Learning Objectives:
- Understand what regeneration is and organisms that can carry it out
- Be able to identify anatomical structures in a Planarian
- Understand Planarian behavior and the receptors involved in the behavior
- Understand how planarians regenerate
• Regeneration is the process of molding tissues into an exact replica of missing parts.
• Many organisms, such as those shown in Figure 9.1, sea stars, lizards, sea slugs and axolotls can regenerate parts of their body.
• But a planarian, such as the one in Figure9.2, can regenerate entire new individuals from even small pieces.
• In fact, regeneration is their main method of reproduction! They normally divide by fission. They stretch their bodies out and they cut themselves in half!
• Planarians are masters at regeneration.
• Planarians are Classified as Triclad turbellarians. These are a group of free living flatworms in which the gut has three branches (Tripartite). Turbellarians are a class of free living flatworms within the phylum Platyhelminths.
• Planarians need to be maintained in Spring or Well water very clean conditions and kept in a dark place. They cannot be maintained in tap water since tap water contains chlorine which is very toxic.
• The water should be changed 1-2 times a week.
• Planarians that are kept for more than 1 week prior to regeneration experimentation should be fed raw beef liver or egg yolks. The food should be left I the dish for 2-3 hours and then removed and the water changed to prevent bacteria from fowling the water.
• 1 week prior to experimentation the Planarians should be starved to empty their gut of foo sans to avoid bacterial contamination during recovery.
• Do not feed the Planarians again until after regeneration is complete
In order to know if a planarian has fully regenerated we must first familiarize ourselves with their behavior and anatomy.
• They are simultaneous hermaphrodites. This means they possess both male and female reproductive structures at the same time.
• Other organisms, like C. elegans and some fish are sequential hermaphrodites(meaning they switch sexes during their life cycle)
• Unlike C. elegans, the Planarian is not capable of self-fertilization, but they are capable of cross fertilizing each other.
• Notice in Figure 9.3 the ovaries. These are the female gonad in which the germ cells form ova. The ova will leave the ovary via the oviduct.
• You can also see the male gonads, the testis where sperm are produced. The sperm leave the testis via the sperm duct.
• In the planarian the region of the reproductive tract that receives the products from both the oviducts and sperm ducts is called the Genital antrum. The copulatory bulb, a muscular organ used to introduce sperm into the reproductive tract of another planarian, protrudes from the genital antrum at the time of copulation.Both the sperm and the fertilized eggs leave through the genital pore.
• The head contains a number of sensory organs including chemoreceptors, photoreceptors and tactile receptors (Figure 9.4).
• The chemoreceptors, used to sense chemicals, are located in the auricles.In the planarian these are ear-like extension of the head containing ciliated grooves where the chemoreceptors are concentrated.
• The photoreceptors (for sensing light) are located in the eye cups (or eye spots). The eye cups exist as a cluster of photoreceptors shaped by a cup of pigmented cells.
• The Tactile receptors respond to touch and are concentrated in the head region. Planarian will respond stronger to being touched on the head than anywhere else on their body.
• The digestive system consists of a mouth that is located on the ventral surface near the middle of the body, not the head.
• Behind the mouth is the pharynx, a muscular structure that can be protruded though the mouth to suck up food.
• The intestine has 3 branches (tripartite). There is a single anterior branch and two posterior branches.
• Undigested material leaves the way it entered, through the mouth.
• Also located in the body is the lateral nerve cord. In a planarian these paired nerve cords run from the anterior to the posterior of the worm and lie along the ventrolateral sides of the animal. There are many cross branches connecting the two cords. Anteriorly the lateral nerve cords meet to form the cerebral ganglion, a primitive brain.
• Planarians will use the photoreceptors in their eye spots to sense the presence of light. They prefer shady spots and will move away from the light. This called negative phototaxis.
• The chemoreceptors in the worm’s auricles are used to detect a wide range of chemicals, including those associated with food. If they sense food is present, they will follow the concentration gradient until they reach the food This is called positive chemotaxis. If they sense toxins in the environment, they will use negative chemotaxis to move away from the toxin.
• If the worms are touched gently on the head, where their tactile receptors are concentrated, they will turn away from the stimulus.
• Interestingly planarian’s cells are in a constant state of turnover so the worms can grow and shrink daily based upon food supply.
• Planarians are masters of regeneration largely because many of their cells act as stem cells.
• In a planarian the stem cells are called neoblasts. These are the only cells that normally divide in the worm.
• Approximately 20% of the worm’s cells are neoblasts. The other 80% are one of the 12-15 histologically distinct differentiated cells.
• When the worm is cut, or pulls itself into 2 pieces, the area of the cut is first limited by a muscular contraction of the body wall at the cut site. This closes the would in about 10 minutes.
• This is followed by the formation a thin would epithelium across the cut surface. This takes an additional 20 minutes to account.
• Once the wound has healed neoblasts accumulate in the area and form a blastema.
• When a planarian is cut in half crosswise the anterior piece will regenerate a posterior half and the posterior piece will regenerate the anterior half (Figure 9.5A).
• If the worm is cut in half lengthwise each piece will regenerate the missing side.(Figure 9.5B).
• There are sometimes mistakes are made. If a thin segment from the middle part of the worm is taken it often forms with 2 anterior or 2 posterior blastemas. If two anterior blastemas form the regenerated will have two heads (Janus head). If 2 posterior blastemas for it will form 2tails (Janus tail) (Figure 9.5C).
• If a T cut is made in the anterior region the worms will generate 2 heads in the cut area that is still sharing the same posterior region (Figure 9.5D).