Histology, Part 1: Epithelial Tissues – Study Guide

Directional and Anatomical Terms Review

Directional Terms

Directional terms are used to describe the locations of structures in relation to other structures or locations in the body.

• Superior: Toward the head or upper part of a structure (e.g., the nose is superior to the lips).

• Inferior: Away from the head or toward the lower part of a structure.

• Anterior (ventral): Toward the front of the body (e.g., the sternum is anterior to the heart).

• Posterior (dorsal): Toward the back of the body.

• Medial: Toward the midline of the body (e.g., the eyes are medial to the ears).

• Lateral: Away from the midline of the body.

• Proximal: Closer to the point of attachment or origin (e.g., the knee is proximal to the ankle).

• Distal: Farther from the point of attachment or origin.

• Superficial: Toward or at the body surface (e.g., veins are superficial to arteries).

• Deep: Away from the body surface; more internal.

Anatomical Planes

Anatomical planes are imaginary lines used to divide the body into sections for descriptive purposes.

• Midsagittal (median) plane: Divides the body into equal right and left halves.

• Sagittal plane: Divides the body into right and left portions (not necessarily equal).

• Frontal (coronal) plane: Divides the body into anterior (front) and posterior (back) portions.

• Transverse (horizontal) plane: Divides the body into superior (upper) and inferior (lower) portions.

Body Cavities

The body contains two major closed body cavities:

• Ventral cavity: Includes the thoracic and abdominopelvic cavities.

• Dorsal cavity: Includes the cranial and spinal cavities.

The diaphragm separates the thoracic cavity from the abdominopelvic cavity.

Histology: The Study of Tissues

Definition and Importance

Histology is the study of the structure and function of tissues. Tissues are groups of cells with similar structure that perform a common or related function, representing the tissue level of structural organization in the body.

Four Primary Tissue Types

The human body is composed of four primary tissue types, each with distinct functions:

Tissue Type	Main Function	Examples/Locations
Nervous tissue	Internal communication	Brain, spinal cord, nerves
Muscle tissue	Contracts to cause movement	Skeletal muscles, cardiac muscle, smooth muscle in walls of hollow organs
Epithelial tissue	Forms boundaries, protects, secretes, absorbs, filters	Skin surface (epidermis), lining of digestive tract organs, glands
Connective tissue	Supports, protects, binds other tissues	Bones, tendons, fat, soft padding tissue

Epithelial Tissue (Epithelium)

Definition and Forms

Epithelial tissue is a sheet of cells that covers a body surface or lines a body cavity. There are two main forms:

• Covering and lining epithelium: Forms the outer layer of the skin, lines open cavities of the urinary, reproductive, digestive, and respiratory systems, and covers the walls and organs of the ventral body cavity.

• Glandular epithelium: Forms all exocrine glands.

Special Characteristics of Epithelial Tissues

• Apical-basal polarity: Epithelial cells have an apical surface (exposed to the exterior or cavity) and a basal surface (attached to the underlying connective tissue via the basement membrane).

• Specialized contacts: Cells are joined by tight junctions and desmosomes to form continuous sheets.

• Supported by connective tissue: The basement membrane anchors the epithelium and provides support.

• Avascular but innervated: Epithelial tissues lack blood vessels but are supplied by nerve fibers.

• High regenerative capacity: Epithelial cells can rapidly replace lost or damaged cells by cell division.

Classification of Epithelial Tissues

Epithelia are classified based on two criteria:

• Number of cell layers:

  • Simple: Single layer of cells

  • Stratified: Two or more layers of cells

• Cell shape:

  • Squamous: Flattened, scale-like cells with a flattened nucleus

  • Cuboidal: Box-like cells with a round nucleus

  • Columnar: Tall, column-shaped cells with an oval nucleus

Types of Epithelial Tissues

Simple Epithelia

• Simple squamous epithelium:

  • Main Functions: Provides a thin, permeable barrier for rapid diffusion and filtration (e.g., exchange of O2 and CO2 in alveoli).

  • Key Locations: Alveoli of lungs, lining of blood vessels (endothelium), lining of body cavities (mesothelium).

• Simple cuboidal epithelium:

  • Main Functions: Secretion and absorption; active transport due to abundant mitochondria.

  • Key Locations: Kidney tubules, thyroid follicles, ducts of small glands, ovary surface.

• Simple columnar epithelium:

  • Main Functions: Absorption (with microvilli), secretion of mucus and enzymes (goblet cells), movement of mucus (with cilia).

  • Key Locations: Lining of stomach, intestines, rectum, oviducts, and some gland ducts.

• Pseudostratified columnar epithelium:

  • Main Functions: Secretion (especially mucus), propulsion of mucus by ciliary action, protective barrier.

  • Key Locations: Trachea and most of the upper respiratory tract (ciliated), male reproductive tract (non-ciliated).

Stratified Epithelia

• Stratified squamous epithelium (non-keratinized):

  • Main Functions: Protects underlying tissues in areas subject to abrasion; less resistant to water loss.

  • Key Locations: Lining of mouth, esophagus, vagina, anal canal.

• Stratified squamous epithelium (keratinized):

  • Main Functions: Protects against abrasion, pathogens, and water loss; forms a tough, water-resistant layer.

  • Key Locations: Epidermis of skin (palms, soles, thin skin elsewhere).

• Transitional epithelium (urothelium):

  • Main Functions: Stretches to permit distension of urinary organs; provides a barrier against urine toxicity.

  • Key Locations: Urinary system only—renal calyces, renal pelvis, ureters, urinary bladder, proximal urethra.

Nervous Tissue (Overview)

Nervous tissue is specialized for communication by electrical and chemical signals.

• Neurons: Transmit electrical impulses from sensory receptors to effectors (muscles and glands).

• Glial cells: Support, protect, and nourish neurons.

• Key Locations: Brain, spinal cord, peripheral nerves.

Summary Table: Epithelial Tissue Types

Type	Main Function	Key Locations
Simple squamous	Diffusion, filtration	Alveoli, blood vessels, serous membranes
Simple cuboidal	Secretion, absorption	Kidney tubules, thyroid, ducts
Simple columnar	Absorption, secretion	Digestive tract, uterus, glands
Pseudostratified columnar	Secretion, movement of mucus	Respiratory tract, male reproductive tract
Stratified squamous (non-keratinized)	Protection	Mouth, esophagus, vagina
Stratified squamous (keratinized)	Protection, water resistance	Skin (epidermis)
Transitional	Stretching, protection	Urinary bladder, ureters

Lab Application: Epithelial Tissue Identification

• Identify epithelial tissue types under the microscope.

• Label apical and basal surfaces, nuclei, and any special structures (microvilli, cilia).

• Associate tissue type with function and location.

Additional info: For exam preparation, students should be able to recognize epithelial tissue types in histological images, describe their functions, and relate their structure to their location in the body.