Epithelial cells are among the most abundant cells lining the skin, body cavities, and blood vessels. They contribute significantly to various aspects of the human life cycle from embryogenesis to adulthood. Its highly specialized histological characteristic is critical for its physiological functions in different organs. Disturbances of epithelial cell morphology and function have been confirmed in multiple clinical conditions, including cancer, organ fibrosis, celiac disease, and bullous pemphigoid.
Epithelial tissue is one of the four main types of body tissue found in organs and covers the internal and external surfaces of the body. It has several different structures and functions depending on where it is located in your body.
What are Epithelial Cells?
Epithelial tissue is made up of epithelial cells. Cells can have different shapes and can be arranged in a single layer or in multiple layers depending on where they are in the body and what types of functions they have.
In biology, a cell is the smallest unit that can live on its own. Cells make up all living organisms and the tissues in your body. More than 30 trillion cells make up your body.
What is Epithelium ?
Epithelium is a type of body tissue that forms the covering of all internal and external surfaces of the body, lines body cavities and hollow organs, and is the main tissue for glands. Epithelial tissue has a variety of functions depending on where it is located in the body, including protection, secretion, and absorption.
Parts of Cell:
A cell has three main parts:
- The cell membrane: The cell membrane surrounds the cell and controls the substances that enter and leave the cell.
- The nucleus: The nucleus is a structure within the cell that contains most of the cell’s DNA (genetic material).
- The cytoplasm: The cytoplasm is the fluid found inside the cell. It contains other parts of the cell that have certain functions.
Types of Epithelial Cells:
There are several different types of epithelial cells because epithelial tissues have many different functions depending on where they are located in the body.
Types of epithelial cells according to their shape:
The different types of epithelial cells depending on their shape include:
- Squamous epithelium: Squamous epithelial cells are flat and lamellar in appearance.
- Cubic epithelium: Cubic epithelial cells look like a cube, meaning they have the same width, height, and depth.
- Columnar epithelium: Columnar epithelial cells have a columnar appearance, meaning they are taller than they are wide.
Types of epithelial cells according to their arrangement:
Epithelial tissue can also vary depending on the arrangement of the cells. Descriptors or adjectives for the way the cells are arranged include:
- Simple: A simple epithelium means there is only one layer of cells.
- Stratified: A stratified epithelium is made up of more than one layer of cells.
- Pseudostratified: A pseudostratified epithelium is made up of very compact cells that appear to be arranged in layers because they are different sizes, but in reality there is a single layer of cells.
Epithelial cells based on specialized functions:
Epithelial cells can also be classified based on the special functions they have, including:
- Transitional epithelium: A transitional epithelium (also known as urothelium) is made up of several layers of cells that flatten when stretched. It lines most of the urinary tract and allows the bladder to expand.
- Glandular epithelium: This type of epithelium is specialized in producing and secreting (releasing) substances. It is found in glands, which are specialized organs that can produce, store and/or release substances such as hormones, proteins and water.
- Olfactory epithelium: The olfactory epithelium, located within the nasal cavity, contains olfactory receptor cells, which have specialized cilia extensions. Cilia trap inhaled odor molecules as they pass over the epithelial surface. Information about the molecules is then transmitted from the receptors to the brain’s olfactory bulb, where the brain interprets the smell.
Where Epithelial cells in your body:
Given the different shapes and types of epithelial cell layers, there may be several types of epithelial tissue, including:
- Simple squamous epithelium: This type of epithelium generally lines blood vessels and body cavities and regulates the passage of substances to the underlying tissue.
- Simple cuboidal epithelium: This type of epithelium is typically found in glandular (secreting) tissue and renal tubules.
- Simple columnar epithelium: This type of epithelium is usually specialized for absorption and usually has cilia or apical microvilli. These cells line the stomach and intestines.
- Stratified squamous epithelium: This type of epithelium often has protective functions, including protection against invasion of microorganisms into the underlying tissue and/or protection against water loss. The outer layer of the skin (the epidermis) is made up of stratified squamous epithelial cells.
