Oogenesis | Zoology Optional Notes for UPSC PDF Download

Oogenesis

• Process of generating female gametes in the ovaries of female animals
• Similarities with spermatogenesis in terms of nuclear changes
• Cytoplasmic specialization differs from spermatogenesis
• Initiates within the fetus before birth
• Steps up to production of primary oocytes occur prenatally
• Primary oocytes become secondary oocytes or degenerate
• Located in the outermost layers of the ovaries

Stages of Oogenesis

• Primary oocyte formation before birth
• Development from birth to puberty
• Development of secondary oocytes from puberty to menopause
• Primary oocyte undergoes growth while arrested in meiosis-I
• Surrounded by granulosa cells that form zona pellucida
• Formation of secondary follicles occurs in monthly cycles

Meiosis in Oogenesis

• Meiosis-I completion triggered by LH surge
• Two haploid cells formed within the follicle: polar body and secondary oocyte
• Meiosis-II involves replication of polar body and arrest of secondary oocyte in metaphase stage
• Oogenesis comprises three phases: Multiplication Phase, Growth Phase, and Maturation Phase
• Maturation Phase involves two meiotic divisions resulting in formation of functional ovum and polar bodies
• First polar body may or may not divide, leading to formation of one functional ovum
• Second polar body forms during fertilization when sperm enters ovum

In Vertebrates

• First polar body is formed after primary oocyte is released from ovary and enters oviduct
• Second polar body forms only during fertilization when sperm enters the ovum.