Histology of Epithelium, Connective Tissue, and Membranes

All organs in the body are composed of what four tissues?

1. Epithelial

2. Connective

3. Muscular

4. Nervous

What are the common locations of epithelial tissue?

Epidermis, linings of the gut, body cavities (pleural, peritoneal, pericardial), and blood vessels.

What are the common features of all epithelia?

1. Close opposition of cells

2. Free (apical) surface facing a space

3. Basal surface adjacent to connective tissue

4. No blood vessels (One exception: inner ear)

5. May form into glandular structures

What are the functions of Epithelia?

A. Protection

B. Lubrication

C. Absorption

D. Secretion

E. Sensory Reception

F. Gas Transfer

G. Ion Transport

What are the two ways to classify Epithelia?

1. By the number of layers

2. By the shape of the cells

What is the difference between simple, stratified, and pseudostratified epithelia?

Simple epithelia only has one layer of cells while stratified epithelia can have 2 or more layer of cells. Pseudostratified and stratified epithelia differ in that the first can have their nuclei arranged at different levels while in stratified the nuclei are all uniform.

What are the 4 shapes/types of epithelia?

1. Squamous (Flattened)

2. Cuboidal (equal height and width)

3. Columnar (taller than they are wide)

4. Transitional (able to change shape)

Where is the simple squamous epithelium found?

Endothelium (lines internal surface of vessels), mesothelium (lines external surface of body cavities)

Where is stratified squamous epithelium found?

Esophagus, cornea, epidermis

Where is pseudostratified columnar epithelium found?

Respiratory tract

Where is simple cuboidal epithelium found?

Glands and ducts

Where is simple columnar epithelia found?

GI tract and collecting ducts

Where is the transitional epithelium found?

Urinary Tract

What is a goblet cell?

A mucus secreting cell found in the GI and respiratory tract (Contains a glycoprotein, mucin, which secretes mucus)

What are the major surface domains of epithelial cells?

1. Apical (free side)

2. Lateral (adjacent to another epithelial cell, has intercellular space)

3. Basal (adjacent to connective tissue)

What is the function of microvilli?

Increase surface area for absorption/secretion; appear as a brush or striated border (unable to see individual microvilli)

What are cilia and what is their function?

Longer than microvilli and function to move fluid or mucus (Not as uniform and regular)

What are stereocilia, what is their function, and where are they found?

They are long-microvilli like structures (you can see individual strands) that are involved in absorption/secretion. Found in epididymis of male reproductive tract and some sensory epithelia (ear)

What is keratin and where is it found?

Keratin is a protective protein found in stratified squamous epithelium (e.g. skin) that protects against abrasion or desiccation.

What defines glandular epithelium?

They are formed by epithelial cells that secrete fluid (different composition than blood or intercellular fluid)

Secreted substance can also be ions, polypeptides/proteins, lipids, or glycoproteins.

What are the two major types of multicellular glandular epithelia?

1. Endocrine glands

2. Exocrine glands

How are endocrine and exocrine glands different from each other?

Endocrine glands are ductless and secret hormone into the bloodstream while Exocrine glands retain their ducts and secrete onto a surface via the ducts.

What are the 3 modes of secretion for glandular epithelia?

1. Merocrine

2. Apocrine

3. Holocrine

How are merocrine, apocrine, and holocrine modes of secretion different?

Merocrine secretion is exocytosis of granules without cytoplasm loss, apocrine secretion is secretion with a potion of apical cytoplasm, and holocrine secretion is the entre cell is secreted into lumen of gland

What are serous secreting cells and where are they found?

Cells that secrete a watery, protein-rich fluid called serous fluid, commonly found in glands as they are a form of glandular epithelia

What type of epithelial membrane do serous cells have?

Simple squamous epithelium

What are key characteristics of serous cells?

Often triangular shaped

Basophilic basal region (dark purple due to rER)

Acidophilic apical region (pink due to secretory granules)

Round nuclei at base

What are mucous secreting cells and where are they found?

Cells that contain mucin while produces and releases mucus. Found in respiratory tract and digestive tract

What type of epithelial shape are most mucus-secreting cells?

Columnar

What are key characteristics of mucus-secreting cells?

Cells often columnar in shaped

Mucin content usually pale staining with most dyes

Flatted nuclei at base

What are the main functions of connective tissue?

Support for epithelia and organs by providing tensile strength to area subjected to mechanical stress

Pack and fill spaces

Encapsulate other tissue

Storage and transport site for water, ions, and inorganic material

Defense and wound repair

What are the 3 basic components of connective tissue?

Cells, fibers, and ground substance (together called the extracellular matrix)

What determines the type of connective tissue?

The cell types, the amount of fibers and/or the amount of ground substance

What is the main function of fibroblasts?

They produce fibers (collagen, elastic, reticular) and the intercellular matrix

What is the appearance of fibroblasts under a light microscope?

Large, oval-shaped nucleus with cytoplasm hard to discern

What other cell types may be found in loose connective tissue?

