C

Histology INTRO

Four primary tissue types

  • Nervous tissue

    • Specialized for rapid communication

    • Located in the brain and spinal cord, but also in the peripheral nervous system (nerves and ganglia)

  • Epithelial tissue

    • Forms lining on surfaces and cavities of the body

    • Examples: epidermis (skin surface), inner lining of GI tract (stomach and intestines), mucous membranes

  • Muscle tissue

    • Specialized for contraction to produce movement

  • Connective tissue

    • Provides support and metabolic support for other tissues

    • Most diverse tissue type

    • Includes bone, arteries (blood vessels), fat, loose connective tissue, tendons (dense fibrous connective tissue)

Muscle tissue: types and features

  • Three major types of muscle tissue: 3 types

    • Skeletal muscle

    • Attached to bones

    • Long cylindrical fibers, multiple nuclei per fiber

    • Striated appearance due to alternating light/dark bands (sarcomeres)

    • Voluntary control (under conscious command)

    • Cardiac muscle

    • Found in the heart

    • Branched fibers connected by intercalated discs

    • Striated like skeletal muscle

    • Typically one nucleus per fiber

    • Involuntary control

    • Intercalated discs contain desmosomes (anchor cytoskeletons) and gap junctions (allow ion passage for synchronized contraction)

    • Smooth muscle

    • Found in walls of hollow organs (e.g., GI tract, blood vessels) and other areas (e.g., iris, ciliary body of the eye)

    • Spindle-shaped cells with a single nucleus

    • No striations (non-striated)

    • Involuntary control

  • Skeletal muscle anatomy (brief)

    • Skeletal muscle fibers are long, cylindrical, multinucleated cells

    • Striation due to arrangement of actin/myosin (motor proteins) within sarcomeres

Epithelial tissue and membranes

  • Epithelial tissue location and roles

    • Found on surfaces and as lining of cavities and organs

    • Example: epidermis (skin) as part of the cutaneous membrane

    • GI tract epithelium lines the stomach and intestines; serves roles in secretion and absorption

  • Membranes (two broad categories)

    • Connective tissue membranes (e.g., synovial membranes)

    • Composed of connective tissue only

    • Example: synovial membrane lining joint cavities and producing synovial fluid for lubrication

    • Epithelial membranes

    • Mucous membranes line digestive, respiratory, urinary, and reproductive tracts; coated with mucus (secreted by glands within the epithelium)

    • Serous membranes line ventral body cavities (e.g., peritoneal, pleural, pericardial membranes)

      • Structure: parietal layer lines cavity walls; visceral layer covers organs; serous fluid between layers

      • Epithelium: simple squamous (mesothelium) over loose connective tissue

  • Epithelial tissue details

    • Simple squamous epithelium: one cell layer; very thin; found in alveoli and endothelium

    • Simple columnar epithelium: one layer of tall cells; often has microvilli in absorptive regions (e.g., small intestine)

    • Simple cuboidal epithelium: one layer of cube-shaped cells

    • Stratified epithelia: more than one layer; named by apical cell shape

    • Stratified squamous epithelium: multiple layers; apical cells flat; keratinized vs nonkeratinized

    • Stratified cuboidal and stratified columnar epithelia: two or more cell layers; secretory roles (exocrine glands)

    • Pseudostratified epithelium: appears layered but all cells touch the basement membrane; often ciliated in respiratory tract (trachea)

    • Transitional epithelium: stratified with apical cells varying from squamous to columnar; stretchable (e.g., ureter, bladder)

  • Specific examples

    • Lung/alveoli and blood vessel endothelium: simple squamous

    • Small intestine mucosa: simple columnar with microvilli; secretes mucus and absorbs nutrients

    • Uterine tubes (fallopian tubes): simple columnar epithelium; some cells are ciliated to move oocytes/zygotes

    • Esophagus: nonkeratinized stratified squamous epithelium

    • Skin epidermis: keratinized stratified squamous epithelium; keratin provides abrasion resistance

    • Dermis: underlying loose areolar tissue (papillary layer) and dense irregular connective tissue (reticular layer)

  • Glandular epithelia

    • Glands contain one or more epithelial cells that produce secretions

    • Endocrine glands: secrete hormones into the bloodstream

    • Exocrine glands: secrete onto surfaces or ducts (e.g., sweat glands, salivary glands)

    • Goblet cells: unicellular exocrine glands that secrete mucus via mucin; integrated in mucous membranes

    • Pancreas: both endocrine (islets secreting insulin and glucagon) and exocrine (acinar cells secreting pancreatic digestive enzymes into the duodenum via the pancreatic duct)

