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 the fertilization envelope formation.

○ 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, forming the fertilization envelope.

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. Equal (isolecithal) 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 4^th 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 send inductive signals to the cell layer immediately above them, inducing the formation of non-skeletogenic mesenchyme (NSM).

② 1^st - 2^nd. The non-skeletogenic mesenchyme (NSM) in turn sends inductive signals to the overlying cell layer.

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: Received signals 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, and separates neural tube

② Decreased cadherin expression initiates mesenchymal cell separation

⑵ 2^nd. After separating, they ingress into the blastocoel to form the mesodermal primary mesenchyme.

9. Stage 8: Invagination

⑴ 1^st. Invagination: after ingression, the presumptive endodermal cells at the very bottom roll inward.

① Primary driving force of invagination: microfilaments.

○ In presumptive endodermal cells, microfilaments are distributed on the outer (apical) side rather than the blastocoel-facing side.

○ Contraction of the microfilaments causes relative constriction on the outer side and relative relaxation on the blastocoel side, leading to invagination.

⑵ 2^nd. Archenteron formation occurs simultaneously with invagination.

① Blastopore: the opening of the archenteron.

② Archenteron = the primitive gut.

⑶ 3^rd. From the tip of the archenteron, additional cells ingress into the blastocoel to form the mesodermal secondary mesenchyme.

⑷ 4^th. Secondary driving force of invagination: filopodia.

① Filopodia: driven by microfilaments; the pseudopodial movement of amoebae is one type of filopodial movement.

② Filopodia extending from the secondary mesenchyme contact the ectoderm of the animal-pole epidermis, bringing the archenteron into contact with the ectoderm.

10. Stage 9: Birth of the Larva

⑴ 1^st. When the archenteron fuses with the coelomic wall, a digestive tract with a mouth and anus is formed.

① The coelom does not give rise to the intestinal lumen; the archenteron becomes the intestinal lumen.

⑵ 2^nd. The mesoderm consists of skeletogenic mesenchyme and non-skeletogenic mesenchyme.

① The skeletogenic mesenchyme differentiates into skeletal rods (spicules).

⑶ 3^rd. The endoderm later becomes the digestive tract.

① The site where the sperm first penetrated becomes the mouth.

② Deuterostomes: the 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