Germ Layers Formation and History

Germ Layers

• There are three primary germ layers: Ectoderm, Endoderm, and Mesoderm
• These germ layers form during organogenesis

History of Germ Layers

• Heinz Christian Pander discovered three primordial germ layers in 1817 while studying chick embryos
• Between 1850 and 1855, Robert Remak refined the germ cell layer concept
• T.H. Huxley introduced the terms "mesoderm" in 1871, and Lankester introduced "ectoderm" and "endoderm" in 1873

Formation of Germ Layers

• Gametogenesis: production of sperms (male) and ova (female)
• Fertilization: transmission of genes from parents
• Cleavage: rapid mitotic cell divisions of the zygote to form the Blastula
• Blastulation
• Gastrulation: rearrangement of cells to form a three-layered embryo (gastrula)
• Neurulation
• Organogenesis: formation of organs

Establishment of Somatic Cells

• Three classes of somatic cells: Ectoderm, Endoderm, and Mesoderm (also known as the three germ layers)

Gastrulation

• The blastula becomes a three-layered embryo (gastrula) through gastrulation
• Cells become arranged in three distinct germ layers: Ectoderm, Endoderm, and Mesoderm
• Each germ layer develops into specific parts of the embryo

Ectoderm

• Forms the outermost layer of the embryo
• Develops into:
  • Central Nervous System
  • Lens of the eye
  • Cranial and sensory
  • Ganglia and nerves
  • Pigment cells
  • Epidermis
  • Hair and mammary glands

Endoderm

• Forms the innermost layer of the embryo
• Develops into:
  • Flattened cells that become columnar

Mesoderm

• Forms the middle layer of the embryo
• Develops into:
  • Coelom (body cavity)
  • Generation of chimeric mice

Types of Animals

• Sponges: single germ layer
• Diploblastic animals (Cnidaria and Ctenophores): two germ layers (endoderm and ectoderm)
• Triploblastic animals: three germ layers (ectoderm, endoderm, and mesoderm)

Tissues

• A group of closely associated, similar cells that carry out specific functions
• Four kinds of animal tissues: epithelial, connective, muscle, and nervous tissue
• Each type of tissue is specialized to perform a specific function or group of functions

Epithelial Tissues

• Consists of cells fitted tightly together to form a continuous layer of cells
• One surface is exposed, while the other surface attaches to underlying tissue by a noncellular basement membrane
• Functions include:
  • Protection (epidermis)
  • Absorption (digestive tract)
  • Secretion (glands)
  • Sensation (sensory receptors)

Learn about the three primary germ layers - Ectoderm, Endoderm, and Mesoderm - their formation during organogenesis and the history of their discovery.