Epithelial Tissue and Cell Junctions

• There are 4 basic tissue types: Epithelial, Connective, Muscle, and Nerve.

Epithelial Tissue

• Covers exposed surfaces such as skin.
• Lines hollow organs, internal passageways, and ducts, including the digestive, urinary, and reproductive tracts.
• Forms glands.
• Composed almost entirely of cells, exhibiting cellularity.
• Forms continuous sheets held together by tight junctions and desmosomes, known as special contacts.
• Has apical and basal surfaces exhibiting polarity.
• Supported by connective tissue via the basement membrane, which includes reticular and basal laminae.
• Avascular, lacking blood vessels but supplied by nerve fibers
• Rapidly replaces lost cells through cell division demonstrating regeneration.

Basement Membrane

• Consists of two parts: basal lamina and reticular lamina.
• Basal lamina is the top layer, encompassing the lamina lucida and lamina densa.
• Reticular lamina is the bottom layer.

Cell Junctions

• Intercellular connections involve either extensive areas of opposing plasma membrane or specialized attachment sites.
• Cell adhesion molecules (CAMs) interconnect two cell membranes via transmembrane proteins.
• Proteoglycans bond two membranes with a thin layer of protein-carbohydrate complex molecules.
• Specialized cell-cell junctions include gap junctions, tight junctions, and desmosomes (spot desmosomes and hemidesmosomes).

Epithelial Tissue Functions

• Provides physical protection against abrasion, dehydration, and chemical damage.
• Facilitates absorption of nutrients and exchange of gases in the gut and respiratory tract.
• Enables excretion through the unique lining of kidney tubules, concentrating excretory products in urine.
• Produces secretions such as hormones, mucus, digestive juices, and sweat via glandular epithelium.
• Provides sensation through sensory nerves in the eyes, ears, nose, and skin.
• Specialized epithelium features microvilli for absorption/secretion, found in digestive and urinary tracts.
• Cilia in respiratory tracts move mucus from the lungs to the throat through synchronized beating.

Types of Epithelium

• Covering and lining epithelium includes the epidermis of skin and the lining of blood vessels, ducts, as well as respiratory, reproductive, urinary, and GI tracts.
• Glandular epithelium forms the secreting portion of glands like thyroid, adrenal, and sweat glands.

Epithelium Classification (Arrangement)

• Simple epithelium is one cell layer thick.
• Stratified epithelium consists of more than one layer, providing protection where mechanical or chemical stresses are present (e.g., skin, mouth).
• Pseudostratified epithelium is a single layer where not all cells reach the apical surface.
• Nuclei are at different levels and appear multilayered.

Epithelium Classification (Shape)

• Squamous cells are flat.
• Cuboidal cells are cube-shaped.
• Columnar cells are tall and column-like.
• Transitional cells' shape varies with tissue stretching.

Epithelium Locations

• Simple Squamous Epithelium: lines the ventral body cavities, heart and blood vessels, alveoli of lungs, and the inner lining of the cornea.
• Simple Cuboidal Epithelium: found in glands, ducts, and portions of kidney tubules.
• Simple Columnar Epithelium: lines the stomach, intestine, gall bladder, and uterine tubes.
• Stratified Squamous Epithelium: found on the surface of the skin (epidermis), lining the mouth, throat, esophagus, rectum, anus, and vagina.
• Stratified Cuboidal Epithelium (rare): lines sweat gland ducts and seminiferous tubules in the testis.
• Stratified Columnar Epithelium (rare): found in small areas of the pharynx, limited areas of the larynx, anal canal, and male urethra.
• Pseudostratified Ciliated Columnar Epithelium: lines the nasal cavity, trachea (windpipe), bronchi, and parts of the male reproductive tract.
• Transitional Epithelium: found in the urinary bladder, ureters, and umbilical cord.

Glandular Epithelium

• Composed of exocrine glands that secrete sweat, tears, milk, ear wax, saliva, and digestive enzymes onto the epithelial surface via tubes (ducts).
• Includes endocrine glands that are ductless, such as the thyroid, pituitary, and adrenal glands, secreting hormones into the bloodstream.

Exocrine Gland Structure

• Unicellular glands are goblet cells.
• Multicellular glands consist of clusters of cells.
• Classified by duct structure: simple glands have a single, unbranched duct while compound glands have branched ducts.
• Classified by secretory portion shape: tubular (forms tubes), alveolar (blind pockets or sacs), and tubuloalveolar (both tubes and sacs).

Glandular Methods of Secretion

• Merocrine glands release products via exocytosis (e.g., saliva, digestive enzymes, sweat).
• Apocrine glands release the upper part of the cell along with the secretion (e.g., milk).
• Holocrine glands release products through whole cells dying and rupturing (e.g., oil glands).

Exocrine Glands by Secretion Type

• Serous glands secrete a watery solution containing enzymes (e.g., parotid gland).
• Mucous glands secrete mucins that hydrate to form mucous, found in salivary glands and submucosal glands of the small intestine.
• Mixed glands such as submandibular glands

Connective Tissues

• Cells are rarely touch due to the presence of an extracellular matrix
• The matrix contains fibers and ground substance secreted by cells.
• Consistency varies from liquid to gel or solid.
• Most CTs have good nerve and blood supply, except cartilage and ligaments

Classification of Connective Tissue

• Connective Tissue Proper: includes Loose (areolar, adipose, reticular) and Dense (dense regular, dense irregular, elastic) types.
• Fluid Connective Tissues: consists of blood and lymph.
• Supporting Connective Tissues: includes cartilage and bone.

