Chapter 4 Histology: Quick Reference Notes

Epithelial Tissue

Epithelial tissue forms boundaries between environments and lines body surfaces. It is avascular, innervated, and has high regenerative capacity due to wear and tear on apical surfaces. Epithelia rest on a nonliving basal lamina and are supported by the connective tissue beneath, with a basement membrane consisting of the basal lamina plus the reticular lamina.

Two main types by location are 2: covering and lining epithelia and glandular epithelia. Covering/lining epithelia line external surfaces (skin) and internal surfaces (open cavities of cardiovascular, digestive, and respiratory systems) and form walls and organs of the ventral body cavity. Glandular epithelia are secretory tissues in glands. Epithelial cells fit tightly to form continuous sheets and join via desmosomes and tight junctions; gap junctions allow small molecule passage between cells.

Cell organization and surfaces

Epithelia have apical (free) and basal (attached) surfaces. The apical surface may bear microvilli (e.g., brush border of intestinal lining) or cilia (e.g., tracheal lining). The basal surface rests on a basal lamina composed of glycoproteins and collagen, with a reticular lamina deep to it.

Classification by layers and shape

Epithelia are named by layers (simple vs stratified) and by cell shape (squamous, cuboidal, columnar). Nuclei often reflect cell shape and help identify the tissue when membranes are not clear.

Simple epithelia

Simple epithelia mainly function in absorption, secretion, and filtration. They consist of a single cell layer.

  • Simple squamous: thin and permeable; rapid diffusion; locations include kidneys (filtration membranes) and lungs (gas exchange).

  • Simple cuboidal: forms smallest ducts of glands and many kidney tubules.

  • Simple columnar: lines most of the digestive tract; may contain goblet cells.

  • Pseudostratified columnar epithelium: cells of varying heights giving a false impression of multiple layers; often ciliated with goblet cells; lines much of the respiratory tract.

Stratified epithelia

Stratified epithelia provide protection and have multiple cell layers.

  • Stratified squamous: apical layer is squamous; beneath are cuboidal or columnar; keratinized (skin) and nonkeratinized (mouth, esophagus, vagina).

  • Stratified cuboidal: rare; found in some ducts.

  • Stratified columnar: limited distribution (pharynx, male urethra, some glandular ducts).

  • Transitional epithelium: lines hollow urinary organs; cells change shape with stretch.

Glandular epithelia

Glandular epithelia are secretory tissues in glands and can be classified by

  • Site of product release: endocrine (ductless, hormones) and exocrine (secretions onto surfaces or cavities).

  • Number of cells: unicellular (e.g., goblet/mucus glands) or multicellular.
    Unicellular glands include goblet cells that secrete mucin to form mucus.

Glandular secretion modes

  • Merocrine: secretions released by exocytosis (e.g., pancreas, sweat, salivary glands).

  • Holocrine: secretions via rupture of gland cells (e.g., sebaceous glands).

Other epithelial tissue notes

Epithelial tissues are avascular, innervated, and regenerative. In lab, the shape of a cell can be inferred from its nucleus when the plasma membrane is not clearly visible. In various tissues, apical surfaces may show microvilli or cilia, and the basement membrane defines the boundary between epithelium and underlying connective tissue.

Connective Tissue

Connective tissue is the most abundant and widely distributed tissue type. It provides binding and support, protection, storage, insulation, and transportation. It arises from the embryonic mesenchyme and ranges from avascular to highly vascular. It consists mainly of nonliving extracellular matrix that separates cells and bears mechanical loads.

There are 四4 major classes of connective tissue: connective tissue proper, cartilage, bone, and blood.

Structural elements

Connective tissue contains:

  • Ground substance: unstructured material filling space between cells; contains interstitial fluid, cell-adhesion proteins, and proteoglycans that trap water, affecting viscosity.

  • Fibers: collagen (high tensile strength), elastic (stretch/recoil), and reticular (support a soft 3D framework).

  • Cells: immature blasts (produce matrix) and mature cytes (maintain tissue, can revert to blasts for repair).

The extracellular matrix (ECM) is Ground Substance + Fibers.

