Embryonic Development: From Zygote to Organism
Development: Zygote to Adult Organism
Development refers to the transformation of the zygote into an adult organism.
Segmentation
Segmentation is the set of cell divisions where a single cell or multicellular body produces a blastula. Initially, the zygote divides through mitosis, forming a compact mass of undifferentiated cells called blastomeres, known as a morula.
Cells reorganize, placing blastomeres around a central cavity called the blastocoel. This structure is the blastula stage of cell cleavage.
Segmentation varies based on yolk quantity and distribution:
Types of Eggs
- Isolecithal or Oligolecithal: Small, evenly distributed yolk. Found in species with short embryonic development requiring few nutrients (coelenterates, echinoderms).
- Heterolecithal: Abundant yolk in the vegetal pole. Nucleus and organelles are in the opposite pole. Found in annelids, molluscs, and amphibians.
- Telolecithal: Yolk fills most of the zygote, with a small cytoplasmic area for the nucleus and organelles. Typical of fish, reptiles, and birds.
- Centrolecithal: Yolk is in the center, surrounding the nucleus and cytoplasm. Characteristic of insects.
Segmentation Types
Cells divide faster with less yolk. Segmentation can be total or partial:
- Total Segmentation: Cytoplasm divides completely.
- Complete and Equal: All blastomeres are the same size. Typical of coelenterates and echinoderms with oligolecithal eggs.
- Complete and Unequal: Blastomeres are different sizes. Found in heterolecithal eggs with more yolk in the vegetal pole, resulting in larger macromeres and smaller micromeres.
- Partial Segmentation: Affects only the germinal pole due to abundant yolk.
- Partial and Discoidal: Occurs in telolecithal eggs, confined to the germinal disc. The vegetal pole is the yolk sac, nourishing the embryo. Typical of reptiles and birds.
- Partial and Superficial: Occurs in centrolecithal eggs where the yolk is central and undivided. The nucleus divides and migrates to the surface, where cytoplasm division begins, placing blastomeres around the yolk mass. Typical of insects.
Gastrulation
Gastrulation forms the gastrula, the first step of cellular differentiation. Movements and folding of the blastula create embryonic layers.
Initially, two layers form:
- Endoderm: Inner layer.
- Ectoderm: Outer layer.
The blastocoel shrinks as these layers meet. A new cavity, the archenteron, forms within the endoderm, becoming the future digestive tract with an opening called the blastopore.
The blastopore’s development defines:
- Protostomes: Blastopore forms the mouth.
- Deuterostomes: Blastopore forms the anus.
Gastrulation methods vary by egg type:
- Invagination: Vegetal pole folds inward, forming two layers.
- Epiboly: Micromeres grow faster, covering macromeres.
- Ingression: Cells from the blastula’s periphery migrate into the blastocoel, forming the endoderm.
- Delamination: Cells in a single-layered blastula divide, creating an inner layer (endoderm).
Primitive animals with ectoderm and endoderm are diploblastic. Others develop a third layer, the mesoderm, becoming triploblastic.
Mesoderm development leads to:
- Acoelomates: Compact mesoderm tissue, no coelom.
- Coelomates: Hollow mesoderm forming a coelom, housing internal organs.