Comprehensive Notes: Weeks 1–3 of Human Development

1st Week of Human Development (Days 1–7)

  • Sperm's parts: What a sperm looks like:

    • The head holds the genetic material (nucleus) and a cap called the acrosome.

    • The neck is a short part connecting the head to the middle.

    • The middle piece is like the engine room, full of energy-producing parts (mitochondria).

    • The principal piece and end piece are parts of the tail that help it swim.

    • It also has a jelly-like inside (cytoplasm-containing region).

    • The egg it's trying to reach has outer covers: follicular cells (corona radiata) and a protective shield called zona pellucida.

  • Fertilization: This is when a sperm and an egg join together. It usually happens in a wide part of the fallopian tube (ampulla) and takes about 24 hours. Here's how it happens:

    • Sperm moving through the outer layer (corona radiata): The sperm needs to become

1st Week of Human Development (Days 1

–7) - Sperm's parts: What a sperm looks like: - The head holds the genetic material (nucleus) and a cap called the acrosome. - The neck is a short part connecting the head to the middle. - The middle piece is like the engine room, full of energy-producing parts (mitochondria). - The principal piece and end piece are parts of the tail that help it swim. - It also has a jelly-like inside (cytoplasm-containing region). - The egg it's trying to reach has outer covers: follicular cells (corona radiata) and a protective shield called zona pellucida. - Fertilization: This is when a sperm and an egg join together. It usually happens in a wide part of the fallopian tube (ampulla) and takes about 24 hours. Here's how it happens: - Sperm moving through the outer layer (corona radiata): The sperm needs to become (1) capacitated (a process where the sperm undergoes changes to become capable of fertilization, usually taking about 7 hours in the female reproductive tract) and (2) undergo the acrosome reaction while passing through the corona radiata. The acrosome reaction involves the release of enzymes (like hyaluronidase) from the acrosome, which helps the sperm penetrate this layer. - Sperm penetrating the zona pellucida: Once through the corona radiata, the sperm binds to specific receptors (ZP3 proteins) on the zona pellucida. This binding triggers the release of more acrosomal enzymes (like acrosin), which create a path through the zona pellucida. - Fusion of sperm and oocyte cell membranes: After penetrating the zona pellucida, the sperm's head and tail enter the oocyte cytoplasm. The sperm's cell membrane fuses with the oocyte's cell membrane. - Completion of the second meiotic division of the oocyte: The entry of the sperm causes the oocyte to complete its second meiotic division, forming a mature ovum and a second polar body. - Formation of male and female pronuclei: The nucleus of the sperm decondenses and swells to form the male pronucleus. Simultaneously, the nucleus of the ovum swells to form the female pronucleus. Both pronuclei are haploid (nn chromosomes, 2c2c DNA). - Syngamy (fusion of pronuclei): The male and female pronuclei move towards each other, and their nuclear envelopes break down. The chromosomes from both pronuclei intermingle, forming a single diploid nucleus. This fusion marks the moment of zygote formation. - Formation of the zygote: The cell that results from the fusion of the male and female pronuclei is called a zygote. It is diploid (2n2n chromosomes, 4c4c DNA) and contains a