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Development in the Sea Urchin (Psammechinus miliaris) - 2. Gastrulation and Larval Stages

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Formal Metadata

Title
Development in the Sea Urchin (Psammechinus miliaris) - 2. Gastrulation and Larval Stages
Alternative Title
Entwicklung beim Seeigel (Psammechinus miliaris) - 2. Gastrulation und Larvenstadien
Author
License
No Open Access License:
German copyright law applies. This film may be used for your own use but it may not be distributed via the internet or passed on to external parties.
Identifiers
IWF SignatureC 1188
Publisher
Release Date
Language
Other Version
Producer
Production Year1973-1975

Technical Metadata

IWF Technical DataFilm, 16 mm, LT, 106 m ; F, 10 min

Content Metadata

Subject Area
Genre
Abstract
Ingrowth of primary mesenchymal cells; invagination; outgrowth of secondary mesenchymal cells; coelom formation; production and growth of spicules; various pluteus larval stages.
Keywords
IWF Classification
Transcript: English(auto-generated)
Sea urchin, gastrulation and larval stages, normal speed, time-lapse.
After the cleavage divisions of the sea urchin egg follows the transformation of the blastula to the gastrula. To observe this process, the specimens had to be immobilized and the cilium movement blocked. Gastrulation begins with the migration of micromia material, that is, cells from the central area of the vegetal pole.
Within the blastocoel, these differentiate to primary mesenchyme cells while producing protoplasmic processes. They spread out apically by creeping along the walls of the blastocoel. Not long afterwards, the invagination of the archenteron begins.
First, a schematic representation of the development from blastula to gastrula. The totally ciliated blastula has escaped from the fertilization membrane and has become flattened at the vegetal pole.
At the animal pole, a tuft of cilia projects from an epithelial thickening. The blastula is a relatively stable hollow sphere with an interior cavity, the blastocoel, filled with gelatinous substances. The animal germ layers 1 and 2,
which originated in the cellular tiers of the 32 and 64 blastomere stages, are colored yellow. The vegetal tiers 1 and 2 are colored green. The red area indicates the micromirs, which are displaced into the blastocoel before the onset of gastrulation. Here in the region of the flattened vegetal pole,
they are differentiated into the primary mesenchyme cells with their long protoplasmic processes. Gastrulation begins with the invagination of the material denoted as vegetal 2.
The infolding gives rise to the primitive intestine cavity, the archenteron, the external opening of which is the blastopore. Before archenteron formation has been completed, secondary mesenchyme cells are already migrating to the roof of the archenteron in the blastocoel. At the base of the intestine, the primary mesenchyme cells are already laying
down the rudimentary larval skeleton in the form of three-pointed spicules originating from calcareous crystals. Here, once again under higher magnification, the migration of the primary mesenchyme cells from the thickened polar plate. The cells issue from the vegetal
pole as if they were being subjected to pressure, and they are then displaced into the blastocoel in disorderly fashion. The archenteron folds itself into the blastocoel.
When it is about one-third invaginated, the secondary mesenchyme cells migrate from the roof of the primitive intestine. These mesenchyme cells also exhibit amoeboid movements and are provided with filamentous processes with which they attach themselves to the walls of the blastocoel.
Gastrulation viewed this time from the blastopore side. Between the walls and the centrally located archenteron roof are the primary mesenchyme cells that have already migrated. As the archenteron invaginates,
the secondary mesenchyme cells emerge from the now deeply depressed archenteron roof. During the course of gastrulation, the appearance of the larva is changed by a flattening in the area of the presumptive stomodeum. This can be seen more clearly as the larva is tilted.
It is gradually assuming bilateral symmetry. From particular aggregations of primary mesenchyme cells, the three-pointed skeletal spicules are secreted. They form the rudimentary calcareous skeleton of the sea urchin larva. During the late gastrula stage, two mesodermal
outpocketings appear on either side of the archenteron roof. They are known as the left and right coelomic sacs. The first skeletal spicules are produced at the center of a triaxial
syncytium. First, a grain of inorganic matter is laid down. The shape of the syncytium determines the accretion pattern of further layers until the characteristic tri-radiated spicules are recognizable. There is one skeletal rudiment on either side of the
archenteron in the blaster seal. The longitudinal growth of the spicules can be followed very clearly at this late stage. The differentiation of the coelome, the secondary body cavity,
from the two mesoderm folds at the roof of the archenteron completes gastrulation. The larva has meanwhile assumed its characteristic prismoidal appearance. The transformation of the late gastrula to a young pluteus larva is demonstrated schematically.
The late gastrula grows faster dorsally than ventrally, thereby taking on a prismoidal appearance. Two mesodermal outpocketings bud off from the archenteron roof, the left sac
developing more noticeably than the right. The archenteron becomes differentiated into three regions by constriction, the esophagus, the stomach, and the intestine opening at the blastopore. Viewed laterally, the more active dorsal growth is apparent. The archenteron roof arches down
towards the future ventral aspect, the animal part of which, colored yellow, will form the so-called oral field. An oral depression appears, which soon merges into the archenteron roof.
In the young pluteus larva, the oral field is girdled with ciliated bands. At the point of contact between the oral depression and the esophagus, the definitive mouth is now formed. The opposed blastopore now becomes the anus. The three regions of the larval intestine can now be recognized more clearly.
The divisions of the archenteron can now be seen very clearly at this prism stage. To the left is the esophagus, followed by the stomach, and on the right the intestine, emerging at the blastopore. The skeletal rods now extend along the whole length of the larva.
The oral depression now unites with the archenteron roof. Then the definitive mouth opens up while the blastopore becomes the anus. This process is characteristic of the morphogenesis of the deuterostomia.
These young pluteus larvae already have the rudiments of the four processes which delimit the oral field. In older pluteus larvae, these rudiments extend as arms.
From the side, the interior organization of the intestine is clearly visible. Mouth, esophagus, stomach, intestine, and anus. The two dorsal arms are almost parallel, being connected by an ectodermal lamella.
The two opposed ventral arms are free and divergent. In this roughly four-day-old pluteus larva, the inner processes leading to metamorphosis have already been initiated.
Not until five or six weeks have elapsed is the development to the young sea urchin complete.