Histology and Epithelial Tissue Overview

Histology Overview

• Histology is the study of tissues in the body
• Tissues develop from the three embryonic layers: ectoderm, mesoderm, and endoderm
• Five basic tissue types emerge from these layers: epithelial, connective, vascular, muscular, and nervous tissue

Levels of Organization

• The levels of organization are Cells, Tissues, Organs, Organ Systems, and Organism
• Cells make up tissues
• Tissues form organs
• Organs work together in organ systems
• Organ systems combine to form an organism

Epithelial Tissue

• Characteristics: Composed mainly of cells; covers body surfaces, lines hollow organs, and forms glands (e.g., outside body surface, digestive system lining).
• Polarity: Has apical, basal, and lateral surfaces
• Basement Membrane: Rests upon this membrane
• Specialized Cell Contacts: Bind adjacent cells together
• Avascular: No blood vessels
• Mitotic Ability: Replaces lost cells through cell division

Epithelial Structure

• Apical Surface: Free surface facing outside or open space
• Basement Membrane: Underlying non-cellular layer that anchors the epithelium

Functions of Epithelia

• Protection: e.g., skin, epithelium lining the mouth
• Barrier: e.g., skin
• Passage Permitting: e.g., cells lining air sacs in lungs, nephrons in kidneys
• Secretion: e.g., pancreatic cells
• Absorption: e.g., lining of stomach and small intestine

Classifications of Epithelia

• Number of Layers: Simple (one layer), stratified (more than one layer), pseudostratified (appears stratified but all cells touch the basement membrane)
• Shape of Cells: Squamous (flat), cuboidal (equal height and width), columnar (taller than wide)

Simple Columnar Epithelium

• Description: Single layer of tall cells with oval/round nuclei, sometimes with cilia or goblet cells (mucus-secreting).
• Function: Absorption, secretion of mucus, enzymes, and other substances. Ciliated type propels mucus.
• Location: Lines most of the digestive tract, gallbladder, excretory ducts, small bronchi, uterine tubes, and some regions of the uterus

Stratified Epithelia

• Characteristics: Contain two or more layers of cells; regenerate from below; major role in protection; named based on apical cell shape.

Stratified Squamous Epithelium

• Function: Protects underlying tissues in areas subject to abrasion.
• Location: Keratinized forms epidermis; non-keratinized lines esophagus and mouth.

Stratified Squamous Epithelium (detail)

• Description: Thick membrane with several cell layers; basal cells are cuboidal/columnar (metabolically active); surface cells are flattened (squamous); keratinized types are full of keratin and dead; basal cells actively multiply and produce more superficial layers.

Epithelium: Glandular

• Glands: One or more cells that makes and secretes an aqueous fluid
• Endocrine Glands: Ductless; release hormones into the bloodstream (e.g., pituitary, thyroid, adrenal, pancreas).
• Exocrine Glands: Have ducts; release secretions to the body surface or into body cavities (e.g., sweat, oil). Classified by structure (unicellular, multicellular) or method of secretion (merocrine, holocrine, apocrine).

Unicellular and Multicellular Glands (classification)

• Unicellular: Goblet cells (e.g. single cells)
• Multicellular: Classified by structure (simple tubular, simple coiled tubular, simple branched alveolar, compound tubular, simple alveolar, compound alveolar, compound tubuloalveolar)

Method of Secretion

• Merocrine: Products released by exocytosis (most glands)
• Holocrine: Cell ruptures and releases the product (sebaceous glands)
• Apocrine: Top of the cell pinches off (possibly in lactating mammary glands)

Connective Tissue

• Functions: Enclose organs, separate organs into layers, connect tissues, support and movement (bones), storage fat, insulation, transport, and protection (bone)

Structural Elements of Connective Tissue

• Ground Substance: Unstructured material fills the space between cells
• Fibers: Collagen, elastic, or reticular
• Cells: Fibroblasts, chondroblasts, osteoblasts, and others

Areolar Connective Tissue

• Description: Gel-like matrix with all three fiber types; cells include fibroblasts, macrophages, mast cells, and some white blood cells.
• Function: Wraps and cushions organs, macrophages phagocytize bacteria, plays a role in inflammation, and holds/conveys tissue fluid.
• Location: Widely distributed under epithelia, lamina propria of mucous membranes, around kidneys and eyeballs, within abdomen, in breasts

Adipose Tissue

• Description: Matrix is sparse; closely packed adipocytes (fat cells) with nucleus pushed to the side by large fat droplet.
• Function: Reserve fuel, insulates, supports and protects organs.
• Location: Under skin, around kidneys and eyeballs, within abdomen, breasts

Hyaline Cartilage

• Description: Amorphous matrix; collagen fibers are imperceptible; chondroblasts produce the matrix and mature into chondrocytes (in lacunae).
• Function: Supports, reinforces, has resilient cushioning properties, resists compressive stress.
• Location: Embryonic skeleton, ends of long bones, costal cartilages, nose, trachea, and larynx

Bone Tissue

• Description: Hard, calcified matrix containing collagen fibers; osteocytes lie in lacunae; very well vascularized.
• Function: Supports and protects (encloses), provides levers for muscles, stores calcium and other minerals and fat, and marrow inside bones is for blood cell formation (hematopoiesis).
• Location: Bones

Muscle Tissue

• Characteristics: Cells called fibers; contract or shorten with force when stimulated; move body and pump blood.
• Types:
  • Skeletal: attached to bones
  • Cardiac: muscle of the heart
  • Smooth: associated with tubular structures and skin; nonstriated and involuntary.

Cardiac Muscle Tissue

• Description: Branching, striated, generally uninucleate cells with specialized junctions (intercalated discs).
• Function: As it contracts, it propels blood into the circulation; involuntary control.
• Location: Walls of the heart

This quiz covers the basics of histology, focusing on various tissue types and their organization within the body. It delves into the specifics of epithelial tissue, including its characteristics, structure, and function. Perfect for students delving into human biology and anatomy.