Endocrine signaling occurs through molecules secreted into the bloodstream. Anabolic hormones such as growth hormones are secreted preferentially during sleep. Noncolligative proteins are secreted by the liver into the bloodstream. Here, digestive enzymes are secreted and a pyloric gland adds further secretions. Proteins without signal peptides can also be secreted by unconventional mechanisms. It may also be secreted by human sebaceous glands. They are secreted as proenzymes and require extracellular activation. Phage particles are secreted from bacterial cells without lysis. PTH is secreted primarily by the chief cells of the parathyroid glands. However, most morphogens are secreted proteins that signal between cells. Glucagon is secreted from alpha cells, while insulin is secreted by beta cells. Pancreatic juice is secreted into the duodenum through duodenal papillae. The digestive enzymes however are mostly secreted as their inactive precursors, the zymogens. These enzymes are secreted in a fluid rich in bicarbonate. Digestive enzymes are secreted from the mouth to begin external digestion.