Connective tissue proper: loose and dense

  • Loose connective tissue: Areolar, Adipose, Reticular

    • Areolar: gel-like matrix; wraps and cushions organs; widely distributed under epithelia; surrounds capillaries.

    • Adipose: sparse matrix; adipocytes store fat; insulates and protects; locations include hypodermis, around kidneys/eyeballs, abdomen, breasts.

    • Reticular: network of reticular fibers forming a soft internal skeleton (stroma) for lymphoid organs.

  • Dense connective tissue: Dense regular, Dense irregular, Elastic

    • Dense regular: parallel collagen fibers; attaches muscles to bones or bones to bones; withstands unidirectional tensile stress; tendons, ligaments.

    • Dense irregular: irregular collagen fiber arrangement; withstands multidirectional tension; dermis, joint capsules.

    • Elastic: dense regular with many elastic fibers; allows recoil (artery walls, ligaments of the spine, bronchial tubes).

Cartilage

Cartilage is avascular and aneural; chondrocytes reside in lacunae and are produced by chondroblasts.

  • Hyaline: firm, resilient; most common; supports embryonic skeleton; covers ends of long bones; costal cartilages; trachea, nasal cartilages.

  • Elastic: includes elastic fibers; maintains shape with flexibility (external ear, epiglottis).

  • Fibrocartilage: dense collagen; withstands heavy pressure (intervertebral discs, pubic symphysis, knee menisci).

Bone (osseous tissue)

Bone provides rigid support and protection; it is highly vascular and innervated. The matrix is mineralized with calcium salts; osteons (concentric lamellae around central canals) form functional units. Cells include osteoblasts (produce matrix) and osteocytes (maintain matrix within lacunae).

Blood

Blood is a connective tissue with a liquid matrix (plasma) and cells (erythrocytes, leukocytes). It transports gases, nutrients, and wastes and resides within vessels.

Muscle Tissue

Muscle tissue is highly cellular and vascular, specialized for movement, and organized around myofilaments (actin and myosin). There are three types:

  • Skeletal: long, cylindrical, multinucleate cells with obvious striations; voluntary control; attaches to bones or skin.

  • Cardiac: branching, striated, usually single nucleus; intercalated discs; involuntary; found in the heart walls.

  • Smooth: spindle-shaped cells with central nuclei; no striations; involuntary; found in walls of hollow organs.

Nervous Tissue

Nervous tissue contains neurons and supporting glial (nonconductive) cells. Neurons generate and conduct electrical impulses; glial cells support, insulate, and protect neurons. Location: brain, spinal cord, and nerves.

Covering and Lining Membranes

  • Cutaneous membrane (skin): keratinized stratified squamous epithelium attached to a thick layer of dense irregular connective tissue; a dry membrane.

  • Mucous membranes: line body cavities opening to the exterior; lines may be stratified squamous or simple columnar epithelia.

  • Serous membranes: simple squamous epithelium on a thin layer of areolar (loose) connective tissue.

Tissue Repair

Tissue repair occurs by two main processes: regeneration (replacement with the same tissue) and fibrosis (replacement with scar tissue). Fibrosis yields a stronger but less flexible tissue.

Steps in tissue repair

1) Inflammation: inflammatory chemicals are released; vessels dilate and become more permeable; clot forms a scab. 2) Organization: clot is replaced by granulation tissue with capillaries; epithelium regenerates; fibroblasts lay down collagen; debris is phagocytized. 3) Regeneration and fibrosis: scab detaches; fibrous tissue matures; epithelium thickens; scar tissue forms under.

Regenerated epithelium with underlying scar tissue is a common outcome. Fibrosis dominates in tissues with poor regenerative capacity and yields strong but less functional tissue.

Regenerative capacity by tissue type

  • High healing capacity: epithelia, bone, areolar connective tissue, dense irregular connective tissue, blood-forming tissue

  • Moderate capacity: smooth muscle and dense regular connective tissue

  • Weak or none: cartilage (avascular), cardiac muscle, nervous tissue

Tissue Development

Embryonic cells differentiate to form the germ layers (ectoderm, mesoderm, endoderm), from which all tissues and organs develop.