Cell Biology: Endoplasmic Reticulum Overview

Smooth Endoplasmic Reticulum (SER)

• Appears as membrane-bound tubules and vesicles without ribosomes.
• Involved in lipid synthesis, including steroids, in cells like those in the adrenal cortex and testes.
• Plays a role in detoxification of drugs, particularly in liver cells.
• Aids in muscle contraction by regulating calcium ion (Ca2+) levels.
• Facilitates intracellular transport of molecules.
• Participates in glycogenolysis, a process that breaks down glycogen in liver cells.
• Contributes to mineral metabolism in the parietal cells of the stomach.

Rough Endoplasmic Reticulum (RER)

• Characterized by flattened cisternae with ribosomes attached to its membrane.
• Ribosomes on the RER synthesize proteins destined for the plasma membrane, extracellular environment, or specific intracellular organelles.
• Highly developed in active secretory cells, like pancreatic acinar cells, plasma cells, and neurons.
• Appears basophilic under a light microscope due to the presence of ribosomes containing RNA.
• Nissl bodies, the large basophilic structures in nerve cells, are composed of RER and free ribosomes.
• Exhibits membrane-bound cisternae with ribosomes and concentrated material within its lumen under an electron microscope.
• Often continuous with the outer membrane of the nuclear envelope.
• Functions as the site for production of membrane and secretory proteins.

Golgi Complex (Golgi Apparatus)

• Consists of a complex network of membrane vesicles and cisternae.
• Modifies and matures proteins and other molecules synthesized in the ER.
• Sorts proteins into specific vesicles for various cell functions.
• Well-developed in secretory cells that release proteins via exocytosis and cells synthesizing large amounts of membrane and membrane-associated proteins.
• Appears as a light-staining or unstained area near the nucleus under a light microscope.
• Shows up as a dark structure after silver or osmic impregnation.
• Located perinuclearly in nerve cells, between the nucleus and secretory surface in secretory cells, and scattered in the cytoplasm of liver cells.
• Exhibits a stack of flattened cisternae with dilated edges and small and large vesicles under an electron microscope.
• The stack of cisternae is curved, with convex and concave sides.
• Contains 3-8 cisternae per Golgi depending on the cell's activity.
• Smaller vesicles originating from the RER carry secretory material towards the formation surface.
• Functions in the condensation and packaging of proteins originating from the RER.
• Forms lysosomes by packaging hydrolytic enzymes.
• Adds carbohydrates to proteins, creating proteoglycans.
• Modifies protein molecules by adding specific chemical groups.

Lysosomes

• Small, membrane-bound vesicles rich in hydrolytic enzymes.
• Act as the digestive apparatus of the cell, degrading obsolete cellular components.
• Contain enzymes like nucleases, proteases, and phosphatases, which are active at an acidic pH (4.5-5.0).
• Maintains an acidic internal environment for hydrolysis through a hydrogen ion pump embedded in its membrane.
• Originate from the Golgi region, with their enzymes synthesized in the RER.
• Particularly abundant in cells with high phagocytic activity, such as macrophages and neutrophils.
• Appear spherical under a light microscope and can be identified using special histochemical methods for detecting their enzymes (e.g., acid phosphatase test).
• Show up as membrane-bound vesicles under an electron microscope.
• Primary lysosomes have a uniformly granular, electron-dense appearance.

Blood Supply of an Individual Bone

• Blood vessels enter and spread through the periosteum, the fibrous sheath surrounding the bone.
• Nutrient vessels penetrate the cortex of the bone, extending into the marrow.
• Nutrient canals provide passageways for these vessels.

Development of an Individual Bone

• The human skeleton is preformed in the fetus, but not initially as bone tissue.
• Two bone types are classified based on their preformed basis: membranous bones and cartilage bones.

Membranous Bones

• Outer skull bones and the clavicle are examples.
• Osteoblasts invade a membrane, forming a center of ossification where bone formation begins.
• Bone formation spreads from this center, eventually forming a complete bone plate.

Cartilage Bones

• Many bones, such as long bones, exist as cartilage models in the fetus.
• Cartilage models develop from mesenchyme during the fetal period, and bone gradually replaces most of the cartilage.

