Cell Secretion: Mechanisms, Types, and Glandular Organization
Cell Secretion: An Overview
Cell secretion is the process by which cells synthesize and release specific substances into their internal or external environment to perform various functions. This process is primarily carried out by epithelial cells, which form glandular epithelia. However, non-epithelial cells can also secrete substances such as the interstitial extracellular matrix, histamine, or antibodies. Some cells produce materials for their own use, and organs like the gonads, bone marrow, and lymphoid tissues also function as secretory organs by releasing their own cells.
Structure of Secretory Cells
The structure of secretory cells varies depending on their functional state. Generally, they are polygonal with membrane differentiations, including junctions for union and communication, as well as outgrowths and invaginations that regulate the entry and exit of materials. These cells are often grouped together to form glands.
The Secretory Cycle
The secretory cycle involves several phases:
- Resting Phase: Functional incorporation of precursors.
- Synthesis Phase: Production of the secretory product.
- Storage Phase: Accumulation of the synthesized product.
- Disposal Phase: Release of the stored product.
Mechanisms of Secretion Release
There are three primary mechanisms by which cells release their secretions:
- Merocrine: The product is released through the fusion of a membrane-bound vesicle with the plasma membrane. This is the most common mechanism.
- Apocrine: The secretion involves the release of cytoplasmic material along with the secretory product, effectively resulting in a “decapitation” of the cell.
- Holocrine: The entire cell is destroyed and released, along with its contents.
Types of Secreting Cells
Secreting cells can be classified based on the type of product they secrete and the destination of the secretion.
1. Based on the Secreted Product
a) Cells Secreting Proteins and Biogenic Amines
Structure: These cells have a highly developed rough endoplasmic reticulum (RER), free polyribosomes, and a Golgi apparatus near the nucleus. They contain basophilic secretion granules that are electron-dense.
Examples: Exocrine pancreas cells, endocrine cells of the adrenal medulla and paraganglia, and cells producing ACTH and STH in the pituitary gland.
b) Cells Secreting Glycoproteins
Structure: These cells have a moderately developed RER and a very clear Golgi apparatus. Their secretion granules are PAS positive and vary in electron density.
Examples: Goblet cells, mucous cells of salivary glands, and cells of the superficial lining of the stomach and endocervix.
c) Steroid-Secreting Cells
Structure: These cells contain numerous mitochondria with tubular cristae, a highly developed smooth endoplasmic reticulum (SER), lipid vesicles, and clumps of lipofuscin. Special stains and frozen sections are required to demonstrate their lipid content.
Examples: Endocrine cells of the adrenal cortex, ovaries, and testes.
d) Cells Secreting Special Products
These cells secrete products such as HCl, milk, sweat, tears, wax, and tallow.
2. Based on the Fate of the Secretion
a) Exocrine Cells
Exocrine cells release their products outside the body or into natural cavities.
b) Endocrine Cells
Endocrine cells release their secretions directly into the bloodstream or, occasionally, the cerebrospinal fluid. The target cells are distant, and hormones are transported via the blood.
c) Paracrine Cells
Paracrine cells release their products into the immediate surroundings without a duct system.
Examples: Cells producing local chemical mediators, growth factors, and cytokines.
d) Autocrine Cells
Autocrine cells secrete products that act on the same cell, stimulating themselves.
Example: Tumor cells.
Biogenesis and Organization of Exocrine and Endocrine Glands
Secretory epithelial cells form glands, which can be unicellular or multicellular and located on the surface or deep within tissues, forming a complete organ or part of an organ. They originate from embryonic epithelia lining following different paths:
- Differential secretory cells remain on the surface, aligned or dispersed between the coating.
- Epithelial ingrowth into surrounding tissues can occur, either retaining the link with the outside or losing the connection.
Exocrine Glands
- Cells Grouped and Arranged on the Surface: These cells perform both coating and secretion functions.
- Example: Surface epithelium of the stomach and endocervix.
- Example: Goblet cells and type II pneumocytes.
The term “acinus” usually refers to a group of exocrine cells that discharge their secretion into a central lumen. Depending on the released product, acini can be serous, mucous, or seromucous. The excretory duct system can be simple, branched, or coiled. Connective tissue stroma surrounds the capsule and forms partitions between lobes and lobules of the parenchyma.
Endocrine Glands
Endocrine glands generally consist of polygonal cells of epithelial appearance, grouped together.