Cell Biology - lecture 15 - Development of Multicellular Organisms

Learning Aims/Outcomes

  • Understand how germ cells form during development.

  • Describe how meiosis ensures gametes have the correct amount of genetic material.

  • Understand fertilisation processes in mammals.

  • Describe asexual reproduction in Hydra.

  • Understand parthenogenesis.

Germ Cell Formation

Overview

  • Germ cells are the precursors to sperm and eggs, distinct from somatic cells that comprise the body.

  • Germline cells are uniquely segregated during early embryonic development.

  • Functional gametes develop during adulthood.

Mouse Germ-Cell Formation

  • Primordial Germ Cells (PGCs):

    • First identified in proximal epiblast (pre-gastrulation).

    • Migrate to posterior end during gastrulation.

    • Eventually migrate to gonads.

  • Human Development:

    • Similar migration occurs during the 4th week.

  • Gene Expression:

    • PGCs express genes essential for pluripotency (nanog) and cell adhesion/migration.

Migration Pathways

  • PGCs enter hindgut endoderm and travel via dorsal mesentery to genital ridges.

  • Differentiation:

    • Migrate and differentiate into eggs or sperm.

  • Rational for migration includes exclusion from body plan formation and selecting for health (localized survival).

  • Environmental chemical signals regulate migration; transplanted PGCs cannot orient correctly.

Meiosis in Germ Cells

Overview

  • Meiosis produces haploid gametes (egg and sperm), ensuring the correct chromosomal count in the zygote.

  • Phases of Meiosis:

    • Prophase: Chromosomes recombine and pair up.

    • Meiosis consists of two divisions; chromosome replication occurs before the first but not before the second, halving the count.

Oogenesis

  • Polar Body:

    • Small product of meiosis, nonembryonic.

  • Mitosis occurs during migration to ovaries; primary oocytes halt at prophase until ovulation.

  • Second meiotic division only occurs post-fertilisation.

Spermatogenesis

  • Germ cells enter embryonic testis, pausing in G1 stage of the cell cycle.

  • Post-birth, they proliferate via mitosis into spermatogonia.

  • Mature sperm develop from differentiating spermatocytes through meiosis.

Impact of Age on Oocyte Numbers

  • Human oocyte numbers decline with age.

  • Initial count: ~7 million germ cells at fetal stage; drops to approximately 1 million at birth; ~300,000 at puberty; only about 400 released during lifetime.

Hypotheses of Germline Death in Mammals

  • Apoptosis Mechanisms:

    • Growth factors suppress apoptosis via germ-cell surface receptors.

    • Meiotic recombination failures can lead to cell death.

    • They can sacrifice themselves to protect other germ cells.

    • Nurse Germ Cells:

    • In cysts, these transport necessary materials to oocytes.

Fertilisation Overview

  • Fertilisation marks the beginning of development, occurring in the fallopian tube.

  • Fusion of the sperm and egg that initiates development.

  • Critical Aspect: Only one sperm should fertilise the egg to prevent chromosomal abnormalities.

Process of Fertilisation

  1. Gametes Fusion:

  • Sperm penetrates sticky layer of hyaluronic acid and somatic follicle cells - cumulus cells

  • Sperm binds to zona pellucida

  • Penetrates the zona pellucida - layer of fibrous glycoproteins via acrosomal reaction

  • Plasma membrane of sperm fuses with egg plasma membrane

  • Sperm nucleus enter the egg cytoplasm

  • Initiates developmental processes.

  1. Mitotic Stages:

  • Series of cell divisions from zygote to morula to blastocyst.

  1. Egg Structure:

  • Zona pellucida and corona radiata protect the oocyte.

Acrosome Reaction

  • Enzymes from the acrosome digest the protective egg coat facilitating sperm entry.

Calcium Wave and Polyspermy Prevention

  • Calcium Wave:

  • Triggered by sperm entry, essential for cortical granule exocytosis and hardening of the vitelline envelope - fertilisation envelope.

  • Prevents more sperm from reaching the egg

Asexual Reproduction in Hydra

  • Budding Process:

  • Buds develop from repeated mitotic divisions of epidermal cells, eventually detaching.

  • Grows connected to parent

  • Then constricts at base and separates

Parthenogenesis

  • Embryos can develop from unfertilised egg cells, observed in specific species, including Komodo dragons and certain bees.

Reproduction in Honey Bees

Types of Bees

  • Worker Bees:

    • Females, non-functional reproductive system through eating phenolic acid.

  • Drone Bees:

    • Males with functional reproductive systems.

  • Queen Bee:

    • Fully developed reproductive capabilities.

Sexual Determination in Bees

  • Haploid-diploid system:

    • Males from unfertilized eggs; females from fertilized diploid eggs.