Tissues, How They Are Organized and Forms of Tissues

Cells exhibit a high degree of organization. However, in multicellular organisms, cells do not operate in isolation. Instead, they collaborate in groups of similar cells known as tissues. A tissue represents a collection of akin cells and their intercellular matrix that share a common embryonic origin and collaborate to perform specialized functions. The scientific field devoted to the study of tissues is known as Histology.

Various tissues within the body can be categorized into four primary types based on their function and structure. These are epithelial, connective, muscular, and nervous tissues.

Epithelial Tissue:

Epithelial tissue covers the body's surfaces, lines its cavities and ducts, and forms glands. It can be further divided into:

1. Covering and Lining Epithelium

2. Glandular Epithelium

Covering and Lining Epithelium:

This type of epithelium forms the outer surface of the body and the external surface of some internal organs. It lines body cavities, the respiratory and gastrointestinal tracts, blood vessels, and ducts. In collaboration with nervous tissue, it also constructs components of sensory organs. It serves as the source of gametes (egg and sperm).

Covering and lining epithelium is classified based on the arrangement of layers and cell shapes:

Arrangement of Layers:

a) Simple epithelium: A single layer specialized for absorption and minimal wear and tear.

b) Stratified epithelium: Multiple layers found in areas subjected to high degrees of wear and tear.

c) Pseudostratified epithelium: Appears to be multilayered but is actually single-layered.

Cell Shapes:

a) Squamous: Flat and scale-like.

b) Cuboidal: Cube-shaped.

c) Columnar: Tall and cylindrical.

d) Transitional: Adaptable shape, found in distensible areas.

Simple Epithelium:

- Simple Squamous Epithelium: Ideal for diffusion and filtration; found in air sacs of lungs, kidneys, blood vessels, and lymph vessels.

- Simple Cuboidal Epithelium: Covers ovaries, lens of the eye, retina, and kidney tubules.

- Simple Columnar Epithelium: Lines the gastrointestinal tract, gallbladder, and excretory ducts of glands; functions in secretion, absorption, protection, and lubrication.

Stratified Epithelium:

- Stratified Squamous Epithelium: Divided into keratinized (skin) and non-keratinized (mouth, tongue, vagina) types.

- Stratified Cuboidal Epithelium: Found in sweat gland ducts, eye conjunctiva, and male urogenital system.

- Stratified Columnar Epithelium: Occurs in mammary gland ducts and anus layers; functions in protection and secretion.

Transitional Epithelium:

Specialized for stretching; found in the urinary bladder, ureters, and urethra.

Pseudostratified Epithelium:

Lines excretory ducts, epididymis, parts of male urethra, and auditory tubes; offers protection and secretion.

Glandular Epithelium:

Mainly involved in secretion, glands can be exocrine (release into ducts) or endocrine (release into blood). Examples include sweat and salivary glands (exocrine) and pituitary, thyroid, and adrenal glands (endocrine).

Connective Tissue:

Connective tissues include embryonic and adult types. Embryonic connective tissue contains mesenchyme and mucous connective tissue. Adult connective tissue encompasses connective tissue proper, cartilage, bone, and blood.

a) Connective Tissue Proper: Various forms include loose (areolar), adipose, dense (collagenous), elastic, and reticular connective tissues, each serving different functions.

b) Cartilage: Types include hyaline, fibro, and elastic cartilage, providing support and maintaining shape.

c) Osseous Tissue (Bone): Comprises osteocytes embedded in mineral-rich intercellular substance; forms the skeletal system.

d) Vascular Tissue (Blood): A liquid connective tissue containing plasma, erythrocytes, leukocytes, and thrombocytes; involved in transport and defense.

Muscle Tissue:

Specialized cells for motion, posture, and heat production. Divided into skeletal, cardiac, and smooth muscle tissues.

- Skeletal Muscle: Attached to bones, voluntary, striated, and multinucleated.

- Cardiac Muscle: In the heart, involuntary, striated, and uninucleated.

- Smooth Muscle: Found in hollow internal structures, involuntary, non-striated.

Nervous Tissue:

Comprises neurons (nerve cells) and neuroglia (supporting cells). Neurons convert stimuli to nerve impulses and consist of cell bodies, axons, and dendrites. Neuroglia protect and nourish neurons.

Membranes:

Thin, flexible layers of epithelial and connective tissues. Three types include mucous, serous, and synovial membranes.

- Mucous Membranes: Line exterior-opening body cavities, secrete mucous.

- Serous Membranes: Cover organs within non-exterior body cavities, secrete lubricating fluid.

- Synovial Membranes: Found in freely movable joints, secrete synovial fluid for lubrication.