Animal Development II: Gastrulation & Organogenesis

Learning Objectives

1. Gastrulation & Organogenesis: Explain their significance, features, and consequences in early animal development.
2. Adult Tissue Types: List and describe major organs arising from each embryonic germ layer (epidermal tissue, nervous system, muscle and skeletal systems, etc.).
3. Extra-Embryonic Membranes: Understand roles in amniotes (birds, reptiles, mammals).
4. Induction and Gene Expression: Describe roles in cell specialization and morphogenesis.
5. Hox Genes: Explain their role in segment identity.

Stages in Early Animal Development

• Fertilization: Sperm and egg combine to form a zygote.
• Cleavage: Rapid mitotic cell division forming a blastula.
• Gastrulation: Cells rearrange to form embryonic tissue layers.
• Organogenesis: Formation of organs and tissues.

Gastrulation

• Blastula becomes a gastrula with three germ layers:
  • Endoderm: Forms the embryonic gut (archenteron).
  • Ectoderm
  • Mesoderm (in triploblasts)
• Body Axes become visible post-gastrulation.
• Protostomes vs. Deuterostomes: Blastopore becomes mouth (protostomes) or anus (deuterostomes).

Germ Layers and Their Derivatives

• Ectoderm: Nervous system, skin epidermis, lining of mouth/rectum.
• Mesoderm: Muscle, skeletal system, circulatory system, reproductive organs.
• Endoderm: Liver, pancreas, lining of digestive and respiratory systems.

Types of Animal Tissues

• Epithelial Tissue: Covers surfaces, lines cavities; involved in protection and absorption.
• Connective Tissue: Supports and connects other tissues.
  • Loose Connective Tissue: Supports organs and blood vessels.
  • Dense Connective Tissue: Found in tendons and ligaments.
  • Other types include adipose tissue, bone, cartilage, blood.
• Muscle Tissue: Involved in movement, maintaining posture.
  • Skeletal Muscle: Voluntary control, attached to bones.
  • Cardiac Muscle: Found in heart, involuntary.
  • Smooth Muscle: Found in walls of organs, involuntary.
• Nervous Tissue: Senses stimuli, processes information.

The Four Extra-Embryonic Tissues in Amniotes

• Amnion: Surrounds embryo, provides protection.
• Chorion: Gas exchange.
• Allantois: Stores waste, aids in respiration.
• Yolk Sac: Transports nutrients.
• Placenta: Fetal and maternal blood components exchange.

Organogenesis

• Nervous System Formation: Ectoderm-derived.
  • Notochord: Induces neural plate to form neural tube.
  • Somites: Develop into bone, muscle, connective tissue.

Hox Genes and Development

• Differential Gene Expression: Determines cell form and function.
• Hox Genes: Regulate body plan, segment identity.
  • Mutations can lead to abnormal body segment development.
• Gene Regulation: Controlled spatially and temporally during development.

Summary

• Gastrulation and Organogenesis are crucial in shaping the embryo.
• Hox Genes play a vital role in segment identity and body plan regulation.