- Stratified cuboidal epithelium: This type of epithelium is not so common and is found in the excretory ducts of the salivary and sweat glands.
- Stratified columnar epithelium: this type of epithelium is not so common and is observed in the mucous membrane (conjunctiva) that lines the eyelids, where it has a protective and mucus-secreting effect.
- Pseudostratified columnar epithelium: This type of epithelium lines the upper respiratory tract and usually has many cilia.
The Functions Epithelial Cells:
Epithelial tissue has several important functions that are essential for life. Since epithelial cells are found throughout the body, their function and purpose change depending on their location.
Epithelial tissue may have one or a combination of the following functions:
| Function | Description | Example |
|---|---|---|
| Protection | Protects deeper tissues from injury, pathogens, and dehydration. | Skin protects underlying tissues; cilia in intestines protect against bacteria. |
| Secretion | Releases enzymes, hormones, and fluids into the body or cavities. | Glandular epithelium secretes enzymes and hormones. |
| Absorption | Allows the uptake of nutrients, gases, and other substances from the external environment or body cavities. | Intestinal epithelium absorbs nutrients from food. |
| Excretion | Removes waste products from the body or body cavities. | Kidney epithelium excretes waste; sweat gland epithelium excretes sweat. |
| Filtration | Filters and cleanses fluids or gases passing through the epithelial layer. | Respiratory epithelium filters air; kidney epithelium filters blood. |
| Diffusion | Facilitates the passive movement of molecules across the epithelial layer. | Simple squamous epithelium allows for selective diffusion. |
| Sensory Reception | Contains sensory nerve endings that detect external stimuli. | Epithelium of the ear with stereocilia for hearing and balance; taste buds in tongue epithelium. |
Clinical Significance:
Epithelial tissue raises significant concerns, primarily due to the risk of developing malignancies such as adenocarcinoma and papillary thyroid carcinoma. Adenocarcinoma, which affects glandular epithelial tissue, is common in several organs, including the prostate, breast, colorectum, pancreas, and lungs. Papillary thyroid carcinoma, which comprises the majority of thyroid cancers, generally progresses slowly and responds well to treatment, with high curability rates.
Apart from cancer, problems related to epithelial tissue manifest themselves in several organs:
- Asthma: Involves changes in the bronchial epithelium that cause narrowing of the airways, swelling, and increased mucus production.
- Celiac disease: Gluten ingestion triggers immunological damage to intestinal epithelial microvilli, making nutrient absorption difficult.
- Human papillomavirus (HPV): HPV infections can cause warts in the squamous epithelial cells of the epidermis.
- Vertigo: Abnormal growth of stratified squamous epithelial cells inside the ear (cholesteatoma) can cause vertigo and hearing problems if left untreated.
Understanding these epithelial tissue-related conditions is crucial for effective diagnosis and treatment.
Laboratory Tests for Epithelial Cell:
Epithelial cells are crucial in various body parts, and various tests examine them for medical conditions. Pathology in medicine involves laboratory examination of cells in body tissue or fluids for diagnostic purposes, conducted by a pathologist.
Some tests that involve epithelial tissue include:
- Epithelial cells in Urine: Part of the urinalysis, a test that measures different substances in the urine, involves checking the epithelial cells in the urine. A lab technician examines your urine sample under a microscope to see if the number of epithelial cells is within the normal range. It is normal to have a small number of epithelial cells in your urine. A large amount may indicate an infection, kidney disease, or another serious medical condition.
- Pap Smear: A Pap smear is a diagnostic procedure that checks for cancerous or potentially cancerous abnormal epithelial cells in the cervix, which are examined under a microscope by a healthcare provider.
- Pap test: A Pap test (also called a Pap test) is a test that looks for abnormal epithelial cells in the cervix that are cancerous or have the potential to become cancerous. During a Pap test, a doctor takes a sample of cells from the cervix to examine them under a microscope.
