Lecture Notes on Organogenesis and Developmental Biology

Lecture Overview

Lecture Focus: Organogenesis in Developmental Biology

Instructor: Prof. Paula Murphy, Trinity College Dublin

Date: February 25, 2025

Key Events in Early Development:

1. Fertilization: This initial event marks the beginning of embryonic development, where a sperm cell penetrates an egg (oocyte), leading to the formation of a zygote.

2. Cleavage: After fertilization, the zygote undergoes several rapid cell divisions known as cleavage, resulting in an increase in cell number while the overall size of the embryo remains constant. This process leads to the formation of a hollow ball of cells known as the blastula or blastoderm, which is crucial for subsequent developmental stages.

3. Gastrulation: This critical phase involves the reorganization of the blastula into a gastrula, characterized by the establishment of three distinct germ layers: ectoderm, mesoderm, and endoderm. Each germ layer is responsible for forming specific tissues and organs in the developing embryo.

4. Neurulation: This specialized form of organogenesis leads to the development of the nervous system. During neurulation, the neural plate folds to form the neural tube, which will eventually give rise to the brain and spinal cord.

5. Organogenesis: This phase refers to the process through which organs and body systems develop from the germ layers established during gastrulation. Organogenesis encompasses the coordination of cell differentiation, migration, and interaction to form functional organs.

Organogenesis Overview:

• Interaction of Cells: Local interactions within and across the germ layers are critical for organ formation. These interactions involve signaling pathways and cellular communications that guide the differentiation and organization of cells into specific structures.

• Germ Layers:

  • Ectoderm: This outermost layer is responsible for forming structures such as skin, hair, nails, and the nervous system, including the brain and spinal cord.

  • Mesoderm: The middle layer forms vital components, including the heart, muscles, bones, kidneys, and blood. It is essential for developing the cardiovascular system and the musculoskeletal system.

  • Endoderm: This innermost layer gives rise to internal organs, such as the gut, liver, pancreas, and lungs. The endoderm is responsible for forming the lining of many digestive and respiratory structures.

Organogenesis involves tissue production and morphogenesis, whereby cells not only grow but also undergo significant shape changes to form organized structures.

Gastrulation Process:

• Converts blastula into a gastrula with three distinct germ layers, setting the foundation for organ development.

Neural Tube Formation:

• The neural plate forms from the ectoderm under the influence of the underlying notochord, a structure that induces the development of the neural tube.

• Neural folds rise and ultimately fuse to create the neural tube, which is essential for the formation of the Central Nervous System (CNS).

Neurulation Details:

• Neural Crest Cells: These specialized cells migrate from the neural tube to various parts of the embryo, contributing to the development of the Peripheral Nervous System (PNS) and other critical tissues such as melanocytes, cartilage, and bones of the face.

• Signaling Molecules:

  • Sonic Hedgehog (Shh): This signaling molecule is vital for the formation of the neural tube and plays a significant role in neural differentiation, helping to establish the dorsal-ventral axis of the CNS.

Mesodermal Organogenesis:

• Develops structures such as somites, which give rise to vertebrae and muscle, and the heart, highlighting the complexity of mesodermal differentiation.

Heart Development:

• The formation of the heart begins with the development of a linear tube, which then undergoes looping and folding to create four chambers: two atria and two ventricles. This process is crucial for establishing proper blood circulation.

• Mesenchymal to Epithelial Transition: A critical process for forming heart tubes from mesodermal cells, enabling the development of functional heart structures.

Molecular Pathways in Heart Development:

• Nkx2.5: This transcription factor is essential for cardiac muscle differentiation, providing direction for the development of cardiac tissues.

• BMP and Wnt Signaling: Two important signaling pathways involved in heart development, influencing cell fate decisions and tissue organization.

Circulatory System Development:

• Blood Vessels: These structures develop from mesodermal progenitor cells and undergo extensive branching morphogenesis to create a functional vascular system.

• Haematopoiesis: The process of blood cell formation begins in the yolk sac and later shifts to the liver and bone marrow, highlighting the dynamic nature of blood cell development throughout embryogenesis.

• Dual Circulatory System: This system is essential for nutrient exchange in the embryo, functioning effectively during development and continuing to support physiological functions post-birth.

Endodermal Organ Development:

• Lungs: Develop from the branching of the respiratory tract as it emerges from the foregut, induced by signals exchanged between mesodermal and endodermal tissues. This process is critical for establishing the gas exchange surface of the lungs.

• Branching Morphogenesis: This intricate process is facilitated by FGF-10 and Shh signaling, allowing the lungs to develop the necessary surface area for effective respiration.

Summary of Organogenesis:

• The organogenesis phase integrates the events of gastrulation and neurulation with organ formation stemming from ectoderm, mesoderm, and endoderm. It is a complex orchestration of cellular events and signaling pathways.

• It emphasizes cellular interactions and regional signaling essential for organ development.

• The role of key molecular signals and transcription factors is highlighted, shaping the embryo's future physiological systems and ensuring proper organ functionality.

Contact Information:

Prof. Paula Murphy: paula.murphy@tcd.ie.

Encourages questions and feedback on resource tools developed for education.