Cell Biology: Lysosomes and Organelles

Cytology

• Cytology is a biological science studying cell structure, histochemistry, and ultrastructure. It also focuses on immunohistochemistry and cell genetics.
• Key cellular components are studied including organelles and inclusions. Cell membranes are also an essential component for study, including their structure and function.

Cell Membrane

• Also known as plasmalemma or plasma membrane, this membrane has a trilaminar structure.
• It's composed of a dense layer of cytoplasm surrounding eukaryotic cells.
• It is made up of two layers of dense protein with phospholipids between them.
• The membrane's thickness ranges from 8 to 10 nm and can only be seen with electron microscopy (EM).
• The structure appears as proteins swimming in a lipid sea.
• Theories of cell membrane structure include linear theory, molecular theory, and the fluid mosaic theory

Mitochondrial Structures

• Mitochondria: plural term, not always a single mitochondrion, or sometimes called a chondriosome.
• Mitochondria are power centers of cells
• They are approximately the size of bacteria but can have varied shapes based on the cell type.
• They are membranous organelles that can be viewed under a light microscope using Janus green stain.
• They are enclosed by two membranes separated by an intermembranous space.
• Morphology may appear as short rods, threads or granules.
• The number of mitochondria in a cell can increase depending on the metabolic activity of that cell (ranging from 2500-3000).
• The structures comprise an outer membrane, inner membrane, cristae, matrix, intermembrane space, ribosomes, granules, and DNA.

Lysosome

• Lysosomes are membranous organelles, containing hydrolytic enzymes, that operate at an acidic pH.
• Lysosomes serve to break down cellular nutrients and destroy the cell after death.
• These structures are often implicated in diseases that cause break down of cell processes in living cells and have a diameter of 800nm.
• Derived from the Golgi complex. Viewed at the light microscope (LM) level, they appear as vesicles in basophils, eosinophils, and neutrophils.
• The EM image of a lysosome shows a homogenous, round vesicle with a thin membrane, containing proteins/enzymes.
• Lysosomes are formed from the rough endoplasmic reticulum (rER) and can fuse with other vesicles, like phagosomes in phagocytosis, or damaged organelles to form secondary lysosomes.

Endoplasmic Reticulum

• The rough endoplasmic reticulum (rER) is a membranous organelle that's a system of membranes, involved in protein and lipid production.

• It is marked by the presence of ribosomes on its surface- hence the name rough ER.

• It also appears as a three-dimensional branching and anastomosing network of membranes.

• The smooth endoplasmic reticulum (SER) is a membranous network lacking ribosomes.

• The SER's cisternae are more tubular and interconnected; they communicate with the rER and nuclear envelope.

• It's thought that smooth ER arises from the rough ER after it has lost its ribosomes.

• Functions for the SER include lipid and fatty acid synthesis, synthesis of cholesterol and steroid hormones, metabolism of cholesterol, drug detoxification, and muscle contraction.

Golgi Apparatus

• The Golgi apparatus, also called the Golgi network, is a membrane-bound organelle composed of plate-like sacs (cisternae) stacked together and connected to the endoplasmic reticulum (ER).
• The Golgi modifies, categorizes, and transports proteins and lipids originating in the ER.
• The Golgi apparatus is considered the "post office" of the cell for its role in packaging and transportation.
• Morphological differences exist depending on the cell type or physiological state.
• The Golgi apparatus is known to vary in shape (ranging from a dense body in intestinal cells to a net-like structure in neuron axon poles).
• Other forms include flattened saccules, vesicles, and cisternae.
• The Golgi apparatus has various functions including the concentration of proteins from transfer vesicle, polysaccharide synthesis ("hexosamine"), protein conjugation to polysaccharide ("glycoproteins"), concentration of hydrolytic enzymes, role in exocytosis, glycocalyx formation and membrane renewal.
• Gerl is an area near Golgi that may connect with ER, serving a function related to protein modification and lysosome production.

Ribosomes

• Free ribosomes are small electron-dense particles (15-20 nm in size) composed of rRNA and protein.
• They exist either on the rough endoplasmic reticulum (RER) or in the cytoplasm as polysomes.
• Ribosomes play a critical role in interpreting cellular information (from the nucleus) to synthesize proteins.
• Ribosomes are composed of two subunits, held together by mRNA.
• The protein and rRNA of ribosomes are synthesized in the nucleoli, then transported to the cytoplasm to form ribosomes.
• Ribosomes are also found in eukaryotic cells, but also in prokaryotic cells (mitochondria and chloroplasts).

Centrioles (Centrosomes)

• Centrioles are non-membranous structures found in most cells (except nerve cells and red blood cells).
• The term "centrosphere" describes the clear dark body that surrounds a centriole.
• A centrosome has two centrioles that are perpendicular to each other and composed of nine triplets of hollow microtubules together with associated fibrils and arms
• Centrioles are involved in cell division (mitosis), sperm tail formation, and cilia formation.

Components of the Cytoplasm

• Cytosol: The cytoplasmic matrix, a solution containing water, proteins, enzymes, nutrients, inorganic substances, and all cytoplasmic organelles.
• Cytoplasmic Inclusions: Non-living, transitory structures in the cell made of accumulated metabolites or deposits of varying natures. These primarily include secretory granules, stored food (glycogen in liver and muscle cells), and pigments (e.g., lipochrome, lipofuscin, melanin).

Cytoskeleton

• Cytoskeleton: A complex three-dimensional network of filamentous structures in the cytoplasm.
• Microtubules, intermediate filaments, and microfilaments are components that help in maintaining cell shape, transport, and cell division.
• Cytoskeleton structures link microtubules to other filament-like structures.

Cell Cycle

• The cell cycle is the sequence of events in a cell's life, divided into stages like gap 1, synthesis, gap 2, and mitosis.
• Different cells have different durations in their cell cycle, regulated by a mechanism that stops mitosis at times of unfavorable cell conditions.