  • Secretory mechanisms in glands

    • Merocrine (exocytosis): vesicles fuse with cell membrane to release product (e.g., goblet cells; pancreatic acinar cells; many sweat glands)

    • Apocrine: apical portion of cell pinches off to release secretion (e.g., mammary glands during milk production)

    • Holocrine: entire cell disintegrates to release product (e.g., sebaceous glands secrete sebum)

  • Additional gland examples

    • Salivary glands: exocrine secretion of saliva via ducts

    • Gastric glands: gastric juice production in stomach walls

    • Liver/gallbladder: bile production and storage

Connective tissue: structure and major types

  • General characteristics

    • Cells are dispersed within a nonliving extracellular matrix

    • Matrix contains fibers (collagen, reticular, elastic) and ground substance

    • Specialized cells include fibroblasts (produce fibers), fibrocytes (maintain fibers), adipocytes, leukocytes, mesenchymal cells (stem cells that differentiate into fibroblasts, adipocytes, etc.)

    • Ground substance ranges from viscous (e.g., in loose connective tissue) to mineralized (bone)

  • Ground substance and fibers

    • Collagen fibers: strong, large bundles; provide tensile strength

    • Reticular fibers: thinner, form networks

    • Elastic fibers: allow stretch and recoil

  • Major connective tissue proper types

    • Areolar connective tissue (loose): loose matrix with all three fiber types; supports epithelia; found beneath epithelia in serous and mucous membranes; papillary layer of the dermis

    • Adipose tissue: predominant adipocytes; stores fat, cushions, insulates; locations include beneath skin (hypodermis), around kidneys, mesenteries, mammary glands

    • Reticular connective tissue: network of reticular fibers forming a soft stroma for organs like liver, bone marrow, lymph nodes, spleen

    • Dense connective tissue

    • Dense regular connective tissue: collagen fibers parallel; predominant cells are fibroblasts/fibrocytes; forms tendons and ligaments; resists unidirectional stress

    • Dense irregular connective tissue: collagen fibers run in multiple directions; resists multi-directional stress; found in reticular layer of dermis and joint capsules

    • Elastic connective tissue: abundant elastic fibers; allows stretch and recoil (e.g., elastic arteries like the aorta; certain ligaments)

  • Cartilage

    • A supportive connective tissue with a firm yet flexible matrix

    • Chondrocytes reside in lacunae within the matrix

    • Matrix contains proteoglycans with chondroitin sulfate

    • Three major cartilage types:

    • Hyaline cartilage: most common; glassy matrix; found in the nose bridge, ribs to sternum (costal cartilage), tracheal rings

    • Fibrocartilage: high density of collagen; strong compression resistance; found in intervertebral discs (annulus fibrosus) and pubic symphysis

    • Elastic cartilage: high elastic fiber content; bent and returns to shape easily; found in the external ear (pinna), epiglottis, and Eustachian tube

  • Bone (osseous tissue)

    • Matrix with collagen fibers mineralized by calcium salts (hydroxyapatite)

    • Osteocytes sit in lacunae within a rigid matrix

    • Provides structure, protection, and supports the body; major component of the skeletal system

  • Blood and lymph (fluid connective tissues)

    • Blood: liquid connective tissue with plasma as the matrix; formed elements include

    • Erythrocytes (red blood cells): transport O2 and CO2

    • Leukocytes (white blood cells): defend against infection and debris

    • Thrombocytes (platelets): essential for hemostasis and clotting

    • Lymph: formed from extracellular fluid; resembles plasma but with lower protein; contains leukocytes; lacks erythrocytes

Membranes in the body

  • Tissue membranes: continuous sheets that cover or line body surfaces

  • Connective tissue membranes (synovial membranes)

    • Composed primarily of connective tissue; line joint cavities and produce synovial fluid to lubricate joints

  • Epithelial membranes

    • Mucous membranes (mucosa): line digestive, respiratory, urinary, and reproductive tracts; secret mucus from glands within epithelium

    • Serous membranes (serosa): line ventral body cavities; include parietal and visceral layers with serous fluid in the cavity

    • Cutaneous membrane: skin; epidermis (keratinized stratified squamous epithelium) over dermis

  • Epithelium specifics in membranes

    • Serous membranes: simple squamous epithelium (mesothelium) with loose connective tissue underneath (areolar)

    • Cutaneous membrane: epidermis is stratified squamous with keratin; dermis includes papillary (areolar) and reticular (dense irregular) layers

Integumentary system specifics

  • Cutaneous membrane (skin)

    • Epidermis: stratified squamous epithelium; keratin production increases as cells move toward surface; forms a protective outer layer

