Page 1: Attendance

• Attendance sheet for the lecture.

Page 2: Lecture Overview

Key Topics Discussed

• Organismal complexity in animal development.

• Differentiation and body plans of organisms.

• Experimental model systems of development.

Cell Potency

• Definition: The capacity of a cell to generate different types of specialized cells.

  • Totipotent: Cells that can develop into all types of cells in an organism.

  • Pluripotent: Cells that can become many types but not all.

  • Terminally differentiated: Cells that are specialized and cannot become other types (e.g., muscle cells, neurons, hepatocytes).

• Differentiation: The process through which a cell becomes specialized.

Page 3: From Zygote to Adult

Comparison of Zygote and Adult

• Zygote: 1 cell.

• Adult: Approximately 100 trillion cells (10^14).

• Achieved through 47 rounds of cell division.

Key Differences

1. Differentiation: Formation of distinct cell types (neurons, muscle cells, etc.).

2. Morphogenesis: Development of shape, pattern, and form.

• Example: Adult sponges vs. complex animals.

Symmetry in Animals

• Complex animals exhibit distinct anterior/posterior and dorsal/ventral orientations with bilateral symmetry, unlike simpler forms like sponges.

Page 4: Developmental Pathways

Overview of Cell Types

• Zygote - Totipotent.

• Blood Stem Cells, Muscle Cells, Neurons, Liver Cells, Lung Cells, Intestinal Cells.

• Adult cells are terminally differentiated, meaning they perform specific functions.

Differentiation Process

• Differentiation occurs in stages, progressively restricting potentials of cells.

Page 5: Understanding Totipotency

Definition of Totipotency

• Cells can form all parts of an organism.

• When is totipotency lost? This occurs at specific developmental stages.

Page 6: Embryogenesis and Twinning

Totipotency in Development

• In mammals, single cells from 2, 4, and 8-cell stages can form an embryo.

• Totipotency remains until the early blastocyst stage.

• After the 8-cell stage, blastomeres may diminish in size restrictively, limiting their ability to develop into functional embryos.

Page 7: Evolution of Bilateria

Axes of Symmetry

• Evolution and classification based on symmetry:

  • Monoblastic: No true tissues (e.g., sponges).

  • Diploblastic: Two layers of cells (e.g., jellyfish) - radial symmetry.

  • Triploblastic: Three layers, bilateral symmetry (e.g., humans).

Page 8: Tissue Development

Classification of Animals

• Monoblasts: Sponges with only one layer of cells, no true tissues.

• Diploblasts: Cnidarians with ectoderm and endoderm.

• Triploblasts: Animals with ectoderm, mesoderm, and endoderm, allowing for a more complex anatomy.

• Gastrulation: The process that forms three distinct layers from the embryonic cells.

Bilaterians

• All bilaterians have bilateral symmetry and organizational complexity from three cellular layers.

Page 9: Body Plans in Animals

Complexity in Body Structures

• Diploblasts have basic dorsal/ventral sides without anterior/posterior.

• Bilaterians introduce anterior/posterior distinctions alongside gut structures.

• Gastrulation contributes to the formation of complex body plans.

Page 10: Body Plans and Germ Layers

Three-Layered Body Plan

• All complex animals share a three-layered body derived from embryonic layers.

• This body plan exists in adult tissues, highlighting evolutionary continuity.

Page 11: Classification of Animals

Evolutionary Lineage

• Major groups:

  • Metazoa: All animals.

  • Eumetazoa: True tissues.

  • Triploblastic vs. Diploblastic Animals: Providing insights into animal evolution.

• Notochord: A precursor to the vertebrate spine.

Page 12: Embryonic Development

Structure of the Triploblastic Embryo

1. Ectoderm: Skin.

2. Endoderm: Digestive system.

3. Mesoderm: Muscle and skeleton.

• This structure is typical in sea urchins, illustrating triploblastic organization.

Page 13: Challenges in Embryonic Development

Evolution and Adaptation

• The evolution of land-dwelling animals requires adaptations in embryogenesis for survival (e.g., amniotic sac, placenta).

Page 14: Inner Cell Mass Development

Processes in Gastrulation

1. Epiblast migration through the primitive streak to form endoderm cells.

2. Mesodermal cells develop between endoderm and epiblast.

3. Ectodermal layer remains uppermost.

Page 15: Development of Embryo Structures

Important Structures

• Amnion: Fluid-filled sac.

• Placenta: Formed from trophoblast and extra-embryonic mesoderm.

• Yolk Sac and Allantois: Structures aiding in nourishment and connection.

Page 16: Understanding Development

The Molecular Basis of Development

• Utilizing model organisms for understanding genetic principles of development (e.g., Drosophila, Caenorhabditis elegans).

Page 17: Model Organisms

Examples of Organisms in research

• Drosophila melanogaster: The fruit fly (5mm).

• Caenorhabditis elegans: The nematode worm (1mm).

• Both are favored due to their short life cycles and prolific offspring.

Page 18: Homeotic Genes

Discoveries and Applications

• Edward B. Lewis discovered homeotic mutants in Drosophila.

• Homeotic genes determine anatomical identity in organisms and play a key role in segment development.

Page 19: Functions of Homeotic Genes

Mechanism and Influence

• Homeotic genes specify body segment identity and influence structure production.

• These genes produce transcription factors that guide embryonic development.

Page 20: Homeobox DNA Motifs

Structure and Function

• Homeotic genes contain the Homeobox motif.

• Regulates developmental gene expression and functions as a global positioning system for anatomical identity.

Page 21: Evolutionary Implications

Hox Clusters in Evolution

• Four Hox clusters observed in mammals signify the diversity of anatomical structures developed through evolution.

Page 22: Genetic Conditions Related to Homeotic Genes

Case Study: Synpolydactyly

• Mutation in human homeotic gene HOXD13 leads to anatomical misconfigurations.

• Highlights the role of homeotic genes in controlling differentiation.

Page 23: Summary of Evolutionary Mechanisms

Developmental Evolution

1. Diploblasts (e.g., Cnidaria): Two tissue types, limited anatomical structures.

2. Triploblasts: Introduces a more complex body plan with anterior/posterior orientation.

3. Amplification of HOX gene families in Bilaterians leads to diversified morphologies and the Cambrian explosion.

• Gastrulation and HOX complexity are integral to the evolution of modern animal forms.