Connective Tissue Characteristics

• Situated throughout the body but not exposed to the outside environment.
• Highly vascular, with the exception of ligaments and cartilage.

Connective Tissue Functions

• Provides structural framework (bones, tendons, ligaments).
• Transports fluids and dissolved materials (blood).
• Protects delicate organs.
• Stores energy reserves, especially in the form of lipids (adipose).
• Defends the body from invading microorganisms (WBCs).
• Key components include specialized cells, extracellular proteins, and ground substance, which together form the matrix.

Connective Tissue Proper Characteristics

• Contains fibers, a viscous ground substance, and a varied cell population
• Cell populations: Fibroblasts, Mesenchymal Stem Cells, Adipocytes, Melanocytes, Macrophages, Mast cells, Lymphocytes

Connective Tissue Fiber Types

• Collagen fibers are tough, unbranched, provide high tensile strength.
• Elastic fibers are long, thin, and allow for stretch.
• Reticular fibers are branched collagenous fibers that form delicate networks.

Areolar Connective Tissue (Loose)

• Features a gel-like matrix with all three connective tissue fiber types.
• Contains fibroblasts, macrophages, mast cells, and some white blood cells.
• Wraps and cushions organs, distorts easily but regains shape readily.
• Widely distributed throughout the body and is vascular.

Adipose Connective Tissue (Loose)

• Comparable to areolar connective tissue in matrix, but with closely packed adipocytes.
• Adipose tissue is highly vascularized,matrix is scanty and cells are closely packed .
• Found under the skin, around kidneys, within the abdomen, and in the buttocks and breasts
• Includes White and Brown adipose tissue types.

Reticular Connective Tissue (Loose)

• Features loose ground substance with reticular fibers.
• Reticular cells lie in a fiber network.
• Forms a soft internal skeleton, or stroma, that supports other cell types.
• Found in lymph nodes, bone marrow, and the spleen.

Dense Connective Tissues

• Includes Dense regular and Irregular Connective Tissue
• Dense Regular Connective Tissue: Located between skeletal muscles and skeleton (tendons and aponeuroses); between bones or stabilizing positions of internal organs (ligaments) Provides firm attachment; conducts pull of muscles; reduces friction between muscles; stabilizes relative positions of bones.
• Dense Irregular Connective Tissue: Capsules of visceral organs; periostea and perichondria; nerve and muscle sheaths; dermis Provides strength to resist forces applied from many directions; helps prevent overexpansion of organs such as the urinary bladder.

Fluid Connective Tissues

• Features cells suspended in a fluid matrix
• Blood: Includes formed elements suspended in plasma. Consists of red blood cells (erythrocytes), white blood cells (leukocytes), and cell fragments (platelets) Arteries carry blood away from the heart, veins carry blood to the heart. Capillaries allow diffusion into interstitial fluid.
• Lymph: Consists of interstitial fluid entering lymphatic vessels.

Compact Bone

• Consists of osteons, which are lamellae (rings) of mineralized matrix with calcium and phosphate for hardness.
• Interwoven collagen fibers provide strength.
• Osteocytes reside in spaces (lacunae) between lamellae.
• Canaliculi (tiny canals) connect cell to cell and form a branching network for the exchange of materials between blood vessels and osteocytes.

Cartilage

• Features a network of fibers in a rubbery ground substance.
• Gel is firm due to polysaccharide derivatives called chondroitin sulfate.
• Resilient, enduring more stress than loose or dense connective tissue.
• Lacks a direct blood supply (avascular).
• Chondrocytes produce anti-angiogenesis factors, inhibiting blood vessel growth within the cartilage.
• Nutrient and waste exchange occurs by diffusion.
• Includes Hyaline, Fibrocartilage, and Elastic types.

Muscle Tissue Overview

• Cells can shorten, lengthen, and remain the same length.
• Provides motion, posture, and heat.
• Includes skeletal, cardiac, and smooth muscle types.

Skeletal Muscle

• Cells are long cylinders with many peripheral nuclei.
• Exhibits visible light and dark banding.
• It is voluntary and under conscious control.

Cardiac Muscle

• Composed of branched cylinders with one central nucleus.
• It is involuntary and striated.
• Cells are attached to and communicate with each other via intercalated discs.

Smooth Muscle

• Composed of spindle-shaped cells with a single central nucleus.
• Forms the walls of hollow organs (e.g., blood vessels, GI tract, bladder).
• Involuntary and nonstriated.

Nerve Tissue

• Includes neurons and neuroglial (supporting) cells.
• A nerve cell structure that is a nucleus & long cell processes conducts nerve signals
• Dendrites carry signals towards the cell body.
• Axons carry signals away from the cell body.

Membranes

• Epithelial layer sits on a thin layer of connective tissue (lamina propria)
• Types: mucous, serous, synovial, and cutaneous (skin).