    • Dermis: beneath epidermis; contains papillary layer (areolar) and reticular layer (dense irregular)

  • Epidermal features

    • Keratin: fibrous protein; accumulates in/mature keratinocytes to create a tough, protective layer

    • Keratinization: formation of layered dead cells at the surface in keratinized regions (e.g., skin) for abrasion resistance

  • Mucous and serous membranes in relation to epithelium

    • Mucous membranes can vary with epithelia: simple columnar (intestines) or stratified squamous (mouth/esophagus)

    • Serous membranes are lined by simple squamous epithelium (mesothelium) with underlying connective tissue

Wound healing and tissue regeneration

  • Four overlapping phases of wound healing (in skin, but similar in other tissues)

    • Hemostasis: blood clotting to stop bleeding and cover wound surface

    • Inflammation: immune activation; leukocytes infiltrate to defend against infection and remove damaged tissue; cardinal signs: redness, swelling, heat, pain

    • Proliferation: cell division and tissue growth; fibroblasts lay down collagen; formation of granulation tissue; edges pulled together by wound contraction; angiogenesis (new blood vessels) occurs

    • Remodeling (tissue maturation): collagen fibers reorganized to increase strength; epithelial tissue grows from edges to cover surface; regeneration vs fibrosis determines outcome

  • Regeneration vs fibrosis

    • Regeneration: replacement by the same cell type, restoring original structure

    • Fibrosis (scar formation): connective tissue fills wound; can impede full restoration of original function

  • Tissue regenerative capacity

    • High regeneration: most epithelia; many connective tissues (e.g., bone)

    • Poor regeneration: cartilage (slow, limited), some connective tissues, CNS

    • PNS nervous tissue can regenerate; CNS (brain/spinal cord) generally cannot

  • Example outcomes

    • Skin wound: potential for regeneration with some fibrosis depending on wound size

    • Cardiac muscle: limited regenerative capacity; tends toward fibrosis after injury

  • Histology references for healing

    • Fibrosis involves collagen deposition by fibroblasts

    • Granulation tissue color and appearance due to neovascularization (angiogenesis)

Epithelial tissue types and classifications (detailed overview)

  • Based on layers and shape

    • Simple epithelia: one cell layer

    • Stratified epithelia: more than one cell layer

    • Shapes in the apical layer determine subtype: squamous (flat), cuboidal (cube-shaped), columnar (tall)

  • Simple epithelia examples

    • Simple squamous: single layer of flat cells; height < width

    • Simple cuboidal: single layer of cube-shaped cells; height ≈ width

    • Simple columnar: single layer of tall cells; height > width; often with microvilli

  • Stratified epithelia examples

    • Stratified squamous: multiple layers; apical surface cells are flat; keratinized in skin, nonkeratinized in mouth/esophagus/rectum/vagina

    • Stratified cuboidal: multiple layers; cells at apical surface are round; ducts of glands (e.g., salivary gland ducts)

    • Stratified columnar: multiple layers; apical cells tall; secretory functions

  • Transitional epithelium

    • Stratified with variable apical cell shapes (squamous, cuboidal, columnar) depending on stretch

    • Found in the ureter and parts of the bladder

  • Pseudostratified epithelium

    • Appears multilayered but is a single layer; all cells reach the basement membrane

    • Often ciliated; located in respiratory tract (trachea, nasal cavity, paranasal sinuses)

  • Endothelium and capillaries

    • Endothelium: simple squamous epithelium lining blood vessels; very thin to allow diffusion

Glandular epithelia: endocrine and exocrine glands

  • Endocrine glands

    • Produce hormones released into the bloodstream

    • Pancreatic islets are endocrine tissue that secretes insulin and glucagon

  • Exocrine glands

    • Produce secretions that exit the body or the lumen of a hollow organ

    • Examples: goblet cells (unicellular exocrine glands) secreting mucus via merocrine exocytosis; salivary glands secreting saliva; gastric glands secreting gastric juice; liver/biliary system and pancreas ducts for digestive secretions

  • Secretory mechanisms (overview)

    • Merocrine: secretions released by exocytosis without loss of cellular material (e.g., goblet cells, pancreatic acini, sweat glands)

    • Apocrine: secretion by loss of apical cytoplasm; cell remains; example: some mammary gland components

    • Holocrine: entire cell disintegrates to release product (e.g., sebaceous glands, certain glandular cells)

  • Additional gland examples in context

    • Pancreas as both endocrine and exocrine organ

    • Mammary glands as example of apocrine secretion in milk production

Histology of connective tissues and supporting structures

  • Areolar connective tissue (loose)

