Epithelial tissue
Epithelial tissue is a protective covering tissue that lines the outer surface of organs, blood vessels, and body cavities. It has little intercellular matrix and rests on a basement membrane.
Functions of Epithelial Tissues
- Protection: Epithelial tissue forms the skin’s outer layer to shield against injuries and infections, lines the mouth and esophagus to prevent damage from food, and covers internal organs to protect them from harm.
- Absorption & Diffusion: It enables nutrient absorption in intestinal villi, reabsorbs useful substances in kidney tubules, and facilitates oxygen exchange in lung alveoli.
- Secretion: This tissue produces sweat in skin glands, digestive enzymes in salivary glands, stomach acid, and hormones in endocrine glands.
- Sensory & Specialized Functions: It contains taste buds and smell receptors, moves mucus via respiratory cilia, and stretches in the urinary bladder.
Types of Epithelial Tissues
Based on shape, layering, and functions, there are following types of epithelial tissue:
Simple epithelial tissue
The cells of this tissue are arranged in a single layer.
- Simple squamous epithelial tissue/pavement epithelium: It is a single layer of thin, flat cells found in locations where rapid diffusion or filtration is needed.
Location: It lines blood and lymphatic vessels (endothelium), the body cavities (mesothelium), and the air sacs of the lungs (alveoli). It also lines the glomeruli and Bowman’s capsules of the kidneys.
Function:
1. Enables rapid diffusion, filtration, and osmosis due to its thin structure.
2. Provides a smooth surface to reduce friction (e.g., in blood vessels). - Simple cuboidal epithelial tissue: It is a type of tissue composed of a single layer of cube-shaped cells, each with a large, spherical nucleus.
Location: It is found lining kidney tubules, thyroid gland follicles, ducts of some glands (like salivary, sweat, and mammary glands), ovaries, and parts of the eye. It’s also present in bronchioles of the lungs.
Function:
1. Secretion: Simple cuboidal epithelium is crucial for the secretion of various substances, including hormones, enzymes, and mucus, depending on the specific location.
2. Absorption: It plays a key role in absorbing nutrients, ions, and other molecules from surrounding fluids or tissues. - Simple columnar epithelial tissue: It is a type of tissue composed of a single layer of tall, column-shaped cells.
Location: It is primarily found lining the digestive tract, including the stomach, small intestine, and large intestine. It’s also present in the uterus and fallopian tubes and can be found lining the gallbladder and some ducts in the kidney.
Function: Those cells can secrete mucus, enzymes, and other substances, while also absorbing nutrients and protecting underlying tissues. - Pseudostratified epithelial tissue: is a type of epithelium consisting of a single layer of cells that gives the appearance of being multiple layers.
Location: It lines the respiratory tract (nasal cavity, trachea, bronchi), male reproductive tract (epididymis, vas deferens), parts of the male urethra, and inner ear.
Function: It functions primarily in protection, secretion, and transportation.
Compound (Stratified) Epithelial Tissue
It is a type of tissue composed of multiple layers of cells. It is usually protective in function. They protect against mechanical and chemical stress.
Types of Compound Epithelium:
- Stratified Squamous Epithelial Tissue (Keratinized): It is a type of tissue characterized by multiple layers of flat, scale-like cells, with a surface layer of dead cells containing keratin.
Location: It is found in the epidermis (outer layer of skin), the lining of the mouth (hard palate, dorsum of the tongue), and parts of the esophagus.
Function: It provides protection against mechanical injury, dehydration, and microbial invasion. - Stratified Squamous Epithelial Tissue (Non-Keratinized): It is a type of tissue characterized by multiple layers of cells, with the surface cells remaining alive and retaining their nuclei, unlike the keratinized type, where the surface cells are dead and filled with keratin.
Location: It lines the oral cavity (mouth), esophagus, vagina, and anus. It’s also present in the lining of the buccal mucosa, soft palate, and ventral surface of the tongue.
Function: It protect underlying tissues from abrasion, chemical damage, and water loss. - Stratified Cuboidal Epithelial Tissue: It is a rare type of epithelial tissue characterized by multiple layers of cube-shaped cells.
Location: It is found in the larger ducts of sweat glands, mammary glands, salivary glands, and the male urethra.
Function: It provides protection and some secretion and excretion functions. - Stratified Columnar Epithelial Tissue: It is a rare type of epithelial tissue characterized by multiple layers of cells, with the outermost layer composed of columnar (tall, rectangular) cells.
Location: It is found in parts of the male urethra, the conjunctiva of the eye, the anus, and the pharynx.
Function: It forms a protective barrier due to its multiple cell layers, and it also facilitates secretion of substances like mucus.
Transitional Epithelial Tissue: It is a type of specialized tissue characterized by its ability to change shape, allowing organs like the bladder to expand and contract.
Location: It is primarily found in the urinary tract, including the bladder, ureters, and part of the urethra.
Function: It allows organs like the urinary bladder to stretch and contract without damaging the underlying tissue.
Specialized Epithelial Tissues:
Specialized epithelial tissues are epithelial tissues that have unique structures and functions beyond the basic functions of protection, secretion, and absorption.
Ciliated epithelium: It is a type of tissue characterized by cells with hair-like structures called cilia on their surface. It is is primarily found in the respiratory system and the female reproductive system. Its primary function is to move substances along a surface using the coordinated beating of cilia.
Sensory Epithelium: It is a specialized type of epithelial tissue containing sensory cells that convert external stimuli into electrical signals, which are then transmitted to neurons. It is found in the olfactory epithelium in the nose, the sensory epithelia of the inner ear (including the cochlea and vestibular system), and specialized epithelial cells in the taste buds of the tongue and the retina of the eye. It act as a transducer, converting signals from the outside world into an electrical form that can be interpreted by the nervous system.
Germinal Epithelium: The germinal epithelium is a specialized layer of cells found on the surface of the ovary or lining the seminiferous tubules of the testes. It’s crucial for the production of gametes (sperm and eggs) through a process called gametogenesis.
Glandular Epithelium: It is a specialized type of epithelial tissue responsible for producing and releasing various substances, such as hormones, enzymes, and other secretions. It is found in various locations, including the skin (sweat glands), digestive system (salivary glands, stomach glands), and endocrine glands (pituitary, thyroid).
Connective Tissue
Connective tissue is the tissue that connects or separates and supports all the other types of tissues in the body. It’s characterized by cells, fibers, and a gel-like substance called ground substance, all embedded in an extracellular matrix.
Functions:
Attaching. Connective tissues like ligaments and tendons can anchor parts in place.
Cushioning. Cartilage in our joints helps absorb impact and keep bones from grinding together. Fatty tissue can also absorb impact and movement forces so they don’t harm more delicate structures.
Protecting. Some connective tissues carry and distribute immune cells. And bones provide armor-like physical protection to our brain and vital organs in our chest.
Supporting. Our bones are the framework that holds our body up.
Supplying. Some connective tissues carry and distribute oxygen and other nutrients to various tissues.
Types of Connective tissues
Proper Connective Tissue
It is a type of connective tissue that includes loose and dense connective tissues. It’s characterized by its abundant extracellular matrix (ECM) and serves as a structural and supportive framework for other tissues and organs. It is further divided into different types:
Loose Connective Tissue
It is a type of connective tissue characterized by its loose arrangement of fibers and abundant ground substance. They are of following types:
- Areolar tissue: Areolar tissue is a loose, flexible connective tissue that provides support, elasticity, and nourishment to surrounding structures. It acts as a packing material between organs, filling spaces and holding them in place.