Embryonic Development Stages

Embryonic Development

• Sea urchin embryonic development consists of several stages:
  1. Fertilization: Sperm fertilizes egg, resulting in a zygote.
  2. Cleavage: Zygote undergoes multiple cell divisions without significant growth, resulting in a multicellular blastula.
  3. Blastula: A hollow ball of cells with a fluid-filled cavity (blastocoel).
  4. Gastrulation: Complex series of cell movements and transformations, resulting in the formation of a gastrula.

Gastrula Formation

Primary Invagination

• Begins with the formation of a small invagination (primary archenteron) on the vegetal pole of the blastula.
• Cells from the blastula's surface (ectoderm) invaginate into the blastocoel, forming a double-layered structure.
• Primary archenteron will eventually give rise to the endoderm and mesoderm.

Secondary Invagination

• A secondary invagination forms on the opposite side of the primary archenteron, near the animal pole.
• This invagination will eventually give rise to the mouth and anus.

Archenteron Formation

• The primary and secondary invaginations eventually meet and fuse, forming the archenteron.
• The archenteron is a double-layered structure, with the inner layer (endoderm) facing the blastocoel and the outer layer (mesoderm) facing the ectoderm.

Gastrula Formation Completion

• The archenteron continues to grow and eventually fills the blastocoel.
• The endoderm and mesoderm differentiate into specific tissues and organs.
• The gastrula is now complete, consisting of three primary germ layers: ectoderm, mesoderm, and endoderm.

Embryonic Development

• Sea urchin embryonic development consists of several stages:
  • Fertilization: Sperm fertilizes egg, resulting in a zygote.
  • Cleavage: Zygote undergoes multiple cell divisions without significant growth, resulting in a multicellular blastula.
  • Blastula: A hollow ball of cells with a fluid-filled cavity (blastocoel).
  • Gastrulation: Complex series of cell movements and transformations, resulting in the formation of a gastrula.

Gastrula Formation

Primary Invagination

• Formation of a small invagination (primary archenteron) on the vegetal pole of the blastula.
• Ectoderm cells invaginate into the blastocoel, forming a double-layered structure.
• Primary archenteron will eventually give rise to the endoderm and mesoderm.

Secondary Invagination

• Formation of a secondary invagination on the opposite side of the primary archenteron, near the animal pole.
• This invagination will eventually give rise to the mouth and anus.

Archenteron Formation

• Primary and secondary invaginations meet and fuse, forming the archenteron.
• The archenteron is a double-layered structure, with the inner layer (endoderm) facing the blastocoel and the outer layer (mesoderm) facing the ectoderm.

Gastrula Formation Completion

• Archenteron continues to grow and eventually fills the blastocoel.
• Endoderm and mesoderm differentiate into specific tissues and organs.
• Gastrula is now complete, consisting of three primary germ layers: ectoderm, mesoderm, and endoderm.