المحاضرة الثانية هستو (قبل التعديل)

Endoplasmic Reticulum (ER)

• A membranous organelle forming a network of interconnected channels called cisternae that extends from the nuclear membrane to the cell membrane.
• Two types: Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).
• RER: Cisternae covered by ribosomes and polysomes giving it a rough, granular surface.
• SER: No attached ribosomes, giving it a smooth appearance.
• Both types are continuous with each other.

Rough Endoplasmic Reticulum (RER)

• Concerned with the synthesis of proteins, primarily for secretion outside the cell.
• Prominent in cells specialized for protein secretion, such as fibroblasts (synthesize collagen), plasma cells (synthesize antibodies).
• Light Microscopy (L/M): Basophilic due to the attached ribosomes.
• Electron Microscopy (E/M): Parallel cisternae continuous with the outer nuclear membrane, flattened in shape, and have attached ribosomes and polyribosomes.

Smooth Endoplasmic Reticulum (SER)

• Endoplasmic reticulum cisternae without attached ribosomes.
• Continuous with RER.
• Prominent in cells specialized for lipid biosynthesis, such as cells of the adrenal glands, testis, and ovary.
• L/M: Not basophilic.
• E/M: Cisternae are either tubular or vesicular in shape (not flattened). Smooth surface with no attached ribosomes.

Functions of the Rough Endoplasmic Reticulum (RER)

• Segregation of proteins synthesized by attached ribosomes.
• Initial glycosylation of proteins (addition of carbohydrates to the synthesized protein).

Functions of the Smooth Endoplasmic Reticulum (SER)

• General: Synthesizes phospholipids of the cell membrane.
• Liver cells: Involved in glycogen metabolism, detoxification of certain drugs and toxic substances (e.g., alcohol).
• Adrenal glands, Testis, and Ovary: Lipid biosynthesis (e.g., synthesis of steroid hormones like cortisone, male and female hormones).
• Muscle cells (Sarcoplasmic reticulum): Releases calcium ions during contraction and reuptakes them during relaxation.

Golgi Apparatus

• A membranous organelle involved in the secretion of proteins synthesized by the RER.
• Functions:
  • Concentration, packaging, and storage of proteins to be secreted.
  • Glycosylation of proteins (begins in RER and is completed in Golgi saccules).
  • Formation of primary lysosomes.
  • Synthesis of many glycosaminoglycans (GAGs) that form the extracellular matrix (ECM) of connective tissue.
• L/M: H&E staining: Unstained area near the nucleus (negative Golgi image). Silver staining: Appears dark brown.
• E/M: 4-10 saccules forming a stack. Each stack has two surfaces:
  • Cis face (immature face, convex): Receives transfer vesicles carrying proteins from the RER.
  • Trans face (mature face, concave): Releases secretory vesicles outside the cell.
• Transfer vesicles: Small vesicles that carry proteins from the RER to the immature face of the Golgi.
• Secretory vesicles: Large vesicles arising from the mature face, either secreted outside the cell (exocytosis) or remain in the cytoplasm as primary lysosomes.

Lysosomes

• Membranous organelles containing about 40 hydrolytic enzymes (acid hydrolases).
• Known as "suicide bags" due to their destructive potential.
• Acid hydrolases: Optimal activity at acidic pH (~5.0). Examples include proteases and phosphatases.
• Formation: Synthesized and segregated in the RER, transferred to the Golgi apparatus to form primary lysosomes.
• Site: Present in the cytoplasm of all cells. More abundant in cells with high phagocytic activity (macrophages).
• L/M: Enzyme histochemistry: Demonstration of acid phosphatase enzyme activity (marker of lysosomes).

Endoplasmic Reticulum (ER)

• A membranous network that extends from the nuclear membrane to the cell membrane.
• Formed of interconnected channels called cisternae (reservoirs).
• Two types: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

Rough Endoplasmic Reticulum (RER)

• Cisternae are covered by ribosomes and polysomes, giving it a rough or granular surface.
• Involved in protein synthesis, mostly for secretion outside the cell.
• Prominent in cells specialized for protein secretion, such as fibroblasts (synthesizing collagen) and plasma cells (synthesizing antibodies).
• Parallel cisternae are continuous with the outer nuclear membrane.
• Cisternae are flattened in shape.
• Appears basophilic under a light microscope due to the attached ribosomes.

Smooth Endoplasmic Reticulum (SER)

• No attached ribosomes.
• Continuous with the RER.
• Cisternae are either tubular or vesicular in shape (not flattened).
• Prominent in cells specialized for lipid biosynthesis, such as cells of the adrenal glands, testis, and ovary. Also found in liver cells and muscle cells (sarcoplasmic reticulum).
• Not basophilic under a light microscope.

RER Functions

• Segregates proteins synthesized by attached ribosomes.
• Initial glycosylation of proteins (addition of carbohydrates to synthesized protein).

SER Functions

• Synthesizes phospholipids of the cell membrane.
• In liver cells:
  • Glycogen metabolism.
  • Detoxification of drugs and toxic substances (e.g., alcohol).
• In adrenal glands, testis, and ovary:
  • Lipid biosynthesis (e.g., synthesis of steroid hormones, cortisone, male and female hormones).
• In muscle cells (sarcoplasmic reticulum):
  • Releases calcium ions during contraction and reuptakes them during relaxation.

RER Products

• Proteins are transported to the Golgi apparatus for secretion outside the cell.
• Form primary lysosomes.
• Form integral proteins of the cell membrane.

Golgi Apparatus

• A membranous organelle involved in protein secretion synthesized by the RER.

Golgi Apparatus Functions

• Concentration, packaging, and storage of proteins for secretion.
• Glycosylation of proteins (begins in the RER and completes in Golgi saccules).
• Forms primary lysosomes.
• Synthesis of glycosaminoglycans (GAGs) that form the extracellular matrix (ECM) of connective tissue.

Golgi Apparatus Appearance

• Under light microscopy (H&E stain): Unstained area near the nucleus (negative image).
• Under light microscopy (Silver stain): Appears dark brown.
• Under electron microscopy: 4-10 saccules forming a stack. Each stack has two surfaces:
  • Cis face: Immature face (convex), receives transfer vesicles carrying proteins from the RER.
  • Trans face: Mature face (concave), releases secretory vesicles outside the cell.

Golgi Apparatus Vesicles

• Transfer vesicles: Small, carry proteins from the RER to the immature face of Golgi.
• Secretory vesicles: Large, arise from the mature face and either:
  • Secreted outside the cell (exocytosis).
  • Remain in the cytoplasm as primary lysosomes.

Lysosomes

• Membranous organelles containing about 40 hydrolytic enzymes (acid hydrolases).
• Called suicide bags.
• Acid hydrolases are optimally active at acidic pH (~5.0). Examples: proteases and phosphatases.
• Synthesized and segregated in the RER.
• Transferred to the Golgi apparatus to form primary lysosomes.

Lysosome Characteristics

• Present in the cytoplasm of all cells.
• More abundant in cells with great phagocytic activity (macrophages).

Lysosome Appearance

• Light microscopy: Enzyme histochemistry demonstrates acid phosphatase enzyme activity (marker of Lysosomes).