Histology: Epithelial Tissue

• Histology studies the composition, structure, and function of tissues.
• The integumentary system will be studied, it is the body's largest organ system.
• The four major tissue types are epithelial, connective, muscle, and nervous.

Epithelial Tissue

• Epithelial tissue lines and covers body surfaces, both inside and out.
• It forms mucous, serous, cutaneous, and synovial membranes.
• Membranous epithelium lines surfaces for protection and absorption.
• Glandular epithelium secretes substances like hormones.
• Some epithelial tissues can stretch and recoil.
• Epithelial tissues are avascular.
• They reproduce rapidly.
• Epithelial tissues exhibit cellularity, with cells tightly packed and minimal extracellular matrix.
• Cells attach to underlying connective tissue via a basement membrane.
• An upper or apical surface and a lower or basal surface is called polarity.
• Shapes of epithelial cells are squamous (flat), cuboidal (cube-shaped), and columnar (taller than wide).
• Simple arrangement involves a single layer of cells attached to the basement membrane.
• Stratified arrangement involves multiple layers, with upper layer cells not attached to the basement membrane.
• Pseudostratified arrangement involves a single layer of cells attached to the basement membrane, but not all reaching the apical surface.

Types of Simple Epithelium

• Simple squamous epithelium consists of a single layer of thin, flat cells that look like tightly fitted floor tiles.
• Their nuclei are in the middle of the cell.
• Simple squamous epithelium lines air sacs of lungs, walls of capillaries, blood vessels, heart, and body cavities.
• They act to reduce friction, provide selective permeability, secrete and absorb.
• Simple cuboidal epithelium is a single layer of cube-shaped cells with nuclei in the center.
• These line kidney tubules and ducts of glands, such as salivary glands.
• Their function is secretion, absorption, and some protection.
• Simple columnar epithelium consists of a single layer of tall, thin cells with nuclei located at the base.
• Simple columnar epithelium lines the uterus, stomach, and intestines.
• Its function is protection, secretion, and absorption.
• They include microvilli that increase surface area for absorption in the small intestines.
• Goblet cells secrete protective mucus in the lining of the stomach and intestines.
• Pseudostratified columnar epithelium contains a single layer of tall, thin cells that appear multilayered due to varying nuclei levels.
• All cells reach the basement membrane, but not all reach the free surface.
• This lines the upper respiratory tract and part of the male reproductive tract.
• They act to protect and secrete.
• Goblet cells secrete mucus to trap foreign particles in the respiratory tract.
• Cilia move mucus up and out of the respiratory tract.

Types of Stratified Epithelium

• Transitional epithelium consists of several layers of cells resembling cuboidal or columnar epithelium.
• Cells on the free surface have a scalloped or dome-shaped appearance due to crowding when recoiled.
• It is located in the urinary bladder and ureters.
• Its function is to allow expansion and recoil after stretching.
• This tissue can distend in response to changing tension in these organs.
• Stratified squamous epithelium is thick tissue consisting of many layers of thin flat cells at the outer layer and cuboidal or columnar shaped cells near the basement membrane.
• Lining the mouth, esophagus, rectum, anus, and vagina (outermost layer of the skin).
• The baseline layer is in ridges because of crowding.
• Function to provide physical protection against abrasion, pathogens, UV radiation, and chemical attack, limits fluid loss, and can produce vitamin D3.
• Keratinized stratified squamous epithelium has protein (keratin) filaments filling the outer skin cells, helping to link them.
• It can be found mainly on the palms of hands and soles of feet, and some on all outer skin.
• The function is to provide a tough, water-resistant outer layer.

Connective Tissue

• Connective tissue is the most abundant tissue in the body.
• It binds structures together, supports, packages, and protects body organs, fills space, and performs specialized functions.
• Connective tissue can reproduce, is vascular (except for cartilage), contains specialized cells.
• Large amounts of extracellular MATRIX consists of fibers and ground substance.
• Types of cells are varied.
• Collagenous fibers (white fibers) stain light pink or blue, are strong and flexible, but not elastic.
• They resist considerable pulling force with tensile strength.
• Elastic fibers (yellow fibers) stain dark purplish, are weaker than collagen, but can stretch and recoil.
• Reticular fibers are thin collagen fibers; highly branched to form a delicate framework.
• Ground substance varies in consistency depending on the type of CT and supports cells and fibers, filling spaces outside of the cells.

