Operating Systems Concepts

• Cells serve as the fundamental building blocks of all living organisms.
• Tissues consist of groups of cells that share similar structures and functions.
• The tissues within the human body are categorized into four primary types: epithelial, connective, muscular, and nervous.

Epithelial Tissues

• Epithelium comprises highly cellular, tightly packed tissue with minimal extracellular matrix.
• It covers all body surfaces exposed to the external environment and lines internal body cavities.
• Epithelial tissues perform protective, secretory, absorptive, and sensory reception functions.
• Polarity is a characteristic of epithelium, featuring an apical surface (near the top) and a basal surface (near the bottom).
• Epithelium consists of closely packed cells held together by strong junctions.
• Connective tissue provides support to the epithelium.
• It exhibits a high regeneration rate.
• Epithelium covers body surfaces, lines body cavities, and forms glands.
• Epithelial tissue is avascular, lacking blood vessels.
• Nerve fibers can traverse between epithelial cells.

Epithelial Tissue Classification:

• Based on structure, epithelial tissues are classified as:
  • Covering and lining epithelium.
  • Glandular epithelium.
  • Special epithelium.
• Based on function, epithelial tissues are classified by:
  • Covering and lining of surfaces.
  • Absorption.
  • Secretion.
  • Sensation.

Classification of Covering and Lining Epithelium Based on Structure

• Number of Layers:
  • Simple: Contains a single layer of cells.
  • Stratified: Contains multiple layers of cells.
• Shape of Cells:
  • Squamous.
  • Cuboidal.
  • Columnar.
• Each Epithelium tissue receives two names:
  • The first name denotes the number of cell layers present.
    • Simple: One layer.
    • Compound (Stratified): More than one layer.
  • The second name describes the cell's shape:
    • Squamous.
    • Cuboidal.
    • Columnar.

Simple Epithelium

• Simple epithelium consists of a single layer of cells.
• Every cell is in direct contact with the basement membrane.
• It separates from the underlying connective tissue.
• Simple epithelium functions in absorption and filtration.
• Three types of simple epithelium are based on cell shape:
  • Simple squamous epithelium.
  • Simple cuboidal epithelium.
  • Simple columnar epithelium.

Simple Squamous Epithelium

• Squamous epithelial cells appear scale-like with flattened nuclei.
• This epithelium lines blood vessels (endothelium), the alveoli of the lungs, body cavities (mesothelium), and renal corpuscles of the kidney.
• It facilitates the exchange of nutrients, wastes, and gases.

Simple Cuboidal Epithelium

• Cuboidal cells possess central spherical nuclei.
• It covers ovaries, lines kidney tubules, and glandular ducts (salivary, thyroid, pancreas, liver).
• It functions in secretion (in glands) and reabsorption (in kidneys).

Simple Columnar Epithelium

• A single layer of tall, slender columnar cells with oval-shaped nuclei located in the basal region.
• Microvilli and cilia may be present.
• Goblet cells are often found in this layer.
• This epithelium is located in the gastrointestinal tract, gallbladder, and uterus.
• Functions include nutrient absorption and mucus production.
• Ciliated type found in the respiratory passage as the trachea.

Pseudostratified Epithelium

• This epithelium is simple yet appears multilayered due to varying nuclei levels.
• All cells attach to the basement membrane, but not all reach the lumen.

Compound Epithelium (Stratified Epithelia)

• Consists of two or more cell layers.
• Only the basal layer connects to the basement membrane.
• The basal layer houses stem cells vital for regeneration.
• Basal layer cells typically display a cuboidal shape.
• Stratified epithelia primarily fulfill a protective role.
• The classification of stratified epithelia is based on the shape and structure of the surface cells (superficial layer).

Stratified Squamous Epithelium

• Types include:
  • Stratified Squamous Keratinized Epithelium.
  • Stratified Squamous Non-Keratinized Epithelium.

