A Continuation of A First Look at Early Development:

More On Rapid Specification

in Snails and Nematodes

Lange

BIOL 370 – Developmental Biology

Topic #7

To understand cleavage, we need additional vocabulary:

•Vegetal pole – the yolk rich region•Animal pole – the yolk devoid region•Isolecithal – roughly equal distribution of yolk (such as in sea urchins)•Holoblastic cleavage – complete cleavage•Meroblastic cleavage – partial cleavage where only some of the cytoplasm is cleaved (insects, fish, reptiles, birds)•Centrolecithal – centrally placed yolk (insects)•Telolecithal – only one area is free of yolk (birds and fish)•Discoidal cleavage – cleavage in the telolecithal eggs that occurs only in the small disk of cytoplasm•Holoblastic cleavage subtypes:

• Radial• Spiral• Bilateral• Rotational

• Vegetal pole – the yolk rich region• Animal pole – the yolk devoid region

• Isolecithal – roughly equal distribution of yolk (such as in sea urchins)

• Centrolecithal – centrally placed yolk (insects)• Telolecithal – only one area is free of yolk (birds and fish)

• Holoblastic cleavage – complete cleavage• Meroblastic cleavage – partial cleavage where only some of the

cytoplasm is cleaved (insects, fish, reptiles, birds)• Discoidal cleavage – cleavage in the telolecithal eggs that occurs

only in the small disk of cytoplasm• Holoblastic cleavage subtypes:

• Radial• Spiral• Bilateral• Rotational

Summary of the main patterns of cleavage

We will focus on each type of cleavage in greater detail AGAIN in the next few slides.

Summary of the main patterns of cleavage (Part 1)

Summary of the main patterns of cleavage (Part 2)

The change in shape of the organism grows much more complicated as you move from the blastula stage to the gastrula stage.

Types of cell movements during gastrulation

In this slide, we are seeing how during gastrulation, there is a noticeable shift in terms of cell movement. Notice the five types shown:•Invagination•Involution•Ingression•Delamination•Epiboly

Axes of a bilaterally symmetrical animal

With this increasing complexity of the whole organism do the in-folding that occurs, the 3 dimensional shape of the organisms will no longer be symmetrical every plane.

Figure 5.6 Cleavage in the sea urchin

• holoblastic cleavage (complete cleavage)• 1st & 2nd cleaveges are meridional ("along a meridian" or "in the north–south

direction“)• 3rd cleavage is equitaorial• 4th cleavage is more specialized… the animal poles divide meridionally into

mesomeres, the vegetal poles divides both equatorially but also unequally (larger cells called macromeres and the smaller called micromeres)

Cleavage in the sea urchin (Part 1)

5th (16 cell stage) cleavage:•mesomeres (animal) equitorially to produce an1 and an2•macromeres divide meridionally•micromeres divide unequally producing four small mircomeres and four large micromeres

6th division animal hemisphere cells divide meridionally and vegetal cells divide equatorially.

7th division the vegetal cells divide meriodionally and the animal cells divide equatorially.

The embryo is now at 120 cells and is called a blastula.

Micrographs of cleavage in live embryos of the sea urchin Lytechinus variegatus, seen from the side

Fate map and cell lineage of the sea urchin Strongylocentrotus purpuratus (Part 1)

Embryo at 60 cell stage.

Oral – towards the mouth

Aboral – away from the mouth

Fate map and cell lineage of the sea urchin Strongylocentrotus purpuratus (Part 2)

Cell lineage and fate map.

Ability of micromeres to induce presumptive ectodermal cells to acquire other fates

In this series we can see how even in the dissected embryo missing its vegetal portion, the micromeres can guide and direct differentiation into a larva.

What does this suggest as the role for micomeres?

Normal sea urchin development, following the fate of the cellular layers of the blastula

The blastula continues development into a gastrula:

•late stage blastula will contain roughly 1,000 cells•shape is in the form of a hollow ball•the blastomeric cells are of different shapes, sizes, and properties

Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar ("three-layered") structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.

Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers. Each layer gives rise to specific tissues and organs in the developing embryo.

Simplified view of the change from blastula to gastrula.

The ectoderm gives rise to epidermis and also tissues that will later form the nervous system.

The mesoderm is found between (meso = middle) the ectoderm and the endoderm and gives rise to the muscular system, cartilages, the dermis, the notochord, blood and blood vessels, bone, and connective tissue.

The endoderm gives rise to the epithelium of the digestive system and respiratory system, and organs associated with the digestive system, such as the liver and pancreas.

Entire sequence of gastrulation in Lytechinus variegatus

Invagination of the vegetal plate

The archenteron is the primitive gut that forms during gastrulation. It develops into the digestive tract of an animal.

Cell rearrangement during extension of the archenteron in sea urchin embryos

The imaginal rudiment growing in the left side of the pluteus larva of a sea urchin

The imaginal rudiment is the precursor tissue for the developing skeletal structure of the urchin.

A related line to the skeletal development seen in the previous slides about the urchin, is the shell development seen in a variety of snails. These molluscs that develop conchae (spiral shaped shells), can develop often develop either a right handed spiral or a left handed spiral.

Spiral cleavage of the mollusc Trochus viewed from the animal pole (A) and from one side (B)

Looking down on the animal pole of left-coiling (A) and right-coiling (B) snails

Fate map of Ilyanassa obsoleta

The glochidium is a microscopic larval stage seen in some freshwater mussels, aquatic bivalve mollusks, and European freshwater pearl mussels.

This larval form has hooks, which enable it to attach itself to fish (for example to the gills of a fish host species) for a period of time prior to when it detaches and falls to the substrate and takes on the typical form of a juvenile mussel.

Ecologically, since fish are active and free-swimming, this process helps distribute the mussel species to potential areas of habitat that it could not reach via its own locomotion.

Formation of a glochidium larva by the modification of spiral cleavage

Be sure to read the “Sidelights & Speculations associated with this work on pp. 167-168.

Phony fish atop the unionid clam Lampsilis altilis

This fake “fish” is actually a brood pouch holding glochidium in this species of clam. It attracts predatory fish and allows the glochidium easy

access to these fish.

The two-step process for specifying the marginal cells of the tunicate embryo

Macho-1 = the gene/RNA materialFGF = fibroblast growth factor

A fibroblast is a type of cell that synthesizes the extracellular matrix and collagen, the structural framework for animal tissues, and plays a critical role in wound healing. Fibroblasts are the most common cells of connective tissue in animals.

Gastrulation in the tunicate

Convergent extension of the tunicate notochord

Caenorhabditis elegans is a free-living, transparent nematode (roundworm), about 1 mm in length, which lives in temperate soil environments. Research into the molecular and developmental biology of C. elegans was begun in 1974 by Sydney Brenner (Nobel Prize in Medicine in 2002) and it has since been used extensively as a model organism.

The nematode Caenorhabditis elegans (Part 1)

In A, we see the hermaphroditic formIn B, we see how the eggs are forced to pass through sperm cells as they mature, which in effect forces fertilization to occurNote that earlier meiotic divisions actually involve sperm production and the spermatheca is only for STORAGE.

The nematode Caenorhabditis elegans (Part 2)

In the embryos formed, the “P” cell represents the stem cells.

The nematode Caenorhabditis elegans (Part 3)

See comparisons with the Development Tree shown below.

PAR proteins and the establishment of polarity

Polarity – the beginning of orientation of cell lineages in an organism. In C. elegans, this is guided by PAR proteins.

PAR proteins and the establishment of polarity (Part 2)

Note the green staining in (F) which shows the PAR proteins fluorescently stained.

Deficiencies of intestine and pharynx in skn-1 mutants of C. elegans

In the images below, we see the effects of the SKN-1 mutation in C. Elegans. This mutation affects most notably the development of musculature of the pharynx and intestine. Therefore, this mutation drastically alters the development of the digestive system in this species.

End.