Considering the number of layers and cell shape, covering and lining epithelium can be classified into:

Simple Epithelium

• Simple Squamous epithelium: Single layer of flat, scale-like cells resembling a tiled floor. Adapted for diffusion, osmosis, and filtration. Lines air sacs of the lung, kidneys, blood vessels, and lymph vessels.
• Simple cuboidal epithelium: Flat polygon that covers the surface of the ovary, lines the anterior surface of the lens of the eye, retina, and tubules of the kidney.
• Simple columnar epithelium: Similar to simple cuboidal, modified depending on location and function. Lines the gastro-intestinal tract, gall bladder, and excretory ducts of many glands. Functions in secretion, absorption, protection, and lubrication.
• Stratified Epithelium:More durable, protects underlying tissues from the external environment and from wear and tear.
  • Stratified Squamous epithelium: Outer cells are flat. Subdivided into Non-Keratinized and Keratinized types.
  • Non-Keratinized stratified squamous epithelium: Found in wet surfaces subjected to considerable wear and tear (e.g., mouth, tongue, and vagina).
  • Keratinized stratified squamous epithelium: The surface cell forms a tough layer of material containing keratin. Example: skin.Keratin is a waterproof protein that resists friction and bacterial invasion.
  • Stratified cuboidal epithelium: A rare type of epithelium found in sweat gland ducts, the conjunctiva of the eye, the cavernous urethra of the male urogenital system, the pharynx, and the epiglottis. Its main function is secretion.
  • Stratified columnar epithelium: Uncommon in the body. Found in the milk ducts of the mammary gland and anal layers. Functions in protection and secretion.

Transitional Epithelium

• The distinction is that cells of the outer layer tend to be large and rounded rather than flat.
• This feature allows the tissue to stretch without breakage. Found in the urinary bladder, part of the ureters, and the urethra.

Pseudo Stratified Epithelium

• Lines the larger excretory ducts of many glands, epididymis, parts of the male urethra, and auditory tubes.Its main function is protection and secretion.

Glandular Epithelium

• Their main function is secretion. A gland may consist of one cell or a group of highly specialized epithelial cells.
• Glands can be classified into exocrine and endocrine according to where they release their secretion.

Exocrine: Glands that empty their secretion into ducts/tubes that empty at the surface of covering. Their main products are mucous, oil, wax, perspiration, and digestive enzymes. Sweat and salivary glands are exocrine glands.

Endocrine: Glands that ultimately secrete their products into the blood system. The secretions of endocrine glands are always hormones. Hormones are chemicals that regulate various physiological activities. Pituitary, thyroid, and adrenal glands are endocrine.

 Classification of Exocrine Glands

They are classified by their structure and the shape of the secretory portion. According to structural classification, they are grouped into:

• Unicellular gland: Single-celled. The best examples are goblet cells in the Respiratory, Gastrointestinal, and Genitourinary systems.
• Multicellular gland: Found in several different forms. By looking into the secretory portion, exocrine glands are grouped into:
  • Tubular gland: If the secretory portion of a gland is tubular.
  • Acinar gland: If the secretory portion is flask-like.
  • Tubulo-acinar: If it contains both tubular and flask-shaped secretory portions.

(II). Connective Tissue

Connective tissues of the body are classified into embryonic connective tissue and adult connective tissue.

Embryonic Connective Tissue

• Mesenchyme: The tissue from which all other connective tissue eventually arises. Located beneath the skin and along the developing bone of the embryo.
• Mucous (Wharton’s Jelly) connective tissue: Found primarily in the fetus and located in the umbilical cord of the fetus where it supports the cord.

