③ 3^rd - 3^rd. Bindin proteins on the protruded acrosomal process of the sperm head bind species-specifically to bindin receptors on the vitelline membrane

⑷ 4^th. Hole forms in vitelline membrane → fusion of sperm and egg membranes → sperm nucleus enters egg cytoplasm

① Membrane fusion is also species-specific

⑸ 5^th. Fast block to polyspermy: Specific to sea urchins

① 5^th - 1^st. Na+ and Ca²⁺ enter the fertilized egg with sperm

② 5^th - 2^nd. Membrane depolarization

③ 5^th - 3^rd. Anions surround the depolarized membrane

④ 5^th - 4^th. Negatively charged membrane repels additional negatively charged sperm

⑹ 6^th. Slow block to polyspermy (Cortical Reaction): Occurs about 1 minute after sperm-egg fusion

① 6^th - 1^st. Separation of vitelline and plasma membranes: blocks secondary sperm from entering the fertilized egg

○ 6^th - 1^st - 1^st. GPCR-PLC Mechanism: Sperm + GPCR → IP₃

○ 6^th - 1^st - 2^nd. Ca²⁺ released from smooth ER of fertilized egg

○ Intracellular calcium release

○ Ca2+ is always involved in vesicle release like neurotransmitters

○ 6^th - 1^st - 3^rd. Calcium wave: Released Ca2+ actively moves to vesicles. Not diffusion

○ When two sperm are artificially fused with one egg, Ca2+ waves occur separately

○ A23187: Compound that transports Ca2+ across lipid bilayers

○ A23187 causes fertilization envelope formation without fertilization; chelating agent BAPTA inhibits it

○ 6^th - 1^st - 4^th. Oligosaccharide vesicles of fertilized egg, i.e., cortical granules, are released between plasma and vitelline membranes

○ Vesicles contain degrading enzymes and glycoproteins

○ 6^th - 1^st - 5^th. Cortical granules draw in water due to high osmotic pressure

○ Mucopolysaccharides draw in water, expanding the space between vitelline membrane and plasma membrane

○ 6^th - 1^st - 6^th. Perivitelline space forms between membranes

② 6^th - 2^nd. Plasma membrane receptors removed: Cortical granule enzymes cleave sperm-binding receptors

③ 6^th - 3^rd. Fertilization envelope formation: Enzymes from cortical granules harden vitelline membrane

⑺ 7^th. Cleavage

3. Stage 2: Cleavage

⑴ 1^st. Before cleavage begins: Dvl (Dsh) already asymmetrically distributed

⑵ 2^nd. Cleavage starts after fertilization

① Feature 1. Holoblastic cleavage: occurs in isolecithal eggs with small and evenly distributed yolk

② Feature 2. Radial cleavage: Cleavage occurs radially

4. Stage 3: 16-cell Stage

⑴ 1^st. After 4th cleavage, vegetal pole divides unequally

⑵ 2^nd. Skeletogenic mesenchymal cells (micromeres) appear at bottom vegetal layer

① Micromeres: Rich in otx (transcription factor) and β-catenin

② Mesenchymal cell: Cells that migrate within the epithelial layer of the embryo

5. Stage 4: Morula Stage

⑴ 1^st. Signaling

① 1^st - 1^st. Micromeres induce the layer above to form non-skeletogenic mesenchymal cells

② 1^st - 2^nd. Non-skeletogenic mesenchymal cells then signal the layer above

⑵ 2^nd. Germ Layer Determination

① Animal pole region becomes ectoderm: Received no signals

② Upper vegetal region becomes endoderm: Received signals from non-skeletogenic mesenchymal cells

③ Lower vegetal region becomes mesoderm: From both skeletogenic and non-skeletogenic mesenchymal cells

④ Order is ectoderm → endoderm → mesoderm

6. Stage 5: Blastocoel Formation

⑴ 1^st. Tight junctions form between cells and pump salts inward

⑵ 2^nd. Blastocoel develops: Water influx due to osmotic pressure from salt entry

⑶ 3^rd. Cilia also develop on fertilization envelope

7. Stage 6: Hatching of the Fertilized Egg

⑴ 1^st. Fertilized egg dissolves envelope and is released

⑵ 2^nd. Fertilized egg becomes free-swimming, essentially a living organism

8. Stage 7: Ingression

⑴ 1^st. Skeletogenic and non-skeletogenic mesenchymal cells at bottom detach from neighbors

① Cadherin: Binds cells together to form blastocoel, separates neural tube

② Decreased cadherin expression initiates mesenchymal cell separation

⑵ 2^nd. After separation, they migrate into blastocoel forming primary mesenchyme

9. Stage 8: Invagination

⑴ 1^st. Invagination: After ingression, endodermal precursor cells at bottom fold inward

① Primary invagination force: Microfilaments

○ Microfilaments in endodermal precursors are on the outer side, not facing the blastocoel

○ Contraction on outer side and relaxation toward blastocoel leads to inward folding (invagination)

⑵ 2^nd. Archenteron forms during invagination

① Blastopore: Opening of archenteron

② Archenteron = Primitive gut

⑶ 3^rd. More cells migrate into blastocoel from tip of archenteron forming secondary mesenchyme

⑷ 4^th. Secondary invagination force: Filopodia

① Filopodia: Driven by microfilaments, also seen in amoeba pseudopodia

② Filopodia from secondary mesenchyme contact ectoderm at animal pole, connecting archenteron and ectoderm

10. Stage 9: Larva Formation

⑴ 1^st. Fusion of archenteron with wall of cleavage cavity forms a digestive tract with mouth and anus

① Cleavage cavity does not become gut lumen: Archenteron becomes gut lumen

⑵ 2^nd. Mesoderm consists of skeletogenic and non-skeletogenic mesenchymal cells

① Skeletogenic mesenchymal cells degenerate into skeletal spicules

⑶ 3^rd. Endoderm later becomes the digestive tract

① Initial sperm entry site becomes the mouth

② Deuterostomes: Blastopore becomes the anus

Input: 2019.02.10 13:26

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Chapter 32-2. Sea Urchin Embryology

1. Overview

2. Stage 1: Fertilization