Endochondral Ossification

• Mesenchymal cells condense and differentiate into chondroblasts, forming a cartilage bone model.
• The cartilage calcifies with calcium salts in the mid-region of the model.
• Periosteal capillaries grow into the calcified cartilage, supplying its interior.
• These capillaries, along with osteogenic cells, form a periosteal bud.
• The capillaries initiate the primary ossification center, where bone tissue replaces most of the cartilage in the main body of the bone model.
• The shaft of a bone ossified from the primary ossification center grows as the bone develops.
• Secondary ossification centers appear in other parts of the bone after birth, forming the epiphyses.
• Chondrocytes in the middle of the epiphysis hypertrophy, and the bone matrix calcifies.
• Epiphysial arteries extend into the developing cavities, accompanied by osteogenic cells.

Synovial Joints

• Classified based on the number of axes of movement allowed.

Uni-Axial Synovial Joints

• Allow movement in one plane.

Hinge Synovial Joints

• Allow movement in one plane, similar to a door hinge.
• Examples include the elbow and knee joints.

Pivot Synovial Joints

• Permit rotation around a central axis.
• Examples include the proximal radioulnar joint and the median atlantoaxial joint.

Bi-Axial Synovial Joints

• Allow movement in two planes.

Condyloid Joints

• Have a rounded or curved surface in two directions.
• Example: metacarpophalangeal joint of the fingers.

Ellipsoid Joints

• Feature an elliptical convex surface articulating with an elliptical concave surface.
• Example: radiocarpal joint of the wrist.

Saddle Joints

• Possess one articular surface that is partly convex and partly concave, while the other surface is reciprocally concave-convex.
• Example: carpometacarpal joint of the thumb.

Multi-Axial Synovial Joints

• Allow movement in all three planes of space.

Ball-and-Socket Joints

• Characterized by a spherical head fitting into a concavity.
• Example: hip joint.

Stability of Joints

• Stabilized by several factors:

Bony Factor

• The shape of articulating surfaces contributes to joint stability.
• Example: the well-adapted fit between the head of the femur and the acetabulum in the hip joint.

Ligamentous Factor

• The strength of the fibrous capsule and surrounding ligaments play a crucial role in joint stability.
• They prevent excessive movement and protect against sudden stresses.

Muscular Factor

• Strong muscles surrounding a joint contribute to its stability.

Intra-articular Pressure

• The pressure within the joint capsule also helps stabilize the joint.

Factors Limiting Joint Movement

• Several factors limit the range of motion at a joint:

Tension in Ligaments

• Limits movement, as seen when attempting to extend the knee.

Contraction of Antagonistic Muscles

• Limits movement, as observed when flexing the hip with the knee extended.

Intermediate Filaments

• Fibrous proteins that provide structural support and mechanical stability to cells.
• Classified into six major classes:

Keratins (Cytokeratins)

• Present in all epithelial cells.
• Form large bundles (tonofibrils) that attach to junctions between epithelial cells.
• Accumulate during keratinization of skin epidermal cells.

Vimentin

• Found in most cells derived from embryonic mesenchyme.

Desmin

• A vimentin-like protein present in almost all muscle cells.

Glial Fibrillary Acidic Protein (GFAP)

• Found primarily in astrocytes, supporting cells of the central nervous system.

Neurofilament Proteins

• Major intermediate filaments of neurons.

Lamins

• Form the nuclear lamina associated with the inner membrane of the nuclear envelope.
• Help maintain nuclear shape, anchor chromatin to the nuclear envelope, and participate in nuclear assembly and disassembly during cell division.

Cytoskeleton

• Network of protein filaments that provides structural support, helps maintain cell shape, enables movement, and facilitates intracellular transport.

Ribosomes

• Abundant in virtually all cells, their number varying based on the level of protein synthesis activity.
• Tiny, non-membranous organelles (20 × 30 nm).
• Composed of two subunits: a small subunit and a large subunit.
• Functionally, a ribosome binds to a strand of mRNA and facilitates translation, the process of converting mRNA into a sequence of amino acids.
• Chemically, ribosomes consist of rRNA (ribosomal RNA) and proteins.