Adult Connective Tissue

• It is differentiated from mesenchyme and does not change after birth. Adult connective tissue comprises connective tissue proper, cartilage, osseous (bone), and vascular (blood) tissue.

a) . Connective Tissue Proper

Connective tissue proper has a more or less fluid intercellular martial and fibroblast. The various forms are:

• • Loose (areolar) connectives tissue: Widely distributed and consists of collagenic, elastic, and reticular fibers and several cells embedded in semi-fluid intercellular substances. It supports tissues, organ blood vessels, and nerves. It also forms the subcutaneous layer/superficial fascia/hypodermis.
  • Adipose tissue: The subcutaneous layer below the skin, specialized for fat storage. Found where there is loose connective tissue.It is common around the kidney, at the base and on the surface of the heart, in the marrow of long bone, as padding around joints, and behind the eyeball. It is a poor conductor of heat,so it decreases heat loss from the body.
  • Dense (Collagenous) connective tissue: Fibers are more closely packed than in loose connective tissue.Exists in areas where tensions are exerted in various directions. In areas where fibers are interwoven without regular orientation, the forces exerted are in many directions.This occurs in most fascia like the deeper region of the dermis, periosteum of bone, and membrane capsules. In other areas, dense connective tissue adapts to tension in one direction,and fibers have a parallel arrangement. Examples are tendons and ligaments. Dense connective tissues provide support and protection and connect muscle to bone.
  • Elastic connective tissue: Possesses freely branching elastic fibers. They stretch and snap back into their original shape.They are components of the walls of arteries, trachea, bronchial tubes, and lungs. It also forms the vocal cord. Elastic connective tissue allows stretching and provides support and suspension.
  • Reticular connective tissue: A lattice of fine, interwoven threads that branch freely, forming a connecting and supporting framework. It helps form a delicate supporting stoma     for many organs, including the liver, spleen, and lymph nodes. It also helps bind together the fibers (cells) of smooth muscle tissue.

b). Cartilage

Unlike other connective tissues, cartilage has no blood vessels and nerves. It consists of a dense network of collagenous fibers and elastic fibers firmly embedded in chondroitin sulfate. The strength is because of collagenous fibers. The cells of mature cartilage are called chondrocytes. The surface of cartilage is surrounded by irregularly arranged dense connective tissue called the perichondrium. Cartilages are classified into hyaline, fibro, and elastic cartilage.

Hyaline cartilage: Called gristle, most abundant, blue-white in color, and able to bear weight. Found at joints over long bones as articular cartilage and forms costal cartilage (at the ventral end of ribs). It also forms the nose, larynx, trachea, bronchi, and bronchial tubes. It forms the embryonic skeleton, reinforces respiration, aids in the free movement of joints, and assists the rib cage to move during breathing.

Fibro cartilage: Found at the symphysis pubis, in the inter-vertebral discs, and knee. It provides support and protection.

Elastic cartilage: In elastic cartilage, the chondrocytes are located in a thread-like network of elastic fibers. Elastic cartilage provides strength and elasticity and maintains the shape of certain organs like the epiglottis, larynx, external part of the ear, and Eustachian tube.

c). Osseous Tissue (Bone)

The mature bone cells, osteocytes, are embedded in the intercellular substance consisting of mineral salts (calcium phosphate and calcium carbonate) with collagenous fibers. The osseous tissue together with cartilage and joints comprises the skeletal system.

d). Vascular Tissue (Blood Tissue)

It is a liquid connective tissue. It contains intercellular substance plasma. Plasma is a straw-colored liquid, consists of water and dissolved material.

The formed elements of the blood are erythrocytes, leukocytes, and thrombocytes. The fibrous characteristics of blood are revealed when clotted.

(III). Muscle Tissue

Muscle tissue consists of highly specialized cells, which provide motion, maintenance of posture, and heat production. Classification of muscles is made by structure and function. Muscle tissues are grouped into skeletal, cardiac, and smooth muscle tissue.

Skeletal muscle tissue: Attached to bones, it is voluntary, cylindrical, multinucleated, and striated.

Cardiac muscle tissue: It forms the wall of the heart; it is involuntary, uninucleated, and striated.

Smooth muscle tissue: Located in the wall of hollow internal structures like blood vessels, stomach, intestine, and urinary bladder. It is involuntary and non-striated.

(IV). Nervous Tissue

• Nervous tissue contains two principal cell types: neurons and neuroglia. Neurons are nerve cells, sensitive to various stimuli.
• They convert stimuli to nerve impulses. Neurons are the structural and functional unit of the nervous system.
• It contains 3 basic portions: cell body, axons, and dendrites. Neuroglia are cells that protect, nourish, and support neurons. Clinically, they are important because they have the potential to replicate and produce cancerous growths.

Membranes