36. Embryonic development – early stages (zygote to neurula)

1. Zygote (Single-Cell Stage):

   • The zygote is the initial single-cell stage formed immediately after fertilization when the sperm cell fuses with the egg cell, combining their genetic material to create a diploid cell with a complete set of chromosomes.

2. Cleavage (Mitotic Divisions):

   • Cleavage is a series of rapid mitotic divisions that follow fertilization. These divisions partition the large volume of cytoplasm in the zygote into smaller cells called blastomeres, without an overall increase in the embryo’s size. This results in the formation of a solid ball of cells.

3. Morula (Compact Multicellular Stage):

   • The morula is an early stage of embryonic development, characterized by a compact cluster of 16-32 blastomeres. This stage resembles a mulberry (hence the name "morula," Latin for mulberry). The morula stage is significant for the compaction process, where cells increase in adhesion to form a tightly packed structure.

4. Blastula (Formation of the First Embryonic Cavity - Blastocoel):

   • As cleavage continues, the blastomeres rearrange to form a hollow ball of cells called the blastula. This stage is marked by the presence of a fluid-filled cavity known as the blastocoel. The blastula structure enables the embryo to prepare for the next phase, gastrulation.

5. Gastrulation (Formation of Germ Layers):

   • Gastrulation is a critical phase during which the blastula reorganizes into a three-layered structure called the gastrula. This process involves extensive cell movements, resulting in the formation of three primary germ layers:

     • Ectoderm (outer layer)

     • Mesoderm (middle layer)

     • Endoderm (inner layer)

   • During gastrulation, a new cavity called the archenteron (primitive gut) forms, and the opening to this cavity is the blastopore. The germ layers established during this stage give rise to all the tissues and organs of the developing organism.

6. Organogenesis (Formation of Organs and Body Systems):

   • Organogenesis is the process where the three germ layers differentiate into various organs and tissues. In vertebrates, a key aspect of organogenesis is neurulation, the formation of the neural tube, which eventually develops into the central nervous system (brain and spinal cord).

   • Other structures such as the notochord (a rod-like structure that provides support) and somites (blocks of mesoderm that give rise to the vertebrae, muscles, and skin) also form during this stage. Organogenesis continues until the development of all body systems and organs is complete, leading to either hatching (in oviparous organisms) or birth (in viviparous organisms).

cleavage

• Cleavage is characterized by rapid cell divisions without growth phases, leading to an increasing number of smaller cells, the blastomeres. The pattern and speed of cleavage depend on:

  • The amount and distribution of yolk within the egg cytoplasm. Yolk is a nutrient-rich substance that influences the cleavage pattern.

  • The presence of cytoplasmic factors that control the position and timing of the mitotic spindle formation.

• The yolk's distribution creates two poles within the egg:

  • Vegetal Pole: The region with a high concentration of yolk, where cleavage occurs more slowly.

  • Animal Pole: The region with a relatively low concentration of yolk, where cleavage occurs more rapidly.

• The transition from fertilization to cleavage is regulated by the activation of the Mitosis Promoting Factor (MPF), a cyclin-dependent kinase complex. MPF is essential for initiating the mitotic cell cycle in early embryonic development. It was first discovered in frog eggs, where it plays a pivotal role in resuming meiosis and regulating early embryonic cell cycles.

• Early Blastomeres: In the early cleavage stages, the blastomeres (cells resulting from cleavage) rapidly progress through the cell cycle phases of M phase (mitosis) and S phase (DNA synthesis). The typical regulatory mechanisms that control cell division in later stages of development are not yet active, allowing the cells to divide rapidly and synchronously.

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