Unit 4: Epithelial Tissue and Connective Tissue

1. Epithelial Tissue: Types, Structure, Location, and Functions
2. Connective Tissue Proper: Loose and Dense
3. Specialised Connective Tissue

1. Epithelial Tissue: Types, Structure, Location, and Functions

Epithelial tissue (or epithelium) covers body surfaces, lines body cavities and hollow organs, and forms glands. Its cells are tightly packed with very little extracellular matrix.

General Functions: Protection, Secretion, Absorption, Excretion, Filtration, Diffusion, and Sensory reception.

Epithelium is classified based on cell shape and the number of layers.

A. Simple Epithelium (One Cell Layer Thick)

Type	Structure (Cell Shape)	Function	Location
Simple Squamous	Single layer of flat, scale-like cells.	Allows for rapid diffusion and filtration.	Air sacs (alveoli) of lungs, lining of blood vessels (endothelium).
Simple Cuboidal	Single layer of cube-shaped cells with a central spherical nucleus.	Secretion and absorption.	Kidney tubules, ducts of small glands, ovary surface.
Simple Columnar	Single layer of tall, column-shaped cells with an oval nucleus (usually basal).	Absorption and secretion. Can be ciliated (to move mucus) or non-ciliated (with microvilli).	Non-ciliated: Lining of digestive tract (stomach, intestine). Ciliated: Lining of bronchi, uterine tubes.
Pseudostratified Columnar	Single layer of cells of differing heights; all cells rest on the basement membrane, but not all reach the surface. Nuclei are at different levels.	Secretion (especially of mucus) and propulsion of mucus by cilia.	Lining of the trachea and most of the upper respiratory tract.

[Image of types of simple epithelial tissue]

B. Stratified Epithelium (Two or More Cell Layers Thick)

Type	Structure	Function	Location
Stratified Squamous	Multiple layers; basal cells are cuboidal/columnar, surface cells are flat (squamous).	Protection against abrasion.	Keratinized: Dry surface, e.g., epidermis of the skin. Non-Keratinized: Moist linings, e.g., mouth, esophagus, vagina.
Stratified Cuboidal	Two or more layers of cube-shaped cells. (Rare)	Protection and secretion.	Ducts of large glands (sweat glands, mammary glands).
Stratified Columnar	Multiple layers; basal cells cuboidal, surface cells columnar. (Rare)	Protection and secretion.	Rare in the body; small amounts in male urethra and in large ducts of some glands.
Transitional Epithelium	Resembles both stratified squamous and cuboidal. Basal cells are cuboidal; surface cells are dome-shaped (when relaxed) or squamous-like (when stretched).	Stretches readily and permits distension of the organ.	Lining of the ureters, bladder, and part of the urethra.

[Image of types of stratified epithelial tissue]

2. Connective Tissue Proper: Loose and Dense

Connective tissue is the most abundant and widely distributed tissue. It provides support, binds structures together, and protects. It is characterized by having abundant extracellular matrix (ECM) composed of fibers (collagen, elastic, reticular) and ground substance, with relatively few cells.

A. Loose Connective Tissue

Fibers are loosely arranged, with more ground substance and cells.

Areolar Tissue: The "model" connective tissue. A gel-like matrix with all three fiber types and various cells (fibroblasts, macrophages, mast cells).
- Function: Wraps and cushions organs; holds and conveys tissue fluid.
- Location: Widely distributed under epithelia; forms lamina propria.
Adipose Tissue: Matrix as in areolar, but dominated by adipocytes (fat cells) that store triglycerides.
- Function: Provides energy reserve, insulates, supports and protects organs.
- Location: Under skin (hypodermis), around kidneys, within abdomen, in breasts.
Reticular Tissue: A network of thin reticular fibers in a loose ground substance.
- Function: Forms a soft internal skeleton (stroma) that supports other cell types.
- Location: Lymphoid organs (lymph nodes, bone marrow, spleen).

B. Dense Connective Tissue

Fibers are the dominant component, densely packed with fewer cells and less ground substance.

Dense Regular: Primarily parallel collagen fibers; a few fibroblasts.
- Function: Attaches muscles to bones (tendons) and bones to bones (ligaments). Withstands great tensile stress in one direction.
- Location: Tendons, most ligaments.
Dense Irregular: Primarily irregularly arranged collagen fibers; some elastic fibers and fibroblasts.
- Function: Withstands tension exerted in many directions; provides structural strength.
- Location: Dermis of the skin, fibrous capsules of organs and joints.

3. Specialised Connective Tissue

This category includes connective tissues with specialized cells and unique ground substances, such as cartilage, bone, and blood.

A. Cartilage

A tough, flexible tissue that provides support and cushioning. It is avascular (lacks blood vessels). Cells are called chondrocytes, which reside in small cavities called lacunae within a gel-like matrix.

Hyaline Cartilage: Amorphous but firm matrix; collagen fibers are not visible. (e.g., embryonic skeleton, ends of long bones, nose, trachea, larynx).
Elastic Cartilage: Similar to hyaline, but with more elastic fibers. (e.g., external ear, epiglottis).
Fibrocartilage: Thick collagen fibers are prominent. (e.g., intervertebral discs, pubic symphysis).

B. Bone (Osseous Tissue)

A hard, calcified matrix containing many collagen fibers and cells called osteocytes, which lie in lacunae. It is well-vascularized. Forms the body's skeleton.

Function: Supports and protects, provides levers for muscles, stores calcium and minerals, and is the site of blood cell formation (marrow).

[Image of the structure of compact bone (Osteon)]

C. Blood

An atypical connective tissue, as it has a fluid matrix (plasma) and no fibers (except during clotting).

Cells:
- Erythrocytes (Red Blood Cells): Transport oxygen.
- Leukocytes (White Blood Cells): Involved in immunity.
- Platelets (Thrombocytes): Cell fragments involved in blood clotting.
Function: Transport of gases, nutrients, wastes, and hormones.