Lecture 9 Animal and Plant development

Neurulation to Organogenesis

Stages of Development

• Ectoderm: Outermost layer forming skin and nervous system.

• Mesoderm: Middle layer forming muscles, circulatory system, and other organs.

• Endoderm: Innermost layer forming gut and associated structures.

• Key Stages:

  • Fertilization: Fusion of sperm and egg.

  • Cleavage: Zygote undergoes rapid cell division.

  • Gastrulation: Formation of the three germ layers from the blastula.

  • Neural Groove Formation: Initial neural development.

  • Notochord Development: Key for axial skeleton and signaling.

  • Neurulation: Formation of the neural tube from the ectoderm.

Organogenesis

• Organogenesis is the process through which organs develop from the germ layers.

• First Rudimentary Organs are the Neural Tube and the Neural Crest

• These are Formed from Ectoderm and give rise to most of nervous system

• Neural Tube: is the developing central nervous system

• Germ Layers Specific Fates:

  • Endoderm: Forms gut and associated organs.

  • Mesoderm: Forms a variety of structures including muscles, the circulatory system, and certain organs.

  • Ectoderm: Forms skin, hair, nails, and nervous system.

Detailed Functions of Germ Layers

• Endoderm (Innermost Layer):

  • Develops into the gut and organs related to digestion, respiratory system innermost linings.

  • Structures include liver, pancreas, and epithelial lining of digestive and respiratory tracts.

• Mesoderm (Middle Layer):

  • Forms muscles, skeleton, circulatory, excretory systems, and various organs.

  • Notable structures include:

    • Notochord

    • Heart

    • Kidneys

    • Gonads

• Ectoderm (Outermost Layer):

  • Develops into skin and nervous system, and various sensory receptors.

  • Key structures include:

    • Skin epidermis

    • Hair follicles

    • Nervous tissue (brain, spinal cord)

Neurulation Process

• Neural Tube Formation:

  • The neural tube is a critical structure that develops from the ectoderm and later forms the central nervous system.

  • The notochord, derived from mesoderm, induces the overlying ectoderm to form the neural plate.

  • Neural plate folds, creating neural groove and subsequently fuses to form the neural tube.

  • The nervous system is the first organ system to develop

  • Notochord grows and induces overlying ectoderm to form the neural plate

  • Cells of the neural plate fold to form the neural groove and the surrounding neural folds

  • Neural folds fuse, forming a hollow neural tube

• Somite Development:

  • Mesodermal cells on the sides of the notochord develop into somites, which differentiate into vertebrae, muscles, and connective tissues.

Anterior-Posterior Development**

• Development proceeds from the anterior (head) to posterior (tail) end.

• Neural Crest Derivatives:

  • Cells from the neural crest migrate and contribute to various structures including:

    • Neurons of the peripheral nervous system

    • Bone and cartilage in the head

    • Melanocytes (pigmented cells)

Neural Tube Defects**

• Disruptions in neurulation can lead to severe developmental issues, such as:

  • Anencephaly: Absence of large portions of the brain.

  • Spina bifida: Incomplete closing of the spinal cord.

  • These defects result from failures in the fusion of neural folds or issues with neural crest cell migration.

Dorsal-Ventral Specification**

• The dorsal-ventral axis of the neural tube is determined by Sonic Hedgehog (Shh) signaling from the notochord and floor plate.

• TGF-beta (BMPs) also play a role in this specification:

  • High Shh levels promote motor neuron development and muscle differentiation in the ventral regions.

  • BMPs are involved in dorsal neural development, creating a balance essential for proper patterning.

Plant Development (Summary)

Four Processes

• Cell Division

• Cell Growth

• Differentiation

• Morphogenesis

Polarity and Establishment of the Root-Shoot Axis (Embryo)

• First Zygotic cell division is asymmetric, generating 2 cells with 2 different fates

  • 1. Apical Cell: becomes all Shoot tissue (SAM)

  • 2. Basal Cell: becomes all Root tissue (RAM) and forms the suspensor (linking embryo to parent)

Apical Meristems

Three Basic Tissue Systems are Established

• Dermal tissue from the protoderm

• Ground tissue from the ground meristem

• Vascular tissue from the procambium
  

Plant Growth – It is all from Meristems

• Plants differ dramatically from animals in growth and developmental patterns

• Animal cells can adjusting to new positions during embryogenesis, but at gastrulation most cells lose this ability and develop autonomously

• Plants, however, can develop organs continuously in the postembryonic phase and most living cells remain totipotent

• Plant cell proliferation and organ development initiates from meristematic regions (SAM & RAM)

Major Stages

1. Embryogenesis: Formation of the embryo from zygote.

2. Vegetative Development: Growth of plant structures such as leaves and stems.

3. Reproductive Development: Formation of flowers and seeds.

Hormonal Influences on Development

• Hormones like auxin affect development by regulating cell elongation, division, and differentiation.

• Reporter studies visualize hormone action and distribution during plant growth.

Measure Hormone Levels

• Direct measurements - for auxin (GC-MS) can be done, but difficult

• Antibodies – rarer, difficult

• Reporters – transgenic lines (AREs-DR5:GFP, PIN:GFP) - Indirect, but simpler, more widely used

Key Structures in Plants Derived from Embryogenesis

• Apical meristems give rise to all above-ground plant structures.

• Root meristems contribute to root developments, maintaining root growth throughout the plant's lifecycle.

Conclusion

• Understanding the processes of neurulation and organogenesis provides insight into both animal and plant development, highlighting similarities and differences in techniques and structures.