Embryology

• Oogenesis


  Development of an immature oogonium into a secondary oocyte which is ready to be fertilized
• Chromosome count


  Humans are diploid organisms, meaning they have two copies of each chromosome in each cell, one from the father and one from the mother
• DNA replication


  1. DNA doubles, but number of chromosomes remain the same

  2. Chromosomes become X-shaped with two sister chromatids

  
• c


  Denotes the amount of DNA in the cell
• Meiosis


  1. One 2n2c mother cell replicates DNA to become 2n4c, then divides into four 1n1c cells

  2. Meiosis I: 2n4c cell divides into two 1n2c cells

  3. Meiosis II: 1n2c cells divide into four 1n1c cells
• Polar bodies


  Three of the four daughter cells from meiosis that receive barely any cytoplasm and degenerate
• Oogenesis in the foetal period


  1. Primordial germ cells migrate to primordial gonads and differentiate into oogonia

  2. Oogonia replicate by mitosis from 4th week to 7th month of gestation

  3. Some oogonia start meiosis I and become primary oocytes, arrested in prophase I
• Oogenesis after the foetal period

  1. Primary oocytes remain arrested in prophase I until ovulation nears

  2. LH surge causes primary oocyte to finish meiosis I, becoming a secondary oocyte

  3. Secondary oocyte continues meiosis II but is arrested in metaphase

  4. Secondary oocyte is ovulated, and if fertilized completes meiosis II to form the mature ovum
• Oocyte maturation


  • Oogonia (2n2c)
  • Primary oocytes (2n4c)
  • Secondary oocytes (1n2c)
  • Mature ovum (1n1c)
• Primordial follicle


  • Primary oocyte surrounded by flat epithelial follicular cells
• Primary follicle


  • Primary oocyte surrounded by zona pellucida, cuboidal granulosa cells, theca interna, and theca externa
• Antral follicle


  • Fluid-filled spaces appear between granulosa cells, forming an antrum and pushing the oocyte off centre
  • Granulosa cells surrounding the oocyte form the cumulus oophorus
• Graafian follicle


  • Antral follicle has matured, reaching 25mm in diameter
• At puberty, females begin to experience their menstrual cycle, with the first cycle called the menarche
• Menstrual cycle


  1. One mature oocyte is released from the ovaries and travels to the uterus

  2. If fertilized, the blastocyst implants in the endometrium

  3. If not fertilized, the oocyte is shed

  
• Menstrual cycle duration


  Approximately 28 days, with 21-35 days being normal
• Components of the menstrual cycle

  • Ovarian cycle
  • Endometrial/uterine cycle

  
• Hypothalamic-pituitary-gonadal axis


  Hypothalamus produces GnRH, which stimulates anterior pituitary to release FSH and LH, which then control the ovaries

  
• Follicle-stimulating hormone (FSH)


  Stimulates maturation of ovarian follicles, which produce oestrogens
• Luteinizing hormone (LH)


  • Causes oocyte to complete meiosis I and initiate meiosis II
  • Stimulates production of progesterone
  • Stimulates enzymes and prostaglandins that cause ovulation
• Follicular phase of ovarian cycle

  FSH stimulates development of 15-20 primary follicles into antral follicles, but only one matures into a graafian follicle
• Desquamation/menstrual phase of endometrial cycle

  Corpus luteum degenerates, causing progesterone levels to decrease and the endometrium to die and be shed
• Proliferation phase of endometrial cycle


  Oestrogens stimulate proliferation of the basal layer of the endometrium to form new spongy and compact layers
• Ovulation


  1. LH surge stimulates enzymes and prostaglandins that cause the secondary oocyte to be extruded from the graafian follicle

  2. The cumulus oophorus cells form the corona radiata around the zona pellucida
• Ovulation


  1. Ovarian follicle reaches graafian stage

  2. LH surge stimulates processes that cause secondary oocyte to be extruded from graafian follicle
• LH


  • Stimulates collagenase to digest collagen fibres surrounding follicle
  • Increases prostaglandins in ovary to cause muscle contractions in ovarian wall
• At day 14 the processes caused by LH surge cause the secondary oocyte to be extruded from the graafian follicle together with the cumulus oophorus
• Corona radiata


  Granulosa cells comprising the cumulus oophorus form a barrier that spermatozoa must penetrate
• Fertilization


  Process by which one spermatozoon fuses with the secondary oocyte
• Fertilization most commonly occurs in the ampullary region of the fallopian tube
• Sperm journey


  1. Sperm remain viable in female reproductive tract for several days

  2. Movement from cervix to uterine tube is mostly due to uterine/tube contractions, not sperm propulsion

  3. Sperm stop in isthmus and become motile again at ovulation to swim to ampulla
• Capacitation


  Sperm interact with uterine tube mucosa, losing glycoprotein coat and seminal plasma proteins, allowing them to pass through corona radiata
• Acrosome reaction


  Enzymes in acrosome of sperm are released, degrading zona pellucida to allow sperm cell membrane to fuse with oocyte membrane
• Zona reaction


  Enzymes released from oocyte plasma membrane change properties of zona pellucida to prevent other sperm from penetrating
• After sperm enters oocyte

  1. Oocyte finishes meiosis II to form mature ovum

  2. Sperm head forms male pronucleus, oocyte nucleus forms female pronucleus

  3. Pronuclei replicate DNA, then fuse to form 2n4C zygote

  4. Zygote divides into two 2n2C blastomeres
• Cleavage


  Rapid division of zygote by mitosis while confined by zona pellucida
• Cleavage stages


  1. Two-cell stage (30 hours after fertilization)

  2. Four-cell stage (40 hours after fertilization)

  3. Morula (16-cell, 3 days after fertilization)
• Blastocyst


  Zygote enters uterine cavity, zona pellucida disintegrates, fluid forms blastocele

  
• Implantation


  Trophoblast cells penetrate endometrium, most commonly in anterior or posterior uterine wall
• Bilaminar disc


  Epiblast and hypoblast cells form, epiblast will form embryo, hypoblast will form yolk sac