Types of Connective Tissue

• Fibrous or Proper CT has a syrupy ground substance.
• Loose fibrous connective tissue (areolar) mainly contains fibroblasts that are widely dispersed.
• Its matrix consists of collagenous and elastic fibers in a loose, disorganized pattern, with syrupy or gel-like ground substance.
• Loose fibrous connective tissue (areolar) is found underlying the skin, filling space between muscles, and underlying most epithelial tissue.
• It forms a layer that separates and anchors skin to underlying tissues.
• Adipose tissue contain adipocytes specialized to store fat droplets which then forces other cells types out of the tissue.
• It has collagen and elastic fibers but hidden by adipocytes.
• Adipose can be found beneath the skin, filling space between muscles, around kidneys, eyeballs, surface of the heart, around certain joints.
• Functions to store energy reserves, cushions, insulates, and can store fat-soluble vitamins D, E, K, A.
• Reticular connective tissue have fibroblast cells with a three-dimensional network of reticular fibers, Located in the liver, spleen, bone marrow, kidneys, and lymph nodes.
• Its function to provide an open, but supportive framework of these "spongy” organs.

Dense Connective Tissue

• Dense regular connective tissue (white fibrous) - contain mostly fibroblasts appearing sandwiched between layers of collagen fibers, its matrix contains many closely packed collagenous fibers in a parallel fashion, with a fine network of elastic fibers.
• It is located in the tendons (connect muscle to bone) and ligaments (connect bone to bone)
• Acts as an attachment and provide stability.
• Elastic Connective Tissue - Contains mostly fibroblasts, with matrix mainly of elastic fibers in wavy parallel arrangement with few collagenous fibers.
• This can be found in hollow organs: stomach, esophagus, intestines, bladder, uterus, aorta, and blood vessels
• Allows for expansion and contraction of organs.
• Dense Irregular (NO SLIDE) - Contains mostly fibroblasts appearing sandwiched between layers of collagen fibers, closely packed collagenous fibers running in many different directions
• Found forming a protective capsule around most organs.
• Supports by way of Protection.

Supporting Connective Tissue

• There are two types of this tissue: Cartilage and Bone.
• The ground substance of cartilage is semi-solid, while that of bone is solid.

Cartilage

• Avascular
• They contain chondrocytes cells
• Chondrocytes occupy small chambers known as lacunae.
• Enclosed by a covering of fibrous connective tissue called perichondrium.
• Receives nutrients and oxygen by diffusion from blood vessels surrounding the perichondrium.

Types of Cartilage Connective Tissue

• Hyaline cartilage has a glassy appearance.
• Contains collagenous fibers but they are not visible.
• The cell type is chondrocyte.
• It is located at the end of the nose, supporting trachea, larynx, and bronchi, in the embryonic skeleton, and between ribs and sternum, as well as covering bone surfaces at synovial joints.
• The cartilage also provides stiff but flexible support, and reduces friction at joints.
• Elastic cartilage contains visible elastic fibers.
• Many chondrocytes in large lacunae.
• Located In the external ear and epiglottis.
• Elastic cartilage supports such that it can bend slightly and return to original shape.
• Fibrocartilage has dense, parallel, wavy collagenous fibers.
• The cell type is chondrocytes.
• Fibrocartilage is located in intervertebral discs, within the knee joint, and between pubic bones of the pelvic girdle.
• Function to limit compression and relative movement (shock absorber)

Bone or Osseous (Support) Connective Tissue

• Composed of two types of osseous tissue – compact and spongy.
• Compact or Osteonic bone is the outer layer of bone while Spongy or Trabecular bone makes up the internal areas of bone
• Compact bone is very dense while spongy bone tissue is very porous thus light weight.

Compact Bone

• The ground substance matrix is solid as a result of mineral salts being deposited in the matrix.
• Fibers are collagenous for flexibility and tensile strength.
• Cell types - osteocytes in lacunae, osteoclasts remove and recycle bone matrix, and osteoblasts produce new bone matrix.
• Location – skeletal system.
• Function - provide framework and protection for the body, attachment sites for tendons and ligaments.
• Specialized features – Bone matrix is deposited in concentric layers called lamellae, lamellae are arranged around the osteonic canal (Haversian canal).
• Each osteonic canal contains blood vessels, canaliculi connect each osteocyte to the osteonic canal.
• Features of spongy bone (NO SLIDE) – Bone matrix is deposited in bone bridges called trabecula separated by open spaces called pores. Blood vessels and nerves meander through the pores; also has canaliculi and lacunae.