Eosinophils, neutrophils, and lymphocytes (underneath the skin + inside organs)

What is the most abundant protein produced by fibroblasts?

Collagen

How do collagen fibers appear in hematoxylin and eosin (H&E) stains?

Eosinophilic (pink-staining; negatively charged)

What is the hierachy of assembly for collagen?

(Smallest) Fibril —> Fiber —> Bundles of fiber (Largest)

What are elastic fibers composed of and what makes them unique?

Made by fibroblasts; Composed of desmosine and isodesmosine which give them rubber-like qualities

How do elastic fibers appear in H&E staining?

Usually unstained but silver stain helps

How are Reticular fibers different than collagen and elastic fibers?

Reticular fibers offer strength as well as a little flexibility

What are key features of a reticular fiber?

Smaller diameter than collagen

Lack periodicity

Argyrophilic (silver) and PAS-positive for staining

Provide a flexible network of support

Where are reticular fibers found?

Glands and lymphoid organs

What produces is ground substance and what is it?

Produced by fibroblasts; fills spaces between fibers and cells

What are the functions of ground substance?

Retention of water and ions

Regulates diffusion rate of substances in loose connective tissue

What is mesenchyme and its role?

Loosely arranged cells from the mesoderm (embryonic tissue) that differentiate int various connective tissue; fills space between other structures and produces various intercellular materials of connective tissue

What is a key feature of mesenchyme?

They are mobile cells and can travel freely

What is mucoid tissue and where is it found?

Undifferentiated fibroblasts that are gelatinous; Found in the umbilical cord and act as a packing material around the vessels (Wharton’s jelly)

What is loose (areolar) connective tissue?

Flexible tissue which contain lots of cells, ground substance, and collagen in many directions; Not resistant to mechanical stress

Where is loose (areolar) tissue found?

Found under epithelium of trachea, esophagus, and gastrointestinal systems

What is dense irregular tissue?

A type of connective tissue arranged in an irregular pattern

Where is dense irregular tissue found?

Dermis of skin and deep fascia (surrounding tends and nerves); Able to withstand some mechanical stress

What is dense regular tissue?

A type of connective tissue arranged in a regular, parallel fashion; Able to withstand lots of mechanical stress

Where is dense regular connective tissue found?

Tendons and ligaments

What is reticular connective tissue?

A type of connective tissue made up of reticular fibers that provide a delicate yet supportive structure to cells

Where is reticular connective tissue found?

Found in glands, lymph nodes, spleen, liver

What is elastic connective tissue?

A type of tissue composed of elastic fibers and collagen fibers that provides resilience and flexibility

Where is elastic connective tissue found?

Found in dermis, walls of arteries, and vocal cords

What is the function of adipose tissue?

Fat-storing tissue for energy and cushioning

What are the types of hard connective tissue?

Bone and 3 types of cartilage

What are body membranes?

Structures that cover external surfaces or line internal surfaces of body

What are body membranes made up of?

Epithelial tissue overlying connective tissue

What are the 3 types of body membranes?

1. Mucous

2. Serous

3. Cutaneous

What is the man function of mucous membranes

Protect and lubricate the body’s internal surface via secretions

Note: Some mucous membranes do not secrete mucous

Where are mucous membranes found?

Line the internal surfaces of visceral (hollow) organs such as in the digestive, respiratory, and urogenital tracts

What are mucous membranes composed of?

Epithelium and underlying connective tissue called lamina propria

Are mucous membranes continuous with the exterior of the body?

Yes, at locations such as the nostrils, lips, ears, genital area, and anus

What is the structure of a serous membrane?

Thin layer of epithelial cells (simple squamous) overlying connective tissue

What is the function of serous membranes?

Secretes serous fluid

Where are serous membranes found?

External surface of internal organs

Examples: Pleural cavity, pericardium cavity, and peritoneal cavity

What is mesothelium?

The epithelial component of serous membranes; simple squamous cells (produces serous fluid)

What is the role of connective tissue in serous membranes?

It provides blood vessels and nerves for the overlying cells and binds the membrane to organs or other structures.

What are the 2 layers of a serous membrane?

Visceral layer: in contact with the organ

Parietal layer: in contact with the body wall

What are the 3 serous cavities in the body?

1. Pleural cavity (lungs)

2. Pericardial cavity (heart)

3. Peritoneal cavity (abdominal organs)

What is mesothelioma and what causes it?

A cancer of the mesothelium, often caused by asbestos exposure, most commonly affecting the pleura or peritoneum

What is the structure of a cutaneous membrane?

Keratinized stratified squamous epithelium (epidermis) overlying a connective tissue layer (dermis).

What is the only example of cutaneous membrane?

Skin

What is the function of the cutaneous membrane?

It serves as an anatomical barrier between the internal and external environments and is important in bodily defense

What is the hypodermis?

Also called the subcutaneous layer or superficial fascia, it lies beneath the dermis and provides insulation and cushioning.