How do blastomeres form during early embryonic development?

Blastomeres are formed during early embryonic development through a process known as cleavage. Following fertilization, the zygote—a single, relatively large cell—undergoes a series of rapid mitotic divisions.123

Key characteristics of this process include:

• Subdivision without Growth: During cleavage, the zygote subdivides into smaller cells called blastomeres. Because there is little to no growth between these divisions, the total mass of the cellular cluster remains approximately the same as the original zygote.12
• Division Patterns: Initially, these divisions occur in a specific sequence—often starting with longitudinal planes followed by equatorial ones—though this synchronicity is eventually lost as the number of cells increases.2
• Cytoplasmic Inheritance: Each blastomere receives a full complement of paternal and maternal chromosomes. Furthermore, blastomeres may inherit different regions of the original egg's cytoplasm, which can contain regulatory substances known as cytoplasmic determinants that influence future cell commitment.14
• Progression: As cleavage continues, the blastomeres form a solid mass called a morula. In mammals, this eventually develops into a blastocyst, a hollow structure containing a fluid-filled cavity called the blastocoel.156