The freshwater polyp Hydra is a small organism known for its regenerative capabilities, able to regrow its entire body from small tissue fragments. It has a tubular body with a foot at one end and a mouth surrounded by tentacles at the other. This mouth structure is the hypostome, a specialized region that appears as a simple mound. Its study offers insights into biological processes like tissue organization, feeding mechanics, and body patterning.
What is the Hydra’s Hypostome Made Of?
The hypostome is a cone-shaped structure at the apical end of the Hydra, serving as the animal’s mouth. It is composed of two primary tissue layers: an outer epidermis and an inner gastrodermis, separated by an extracellular matrix called the mesoglea. These layers are made of epitheliomuscular cells, which have a dual function in forming the creature’s covering and enabling movement. The arrangement of these cells allows the hypostome to change shape, particularly during feeding.
Within these layers are specialized cell types. The epidermis contains a high concentration of nerve cells that form a dense network, including a distinct nerve ring above the tentacles. This nerve ring is a more concentrated structure compared to the diffuse nerve net found elsewhere and helps coordinate hypostome activities. Sensory cells are also present, providing the animal with information about its environment.
The gastrodermis, lining the internal cavity, is rich in gland cells that secrete mucus and digestive enzymes to facilitate ingestion. Both the external and internal surfaces of the hypostome feature unique cellular structures. The outer surface has sensory hairs and protrusions around the mouth, while the inner surface is covered in microvilli and flagella to help manipulate food particles. This cellular architecture makes the hypostome a specialized region for sensory input and physiological processes.
The Hypostome’s Role in Hydra’s Mealtime
The feeding process begins after the tentacles capture and immobilize a food item, such as a small crustacean, using their stinging cells (nematocysts). The release of chemicals from the captured prey, specifically the peptide glutathione, triggers a feeding response in the tentacles and the hypostome. The tentacles bend inward, bringing the prey towards the mouth.
In response to these chemical cues, the hypostome shows great flexibility. It widens significantly, stretching to accommodate prey that can be nearly as large as the Hydra itself. This mouth opening is a coordinated event, controlled by the dense network of nerve cells in the hypostome region. The peptide Allatotropin, for instance, has been shown to induce the opening of the hypostome, mimicking the effect of food.
Once the mouth is open, gland cells within the gastrodermis secrete mucus, which lubricates the prey and aids in swallowing it into the gastric cavity. After digestion is complete, the hypostome reopens to expel any indigestible materials, such as the exoskeletons of crustaceans. This entire sequence showcases the hypostome’s role in nutritional intake and waste management.
Master Regulator: The Hypostome in Hydra Regeneration and Asexual Reproduction
The hypostome also functions as a “head organizer,” a concept identified through grafting experiments. These studies showed that a small piece of hypostome tissue, when transplanted into another Hydra’s body, can induce the formation of a new head and body axis. This capacity is key to Hydra’s regenerative abilities. When a Hydra is cut, the segment closest to the original head regenerates a new hypostome, which then directs the patterning of the rest of the body.
This organizing function is also part of Hydra’s primary mode of reproduction: budding. A new bud begins as an outpouching of the body wall, and a new head organizer forms at its tip. This new organizer establishes the oral-aboral axis for the new individual. It dictates the formation of a hypostome and tentacles, ensuring a complete offspring develops before detaching from the parent.
The molecular basis for the hypostome’s organizing ability lies in the Wnt signaling pathway. Genes associated with this pathway are expressed in the hypostome and at the tip of a regenerating head or a developing bud. This pathway directs surrounding cells what to become, establishing the polarity and structure of the animal. The continuous activity of the Wnt pathway in the hypostome maintains the organism’s form.
Maintaining the Mouth: Cellular Renewal in the Hypostome
The Hydra’s body is in a constant state of flux, with cells continuously dividing in the body column and being displaced toward the foot and the head. This process of cell turnover means that the tissues of the hypostome are constantly being renewed. Epithelial cells from the upper body column migrate into the hypostome region. There, they differentiate to replace older cells that are sloughed off from the tip.
This steady state of cell production and loss is regulated. As epithelial cells are displaced into the hypostome, their developmental fate is altered by organizer signals from the hypostome itself. They stop dividing and differentiate into the specialized cells that characterize the head region. Interstitial stem cells between the epithelial cells also differentiate into nerve cells and other cell types as needed.
The continuous proliferation, migration, and differentiation of cells allow the hypostome to persist and act as the command center for the organism. This constant renewal is linked to its regenerative prowess and its ability to orchestrate the formation of new individuals.