Invertebrate endocrine systems

By | 2013-07-24

Invertebrate animals rely heavily upon neuroendocrine control systems as opposed to classical endocrine systems. However, as invertebrates have become more and more complex, both in structure and physiology, so have the physiological functions controlled by the endocrine system. This is partly because higher invertebrates, e.g. molluscs, have better-developed circulatory systems than lower invertebrates such as flatworms, and they therefore have a more efficient mechanism for distributing the necessary hormones. A brief summary of endocrine function in selected invertebrate phyla follows.

Coelenterates

Coelenterates, such as Hydra, have cells which secrete substances involved with reproduction, growth and regeneration. If the head of a Hydra is removed a peptide molecule is secreted by the rest of the body. This substance is called the ‘head activator’. Its effect is to cause the remainder of the body to regenerate the mouth and tentacles which make up the head region.

Platyhelminthes

In a similar manner to the coelenterates, substances have been found in the flatworms which are involved in the processes of regeneration. It has also been suggested that hormones are involved in osmotic and ionic regulation as well as in reproductive processes.

Nematodes

Evidence for a role for neuroendocrine control in the nematodes has come from those members of the phyla which are parasitic, As with many parasitic organisms, different stages of the life cycle are quite often completed in different hosts. This means, therefore, that developmental changes in the nematode must coincide with the movement of the nematode to a new environment or new host. Some nematode worms may need up to four moults of their cuticle in order to complete their life cycle. It has been established that nematodes have neuroendocrine secretory structures associated with their nervous systems, in the ganglia in the head region and some of the nerve cords which run the entire length of the body. It is possible that changes in the immediate environment of the worm are the triggers for activity in this neuroendocrine control system and the consequent changes in physiology which are necessary for these animals.

Annelids

Annelid worms, such as polychaetes (e.g. Neris), oligochaetes {e.g. Lumbricus) and the hirudinae (e.g. leeches) show a reasonable degree of cephalization. The brain has been shown to contain large numbers of neurosecretory neurons, and these animals also have a reasonably well developed circulatory system. Therefore, the importance of endocrine control in these animals is enhanced. The endocrine systems in these animals are associated with such activities as growth and development, regeneration and development of the reproductive system. A good example of this is the rnetamorphic transformation of adult polychaete worms, known as epitoky, whereby some body segments become reproductive structures. Some of these transformed body segments break off altogether to become free-living organisms, as seen in some annelids. This process is known as stolonization. Epitoky is controlled by a neuroendocrine control system, but the hormone which is released actually inhibits this process ― Therefore, during epitoky the levels of this hormone must be reduced, otherwise epitoky will be inhibited. How this works is unclear, but secretions may be regulated by environmental cues as some worms have breeding seasons. Similarly, there is a hormone that inhibits the development of gametes, and it is possible that it is the same hormone that controls epitoky.

Molluscs

It is known that there are large numbers of neuroendocrine cells in the ganglia which constitute the central nervous system of molluscs, particularly in the snails. It is also likely that there are some classical endocrine organs in addition to the neuroendocrine cells. Many of the substances released appear to be protein-like. They control many functions, such as osmotic and ionic regulatory processes, the regulation of growth and reproduction. Much of what is known about the molluscan endocrine system has come from studies on the reproductive systems of these animals. Reproduction in molluscs is complicated since many are hermaphroditic, i.e. are male and female simultaneously. A further complication is that some species display protandry, whereby the male gametes appear before the female gametes. Some of the substances utilized as hormones have been identified; for example, there is a hormone which stimulates the release of eggs from gonadal tissue and egg laying. In the cephalopods, where there are distinct males and females, reproduction is also under endocrine control. In this case, though, the role of classical endocrine glands, particularly structures called the optic glands, is thought to be important. The optic glands are thought to secrete several hormones which are required for sperm and egg development.

