Cytology · MU Sofia

75. Fertilization.

Explains how a sperm and an oocyte combine to form a zygote, following the stages in order: ovulation, sperm capacitation, transport and penetration through the corona radiata and zona pellucida via the acrosomal reaction, sperm-egg fusion, the cortical reaction against polyspermy, and zygote formation. It also covers the key structures, the hormones LH, FSH, oestrogen and progesterone, and the significance of fertilization for genetic diversity and early pregnancy.

Inside: 5 sections · 8 labelled pictures · 14 flashcards · 5 min

Topic 75 of 127 in the MU Sofia syllabus for Cytology (Dental Medicine, Year 1). The free AceMed note covers it in 5 sections, about 659 words (5 min of reading), with 14 flashcards.

In this note

  1. Introduction
  2. Key Stages in Fertilization:
  3. Key Structures Involved in Fertilization:
  4. Hormonal Regulation of Fertilization:
  5. Significance of Fertilization:

Introduction

Fertilization is the process by which a sperm from the male and an egg (oocyte) from the female combine to form a zygote, initiating the development of a new organism. It typically takes place in the fallopian tube and involves several critical steps to ensure successful union of the gametes.

Key Stages in Fertilization:

  1. Ovulation:
  • Ovulation is the release of a mature oocyte (egg) from the ovary, usually around day 14 of the menstrual cycle.
  • The egg is captured by the fimbriae (finger-like projections) of the fallopian tube and then transported into the ampulla, the most common site for fertilization.
  1. Sperm Capacitation:
  • Before sperm can fertilize an egg, they must undergo a process called capacitation. This occurs after sperm enters the female reproductive tract.
  • Capacitation involves biochemical changes that allow the sperm to penetrate the egg's protective layers. These changes include alterations in the sperm's membrane and an increase in motility.
  1. Sperm Transport and Penetration:
  • Sperm swim through the cervix, the uterus, and into the fallopian tubes, guided by chemotactic signals from the egg.
  • Upon reaching the egg, sperm must navigate through several layers:
  • Corona radiata: A layer of follicular cells surrounding the egg.
  • Zona pellucida: A glycoprotein layer surrounding the egg, which is essential for species-specific recognition.
  • Acrosomal reaction: The sperm's acrosome (a cap-like structure on its head) releases enzymes that break down the zona pellucida,

allowing the sperm to penetrate and reach the egg's plasma membrane.

  1. Sperm-Egg Fusion:
  • Once the sperm reaches the egg's plasma membrane, the sperm and egg membranes fuse.
  • This fusion allows the sperm's genetic material (nucleus) to enter the egg, and triggers the completion of meiosis in the egg.
  • The sperm's nucleus and the egg's nucleus each contribute half of the chromosomes, resulting in the formation of a zygote with a full set of chromosomes (46 in humans).
  1. Cortical Reaction:
  • After the sperm enters the egg, the egg undergoes a cortical reaction to prevent polyspermy (fertilization by more than one sperm).
  • Cortical granules in the egg's cytoplasm release enzymes that modify the zona pellucida, making it impenetrable to other sperm.
  1. 6. Formation of the Zygote:
  • The sperm's and egg's genetic material (haploid nuclei) merge, forming a diploid set of chromosomes (46 in humans). This fusion creates the zygote.
  • The zygote is now ready to begin cleavage, a series of mitotic cell divisions that will eventually lead to the formation of the blastocyst.

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