Final answer:
The secretory chain, known as the Secretory Component (SC), is produced by cells with significant secretory activity such as the salivary glands and immune system, and is involved in the secretion of IgA. The production of digestive secretions is regulated by hormones like secretin and cholecystokinin. The Signal Hypothesis explains the synthesis and direction of secretory proteins within the RER.
Step-by-step explanation:
The secretory component of secretory IgA involves a polypeptide chain known as the Secretory Component (SC), which is distinct in amino acid composition from the J chain as well as from the light and heavy chains of antibodies. Cells with extensive secretory functions, such as those in the salivary glands or the immune system, possess a significant amount of Golgi apparatus to handle the production and secretion of enzymes and antibodies. The secretory process of proteins involves the synthesis of an initial polypeptide, which becomes enclosed within the cisternae of the rough endoplasmic reticulum (RER) for secretory proteins.
In the digestive system, particularly in the duodenum, the release of hormones like secretin and cholecystokinin (CCK) triggers the production of digestive secretions from the liver, pancreas, and gallbladder to aid in digestion. The hormone secretin specifically stimulates the pancreas to produce an alkaline bicarbonate solution to neutralize the acidity of chyme.
The synthesis of secretory proteins is explained by the Signal Hypothesis, which suggests that secretory protein genes encode an amino-terminal signal peptide that leads the polypeptide to the RER where it eventually becomes a mature, secreted polypeptide.