Fluid CT

• There are two types: blood and lymph.
• Unlike other connective tissues, the ground substance is fluid, allowing cells and fibers of the matrix to move freely.
• CT cells producing the matrix are found in bone marrow and are called hemocytoblasts.

Blood or Vascular Connective Tissue

• Its fluid matrix is called plasma.
• Cell types include red blood cells (RBC), white blood cells (WBC), and platelets.
• Fiber – Fibrinogen, a protein fiber that helps to form a framework for blood to clot
• Location – Circulatory system.
• RBC’s transport gases, plasma moves nutrients, metabolic wastes, and chemical messengers throughout the body, WBC’s are part of the immune system; platelets aid blood clotting.

Lymph

• The matrix is a fluid called lymph derived from blood plasma.
• It contains soluble protein fibers.
• Cell type is WBC.
• The location is the lymphatic system.
• Helps to maintain blood volume and aids immune system.

Muscle Tissue

• Tightly packed and contractile (able to shorten and thicken).
• This leads to movement of limbs or organs.
• Contraction is isotonic (resulting in movement) or isometric (resulting in no movement).
• Isometric contractions are key for maintaining posture and muscle tone (a muscle's resting tension).

Types of Muscle Tissue

Skeletal Muscle Tissue

• Cell Characteristics - multi-nucleated, striated, non-branched, long thin (cylindrical) cellsLocation
• Attached to bone by tendons, entrance and exit of digestive system.
• Function - voluntary (conscious) movement of skeleton; regulates openings at beginning and end of digestive tract (mouth and anus).

Cardiac Muscle Tissue

• Cell Characteristics - mono-nucleated, striated, branched.
• Location – only in walls forming the heart chambers (atria and ventricles).
• Function – involuntary (unconscious) movement (circulates blood and maintains blood pressure).
• Special Feature - intercalated discs (special junction between cells).

Smooth Muscle Tissue

• Cell Characteristics - mono-nucleated, non-striated, non-branched, short, spindle-shaped.
• Location - walls of hollow organs.
• Function - involuntary (unconscious) movement; (moves substances through organs, regulates the diameter of organs).

Neural (Nervous) Tissue

• Two types of cells are found here: neurons and neuroglia cells.
• There is a large amount of extracellular matrix (space between cells).
• Cell types - neurons or neuroglia.
• Neurons transmit electrical impulses (conductivity) and have axons and dendrites extend from the neuron cell body (soma).
• Neuroglia cells provide support for neurons.
• Located in the brain, spinal cord, and peripheral nerves
• Function - conduction of electrical impulses from one region of the body to another, support cells for neurons
• Special Feature - Neuromuscular Junction, the region where the nerve communicates with the muscle cell.

Integumentary System

• Includes the skin and its accessory organs
• Accessory organs - nails, sweat and sebaceous glands, sensory receptors, and hair (with follicle and arrector pili muscle).
• Skin - a cutaneous membrane with an epithelial layer called the epidermis and a connective tissue layer called the dermis.
• A subcutaneous layer of areolar and adipose tissues is associated with the skin, beneath the dermis.
• Epidermis - stratified squamous epithelia tissue is flexible, avascular, and self-repairing.
• It limits fluid loss, provides UV radiation protection, produces vitamin D3, and provides physical protection.
• Divided into layers called strata:

Strata of the Epidermis

• Stratum basale (germinativum) is the deepest layer attached to the basement membrane.
• Mitotic cells give rise to other layers. Melanocytes produce melanin which pigments the skin.
• Stratum spinosum consists of "prickly" cells (keratinocytes) with bundles of pre-keratin filaments joined by desmosomes.
• Stratum granulosum - the cell keratinization begins, 2 to 5 layers thick.
• Cell organelles break down, fills the cell with keratin and special granules, plasma membrane thickens
• Stratum lucidum consists of a thin, clear band of dead keratinocytes not found in areas with thin skin
• Stratum corneum- the most superficial layer; a broad layer of dead cells filled with keratin.
• Layers slough in a range between every 30 to 45 days.
• Glycolipids between cells is what creates a water resistant feature.
• Dermis - made up of both loose and dense connective tissue
• an upper papillary layer with Areolar connective tissue and a lower reticular layer of dense irregular connective tissue.
• Dermis houses the accessory organs.
• Nourishes the epidermis via diffusion and anchors it to underlying tissue.