Stratified Squamous Keratinized Epithelium

• In keratinized epithelia, the top layers are dead, losing nuclei and cytoplasm.
• Possesses the tough protein keratin, rendering the epithelium waterproof.
• The basal layer transitions from cuboidal to columnar.
• The uppermost layers consist of squamous epithelium.
• Intermediate layers contain polyhedral cells with round nuclei.
• Examples include palms and soles (skin).

Stratified Squamous Non-Keratinized Epithelium

• It resembles keratinized epithelium without the keratin layer.
• Found lining the oral cavity and esophagus.

Stratified Cuboidal Epithelium

• It includes cuboidal cells arranged in 2-3 layers.
• It lines esophageal glands, the pancreas, and sweat glands.
• Its primary function is protection.

Stratified Columnar Epithelium

• Composed of two or more layers of cells.
• Superficial cells are elongated, while deeper (basal) layers are cuboidal.
• Found in parts of the pharynx.
• It is rare within the human body.

Transitional Epithelium

• A specialized type of stratified epithelium lining the majority of urinary passages (ureters, bladder).
• The surface cells appear domelike when the organ is empty.
• The surface cells stretch and flatten when the organ is full.

Glandular Epithelium

• Epithelial cells specializing in secretion form it.
• It originates from epithelium.
• During fetal development, glands arise from covering epithelium via cell proliferation and invasion of adjacent connective tissue, followed by differentiation.
• Glands are categorized based on the presence/absence of a duct system into:
  • Exocrine glands.
  • Endocrine glands.
  • Paracrine glands.
  • Mixed glands.

Exocrine Glands

• They secrete products through ducts connected to a surface like salivary glands.

Endocrine Glands

• They lack ducts.
• They secrete products into the bloodstream to reach target cells.
• The products of endocrine glands are hormones like adrenal and pituitary glands.

Paracrine Glands

• Similar to endocrine glands.
• Secretions reach target cells through diffusion or immediately subjacent connective tissue.

Mixed Glands

• Possess both exocrine and endocrine functions, such as the pancreas.
• Glands are classified by the number of secretory cells:
  • Unicellular glands.
  • Multicellular glands.

Unicellular Glands

• Consist of isolated glandular cells.
• Goblet cells are unicellular, mucus-secreting cells among columnar cells.

Multicellular Glands

• Composed of clusters of cells.
• These form glands of the body, such as salivary glands.

Glands Classified by Nature of Secretion:

• Mucus glands.
• Serous glands.

Mucus Glands

• Have wide lumen acini.
• Consist of tall columnar cells.
• They features pale cytoplasm (foamy appearance).
• Periphery flattened dense nuclei.
• Located in sublingual glands.

Serous Glands

• The secretion is watery fluid.
• Have narrow lumen acini.
• Pyramidal cells.
• Possess rounded basally located nuclei.
• Located in parotid glands.

Mixed Glands (Mixed Acinus)

• Serous cells manifest as crescents capping mucous cells.
• Crescents are known as Demilune's of Von Ebner or crescents of Giannuzi.

Classification by Secretion Mechanism:

• Merocrine glands.
• Apocrine glands.
• Holocrine glands.

Merocrine Glands

• Exhibit no cellular changes in secretory cells.
• Secretion occurs via exocytosis, as in salivary glands.

Apocrine Glands

• Apical portions of secretory cells are lost with the secretion, such as mammary glands.

Holocrine Glands

• The entire cell is secreted.

• The entire secretory cells are destroyed and come out with the secretion like sebaceous glands.

• Classified by Arrangement of Secretory Cells and Duct Branching:

  • Ducts.
  • Simple glands.

Ducts

• The gland is simple if the duct is unbranched.

Simple Glands

• Include
  • Simple tubular gland, e.g., colon.
  • Simple tubular coiled glands, e.g., sweat gland.
  • Simple acinar glands.
  • Simple branched glands.

Simple Branched Glands

• Have several secretory units empty into one unbranched excretory duct.
• For example:
  • Simple tubular branched glands (pyloric Stomach).
  • Simple branched acinar gland (sebaceous glands).

