Early Development

Fertilization

Spawning

Cleavage

Gastrulation

Parastichopus parvimensis are a gonochoric species. Individuals spawn synchronously and seasonally in the wild, however, specific spawning cues are generally unknown (Cameron, 1980). In our experimental setting, individuals spawned anywhere between 30 minutes to 2 hours after injection of NGLWY-amide near the gonad (see Methods for more information on spawning protocol). Males began spawning before females, and spawned for a longer duration. Gametes flowed in a generally constant stream from a single gonopore, connected to the gonad.

Spawning P. parvimensis male

P. parvimensis Gametes
sperm (above) and egg (below)

The oocytes of Parastichopus parvimensis are translucent spheres. Within this egg, the germinal vesicle and nucleolus are visible, and arrested in Meiosis I. Once the germinal vesicle disappears then the eggs are ready to be fertilized.

Somewhere between the degeneration of the germinal vesicle and fertilization, the egg undergoes Meiosis II and ejects two polar bodies that mark the animal pole. This forms the very important animal-vegetal axis, which dictates planes of cleavage and other spatial features of development from fertilization up through gastrulation.
A dilute concentration of sperm was added to the eggs to avoid polyspermy. Unlike most of its other echinoderm relatives, Parastichopus parvimensis eggs do not raise a vitelline envelope after fertilization. That being said, the first sign of successful fertilization is first cleavage along the animal vegetal axis.

Recently fertilized egg

First plane of cleavage; arrows indicate direction of bipolar cleavage

Morula - note the uneven cell sizes

Shifting cell phase between
morula and blastula

Blastula

Gastrulation is when the developing embryo is reorganized into three germ layers – ectoderm, mesoderm, and endoderm. The ectoderm gives rise to the epidermis, the mesoderm gives rise to the muscle layers, and the endoderm gives rise to digestive system.

Around gastrulation, a ciliary band begins to form around the now developing gastrula, and the tripartate gut begins to form.

The zygote undergoes holoblastic, even, bipolar, radial cleavage – typical of most deuterostomes.

Holoblastic cleavage: the entire zygote is divided (versus meriblastic cleavage: whole egg is not completely divided, divisions happen in layers; occurs in yolky eggs)

Even cleavage: the dividing cell is split into two equal parts (versus uneven cleavage)

Bipolar cleavage: cytokinesis occurs from both ends, like a pinching effect (versus unipolar cleavage: cytokinesis occurs from one end, like a zipping effect)

Radial cleavage: cleavage planes occur directly in line with one another (versus spiral cleavage: cleavage occurs as if the egg were rotating)

As previously mentioned, the first plane of cleavage occurs along the animal vegetal axis. The second plane of cleavage occurs perpendicularly from the first. After the first eight divisions, the forming blastula begins uneven cleavage. After about 16 hours, the morula, ball of cells, forms.

Cells then begin to shift until a hollow ball of cells, called a blastula, is formed. Parastichopus parvimensis blastula form a thicker layer of cells near the vegetal pole compared to the animal pole. No growth from the zygote to blastula stage takes place.
Afterwards, ingression takes place. Ingression is the movement of cells from the periphery to the inside of the blastula (different from delamination). Simultaneously, the archenteron invaginates at the vegetal pole in a process called gastrulation.n spawning protocol). Males began spawning before females, and spawned for a longer duration. Gametes flowed in a generally constant stream from a single gonopore, connected to the gonad.

Blastula beginning gastrulation; blastopore just beginning to invaginate

Early Gastrula - labeled parts

Derm layers