Write on the following in about 150 words each: 5.(b) Fate map of frog embryo

Fate Map of Frog Embryo

The frog embryo, typically studied at the blastula stage, exhibits distinct regions that are destined to form specific structures. The construction of a fate map involves marking specific cells or regions and tracking their development. This reveals how various parts of the early embryo contribute to the future organism.

The blastula of a frog can be broadly divided into three main prospective regions:

• Animal Pole: This region, characterized by smaller, less yolky cells (micromeres), primarily gives rise to the Ectoderm. The ectoderm further differentiates into:
  • Neural Ectoderm: Forms the entire nervous system, including the brain, spinal cord, and associated structures [12].
  • Epidermal Ectoderm: Develops into the outer layers of the skin (epidermis) and its derivatives [12].
  • Neural Crest Cells: These cells, originating from the neural ectoderm, migrate extensively to form a diverse array of structures such as cranial nerves, pigment cells, and parts of the head skeleton [12].
  • Sense Organs: The ectoderm also contributes to the formation of sensory organs like eyes and ears [12].
• Vegetal Pole: Composed of larger, yolk-rich cells (macromeres), this region is primarily destined to form the Endoderm. The endoderm gives rise to:
  • The lining of the digestive tract (gut) and associated glands (e.g., liver, pancreas) [10].
  • Lining of the respiratory system [10].
• Marginal Zone (Equatorial Region): Located between the animal and vegetal poles, this critical region gives rise to the Mesoderm. The mesoderm differentiates into:
  • Notochord: A rod-like structure providing axial support, crucial for inducing neural plate formation [12].
  • Somites: Blocks of mesoderm that form vertebrae, ribs, and skeletal muscles [12].
  • Intermediate Mesoderm: Develops into the kidneys and gonads (urogenital system) [12].
  • Lateral Plate Mesoderm: Forms the heart, blood vessels, smooth muscle, and connective tissues [9, 10].

During gastrulation, these presumptive areas undergo significant morphogenetic movements, such as epiboly, invagination, and involution, where cells actively migrate and rearrange to establish the three germ layers in their definitive positions [6, 7, 8, 12]. For instance, cells from the dorsal marginal zone involute to form the notochord and prechordal mesoderm, while other marginal zone cells involute to form the lateral and ventral mesoderm [9]. The vegetal cells invaginate to form the endoderm, which lines the archenteron (primitive gut) [7].

Key Features of Frog Fate Map

• Gray Crescent: A crescent-shaped area formed opposite the point of sperm entry, rich in developmental determinants. The dorsal lip of the blastopore forms in this region, which is crucial for initiating gastrulation and inducing the formation of the notochord and neural tube [9, 16].
• Dynamic Nature: Fate maps are not static; they change over time as cells divide and move. Therefore, a fate map typically represents the prospective fates at a specific developmental stage (e.g., blastula) [5].