Compound Glands

• Glands feature a branched duct system and branched secretory portions.
• If shaped like a tube, it is a mammary gland.
• If shaped like a flask, it is a Duodenum’s (Brunner's gland), alveolar or acinar in shape.
• If the structure is a combination of tube ends & a sac-like dilatation, it’s tubuloalveolar like submandibular salivary glands.

Special Epithelium Classification:

• Neuroepithelium.
• Myoepithelial cells.
• Germinal epithelium.

Neuroepithelium

• Epithelial cells are designed to receive sensory stimuli and present in taste buds of the tongue, retina of the eye, and olfactory mucosa in the nose.

Myoepithelial Cells

• Contract around the secretory or conducting portion of glands to propel secretory products, as in mammary, sweat, and salivary glands.

Germinal Epithelium

• A specialized epithelium designed for reproduction, as in testis forming spermatozoa and ovary forming ovum.

Cell Surface Specializations:

• Apical surface.
• Basal surface.
• Lateral surface.

Apical Cell Surface Specializations:

• Apical surface cells incorporate specialized structures to increase surface area or move substances, including:
  • Microvilli.
  • Stereocilia.
  • Cilia.
  • Flagella.

Microvilli

• Fingerlike cytoplasmic projections present on the apical surface.
• Closely packed, numerous structures giving a "brush border" appearance increasing surface absorptions in the small intestine.

Stereocilia

• Long, branched immotile microvilli.
• Serve as mechanoreceptors in sense organs (hair cells).
• Exhibits absorptive function in the male reproductive system (epididymis).

Cilia

• These are motile structures.
• They are longer than microvilli.
• They are visible by light microscope.
• These are found in the respiratory tract.

Flagella

• Only in spermatozoa.
• Structurally similar to cilia but longer; typically, one flagellum per cell for movement.

Basal Cell Surface Specialization

• The Basement membrane (LM) is a non-cellular layer connecting epithelium to underlying tissue.
  • Periodic acid-Schiff (PAS)-positive.
  • Is visible with light microscope.
  • Thicker than the basal lamina.
  • Formed by either two basal laminae or a basal lamina and reticular lamina.

Basal Lamina (EM)

• A sheet-like extracellular structure that consists of:
  • Lamina lucida: electron lucent.
  • Lamina densa: electron dense.
• Collagen IV, glycoprotein, and proteoglycan are its composition.
• Fibers of collagen VII and micro fibril bundles connect to underlying connective tissue.
• Reticular lamina connects reticular fibers of the underlying connective tissue.

Lateral Cell Surface Specialization:

• Impermeable junction: Zonula occludens (tight junction).
• Adhering junction: Zonula adherens, Desmosomes, Hemidesmosomes.
• Communication junction: Gap junction.

Tight junctions (zonula occludens):

• They are at the most apical junction.
• These form bands that completely encircle the cell, sealing off the intercellular space.
• Seen with EM, the two outer leaflets of neighboring cells fuse.
• It prevents the passage of material between epithelial cells.
• For example, in the kidney and urinary bladder, urine stays within kidney tubules due to columnar epithelium cells.

Adhering Junction:

• Zonula Adherens is located directly beneath the occludens junction.
• Encircles the cell, promoting adhesion between neighboring cells.

Desmosome (Macula Adherens)

• Disk-shaped structure
• Exhibits very straight cell membranes.
• Is located along lateral membranes of epithelial cells.
• Does not allow adjacent cells to touch but holds protein filaments together.
• It presents in stratified squamous epithelium of the skin, stomach, and urinary bladder.

Hemidesmosomes

• This is located at the contact zone of the basal lamina and epithelial cells.
• Appears as half desmosomes.
• Serves a basal adhesion function.

Gap Junctions

• Adjacent cells connect through narrow protein channels; ions and nerve impulses pass through.
• Commonly present in the heart and smooth muscle.