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Related Concept Videos

Neurulation01:30

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Neurulation is the embryological process which forms the precursors of the central nervous system and occurs after gastrulation has established the three primary cell layers of the embryo: ectoderm, mesoderm, and endoderm. In humans, the majority of this system is formed via primary neurulation, in which the central portion of the ectoderm—originally appearing as a flat sheet of cells—folds upwards and inwards, sealing off to form a hollow neural tube. As development proceeds, the...
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Changes in the Appendicular Skeleton with Age01:09

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The upper and lower limb initially develops as a small bulge called a limb bud, which appears on the lateral side of the early embryo. The upper limb bud appears near the end of the fourth week of development, with the lower limb bud appearing shortly after.
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Bone Formation by Endochondral Ossification01:24

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Bone formation, or ossification, begins around the sixth to seventh week of embryonic development. Most bones develop from a cartilaginous template through the process of endochondral ossification. Cartilage formation begins when clusters of mesenchymal cells differentiate into chondrocytes. These chondrocytes proliferate rapidly and secrete an extracellular matrix that becomes encased in a membrane called the perichondrium. The resulting cartilage model provides a template that resembles the...
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Gastrulation01:56

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Gastrulation establishes the three primary tissues of an embryo: the ectoderm, mesoderm, and endoderm. This developmental process relies on a series of intricate cellular movements, which in humans transforms a flat, “bilaminar disc” composed of two cell sheets into a three-tiered structure. In the resulting embryo, the endoderm serves as the bottom layer, and stacked directly above it is the intermediate mesoderm, and then the uppermost ectoderm. Respectively, these tissue strata...
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Development of the Limb Synovial Joints01:07

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Joints form during embryonic development in conjunction with the formation and growth of the associated bones. The embryonic tissue that gives rise to all bones, cartilage, and connective tissues of the body is called mesenchyme.
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Spinal Cord: Gross Anatomy01:15

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The spinal cord resides within the protective confines of the vertebral column. It is the main pathway for information traveling between the brain and the body. It plays a fundamental role in nearly all bodily functions, from simple reflexes to complex motor movements. The spinal cord begins at the medulla oblongata at the base of the brainstem and extends downward, terminating at the conus medullaris near the first and second lumbar vertebrae. The spinal cord's length in adults is...
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Related Experiment Video

Updated: Apr 1, 2026

Embryo Microinjection and Electroporation in the Chordate Ciona intestinalis
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Evolution of the notochord.

Giovanni Annona1, Nicholas D Holland2, Salvatore D'Aniello1

  • 1Department of Biology and Evolution of Marine Organisms, Stazione Zoologica Anton Dohrn, Villa Comunale, 80121 Naples, Italy.

Evodevo
|October 9, 2015
PubMed
Summary

The evolutionary origin of the notochord, a key chordate structure, remains debated. Current research revives theories of its novelty within chordates or derivation from ancestral structures, highlighting the need for further genetic studies.

Keywords:
Annelid scenarioAxochordEnteropneust scenarioNotochordPygochordStomochord

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Area of Science:

  • Evolutionary developmental biology
  • Comparative genomics
  • Phylogenetics

Background:

  • The notochord is a defining feature of chordates, but its evolutionary origins are poorly understood.
  • Existing hypotheses for notochord evolution lack comprehensive review and molecular support.
  • Recent advances in molecular phylogenetics and developmental genetics offer new perspectives.

Purpose of the Study:

  • To survey and critically evaluate proposed evolutionary origins of the notochord.
  • To explore the potential homology between the notochord and structures in related taxa.
  • To identify future research directions for resolving the notochord's evolutionary history.

Main Methods:

  • Review of existing literature on notochord development and evolution.
  • Analysis of molecular phylogenetic data.
  • Examination of developmental genetic studies.

Main Results:

  • Many historical hypotheses for notochord origin are not supported by current molecular data.
  • Molecular data have revived two main hypotheses: notochord as a chordate novelty, or derived from an axochord in annelid-like ancestors.
  • Enteropneust hemichordates are proposed as a key group for future research.

Conclusions:

  • The evolutionary origin of the notochord requires further investigation using gene regulatory network analysis.
  • Comparative studies across a wide range of animals, particularly hemichordates, are crucial.
  • Resolving whether the notochord is a chordate novelty or an ancient inherited structure is a primary goal.