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Chapter 32-4. Amphibian Embryology

Recommended Post: 【Biology】 Chapter 32. Embryology

1. Overview

2. Step 1: Fertilization

3. Step 2: Cortical Rotation

4. Step 3: Cleavage

5. Step 4: Blastula Stage

6. Step 5: Blastocoel Stage

7. Step 6: Invagination Induction

8. Step 7: Gastrulation Stage

9. Step 8: Nervous System Formation

10. Step 9: Lens Formation

11. Step 10: Body Formation

1. Overview

⑴ Model organism for amphibian development: Xenopus

⑵ Organizer: Spemann organizer (dorsal lip of the blastopore)

① Spemann-Mangold experiment: When presumptive neural regions of early gastrula embryos are transplanted into presumptive epidermal regions, they become epidermis

⑶ No gray crescent = no notochord formation

⑷ Dorsal lip of the blastopore differentiates into dorsal mesoderm

2. Step 1: Fertilization

⑴ 1^st. Before fertilization: Cytoplasmic determinants and yolk distribution are already established

① Animal pole: Melanin pigment layer present in the egg cortex

② Vegetal pole

○ Vegetal pole cytoplasm: Contains dorsal morphogens (β-catenin mRNA, GSK-3)

○ Vegetal pole base: Contains Dsh protein, Wnt11 mRNA

③ Anterior-posterior axis determination: Animal pole is top, vegetal pole is bottom

⑵ 2^nd. Fertilization

① Fast polyspermy block mechanism applies only to sea urchins

② 2^nd - 1^st. Sperm enters through animal pole

③ 2^nd - 2^nd. Sperm pronucleus and centriole enter the egg

3. Step 2: Cortical Rotation

⑴ 1^st. Cortical rotation: Rotation of the anterior-posterior axis toward the sperm entry point

① 1^st - 1^st. Sperm-derived centriole forms sperm-derived centrosome

② 1^st - 2^nd. Sperm-derived centrosome rearranges microtubules in the cytoplasm

③ 1^st - 3^rd. Motor protein kinesin moves from (-) to (+), opposite to sperm entry

④ 1^st - 4^th. Microtubule rearrangement causes rearrangement of cytoplasmic determinants

⑤ 1^st - 5^th. Egg cortical cytoplasm rotates 30° toward sperm entry direction

⑵ 2^nd. Gray crescent formation

① Gray crescent: Crescent-shaped gray band opposite sperm entry point appears as melanin pigment rotates

② Some cortical pigment remains, appearing gray

⑶ 3^rd. Vesicles in vegetal pole move along microtubules to gray crescent area

⑷ 4^th. Moved vesicles release Dsh into gray crescent cytoplasm

⑸ 5^th. Autonomous specification: Gray crescent determines dorsal side

① 5^th - 1^st. GSK-3 inhibitor (Dsh) inhibits GSK-3

② 5^th - 2^nd. GSK-3 inhibits β-catenin, so inhibition of GSK-3 prevents β-catenin degradation

③ 5^th - 3^rd. β-catenin acts as cytoplasmic determinant and transcription factor, signaling dorsal development

④ 5^th - 4^th. Although β-catenin is spread throughout cytoplasm, it’s restricted to gray crescent due to GSK-3

⑤ Cytoplasmic determinants in gray crescent confer totipotency

4. Step 3: Cleavage

⑴ Characteristics of amphibian cleavage

① Unequal cleavage: Occurs in mesolecithal eggs with moderate yolk concentrated in vegetal pole

② Due to mesolecithal yolk, cleavage is displaced radial

③ Moderately telolecithal yolk concentrated in vegetal pole

⑵ 1^st. First cleavage (vertical): Meridional cleavage

① First meridional cleavage divides gray crescent equally

② Both blastomeres from first cleavage are totipotent

⑶ 2^nd. Second cleavage (vertical): Meridional cleavage

① Second cleavage occurs at a right angle to first

② Only 2 of the 4 blastomeres from second cleavage are totipotent: Without gray crescent = no totipotency

⑷ 3^rd. Third cleavage (horizontal): Equatorial cleavage, offset

5. Step 4: Blastula Stage

⑴ 1^st. Yolk-rich vegetal pole causes differences in division speed from fourth cleavage

⑵ 2^nd. Nieuwkoop center: Area with highest β-catenin concentration

① Nieuwkoop center induces the cell layer above it into the organizer (dorsal lip)

② Wnt11 translated in vegetal pole below Nieuwkoop center is secreted between fertilization membranes and signals back

○ To accumulate high concentration of Wnt11

6. Step 5: Blastocoel Stage

⑴ 1^st. Entering blastocoel stage, blastocoel forms toward animal pole

⑵ 2^nd. TGF-β mRNA (Vg1, VegT) from vegetal pole diffuses upward through space between shell and vegetal pole

① 2^nd - 1^st. Opposite side of Nieuwkoop center starts translating Vg1

② 2^nd - 2^nd. Side near Nieuwkoop center starts translating VegT

③ 2^nd - 3^rd. Activin, Derriere, Nodal (Xnr, Xenopus nodal-related) translated by VegT

⑶ 3^rd. Mesoderm determination

① Vg1, activin, Derriere, Nodal determine mesoderm

② Animal pole → ectoderm (top)

