Developmental Biology Laboratory: C. elegans and Xenopus laevis

Laboratory Experiments in Developmental Biology

Course Overview

This laboratory course introduces students to key experimental techniques and model organisms in developmental biology, focusing on Caenorhabditis elegans and Xenopus laevis. Students will learn practical skills in organism handling, microscopy, gene expression analysis, and molecular biology methods.

Timetable and Main Topics

Day	Topics
Day 1 Tuesday	Development of Caenorhabditis elegans Preparation of NGM agar and M9 buffer Pouring NGM agar plates and seeding plates Bleaching and chunking Developmental stages Epithelial seam cell lineage
Day 2 Wednesday	Development of Xenopus laevis Microscopic observation and drawings until blastula stage Harvesting oocytes at different stages in early development
Day 3 Thursday	Early gene expression in Xenopus laevis RNA isolation from oocytes RNA quantification Reverse transcription (cDNA synthesis) qPCR

Model Organism: Caenorhabditis elegans

Introduction

Caenorhabditis elegans is a transparent nematode about 1 mm long, widely used as a model organism in developmental biology, genetics, and neurobiology. It is easy to cultivate and study in laboratory settings, especially on agar plates. Most individuals are hermaphrodites, with males comprising only about 0.2% of the population.

Life Cycle and Developmental Stages

• Egg: Fertilization occurs internally; eggs are laid on agar plates.

• Larval Stages: Four larval stages (L1-L4) precede adulthood. Under adverse conditions, larvae can enter a "dauer" stage, which is resistant to environmental stress and can survive for months.

• Adult: Hermaphrodites and rare males; adults can lay eggs and continue the cycle.

Example: Under starvation, L1 larvae arrest development; under crowding or low food, L2 larvae may enter the dauer stage.

Anatomy and Cell Lineage

• Transparent body allows for easy observation of internal structures.

• Seam cells: Lateral rows of stem-like cells that divide asymmetrically and symmetrically, contributing to epidermal development.

• Cell lineage: The entire cell lineage from zygote to adult is mapped, making C. elegans a key organism for studying cell division and differentiation.

Example: Seam cells are visualized using fluorescent markers, allowing study of division patterns and fate decisions.

Model Organism: Xenopus laevis

Introduction

Xenopus laevis is a species of African clawed frog, commonly used in developmental biology for studying early embryogenesis and gene expression. Its large oocytes and embryos are ideal for manipulation and observation.

Developmental Stages

• Oocyte: Large, easily harvested for molecular studies.

• Fertilization and Cleavage: Early cell divisions are easily observed under a microscope.

• Blastula: Stage at which cell fate decisions begin to be established.

Example: Drawings and observations of embryos at different stages help students understand the progression of development.

Gene Expression Analysis in Xenopus laevis

RNA Isolation and Quantification

• RNA isolation: Extraction of total RNA from oocytes or embryos for downstream analysis.

• Quantification: Measurement of RNA concentration using UV/VIS spectroscopy.

Equation:

$\text{Concentration} = \frac{A_{260} \times \text{dilution factor} \times \text{volume}}{\text{path length}}$

Reverse Transcription and qPCR

• Reverse Transcription: Synthesis of complementary DNA (cDNA) from RNA template using reverse transcriptase.

• qPCR (quantitative PCR): Amplification and quantification of specific cDNA sequences to measure gene expression levels.

Equation:

$\text{Fold Change} = 2^{-\Delta\Delta C_t}$

Example: Comparing gene expression in oocytes at different developmental stages using qPCR.

Laboratory Skills and Requirements

Lab Equipment and Materials

• Lab coat

• Pipettes

• White plain paper (non-ruled, non-squared) for drawings

• Pencil, eraser, permanent marker

Lab Book and Reporting

• Record all experimental procedures, observations, and results.

• Drawings of C. elegans and Xenopus laevis at specified stages.

• Lab book must be submitted digitally or as a paper copy by the specified deadline.

Assessment and Evaluation

• Pass entrance test before participating in practicals.

• Submit lab book/journal and pass all practical course dates.

• Evaluation includes lab book content, drawings, and analysis of gene expression data.

Summary Table: Comparison of Model Organisms

Feature	C. elegans	Xenopus laevis
Type	Nematode (worm)	Amphibian (frog)
Size	~1 mm	Oocytes ~1 mm; embryos larger
Developmental Stages	Egg, L1-L4 larva, adult, dauer	Oocyte, fertilization, cleavage, blastula, gastrula
Key Use	Cell lineage, genetics, neurobiology	Embryogenesis, gene expression
Transparency	Yes	No

Additional info:

• Students are expected to understand the principles of cell division, differentiation, and gene expression as applied to model organisms.

• Lab book entries should include both raw data and interpreted results, with references to relevant literature where appropriate.

• Drawings and observations are a key part of the assessment, emphasizing the importance of visual documentation in developmental biology.