Cell Structure and Histology

• Histology is the study of tissues

Cell Structures

• Cytoskeleton supports organelles and cell shape; it also plays a role in cell motion.
• Microtubules are tubes of protein molecules and are present in the cytoplasm, centrioles, cilia, and flagella.
• Intermediate filaments are intertwined protein fibers that provide support and strength.
• Actin filaments are twisted protein fibers responsible for cell movement.
• Centrioles are complex assemblies of microtubules that occur in pairs.
• Cytoplasm is a semifluid matrix that contains the nucleus and other organelles.
• The smooth endoplasmic reticulum is a system of internal membranes that aids in the manufacture of carbohydrates and lipids.
• The rough endoplasmic reticulum has internal membranes studded with ribosomes that carry out protein synthesis.
• The nucleus is the command center of the cell.
• The nucleolus is the site where ribosomes are produced.
• The nuclear envelope is the double membrane between the nucleus and the cytoplasm.
• Nuclear pores are openings embedded with proteins that regulate passage into and out of the nucleus.
• Ribosomes are small complexes of RNA and protein and are the sites of protein synthesis.
• Peroxisomes are vesicles containing enzymes that carry out particular reactions, such as detoxifying potentially harmful molecules.
• Mitochondria are organelles in which energy is extracted from food during oxidative metabolism.
• Secretory vesicles fuse with the plasma membrane and release materials to be secreted from the cell.
• Lysosomes are vesicles that break down macromolecules and digest worn-out cell components.
• The Golgi complex collects, packages, and distributes molecules manufactured in the cell.
• The plasma membrane is a lipid bilayer in which proteins are embedded.

Tissue Types and Functions

• Tissue is a group of cells with similar structure working together for a particular function.
• Epithelial tissue covers surfaces, lines cavities, and forms glands, providing protection.
• Connective tissue provides support to other tissues, insulates, and stores energy reserves.
• Muscle tissue facilitates movement.
• Nervous tissue detects changes, sends, and receives impulses for reception and transmission of signals.
• The digestive system coordinates organs like the mouth, esophagus, stomach, intestines, liver, pancreas, rectum, and anus.
• The digestive system converts ingested nutrients into absorbable molecules and ions.
• The digestive system eliminates undigested matter.
• The digestive system helps regulate the body's water content.

Epithelial Tissue

• Functions for protection of the body by covering external and lining internal surfaces.
• Absorption and secretion are other functions.
• Epithelium rests on a basement membrane, separating epithelial cells from underlying connective tissue.
• Epithelial tissue is avascular, so cells receive nutrients, water, and oxygen through diffusion.
• Epithelial cells fit together tightly with little to no intercellular substance.
• Epithelial cells are polarized and possess apical, basal, and lateral surfaces.
• The free (apical) surface is exposed to the outside or to the lumen.
• The basal surface rests on the basement membrane.
• Epithelial cells regenerate easily thanks to rapid and constant mitosis in basal cells.

Classification of Epithelial Tissue

• The first name classifies the number of layers.
• One layer is simple.
• More than one layer is stratified.
• The second name classifies the shape of the cells
• Squamous cells are flattened.
• Cuboidal cells are cube-shaped.
• Columnar cells are column-shaped.
• Stratified epithelia are squamous at the surface.
• Non-keratinized stratified squamous epithelium is found in the oesophagus.
• Keratinized stratified squamous epithelium is found in the skin (epidermis).
• Flattened cells are in the superficial layer.
• Epithelial cells appear in areas where mechanical insults are frequent.
• Fastness is stratification.
• Projections of connective tissue are papillae.
• Cells supported by a firm cytoskeleton, keratin filaments.
• Firm intercellular junctions are connections - desmosomes.
• Hemidesmosomes are at the border of epithelium and connective tissue
• Epithelium cells are avascular and nourished by the diffusion of nutrients from capillaries in the connective tissue.
• Pseudostratified epithelia look stratified but are not.
• Only one layer of cells is present; all cells touch the basement membrane.
• Transitional epithelium can stretch without breaking.
• Transitional epithelium can be called umbrella cells.
• Stratified cuboidal form if bladder is empty.
• Stratified squamous form if bladder is full.

Membrane Specializations

• Surface epithelia form continuous sheets of one or more layers.
• Epithelial cells are closely bound to one another and are polarized.
• Apical specialization = membrane projections of the free surface that lines the lumen.
• Lateral specialization = Junctions (Tight Junctions, Adherens Junctions, Desmosomes, and Gap Junctions) that occur at the interface between two adjacent cells.
• Apical specializations: microvilli, cilia, and stereocilia.

Lateral Specializations - Junctional Complexes

• Junctional complexes include tight junctions, zonula adherens, and desmosomes.
• Tight junctions seal neighboring cells together, preventing molecule leakage between them.
• Zonula adherens joins an actin bundle in one cell to a similar bundle in a neighboring cell.
• Desmosomes join intermediate filaments in one cell to those in a neighboring cell.
• Gap junctions allow the passage of small water-soluble ions and molecules.
• Plaque-like structures that are composed of connexons.
• A connexon is composed of six subunits of a protein called connexin.
• Connexins are arranged radially to form a central channel so ions and small molecules can pass.

Apical Specializations

• Microvilli are small finger-like projections on the apical surface which help increase surface area
• The core of microvilli contains cytoplasm with actin filaments.
• Actin filaments anchor to a network known as the "terminal web" and they form the brush border
• Cilia are longer than microvilli and are typically 7-10 µm in length.
• The core of the cilium is called the axoneme and has 9+2 microtubules organized from a basal body.
• Supported by ATP, cilia movement is based on longitudinal sliding of microtubules in the peripheral doublets.
• Cilia undergo regular synchronous movement, with rapid forward movement during the effective stroke, followed by a slower recovery stroke.

Glandular Epithelium

• Glandular epithelium is involved in secretion and forms secretory organs, known as glands.
• Exocrine glands release secretion through a duct onto internal or external surfaces.
• Endocrine glands release secretion directly into the blood.
• Exocrine gland classification is based on morphology, ways of secretion, and type of secreted products.
• Exocrine gland morphology can be simple/compound, tubular, alveolar, or tubuloalveolar.
• Exocrine secretion methods include merocrine, apocrine, and holocrine.
• Exocrine secreted products include mucous, serous, and mixed secretions.
• Holocrine glands release secretions via disintegration of the entire cell (e.g., sebaceous glands).
• Merocrine glands release secretions via exocytosis of secretory vesicles (e.g., pancreas, gastric glands).
• Apocrine glands release secretions when the apical portion of the cell pinches off (e.g., lipid component of milk from mammary gland).
• Serous glands secrete thin, watery fluids with proteinaceous secretions and a central rounded nucleus.
• Mucous glands secrete thick, viscous fluids with mucopolysaccharides and a nucleus that is flat and peripheral.
• Serous demilunes consist of serous cells forming a semilunar cap at the terminal portion of a mucous acinus.