Cell Structure and Organelles

• The student should be able to explain cell structure, enumerate cytoplasm components, define cytoplasmic organelles, and identify cell/organelle inclusions.
• The student will also be able to compare organelles and inclusions, and membranous and non-membranous organelles.

Cell Structure

• The cell is the structural and functional unit of tissue. Cells consist of the nucleus and cytoplasm.

Cytoplasm Components

• Cytoplasm includes cytosol (cytoplasmic matrix), cell organelles, and inclusions.
• Cytosol/cytoplasmic matrix refers to the part of the cytoplasm found between organelles and inclusions.

Cell Organelles

• Cell organelles are intracellular living structures essential for cell functions
• Cell organelles are classified into membranous and non-membranous.

Membranous Organelles

• Membranous organelles are bounded by a membrane.
• Examples are the cell membrane, mitochondria, endoplasmic reticulum (rER, sER), Golgi apparatus, and lysosomes.

The Cell Membrane

• The cell membrane (plasmalemma or plasma membrane) is the outermost cell covering
• The cell membrane separates the cytoplasm from the extracellular environment.
• With a light microscope (LM), the cell membrane is too thin to be visible unless stained with PAS stain, which stains carbohydrates of the cell coat.
• With an electron microscope (EM), it has a trilaminar appearance with outer and inner dark layers separated by a middle light layer, with a thickness of 8-10 nm. This arrangement is called a unit membrane.
• With an electron microscope (EM), a fuzzy, electron-dense layer is found on the outer surface of the cell membrane, representing the cell coat (glycocalyx).
• The molecular structure includes a lipid bilayer (30%), proteins (60%) and carbohydrates (10%).
  • The lipid part is formed of phospholipid molecules which have hydrophilic (polar) heads that directs outwards and hydrophobic (non-polar) tails that directs inwards, and cholesterol molecules
  • The protein component is divided into intrinsic/integral membrane proteins (small molecules embedded in the lipid bilayer and large molecules extending across the cell and are called trans membrane proteins) and extrinsic/peripheral membrane proteins (loosely attached to both inner and outer surfaces of the cell membrane)
  • Cell coat (glycocalyx) is formed by the carbohydrate component (glycoprotein and glycolipids), and is responsible for cell recognition, protection and intercellular adhesion.
• Cell membrane functions include protecting the cell and exchanging materials (passive diffusion, active transport, selective transport, and bulk transport) between the cell and its environment.
  • Passive diffusion: Allows materials to enter according to the concentration gradient (e.g., water and gases).
  • Active transport: Requires energy and transports amino acids and fatty acids.
  • Selective transport: Uses specific receptors for certain substances on the cell membrane's outer surface.
  • Bulk transport involves endocytosis and exocytosis
• Endocytosis is the engulfing of material from the surrounding environment into the cytoplasm, which includes phagocytosis and pinocytosis
  • Phagocytosis: The membrane engulfs solid particles, forming a phagosome vesicle.
  • Pinocytosis: The cell engulfs a small amount of fluid, forming a pinocytotic vesicle.
• Exocytosis is the extrusion of the contents of a membranous vesicle from inside the cell to the extracellular fluid, adding the vesicle's membrane to the cell membrane.

Mitochondria

• Mitochondria are membranous organelles considered the powerhouses of the cell.
• Under a light microscope (LM), special staining is needed like iron hematoxylin (dark blue) or Janus green (green).
• With a light microscope (LM), their number varies with cellular activity, being high in liver cells and low in lymphocytes.
• With a light microscope (LM), their appearance is granules, rods, and filaments.
• With an electron microscope (EM), each mitochondrion is bounded by two membranes separated by an inter-membrane space
  • The outer membrane is smooth, and the inner membrane forms cristae/folds.
  • Cristae increase the surface area for ATP synthase and the respiratory chain.
  • The interior is filled with a matrix that contains electron-dense granules (calcium ion accumulation), DNA, RNA, and ribosomes.
• Mitochondria are sites of energy production (ATP), regulate calcium levels, and facilitate cell respiration.
• Self-renewal occurs through simple division, due to the presence of DNA.

Endoplasmic Reticulum

• Endoplasmic reticulum is a membranous cell organelle consisting of interconnected membranous channels called cisternae.
• There are two types: rough endoplasmic reticulum (rER) and smooth endoplasmic reticulum (sER).