③ Vegetal pole with high exposure to Vg1 etc. → endoderm (bottom)

④ Vegetal pole with low exposure to Vg1 etc. → mesoderm (middle)

⑷ 4^th - 1^st. General mesoderm cells

① 4^th - 1^st. Mesoderm cells secrete and diffuse BMP4 (TGF-β)

② 4^th - 2^nd. BMP4 induces epidermal tissue

③ 4^th - 3^rd. BMP4 diffuses to animal pole; exposed ectoderm becomes epidermal ectoderm

⑸ 5^th. Organizer mesoderm cells

7. Step 6: Invagination Induction

⑴ 1^st. Vegetal pole near Nieuwkoop center with high Xnr becomes Spemann organizer

① Significance: Dorsal-ventral axis determination

⑵ 2^nd. Spemann organizer secretes noggin, chordin

⑶ 3^rd. Noggin, chordin inhibit BMP4

⑷ 4^th. Inhibition of BMP4 triggers internalization signal

⑸ 5^th. Ectoderm not receiving BMP4 signal becomes neural ectoderm

① Artificial BMP4 to neural ectoderm → epidermal tissue

8. Step 7: Gastrulation Stage

⑴ 1^st. Lower part of Spemann organizer invaginates to form blastopore

① Archenteron: Space newly formed by cell invagination and migration

② Blastopore: Opening of the archenteron, site of invagination

③ Dorsal lip of blastopore: Refers to Spemann organizer

④ Blastopore forming region: Opposite to sperm-egg fusion point

⑤ Bottle cells: Initiating invagination

⑵ 2^nd. Cells initially at dorsal lip become innermost, later ones layer outside

○ Early and late dorsal lip cells are different

⑶ 3^rd. Bottle cells with high Wnt11 invaginate, creating Wnt gradient in presumptive neural ectoderm

① Local Wnt factors: Major players in posterior neural tube development

② High Wnt11 (near blastopore): Tail

③ Intermediate Wnt11: Spinal cord

④ Low Wnt11 (far from blastopore): Brain

9. Step 8: Nervous System Formation

Figure 1. Formation of neural plate, neural crest, and neural tube

⑴ Ectodermal origin, common in vertebrates

⑵ 1^st. Dorsal mesoderm cells aggregate to form notochord

⑶ 2^nd. Signals (e.g. Wnt11) from notochord induce ectoderm above to become neural plate (neural crest)

① 2^nd - 1^st. E-cadherin expressed in original ectoderm

② 2^nd - 2^nd. N-cadherin expressed via Wnt11, etc.

③ 2^nd - 3^rd. Cells destined to be epidermis induced into neural plate

⑷ 3^rd. Hinge structure forms: Neural plate microfilaments contract (invagination), microtubules elongate

⑸ 4^th. Neural plate separates from epidermis and rolls to form neural tube

① Post-tube embryo structure: Notochord (mesoderm), somites (mesoderm), neural tube (ectoderm), neural crest cells (ectoderm)

⑹ 5^th. Neural crest (neural fold) cells separate

① 5^th - 1^st. Neural tube cells with lower N-cadherin expression become neural crest

② 5^th - 2^nd. Neural crest cells: Migrate from neural tube to various embryo regions

③ Sonic Hedgehog

⑺ 6^th. Neural crest cell differentiation

① 6^th - 1^st. Cranial neural crest: From anterior embryo, migrate to head and neck

○ Form cartilage, bones, teeth, and connective tissue of face and head

② 6^th - 2^nd. Trunk neural crest: From posterior body

○ 6^th - 2^nd - 1^st. Ventral path: Move through anterior somites to abdomen

○ Form DRG sensory neurons, Schwann cells, sympathetic postganglionic neurons, adrenal medulla

○ 6^th - 2^nd - 2^nd. Lateral path: Move between dorsal skin and somites

○ Differentiate into melanocytes in skin

⑻ 7^th. Notochord: Eventually degenerates, leaves vertebral remnant

10. Step 9: Lens Formation

Figure. 2. Optic vesicle and lens development

⑴ Overview

① Primary organizer (dorsal lip) → notochord, mesoderm

② Primary organizer (dorsal lip): Induces 「ectoderm → neural tube」

③ Neural tube → brain vesicle → secondary organizer (optic vesicle)

④ Organizer grafting experiment: Lens only develops from head ectoderm

⑵ 1^st. Optic vesicle protrudes from forebrain

⑶ 2^nd. Secondary organizer (optic vesicle): Induces 「epidermis → lens」, i.e., overlying epidermis forms lens placode

⑷ 3^rd. Optic cup forms, induces lens formation

⑸ 4^th. Developing lens separates from surface tissue

⑹ 5^th. Tertiary organizer (lens): Induces 「epidermis → cornea」

11. Step 10: Body Formation

⑴ Ectoderm

Figure 3. Ectoderm-derived tissues

⑵ Mesoderm

① Sperm entry point becomes mouth

② Deuterostome: Blastopore becomes anus

⑶ Endoderm

① Pharyngeal endoderm: Initially formed as bottle cells drag in endoderm

Entered: 2019.02.10 15:59