Gametogenesis and Fertilization Overview : lecture 9

Gametogenesis and Fertilization

• Somatic cells divide by mitosis to create identical copies for tissue growth and repair
• Gametes are produced by meiosis, which creates non-identical cells with half the DNA content
• Meiosis halves the DNA content to ensure a zygote with the correct chromosome number (46) after fertilization

Oogenesis

• Germinal epithelium becomes inactive after birth and no longer divides
• Primary oocyte grows in size after puberty but remains arrested in metaphase I until ovulation
• Menstrual cycle:
  • A secondary follicle develops, and one primary oocyte completes meiosis I to become a secondary oocyte
  • The secondary oocyte pauses in prophase II of meiosis

Ovarian and Menstrual Cycles

• Ovarian cycle:

  • Follicular phase (days 1-14)
  • Luteal phase (days 15-28)

• Corpus luteum secretes progesterone to prepare the uterus for implantation

• Menstrual cycle:

  • Menstrual phase (days 1-5): Uterine lining is shed
  • Proliferative phase (days 6-14): Lining regrows and glands proliferate
  • Secretory phase (days 15-28): Uterine glands secrete compounds for implantation

Spermatogenesis

• Occurs in seminiferous tubules of the testes
• Primary spermatocytes undergo meiosis I to produce haploid secondary spermatocytes
• Spermatids mature into spermatozoa during spermiogenesis

Events Leading to Fertilization

• Corona radiata: a layer of granulosa cells surrounding the oocyte
• Sperm penetrates the zona pellucida using enzymes from the acrosome
• Polyspermy is prevented by oocyte activation, which triggers cortical granule release, hardening the zona pellucida and digesting remaining sperm receptors

Fertilization and Early Development

• Amphimixis: mixing of paternal and maternal DNA during the first mitotic division of the zygote
• Cleavage: division of the zygote into smaller cells called blastomeres
• Implantation: the blastocyst embeds into the uterine lining

Development Stages and Twins

• Prenatal development stages: pre-embryo, embryo, and fetus
• Identical twins form from splitting a single zygote into two inner cell masses
• Fraternal twins form from the fertilization of two separate oocytes by two different sperm

Germ Layers and Organ Formation

• Ectoderm:
  • Brain
  • Spinal cord
  • Nerves
  • Skin
• Mesoderm:
  • Muscles
  • Bones
  • Heart
  • Kidneys
  • Connective tissue
• Endoderm:
  • Gastrointestinal tract
  • Lungs
  • Various glands

Placental Circulation and Fetal Development

• Placenta: supports fetal development by exchanging nutrients and waste between fetus and mother
• Umbilical arteries: carry deoxygenated blood
• Umbilical vein: carries oxygenated blood to the fetus

Neural Tube and Congenital Disorders

• Spina bifida: neural tube defect where spinal cord content protrudes outside the vertebrae
• Hydrocephalus: excess cerebrospinal fluid accumulates, often causing mental deficits and nerve dysfunction

Congenital Heart Defects and Risks

• Leading cause of neonatal death after prematurity
• Maternal factors increasing risk: rubella, diabetes, medication use during pregnancy
• Types of congenital heart defects:
  • Lesions increasing pulmonary blood flow
  • Lesions decreasing pulmonary blood flow
  • Obstructive lesions
  • Mixed lesions

Lung Development and Premature Births

• Canalicular phase: (weeks 17-27) immature alveoli form and lung vascularization increases
• Insufficient surfactant causes infant respiratory distress syndrome (IRDS) due to alveolar collapse

Postnatal Changes and Transition to Neonatal Circulation

• Ductus arteriosus and foramen ovale close after birth to separate pulmonary and systemic circulations
• IRDS symptoms: rapid breathing, cyanosis, nasal flaring, fatigue
• IRDS treatment: artificial surfactants, mechanical ventilation, oxygen therapy

This quiz focuses on the processes of gametogenesis and fertilization, detailing how somatic cells and gametes are formed. It explains oogenesis and the ovarian and menstrual cycles, highlighting the key phases and hormonal influences involved in these reproductive processes.