Rough Endoplasmic Reticulum

• Rough Endoplasmic Reticulum (rER) appears as basophilic areas under a light microscope.
• The rER is abundant in protein-secreting cells such as fibroblasts, liver cells, and plasma cells.
• With an electron micrscope (EM), rER consists of parallel, flattened, communicating tubules forming a reticulum with a rough surface due to the presence of ribosomes, it connects to the cells nuclear envelope.
• Rough Endoplasmic Reticulum (rER) synthesizes proteins via attached ribosomes.

Smooth Endoplasmic Reticulum

• Smooth Endoplasmic Reticulum (sER) is not visible by H&E staining under a light microscope.
• With the electron microscope (EM), it is composed of branching and anastomosing tubules with a smooth membrane (no attached ribosomes).
• Smooth Endoplasmic Reticulum (sER) has variable shape and size.
• The Smooth Endoplasmic Reticulum (sER) functions in -Lipid synthesis (cholesterol and steroid hormones, like testosterone), drug detoxification (in liver cells), glycogen synthesis (in liver and muscle), and Calcium storage/sequestration (essential for skeletal muscle contraction).

Golgi Apparatus

• Golgi apparatus is a membranous organelle involved in secretion and well-developed in secretory cells.
• The site under a light microscope (LM): surrounds the nucleus in nerve cells and, in secretory cells like epididymis, is located between the nucleus and the free border.
• With H&E staining, it appears as an unstained area in the cytoplasm. This is called a negative Golgi image and can be seen in plasma cells
• With silver stain it appears as dark brown granules or fibrils.
• When viewed with an electron micrscope (EM), the Golgi apparatus consists of Golgi saccules, transfer vesicles, and secretory vesicles.
• Golgi stacks are flattened sacs with a saucer-like shape, and are arranged in stacks one above the other. Each stack has a convex (cis) face that is the immature face associated with rER and receives transfer vesicles budding from rER. It also has a concave (trans) face that is the mature face from which secretory vesicles bud.
• Transfer vesicles lie in association with the immature face. They are delivered from the endoplasmic reticulum (ER), usually carrying proteins and fuse into the periphery of the cis Golgi.
• Secretory vesicles originate as budding vesicles from the saccules at the mature face
  • These vesicles may be extruded outside the cell as secretory products or remain as lysosomes.
• The Golgi apparatus accumulates, concentrates, and packs secretory products which are synthesized by the ribosomes of the rER
  • These are then delivered to the cis face of the Golgi via the transfer vesicles.
• Golgi apparatus packages secretory products into membranes as secretory vesicles or lysosomes.
• Golgi apparatus plays an important role in maintaining the cell's membrane condition.
• Golgi apparatus does not synthesize proteins because it lacks ribosomes.

Lysosomes

• Lysosomes are spherical, membranous cell organelles containing hydrolytic enzymes for intracellular digestion
• Lysosomes cannot be seen by LM.
• The number of lysosomes varies depending on the phagocytic activity of the cell
  • They're numerous in macrophages.
• When viewed with an electron microscope (EM), lysosomes are spherical, membrane-bound vesicles.
• Primary lysosomes are freshly produced from the Golgi apparatus and appear as homogenous, spherical, dense bodies surrounded by a single membrane
• Secondary lysosomes form when a primary lysosome fuses with a phagosome. They appear as heterogeneous bodies circled by a membrane
• Types of secondary lysosomes (Fate of lysosomes):
  • Heterolysosomes: primary lysosome + phagosome = heterophagy.
  • Multivesicular body: primary lysosome + pinocytic vesicles= large vesicles containing numerous small vesicles.
  • Autolysosomes: primary lysosome+ autophagic vacuoles (mitochondria, fragments of rER, or other redundant organelles) = autophagy.
  • Residual bodies (telolysosomes): lysosomes that have completed digestion
• After a lysosome has digested its contents, the digested material is transported back to the cytoplasm
• Residual bodies are undigested material retained within vesicles.
• Fate of residual bodies:
  • They release their content outside the cell by exocytosis
  • Accumulate as age pigment as lipofuscin granules, in nerve cell, cardiac muscle.
• Lysosome functions:
  • Intracellular digestion of microorganisms via hydrolytic enzymes.
  • Removing old, nonfunctioning organelles
  • Transferring inactive hormones into active hormones
  • Protecting the body against microorganisms

Non-Membranous Organelles

• Non-membranous organelles include ribosomes, the cytoskeleton (filaments and microtubules), centrioles, and cilia/flagella.

