Talk:Lecture - Fertilization

2011 Audio Lecture 2 Audio

Fertilization

Gamete formation, menstrual cycle and fertilization will be covered in detail in this week's Laboratory. 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

• 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

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

• 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)

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.