Physiology of Finfish and Shellfish (2+1)

5.2.1. Introduction

A gland is a group of cells that produces and secretes or gives off chemicals. A gland selects and removes materials from the blood, processes them and secretes the finished chemical product for use somewhere in the body. Some types of glands release their secretions in specific areas. There are 2 types of glands. They are exocrine glands are glands that secrete their products (enzymes excluding hormones and other chemical messengers like Neurotransmitter - communicates to adjacent cells, Neuropeptide - a protein sequence which acts as a hormone or neurotransmitter, Pheromone - a chemical factor that triggers a social response in members of the same species ) into ducts (duct glands- Any tube or canal by which a fluid or other substance is conducted or conveyed. Or one of the vessels of an animal body by which the products of glandular secretion are conveyed to their destination). They are the counterparts to endocrine glands, which secrete their products (hormones) directly into the bloodstream (ductless glands).

Exocrine glands release their secretions through ducts into the external environment (Silverthorn 2004) or directly onto the exterior surface or an area contiguous with the exterior surface, not utilizing the blood to deliver the product. Most utilize ducts, but some glands classified as exocrine glands, such as the digestive glands in the intestine and the sebaceous glands of the skin (The sebaceous glands are microscopic glands in the skin that secrete an oily/waxy matter, called sebum, to lubricate the skin and hair of mammals) actually open directly onto the body surface. Typical exocrine glands include sweat glands, salivary glands, stomach, liver and pancreas. The chemical substances released by exocrine glands include sweat, digestive enzymes and tears (through tear ducts).

Endocrine glands on the other hand, release or secrete more than 20 major hormones or chemicals directly into the bloodstream where they can be transported to cells in other parts of the body. The endocrine system consists of a set of glands which are ductless. Hormones are chemical messengers. The hormones play important role in the chemical co-ordination in animal body. In fishes many endocrine gland are present which secrete many hormones and control various process of body. The endocrine system acts with nervous system to coordinate the body's activities. The endocrine system is slower than the nervous system because hormones must travel through the circulatory system to reach their target. But the nervous system conducts signals much quicker than the endocrine system.

Chemicals that interfere with the normal functioning of this complex system are known as "endocrine disruptors." Disruption of the endocrine system can occur in various ways.

For example, some chemicals may mimic a natural hormone, fooling the body into over-responding to the hormone. Other chemicals may block the effects of a hormone in parts of the body normally sensitive to it. Still others may directly stimulate or inhibit the endocrine system, leading to overproduction or underproduction of hormones. Certain drugs are used to intentionally cause some of these effects such as birth control pills.