Amphibians and Fish: Early Development and Axis Formation

Lange

BIOL 370 – Developmental Biology

Topic #9

Lazzaro Spallanzani – (1729 -1799) biologist and physiologist who made important

contributions to the experimental study of bodily functions and animal reproduction.

Spallanzani’s most famous work examines the process of fertilization, and he mechanically isloated male gametes from female gametes and was able to induce fertilization in-vitro.

Various stages of development in the typical amphibian.

Reorganization of the cytoplasm and cortical rotation produce the gray crescent in frog eggs

The Grey Crescent in frog eggs:

Due to the reorganization of the cytoplasm and rotation of the cortex

In a) 50% of the cell cycle is complete, but no polarity

In b) (70 % of the cell cycle complete) we see how the microtubules in the cells become parallel in the vegetal hemisphere.

Together, these movements create the grey crescent.

Reorganization of the cytoplasm and cortical rotation produce the gray crescent in frog eggs (Part 2)

The gray crescent is a region of intermediate pigmentation where the first identifiable aspects of gastrulation will be seen. (two slides from now there is an even better rendition of this crescent.

Reorganization of the cytoplasm and cortical rotation produce the gray crescent in frog eggs (Part 3)

Cleavage of a frog egg

Scanning electron micrographs of frog egg cleavage

Animal and vegetal pole cell size differences seen by the fourth division c).

Depletion of EP-cadherin mRNA in the Xenopus oocyte results in the loss of adhesion between blastomeres and the obliteration of the blastocoel

The EP-cadherin (named because it appeared initially similar to both the E-cadherin and the P-cadherin)

is required for adhesion in the blastomere Without these proteins, the formation of the blastocoel is not possible.

Standardized Color Scheme:

Ectoderm – outer germ layer… will become nervous system, tooth enamel, epidermis, lining of the mouth, anus, nostrils, sweat glands, hair and nails.

Mesoderm – middle germ layer… will become the muscle (smooth, cardiac and skeletal), connective tissues, dermis, hypodermis (subcutaneous layer of the skin), bone, cartilage, red blood cells, white blood cells, kidneys, and the adrenal cortex.

Endoderm – inner germ layer… will become a variety of epithelia including the alimentary canal (excluding specialized parts of the mouth, pharynx & rectum), the lining cells of all the glands, trachea, bronchi, and alveoli of the lungs, endocrine glands, auditory tube, urinary bladder and parts of the urethra.

Cell movements during frog gastrulation

I will split this diagram up to highlight specifics.

Cell movements during frog gastrulation (Part 1)

EARLY GASTRULATION

Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula.

Cell movements during frog gastrulation (Part 2)

MID-GASTRULATION

Identified by the formation of the archenteron which replaces the blastocoel.

Note the development in orange, this endodermal tissue will become the BLASTOPORE.

Cell movements during frog gastrulation (Part 3)

X

Later Gastrulation…. note the elimination of the blastocoel.

Cell movements during frog gastrulation (Part 4)

Final Stage of gastrulation….. the design is now called the GASTRULA.

Surface view of an early dorsal blastopore lip of Xenopus

In this Xenopus example, which side is the vegetal and which side is the animal region?

Why did you select the positions you did?

Early movements of Xenopus gastrulation

Focus on cell movement/migration that leads to the formation of the blastopore.

Epiboly of the ectoderm

Epiboly •a cell movement that occurs in the early embryo, at the same time as gastrulation. •It is one of many movements in the early embryo that allow for dramatic physical restructuring.•Movement is characterized as being a thinning and spreading of cell layers.

Epiboly has been most extensively studied in zebrafish as their development allows for an easy visualization of the process.

Xenopus gastrulation continues

Xenopus gastrulation continues (Part 1)

The archenteron is the primitive gut that forms during gastrulation in the developing embryo is known as the archenteron. It develops into the digestive tract of an animal.

The most common place you may have heard this term is in regard to the intercalated discs in cardiac

muscle tissue.

Xenopus gastrulation continues (Part 2)

Radial intercalation - part of the process of epiboly involves radial intercalation. Interior cells of the blastoderm move towards the outer cells, thus "intercalating" with each other. The blastoderm begins to thin as it spreads toward the vegetal pole of the embryo until it has completely engulfed the yolk cell.

To “intercalate” means to insert (something) between layers

Epiboly of the ectoderm is accomplished by cell division and intercalation

Spemann’s demonstration of nuclear equivalence in newt cleavage

Hans Spemann’s work in 1903 demonstrated the concept of nuclear equivalence in this elegant experiment partially constricting the fertilized egg. The resultant development is associated with twinning.

Asymmetry in the amphibian egg

Notice how normal development only proceeds when the cellular constriction occurs along the correct plane…. because the embryo is already asymetrical (as seen with the grey crescent).

Determination of ectoderm during newt gastrulation

Notice how in the early gastrula the neural ectoderm transplant retains plasticity in development and becomes epidermis.

By the time the embryo reaches the late gastrula stage this plasticity is lost.

Organization of a secondary axis by dorsal blastopore lip tissue

Speeman & Mangold, in 1924 differentially colored embryos and then studied the organization of a secondary axis by transferring dorsal lip tissues.

This further shows how a “twinning” may arise.

Transplantation and recombination experiments on Xenopus embryos

Vegetal cells lying under the prospective blastopore lip begin gastrulation.

Transplanting a slice of very dorsal vegetal cell in the 64-cell stage leads to twinning.

End.