorl C, Sedivy JM. Methods. 2007 Feb;41(2):143-50. Review. PMID: 17189856
- JCB - Movie Collection Mitosis | Cytokinesis
- 2008 ANAT3231 Lecture PDF lecture14-15 1 slide/page (view) | 4 slides/page (print) | 6 slides/page (print) | text (print)
- McGraw-Hill Animation comparing Mitosis and Meiosis
- Salmon Lab Mitosis Movies
Dr Mark Hill
|ANAT2341 Embryology S2 2011
Course Content 2011
2011 Timetable: | Embryology Introduction | Fertilization | Cell Division/Fertilization | Week 1 and 2 Development | Week 3 Development | Week 1 to 3 | Mesoderm Development | Ectoderm, Early Neural, Neural Crest | Trilaminar Embryo to Early Embryo | Early Vascular Development | Placenta | Vascular and Placenta | Endoderm, Early Gastrointestinal | Respiratory Development | Endoderm and Respiratory | Head Development | Neural Crest Development | Head and Neural Crest | Musculoskeletal Development | Limb Development | Musculoskeletal | Renal Development | Genital | Kidney and Genital | Sensory | Stem Cells | Stem Cells | Endocrine Development | Endocrine | Heart | Integumentary Development | Heart and Integumentary | Fetal | Birth and Revision | Fetal
- Glossary: A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z | Numbers | Symbols | Term Link
Cite this page: Hill, M.A. (2021, May 9) Embryology Lecture - Fertilization. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Lecture_-_Fertilization
- © Dr Mark Hill 2021, UNSW Embryology ISBN: 978 0 7334 2609 4 - UNSW CRICOS Provider Code No. 00098G