    • Loose matrix with all three fiber types (collagen, reticular, elastic)

    • Found supporting epithelia and lining serous/mucous membranes; under the epidermis (papillary dermis)

  • Adipose tissue

    • Predominant adipocytes; stores fat; cushions and insulates; energy reserve

    • Locations: subcutaneous layer (hypodermis), around kidneys, mesenteries, breast

  • Reticular connective tissue

    • Rich in reticular fibers forming a supportive network (reticular stroma) for organs like liver, bone marrow, lymph nodes, spleen

  • Dense connective tissue

    • Dense regular: tightly packed parallel collagen fibers; resists unidirectional stress; examples: tendons and ligaments

    • Dense irregular: irregularly arranged collagen fibers; resists multi-directional stress; found in reticular dermis and joint capsules

    • Elastic connective tissue: abundant elastic fibers; allows stretch and recoil (e.g., elastic arteries like the aorta)

  • Cartilage

    • Hyaline cartilage: glassy matrix; covers bone surfaces and forms costal cartilage; tracheal rings; nose bridge

    • Fibrocartilage: dense collagen; high compression resistance; intervertebral discs, pubic symphysis

    • Elastic cartilage: abundant elastic fibers; flexible and returns to shape; external ear (pinna), epiglottis, Eustachian tube

  • Bone tissue

    • Osseous tissue with mineralized matrix (hydroxyapatite) and collagen fibers

    • Osteocytes reside in lacunae; provide rigid support and protection; part of the skeletal system

  • Blood and lymph as connective tissues

    • Blood: plasma matrix with formed elements (RBCs, WBCs, platelets)

    • Lymph: derived from extracellular fluid; resembles plasma but with lower protein content; contains leukocytes but no erythrocytes

Nervous tissue (brief recap)

  • Neurons

    • Primary signaling cells for rapid communication

  • Glial cells (neuroglia)

    • Support, protect, and nourish neurons

  • Histology visuals (from the transcript)

    • Photomicrographs show large neurons and smaller surrounding glial cells in nervous tissue sections

Key histological features and terms to remember

  • Cell junctions in cardiac tissue

    • Intercalated discs contain desmosomes (mechanical junctions) and gap junctions (electrical coupling for coordinated contraction)

  • Capillaries and endothelium

    • Capillaries are tiny vessels lined by simple squamous endothelium, allowing rapid diffusion of nutrients and gases

  • Epithelium-dermis relationship in skin

    • Papillary layer (areolar) under epidermis; reticular layer (dense irregular) beneath the papillary layer

  • Keratinization in epidermis

    • Keratin provides abrasion resistance; keratinized stratified squamous epithelium forms the epidermis

  • Healing terminology of fibrosis and regeneration

    • Fibrosis = scar formation with dense connective tissue; may occur when regeneration is incomplete or wound is large

  • Organ-level examples linking structure to function

    • Tracheal mucosa: ciliated pseudostratified columnar epithelium with mucus movement via cilia

    • Ureter: transitional epithelium allows stretching

    • Uterine tubes: ciliated simple columnar epithelium facilitates movement of oocyte/zygote

    • Lungs: alveolar epithelium (simple squamous) enables gas exchange

Quick reference: common types and examples

  • Epithelial: simple squamous (alveoli, capillaries), simple columnar (intestines), pseudostratified columnar (trachea), stratified squamous keratinized (epidermis), stratified squamous nonkeratinized (mouth/esophagus), stratified cuboidal (ducts), stratified columnar (rare, glands)

  • Connective tissue: areolar (support under epithelia), adipose (fat storage), reticular (soft skeleton), dense regular (tendons/ligaments), dense irregular (dermis/joint capsules), elastic (elastic arteries), cartilage (hyaline, fibrocartilage, elastic), bone, blood, lymph

  • Muscle: skeletal (voluntary, striated), cardiac (involuntary, branched, intercalated discs), smooth (involuntary, nonstriated)

  • Glands and secretion: merocrine, apocrine, holocrine; goblet cells; endocrine vs exocrine pancreas; ducts and secretions

  • Healing: hemostasis, inflammation, proliferation, remodeling; regeneration vs fibrosis; CNS vs PNS regenerative capacity

Notation and formulas

  • Cellular and tissue counts cited in the transcript

    • Number of muscle tissue types: 3

    • Parietal and visceral serous layers: 2 layers

    • Simple epithelium layers: 1 layer (in simple epithelia)

    • Glandular secretion mechanisms: merocrine, apocrine, holocrine (three mechanisms)

  • Concepts that involve numbers or counts are presented as above in the notes where appropriate.