- Certain biopsy tests: A biopsy is an examination of tissue, usually under a microscope, that has been removed from a part of your body to look for signs of disease. Several different biopsies analyze the epithelial cells. For example, a prostate biopsy involves removing epithelial cells from the prostate with a needle to look for signs of cancer. A celiac disease biopsy involves an endoscopy to remove epithelial cells from the lining of the small intestine to look for damage to the microvilli of the cells.
- Certain cytology tests: Cytology (also known as cytopathology) involves examining cells from tissues or body fluids to determine a diagnosis. Many types of cytology tests involve epithelial cells, such as gynecologic cytology, thyroid cytology, respiratory cytology, and ocular cytology.
Differences between Epithelium, Endothelium and Mesothelium:
| Characteristic | Epithelium | Endothelium | Mesothelium |
|---|---|---|---|
| Location | Covers body surfaces and lines cavities and organs. | Lines the interior surface of blood vessels and lymphatic vessels. | Lines the walls and covers the organs within body cavities (e.g., pleural, pericardial, peritoneal). |
| Function | Provides protection, absorption, secretion, and sensation. | Facilitates exchange of substances between blood or lymph and surrounding tissues. | Produces a lubricating fluid that reduces friction between organs and body cavity walls. |
| Structure | Consists of closely packed cells with little to no extracellular matrix. | Comprised of a single layer of endothelial cells supported by a basement membrane. | Made up of a single layer of mesothelial cells resting on a basement membrane. |
| Specializations | Can be classified based on cell shape and layering (e.g., squamous, cuboidal, columnar; simple, stratified). | Lacks glands and specialized structures but may have fenestrations or tight junctions for selective permeability. | Contains specialized cells called mesothelial cells that secrete lubricating fluid. |
| Examples | Skin epithelium, gastrointestinal epithelium, respiratory epithelium. | Endothelium of blood vessels, lymphatic endothelium. | Pleural mesothelium, peritoneal mesothelium, pericardial mesothelium. |
FAQs:
What are epithelial cells?
Epithelial cells are a type of animal tissue that forms the linings and coverings of various surfaces in the body.
Where are epithelial cells found?
They are found throughout the body, lining the skin, organs, and body cavities, as well as forming the inner lining of blood vessels and the respiratory, digestive, and urinary tracts.
What are the functions of epithelial cells?
Epithelial cells serve functions such as protection, absorption, secretion, excretion, filtration, and sensory reception.
What are the different types of epithelial cells?
Epithelial cells can be classified based on their shape (squamous, cuboidal, columnar) and the number of layers (simple, stratified, pseudostratified).
What is the importance of epithelial tissue?
Epithelial tissue acts as a barrier against pathogens, absorbs nutrients, secretes hormones and enzymes, and facilitates various physiological processes.
What is the role of epithelial cells in cancer?
Epithelial cells can give rise to cancers known as carcinomas. Common examples include lung cancer, breast cancer, prostate cancer, and colorectal cancer.
How do epithelial cells regenerate?
Epithelial cells have a high capacity for regeneration and repair, facilitated by the presence of stem cells and their continual turnover.
What are some diseases associated with epithelial cells?
Diseases such as asthma, celiac disease, and certain viral infections affect epithelial tissues in organs like the lungs, intestines, and skin.
How are epithelial cells studied in the laboratory?
Epithelial cells can be cultured in vitro for experimental studies, and techniques such as immunohistochemistry and electron microscopy are used to analyze their structure and function.
What are the characteristics of cancerous epithelial cells?
Cancerous epithelial cells often exhibit abnormal growth, loss of cell polarity, increased proliferation, invasion of surrounding tissues, and metastasis to distant organs.
Can epithelial cells be used in regenerative medicine?
Yes, epithelial cells have been explored for tissue engineering and regenerative medicine applications, particularly in skin grafts and organ transplantation.
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