Embriogênese de equinodermos | Ouriço-do-mar

Development of Sea Urchin Embryos

Introduction to Echinoderms

• The presentation discusses the embryonic development of sea urchins, presented by students Ana Beatriz, Maria Fernanda, Mariana, and Rafael.

• Echinoderms are marine animals characterized by a skeleton made of calcareous plates and spines for defense against predators.

• They are triploblastic organisms with three germ layers: endoderm, mesoderm, and ectoderm. They possess a coelom (body cavity) lined with mesodermal tissue.

Symmetry and Classification

• Adult echinoderms exhibit radial symmetry while their larvae show bilateral symmetry. This distinction is crucial in their classification as deuterostomes.

• Approximately 1,000 species of sea urchins have been documented since the 19th century; they serve as model organisms for developmental studies.

Evolutionary Background

• Echinoderms are closely related to sea cucumbers and have existed since the Ordovician period (around 450 million years ago).

• Some species can pose risks due to venomous spines; they are also preyed upon by various marine animals including starfish and fish.

Fertilization Process

• Gametes released into the environment initiate fertilization when sperm contacts the egg's gelatinous layer, which contains proteins that attract sperm.

• After fertilization, there is a rapid synthesis of proteins using mRNA present in the egg cytoplasm.

Cleavage Stages

• The cleavage process involves mitotic divisions creating smaller cells called blastomeres through holoblastic radial cleavage.

• The first cleavage is meridional (dividing from animal to vegetal pole), followed by subsequent cleavages forming an eight-cell stage.

Formation of Morula Stage

• As development progresses to the morula stage, cells vary in size—macromeres (largest), mesomeres (medium), and micromeres (smallest).

• By the fourth cleavage, differences arise between divisions at animal versus vegetal poles leading to distinct cell sizes.

Transition to Blastula Stage

• The sixth cleavage results in an embryo with 64 cells; further divisions lead towards forming a blastula—a hollow sphere surrounding a central cavity.

• During this stage, micromeres contribute significantly to forming structures within the developing embryo.

Developmental Stages of Sea Urchins

Early Development and Cell Division

• In the initial phase of development, all cells are uniform in size. As development progresses, cells begin to form a thinner layer, leading to the formation of a blastula with a single layer of approximately 1,000 cells and a central cavity.

• The activation of the genome occurs, initiating gene expression. Cells at the animal pole express genes coding for protease enzymes that break down the fertilization membrane, allowing the blastula to exit its protective envelope.

Gastrulation Process

• Gastrulation begins with cell migration from the vegetal pole, leading to invagination and forming the blastopore, which will develop into the anus in sea urchins.

• Three germ layers are formed during this stage: ectoderm, mesoderm, and endoderm. These layers provide structural foundations for future larval development.

Larval Formation and Metamorphosis

• After gastrulation, primary mesenchyme cells move via projections called filopodia towards specific regions to form an endoskeleton. This process is followed by further invagination from vegetative cells contributing to primitive gut formation.

• The larvae (called "buzios") are mobile and feed on marine plankton; they develop within 24 to 72 hours post-fertilization. The larval stage lasts between four to six weeks before metamorphosis transforms them into smaller adult forms.

Organ Differentiation

• The stages of organogenesis and morphogenesis involve differentiation of organs from embryonic layers. Ectoderm gives rise to nervous systems and outer coverings while mesodermal cells secrete calcium carbonate for skeletal structures.