Experiment: Transverse Section (T.S.) through the Pharyngeal Region of Amphioxus

Aim:

To prepare a transverse section through the pharyngeal region of amphioxus and identify the key anatomical structures.

Materials:

1. Preserved specimens of amphioxus

2. Dissecting microscope

3. Scalpel and dissecting scissors

4. Microtome

5. Glass slides and coverslips

6. Staining solutions (e.g., Hematoxylin and Eosin)

7. Microscope

Experimental Procedure:

1. Obtain a preserved specimen of amphioxus and place it on a dissecting tray under a dissecting microscope.

2. Use a scalpel and dissecting scissors to carefully remove excess tissue around the pharyngeal region, ensuring not to damage the structures of interest.

3. Embed the cleaned specimen in paraffin wax or agarose to provide support during sectioning.

4. Use a microtome to prepare transverse sections of the pharyngeal region, cutting slices of approximately 10-20 micrometers in thickness.

5. Transfer the sections to glass slides and stain them using suitable staining solutions, such as Hematoxylin and Eosin, to enhance contrast and visibility of cellular structures.

6. Carefully place coverslips over the stained sections and observe them under a compound microscope.

7. Identify and annotate the key anatomical structures present in the transverse section, including the pharyngeal gill slits, endostyle, pharyngeal muscles, notochord, and nerve cord.

8. Take photomicrographs of the sections to document your observations.

Observations:

1. Pharyngeal Gill Slits: The transverse section reveals multiple pairs of elongated openings, corresponding to the pharyngeal gill slits. These slits are arranged in parallel rows along the lateral walls of the pharynx. Each slit is lined with epithelial tissue and surrounded by supportive cartilage-like structures.

2. Endostyle: A prominent longitudinal glandular structure, known as the endostyle, is observed along the ventral midline of the pharynx. The endostyle appears as a dark-staining band of cells, extending longitudinally from the anterior to the posterior end of the pharyngeal region.

3. Pharyngeal Muscles: Thin bands of striated muscle tissue are evident surrounding the pharynx, providing support and enabling rhythmic contractions during feeding and respiration. The muscles exhibit a cross-striated pattern under the microscope, indicating their skeletal nature.

4. Notochord: Running dorsally along the midline of the section, the notochord appears as a rod-like structure with a distinctive lighter staining compared to surrounding tissues. It provides structural support and serves as a primitive axial skeleton in amphioxus.

5. Nerve Cord: The nerve cord is observed as a dorsal tube-like structure located above the notochord. Although not as distinct as other structures, it is identifiable by its location and association with the notochord.

Results and Analysis:

– The transverse section through the pharyngeal region of amphioxus provides valuable insights into the anatomical organization of this primitive chordate.

– The presence of pharyngeal gill slits confirms the filter-feeding nature of amphioxus, allowing water to enter the pharyngeal cavity and pass through these slits for gas exchange and food capture.

– The endostyle is implicated in the production of mucus, which aids in trapping food particles and facilitating their transport to the digestive tract.

– Pharyngeal muscles play a crucial role in the movement of water and food particles through the pharyngeal region, contributing to feeding and respiration.

– The notochord serves as a key structural element, providing support and flexibility to the body of amphioxus, while the nerve cord is associated with coordinating motor and sensory functions.

– Overall, the observed anatomical features reflect adaptations to the filter-feeding lifestyle and primitive vertebrate body plan of amphioxus.