One or more cells that makes and secretes an aqueous fluid

These modes differ by how much the cell is affected by the excretion

• Merocrine (no damage): products to be excreted are contained in vesicles and are secreted by exocytosis. No part of the gland is damaged or lost. (e.g. a sweat gland)
• Apocrine (Some membrane lost): A portion of the plasma membrane buds of the cell, continting the secrition. (e.g. lipid secretionfrommamary  gland
• Holocrine (Cell ruptures): In order to release the secretionthecell ruptures. (e.g. sebaceous glands of the skin)

• More numerous than endocrine glands.
• Secrete their products: onto body surfaces (i.e. skin) ((e.g. sweat)), into body cavities (pancreas), or into the lumen of an organ.
• Exocrine glands include mucous and goblet cells which are unicellular composed of simple columnar epithelial cells that function to secrete mucus using both apocrine and merocrine methods of secretion.
• Multicellular exocrine glands are composed of a duct and secretory unit.
• Other types of glands:
• sweat glands
• salivary glands
• mammary glands
• stomach-pyloric glands, gastric chief cells
• pancreas
• liver

Any chemical, irrespective of whether it is produced by a special gland or not, for export or cellular use that "controls andd regulates the activity of certain cells or organs"

• Old/Classic definition: A substance released by an endocrine andtransportedthrough the bloodstream to a target issue where it has effects on regulating the function of that tissue.

• Reproduction
• Growth and development
• Mobilizationof body defneses
• Maintenance of electrolyte, water, and nutrient balance
• Regulation of cellular metabolism
• Regulation of energy balance

• Autocrine Hormone Actions
• Auto - self.The hormon acts on the same cell type in which it was produced.
• E.g. Insulin - pancreatic islet B cells
• Intracine Hormone Actions
• intra - within. The hormone acts within the specific cell in which it was produced (within the cell's cytoplasm) without ever being released
• E.g. Adipocyte
• Paracrine Hormone Actions
• The hormonehasits effects locally on cells other thanthose in which the hormone was produced. These hormones areproducedwithinone tissue yet regulate a differenttissueofthesameorgan

Where the hormones primary target is the cell memberane.

• Activation/repressionofpre-existing cellproteins
• Rapidonsetofaction
• Rapid adaptationtochangeinthemilieu.
• Effects:
• Binding of hormoneto membrane receptor activates a cascade of second messengers
• 
  

1. A hormone (1st messenger) binds to the receptor.
2. The receptor activates G protein
3. G-protein activates the transmembrane protein adenylate cyclase.
4. Adenylate cyclase converts ATP to cAMP (2nd messenger.
5. cAMP activates protein kinases which triggers the responses of the target cell.

Primary action through specific intracellular receptors. Hormones are circulated to all tissues but only activate cells referred toa s target cells.

• Sent to nucleus
• Gene expression and new protein synthesis
• Relatively long latency of onset
• Organization of cell networks for complex functions
• Hormon response receptors

1. The steroid hormone diffuses through the plasma membrane and binds to an intracellular receptor.
2. The receptor-hormone complex enters the nucleus.
3. The receptor-hormone complex binds to a hormone response element-a specific DNA sequence.
4. This binding initiates transcription of the gene to mRNA.
5. The mRNA then directions protein synthesis.

• In most cases it is the free hormone that is biologically active.
• Hormones circulating bound to plasma protein:
• Cortiscosteroid-binding protein - cortisol, progesterone
• Sex-hormone-binding protein - testosterone, estradiol
• Vitamin D binding protein - vitamin D

• Hormones circulate to all tissues but onl

1. Low Ca² levels (Hypocalcemia) stimulates Chief cells in the parathyroid gland to release PTH.
2. PTH activates osteoclast in the bone and calcium and phosphate ions are released from the bone into the blood.
3. PTH stimulates the kidney to increase calcium reabsorption-->decrease urinary Ca loss.
4. PTH stimulates the kidney to increase the enzyme that makes the active form of vitamin D, 1,25(OH)2D.
5. ActiveVitamin D causes increased calcium absorption from food.

• Produced in the kidneys with the stimulation of the enzyme 1-alpha hydroxylase.
• Stimulates the gup to increase Ca absorption from diet and thus increase Ca in the blood.
• Also stimulates bones to breakdown to release calciu

1. Make D₃ in skin or ingest D₂ or D₃ in diet.
2. It binds to a Vitamin-D Binding Protein (DBP) and travels to the liver.
3. It is then hydroxylated in the liver at 25-position by 25-OHase to form 25(OH)D.
4. 25(OH)D travels bound to DBP to the kidneys where it is hydroxylated at 1-position by 1-aslphOHase to form 1,25(OH)₂D.
5. This then binds to DBP (some is free) and travels to target tissues wehre it has its affects through genomic and non-genomic mechanisms

• A disease of impaire bone mineralization in children (before epiphyses of long bones fuse) caused by too little Ca or P in the blood, vitamin D deficiency, genetic disorder of Ca, D, or P metabolism.
• Symptoms are bowing of legs and growth retardation.
• UV radiation therapy is often used for rickets to stimulate bone mineralization.
• Vitamin D Rickets (VDR) Mutations  - Type 1
• A defect in the 1-alpha hydroxylase enzyme which converts 25(OH)D into the biologically active 1,25(OH)₂D.
• Normal  25(OH)D, low 1,25(OH)₂D
• VDR Mutations - Type II (Genomic)
• Defectin the hormone receptor (HR) interaction
• Hormone binding defects
• Deficient nucleur localization
• Normal binding to receptor but abnormal binding of HR complex to DNA
• Normal 25(OH)D and normal or high 1,25(OH)₂D