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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 is 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.Formation of the Gray Crescent

① Gray crescent: A crescent-shaped gray band that appears on the side opposite the sperm entry point as the pigmented animal hemisphere rotates (cortical rotation).

② It appears gray because a small amount of cortical pigment remains.

⑶ 3^rd. During cortical rotation, vesicles in the vegetal pole are transported along microtubules to the gray crescent region.

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 a moderate amount of yolk concentrated around the vegetal pole.

② Because the egg is mesolecithal, cleavage is displaced radial.

③ The yolk membranes are preferentially localized to the vegetal pole; the egg is subtelolecithal (mesolecithal).

⑵ 1^st. (vertical) cleavage: meridional cleavage

① The first meridional furrow bisects the gray crescent exactly.

② Both blastomeres are totipotent after the first cleavage.

⑶ 2^nd. (vertical) cleavage: meridional cleavage

① The second meridional furrow forms at right angles to the first cleavage plane.

② Of the four blastomeres produced, only two are totipotent; those lacking the gray crescent are not.

⑷ 3^rd. (horizontal) cleavage: equatorial cleavage

① Cleavage is displaced toward one side.

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 The Nieuwkoop center induces the right above cell layer to become the organizer (the dorsal lip of the blastopore).

② In the vegetal pole beneath the Nieuwkoop center, Wnt11 is translated and secreted into the space between the egg and the fertilization envelope (perivitelline space), after which it signals back to the same cells (autocrine signaling).

○ Purpose: to accumulate a 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) diffuses upward through space between shell and vegetal pole

① 2^nd - 1^st. In the vegetal pole, on the side opposite the Nieuwkoop center, Vg1 begins to be translated.

② 2^nd - 2^nd. In the vegetal pole, on the side toward the Nieuwkoop center, VegT begins to be translated.

③ 2^nd - 3^rd. Under the action of VegT, activin, Derriere, and Nodal (Xnr, Xenopus nodal-related) begin to be translated.

⑶ 3^rd. Germ Layer Specification

① Germ-layer identity is specified by Vg1, activin, Derriere, and Nodal.

② The animal pole is specified as ectoderm (upper side).

③ Regions of the vegetal pole highly exposed to Vg1, activin, Derriere, and Nodal are specified as endoderm (lower side).

④ Regions of the vegetal pole exposed to lower levels of these factors are specified as 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. Upon BMP4 inhibition, a signal for inward invagination is generated.

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

① If BMP4 is applied exogenously to the region specified as neural ectoderm, it develops into 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: Cells initiating invagination

⑵ 2^nd. Cells that were at the dorsal lip earlier end up in the innermost positions, whereas cells that reach the dorsal lip later are arranged next innermost (not the very deepest).

① The dorsal lip cells of an early gastrula are not the same as those of a late gastrula.

⑶ 3^rd. As bottle cells with high Wnt11 levels begin to involute, a Wnt concentration gradient is established across the presumptive neuroectoderm.

① Wnt-family paracrine factors: key signals involved in posteriorization of the neural tube.

② High exposure to Wnt11 (close to the blastopore): specified as tail.

③ Intermediate exposure to Wnt11: specified as spinal cord.

④ Low exposure to Wnt11 (far from the blastopore): specified as 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 mesodermal cells aggregate to form the notochord.

⑶ 2^nd. Signals from the notochord (e.g., Wnt11) induce the ectoderm directly above the notochord to differentiate into the neural plate (neural crest).

① 2^nd-1^st. E-cadherin is expressed in the pre-existing ectoderm.

② 2^nd-2^nd. N-cadherin expression is induced by Wnt11, etc.

③ 2^nd-3^rd. Cells that would have become epidermal ectoderm are induced to differentiate into the neural plate.

⑷ 3^rd. Hinge points form: contraction of microfilaments (invagination) and elongation of microtubules.

⑸ 4^th. The neural plate separates from the epidermis and rolls up to form a single long tube (neural tube).

① Embroy structure observed after neural tube formation: notochord (mesoderm), somites (mesoderm), neural tube (ectoderm), and neural crest cells (ectoderm).

⑹ 5^th. Separation of neural crest (neural fold) cells.

① 5^th-1^st. Among neural tube cells, those expressing less N-cadherin than neighboring cells develop into neural crest cells.

② 5^th-2^nd. Neural crest cells: a subset of cells that separate from the neural tube and migrate to various regions of the embryo.

③ Sonic hedgehog (shh)

⑺ 6^th. Differentiation of neural crest cells

① 6^th-1^st. Cranial neural crest cells: located anteriorly; migrate to the head and neck and form facial and cranial cartilage, bone, teeth, and other connective tissues.

② 6^th-2^nd. Trunk neural crest cells: located in the posterior body.

○ 6^th-2^nd-1^st. Ventral pathway: migrate through the anterior half of each somite into the ventral body; give rise to sensory neurons of the dorsal root ganglia, Schwann cells, postganglionic neurons of the sympathetic ganglia, and the adrenal medulla, etc.

 ○ 6^th-2^nd-2^nd. Lateral pathway: migrate between the dorsal epidermis and the somite; differentiate into melanocytes of the skin.

⑻ 7^th. Notochord: degenerates, persisting only as vestigial remnants within the vertebral column.

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., the optic vesicle induces the overlying surface ectoderm to become the lens placode.

⑷ 3^rd. The optic cup forms and induces lens formation.

⑸ 4^th. The developing lens separates from the surface ectoderm.

⑹ 5^th. Tertiary organizer (lens): induces the 「epidermis → cornea」 transformation—that is, it induces the surface ectoderm to form the 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: the endoderm formed early as bottle cells pull the tissue inward.

Entered: 2019.02.10 15:59