Crustaceans

Like the invertebrates described so far, the crustaceans have a predominantly neuroendocrine control system. However, they also have classical endocrine organs. The range of physiological functions controlled by the endocrine system is more varied and includes such aspects as osmotic and ionic regulation, heart rate regulation, blood composition, growth and moulting, Neuroendocrine control is best developed in the malacostrans (e.g. crabs, lobsters, shrimps). The neuroendocrine cells of these crustaceans are located in three major regions:

(i) the sinus gland complex, sometimes called the X organ-sinus gland complex ― this receives neuroendocrine axons from the ganglia of the head and optic lobes and is located in the eye stalks;

(ii) the postcommissural organ ― again, this receives axons from the brain which terminate at the beginning of the esophagus;

(iii) the pericardial organ ― this receives axons from the thoracic ganglia, and is located in close proximity to the heart.

In addition, there are a small number of classical endocrine organs. The Y organ is a pair of glands, located in the thoracic region of the animal on the maxillary or antennary segment. The secretions of the Y gland are thought to be involved in the process of molting ― the periodic shedding of the exoskeleton as the animal grows. The mandibular gland located near to the Y gland is also thought to serve an endocrine function. There are also endocrine structures associated with the reproductive organs of these animals. For example, the androgenic gland is believed to be involved in the development of the testes and sperm production.

An example of endocrine control in crustaceans is the ability of many crustaceans to change color. The advantage of this is that it enables them to assume the color of their background, thus helping them to avoid detection by predators, for example. The ability to change color varies from species to species; for example, some can only alter the shade of their coloration, e.g. from light to dark, whilst others can assume up to six different background colors. Color change is effected by the dispersal of pigments in cells called chromatophores. These cells are mainly located in the body covering, but they may also be found in some deeper organs. A variety of endocrine substances have been shown to alter chromatophore function. It is known, for example, that the sinus gland complex can release a pigment-concentrating hormone and a pigment-dispersing hormone, both of which are peptide hormones. It is also thought that some hormones are released from the pericardial organ which influence chromatophore function.

Insects

Insects are similar to crustaceans in the wide variety of physiological functions that are controlled by endocrine organs (in comparison to other invertebrate phyla) and the predominance of neuroendocrine control systems. There are three main groups of neuroendocrine cells in the nervous system of insects:

(i) the median neurosecretory cells, which send axons down to the paired corpora cardiaca which act as storage and release sites for the neurohormones;

(ii) the lateral group of neurosecretory cells, which also send their axons down to the corpora cardiaca;

(iii) the subesophageal neurosecretory cells, which send their axons down to the corpora allata, which are classical endocrine glands.

Insects also possess classical endocrine glands. The corpora allata are an example of classical endocrine organs, even though they are under neural control as described above. Another classical endocrine gland of importance is the prothoracic gland, which is located in the thorax of the more advanced insects and in the head region in less advanced insects.

Growth is one of the physiological functions controlled by the endocrine system in insects. The growth of insects occurs in a stepwise fashion that requires a series of molts of the exoskeleton. In some insects, e.g. the collembola, molting continues even when the animal has reached maturity. In the majority of insects, however, there is a final molt when the juvenile is transformed into the adult. This process is known as metamorphosis. In some insects, e.g. the cockroach, the juvenile stages are similar in appearance to the adult animal. This type of development is called hemimetabolous development. In other insects, such as butterflies and moths, the juvenile stages are quite different from the adult animal, and this is known as holometabolous development. The process of molting involves several stages. First, the epidermal cells of the old cuticle withdraw and increase in number by the process of mitotic cell division. A new cuticle is produced whilst the old cuticle is digested and absorbed by the epidermal cells. The new insect then appears from the old cuticle, a process known as ecdysis, and, finally, the new cuticle hardens. This whole process is under endocrine control. The median neurosecretory cells produce a peptide molecule called prothoracicotropic hormone (PTTH, of which there are several types), which is released from the corpus cardiaca. This in turn stimulates the prothoracic gland to release the hormone ecdysone, which causes moulting. Another hormone, known as juvenile hormone, which is responsible for controlling metamorphosis is released from the corpus allata. High levels of this hormone result in the development of an immature animal, which indicates that there must be a close association between ecdysone and juvenile hormone if development is to proceed correctly ― juvenile hormone secretion must be inhibited before the final molt in animals undergoing metamorphosis, although how this is achieved is not fully understood.