Ribosomes

• Ribosomes are non-membranous cell organelles made of rRNA and proteins
• These are synthesized in the nucleolus and are increased in protein-synthesizing cells
• Ribosomes are responsible for the basophilia of the cytoplasm, viewed under LM.
• Viewed under the electron micrscope (EM), Ribosomes appear as electron-dense particles.
• Each ribosome is formed of a large and small subunit.
• Free ribosomes are free particles within the cytoplasm
• Attached ribosomes are attached to the outer surface of the rER by their large subunits.
• The free ribosomes synthesize proteins that are used by the cell (e.g., hemoglobin).
• The attached ribosomes to the rER synthesize proteins packed by the Golgi apparatus and then secreted outside the cells (hormones and enzymes).

Cytoskeleton

• The cytoskeleton is a network of tubules and filaments distributed in the cytoplasm.
• It provides shape for cells.
• It helps in movement of the organelles.
• It consists of filaments and microtubules.

Filaments

• Filaments are thread-like structures in the cytoplasm.
• There are three types:
  • Microfilaments: Thin filaments or actin filaments.
  • Thick filaments: Myosin filaments, present in muscles, not part of the cytoskeleton.
  • Intermediate filaments.

Microtubules

• Microtubules are formed of protein subunits called tubulin.
• They support the cell, determine its shape, form the mitotic spindle during cell division, and form centrioles, cilia, and flagella.

Centrioles

• Centrioles are non-membranous cell organelles with a centrosome, which consists of a pair of centrioles.
• Under alight microscope (LM), centrioles appear as short, rod-like cytoplasmic cylinders near the nucleus in a pale area called the centrosome.
• Under an electron microscope (EM), they Appear as 2 cylindrical structures arranged at right angles to each other in resting cells
  • A longitudinal section each centriole appears as a hollow cylinder.
  • The wall of each cylinder consists of 27 microtubules arranged in 9 bundles called triplets.
• Centriole functions involve forming the mitotic spindle during cell division and basal bodies of cilia and flagella.

Cilia

• Cilia are motile, hair-like processes covered by the cell membrane that extend from the free surface of certain cells (e.g., trachea).
• Under a light microscope (LM), they appear as fine acidophilic striations extending from the free surface of ciliated cells.
• Under and electron microscope (EM), each cilium is formed of a basal body, a shaft, and rootlets.
• The basal body's structure is identical to that of a centriole (27 microtubules arranged as 9 triplets).
• The shaft, or axoneme, consists of 9 bundles (18 microtubules), bundles consisting of two microtubules (doublets).
  • The central part of the shaft contains 2 microtubules called singlet. It is covered by a cell membrane.
• Rootlets anchor the basal body to the cytoplasm.
• Cilia beat over the surface of epithelium in one direction, pushing fluid or small particles.

Flagella

• Flagella share the same structure as cilia, but are single and longer.
• They form the tail of sperm and help in sperm movement.

Cytoplasmic Inclusions

• Cytoplasmic inclusions are non-living, temporary, non-essential components of the cytoplasm
• They may be products of cell metabolism or substances taken into the cell from the outside.
• These includes stored food, pigments, and crystals.

Stored Food

• Glycogen and fats are stored as inclusions
• Glycogen appears as unstained vacuoles in H & E stained sections
  • Stains purple with PAS and red with Best's carmine.
  • Under the electron microscope (EM), glycogen appears as electron-dense rosette-shaped granules (e.g., in liver cells).
• Fats or lipids can be stained with Sudan III (orange) and Sudan black (black)
  • Under the electron microscope (EM), they appear as large round electron-dense droplets without a limiting membrane.

Pigments

• Pigments are either exogenous or endogenous
  • Exogenous pigments are introduced into the body from the outside such as carotene, dust particles, carbon, and tattoo marks
  • Endogenous pigments are synthesized inside the body, such as hemoglobin (carries O2 & CO2 in RBCs), melanin (gives skin its color and protects from UV rays), and lipofuscin pigment (waste product in the cardiac muscle and nerve cells, which accumulate and increase with age).

Crystalline Inclusions

• Crystalline inclusions are composed of calcium oxalate, phosphates, and carbonates, and are usually found in pathological conditions/ not commonly found in cells.

Organelles vs. Inclusions

• Organelles: permanent, living/vital, essential, membranous or non-membranous,
• Inclusions: temporary, non-living, non-essential, and include stored food, pigments, and crystals.