Fertilization-lec-notes

Key Events in Animal Development

  • Gamete Formation

    • Sperm and eggs form and mature.

  • Fertilization

    • Fusion of egg and sperm to form a zygote.

  • Cleavage

    • The zygote subdivides, cytoplasm partitioned into blastomeres.

  • Gastrulation

    • Formation of germ layers.

  • Organogenesis

    • Body organs form, cells interact and differentiate.

    • Organs increase in size, adult body form attained.

Fertilization Overview

  • Definition: The initial event in development, the union of male and female gametes to form a zygote.

  • Functions of Fertilization:

    1. Restores the normal number of chromosomes by combining haploid genomes from the mother and father.

    2. Activates the egg to begin development.

Research on Fertilization

  • Modern understanding largely derives from studies on marine invertebrates, especially sea urchins.

  • Sea urchins produce large quantities of eggs and sperm, allowing laboratory studies of fertilization.

Fertilization in Vertebrates

  • Studied in many vertebrates, including mammals (mice, hamsters, rabbits).

Contact and Recognition Between Egg and Sperm

  • Spawning: Release of eggs and sperm into the ocean by marine invertebrates and fish.

  • Chemotactic Factors: Released by eggs to attract sperm of the same species, improving the likelihood of fertilization.

Sea Urchin Fertilization

  • Sperm penetrate the jelly layer surrounding the egg, then contact the vitelline envelope (thin membrane above the plasma membrane).

  • Egg-recognition proteins on the sperm bind to species-specific receptors on the vitelline envelope.

Structure of the Sea Urchin Egg

  • Components:

    • Jelly Layer:

      • Transparent, gelatinous matrix containing chemical attractants for sperm and induces sperm acrosome reaction.

    • Vitelline Envelope:

      • Snugly fits against the egg membrane, protects the egg cell.

      • Works with the jelly layer to attract and protect the egg.

    • Plasma Membrane: Surrounds the egg cell, housing vital cellular components (nucleus, mitochondria, etc.).

Sperm Entry and Fusion


    1. Chemical signals attract sperm to the egg.


    1. Binding occurs at the jelly layer, initiates the acrosome reaction leading to fusion.


    1. Fertilization Cone: Forms at the point of sperm contact with the egg’s vitelline envelope.


    1. Electrical Potential Change: After the first sperm binds, prevents additional sperm from entering (fast and slow block mechanisms).

Fertilization in Mammals

  • No fertilization membrane forms; the cortical reaction modifies egg surface to prevent polyspermy.

  • Sperm loses flagellum after fusion; the sperm nucleus becomes the pronucleus, which migrates to contact the female pronucleus, forming a diploid zygote.

Changes in the Zygote After Fertilization


    1. Removal of metabolic inhibitors, initiating metabolism.


    1. Burst in DNA and protein synthesis.


    1. Complete reorganization of cytoplasm, positioning morphogenetic determinants critical for development.

Amphibian Fertilization Steps

  1. Amplexus: Male grasps female, aligning cloacae.

  2. Oviposition: Female lays eggs in clusters.

  3. Sperm Deposition: Male ejects sperm onto eggs.

  4. Fertilization: Sperm swim to fertilize eggs.

Amphibian Fertilization Mechanism

  • Primarily occurs externally, with sperm factors activating egg processes (increased intracellular calcium).

  • Gray Crescent: A light-colored band appearing post-fertilization indicating embryonic patterning.

Timing of Key Events in Fertilization

  • 12 min: Fusion of sperm and egg pronuclei.

  • 12 hrs: Maturation in mammals.

  • Various timing sequences for divisions and membrane formation.

Human Fertilization Process

  • Sperm fertilizes an egg by attaching to the zona pellucida, releasing enzymes for membrane fusion. Steps include:

    • Capacitation in the reproductive tract.

    • Attraction to progesterone from cumulus cells.

    • Attachment and penetration through zona pellucida.

    • Fusion leading to zygote formation and subsequent implantation in the uterus.

Ovarian Structure

  • Ovaries: Primary female reproductive organs, almond-shaped, located in the pelvic cavity supported by ligaments.

  • Structure: Composed of outer cortex (with follicles) and inner medulla (connective tissue, blood vessels).

Oogenesis Process

  • Sequence of events resembling spermatogenesis but with distinct outcomes.

  • Initial Development: Oogonia create primary oocytes, halted in prophase of meiosis until puberty.

Oocyte Availability

  • Approximately 700,000 oocytes present at birth; reduced to 400,000 by puberty.

Follicle Development

  • Follicles: Structures surrounding developing oocytes, change from primordial to mature vesicular follicles under hormonal influence.

Ovulation Process

  • Triggered by luteinizing hormone, releasing secondary oocyte.

  • If fertilized, completes meiotic division; otherwise, degenerates shortly after.

Post-Ovulation

  • Corpus Luteum Formation: Remains functional post-fertilization, secreting hormones until placenta develops.

  • Without fertilization, degenerates to corpus albicans.

Fertilization in Various Species

  • Fertilization occurring mostly in the ampulla region of oviducts, depending on specific molecular interactions (like Izumo1-Juno).

Anatomy of the Chicken Egg

  • Composed of several layers including shell, inner and outer membranes, albumen, chalazae, and yolk, each serving specific protective and nutritional functions.

Amniotic Egg Characteristics

  • Found in reptiles (birds) and mammals, providing a support system for embryos.

  • Contains membranes such as amnion, yolk sac, allantois, and chorion, facilitating respiration and waste management.

Summary of Ovarian Follicle Development

  • Includes multiple stages from primordial to mature follicles, each characterized by structural and hormonal changes leading to ovulation and potential fertilization.