Planarian Biology: Anatomy, Regeneration, and Experiments

Regeneration

The process of molding tissues into an exact replica of missing parts.

Ultimate prize of studying regeneration

Understanding regeneration in enough detail to be able to trick a normally nonregenerating system, such as the human hand, into regenerating parts tragically lost in accidents.

Fission

A form of asexual reproduction where the planarian adheres tightly to the substrate with its posterior end, pulls forward with its anterior end, and literally tears itself in two, producing genetically identical clones.

Planarian sexual reproduction mode

Hermaphroditic with internal fertilization where paired organisms reciprocally inseminate each other using a copulatory bulb inserted into a genital pore.

Classification of planarians

Free-living, freshwater flatworms classified as triclad turbellarians.

Common genera of planarians used for studies

Planaria, Dugesia, Polycelis, and Phagocata.

Species commonly used for classroom studies

Dugesia dorotocephala and D. tigrina (brown planarian).

Chemotaxis in planarian collection

Using small pieces of fresh liver placed at the edges of flowing water to attract planarians to the shaded underside within 15-20 minutes.

Water requirements for maintaining planarians in the lab

Spring, well, or collecting-site water must be used because chlorine in treated tap water is toxic.

Maintenance cleaning requirement

Dishes must be changed or rubbed down 1-2 times a week to remove accumulated scuzz from copious mucus secretions.

Feeding protocol for long term maintenance

Fed raw beef liver or egg yolks once a week (or twice a week to increase stock quickly), leaving food out for no more than 2-3 hours before changing water.

Pre

experiment starvation protocol - Animals should be starved for a week prior to regeneration experiments to empty the gut and avoid bacterial contamination during recovery.

Planarian squeeze preparation

A ring of petroleum jelly on a coverslip with the planarian and a drop of water, compressed flat under a second coverslip and observed under dissecting and compound microscopes.

Dorsal vs. ventral appearance

The dorsal surface is pigmented and darker, whereas the ventral surface is ciliated and lighter.

Pharynx function and location

A highly muscular, extrusible organ opening ventrally in the midline, used to suck food into the gut.

Tripartite gut structure

A gut with a single anterior branch, two posterior branches, and numerous subbranches, lacking an anus (undigested food exits via the mouth).

Nervous system anatomy

A bilobed brain (cerebral ganglion) with two lateral nerve cords extending posteriorly and numerous transverse connections.

Eye cups (eyespots)

Clusters of photoreceptors shaded by a cup of pigmented cells that ensure light is received from only one direction.

Reproductive organs timing

Paired ovaries, oviducts, multiple testes, sperm ducts, and copulatory bulb typically develop only during spring/summer and regress otherwise.

Locomotion mechanisms

Gliding via a ciliated ventral surface and crawling via longitudinal and circular musculature over a secreted mucus path.

Tactile and chemoreceptive locations

Tactile receptors are concentrated in the head; chemoreceptors are concentrated in ciliated grooves on the auricles.

Negative phototaxis

The behavioral response of moving away from light sensed by photoreceptors.

Wound healing timeline

Wound closure by body wall muscular contraction takes about 10 minutes; epithelium spreading over it takes an additional 20 minutes.

Blastema formation

An accumulation of undifferentiated neoblasts at the cut site forming an unpigmented region within 1-2 days (clearly visible by 3-4 days at 22−24 ∘C).

Neoblasts

Embryonic-like, highly mitotic reserve cells found throughout the body that coalesce to form the blastema.

Temperature effect on regeneration timing

Timing is highly dependent on temperature and is slower at lower temperatures (at 22−24∘C, differentiation takes 4-6 days and completion takes 2-3 weeks).

Epimorphosis vs. morphallaxis

Epimorphosis builds lost parts anew from undifferentiated cells; morphallaxis remodels existing differentiated cells into new parts.

Pattern formation and positional information

Pattern formation is creating specific shapes from tissues; positional information is the cellular coordinate system directing it.

Diffusion Gradient Model

A model proposing that positional information is established along an axis by concentration levels of a diffusible chemical called a morphogen.

Polar Coordinate Model

A 3D positional information model where cells receive zip codes based on anterior-posterior position (A-E) and circumferential position (1-12 clockface).

Intercalation

To insert missing positional values during regeneration to restore structural continuity when mismatched tissues are juxtaposed.

Rule One of Intercalation

Intercalation will always proceed by the shortest route along positional values to restore positional continuity.

Rule Two of Intercalation

Regeneration restores proximal structures first, completing the positional circle at the cut face before extending distal structures.

Graft Inversion

Rotating a tissue graft to create mismatched circumferential clockface values, predicting the duplication of structures around the graft.

T Cut Experiment

Removing the head and making a longitudinal split down the center of the remaining stump, resulting in two regenerated heads.

Janus heads

An abnormal regeneration pattern resulting in two heads joined back-to-back, formed when a very narrow slice is removed from the anterior region.

Janus tails

An abnormal regeneration pattern resulting in two tails joined with opposite polarity, formed when a narrow slice is removed from the posterior region.

Length to Width Ratio Rule

Principle established by Mead & Krump showing that a slice's length-to-width ratio (below 1.0) determines abnormal regeneration, rather than absolute size.

Time Delay Effect

Finding by Mead (1991) that a 5-12 minute delay between cuts yields peak abnormal regenerates, whereas a 2+ hour delay yields normal regeneration.

Homeobox Genes (Plox 4 / Plox 5)

Conserved regulatory genes expressed in an increasing gradient along the anterior-posterior axis involved in pattern formation.

Dorso Ventral Interaction

Discovery by Kato et al. (1999) that the direct juxtaposition of dorsal and ventral epithelia during wound healing triggers regeneration onset.

Microknife / Microneedle

Precision microdissection tools constructed from razor blade chips, insect pins, wooden dowels, and superglue.

Pasteur pipette tool modification

Breaking off the narrow bill of a pipette to use the unpolished end as a cork-borer-like cutting device for punching out standard circular tissue holes.

Copulatory Bulb

A muscular male reproductive structure used to transfer sperm into a mate's genital pore during copulation.

Genital Atrium

The internal cavity in the planarian reproductive system that receives products from both oviducts and sperm ducts.