Epithelial Tissue and Tumors Arising in Epithelial Tissue
The human body has four basic tissue types: epithelial tissue, connective tissue, muscle tissue and nervous tissue. Epithelial tissue, or simply epithelium, is composed of a large number of regularly shaped, closely packed epithelial cells with a small amount of intercellular substance, and functions in protection, secretion, absorption and excretion. The skin on our body surface and the inner surface of hollow organs all belong to epithelial tissue.
Epithelial cells have distinct polarity. The surface facing the body surface or the lumen of an organ is called the free surface. On the free surface of epithelial tissue there are structures such as microvilli and cilia. Microvilli mainly increase the surface area of the cell, facilitating its absorptive function; cilia, on the other hand, perform directional, rhythmic beating, clearing away secretions or dust and bacteria adhering to their surface.
The surface opposite the free surface is called the basal surface. The basal surface usually does not directly contact the external environment but is connected to connective tissue through a basement membrane. The basal surface has a basement membrane and plasma membrane infoldings. The basement membrane, also called basal lamina, is a thin membrane between the basal surface of epithelial cells and the deeper connective tissue; it supports and connects, has a semipermeable nature, and plays an important role in the metabolism of epithelial cells. The basement membrane can also guide the movement of epithelial cells and influence cell proliferation and differentiation.
Besides the free surface and basal surface, epithelial cells also have a lateral surface, which includes tight junctions, adherens belts, desmosomes and gap junctions. Tight junctions, also called zonula occludens, lie between simple columnar cells; they provide mechanical connection, effectively seal the apical side of adjacent cells, prevent substances in the intercellular space from leaking out, and block macromolecules from entering the tissue, thereby protecting the body from harm. The adherens belt, also called intermediate junction, lies below the tight junction; it tenses the terminal web, maintains cell shape, and is closely related to cell contraction and relaxation. Desmosomes, also called macula adherens, are relatively firm junctions between epithelial cells, protecting areas prone to friction or mechanical stimulation; desmosomes are distributed in places such as the esophagus and skin, where they play an important protective role. Gap junctions are mainly distributed between epithelial cells, muscle cells, nerve cells and bone cells; they allow cells to exchange certain small molecules and ions with one another.
By distribution and function, epithelial tissue can be divided into three types: covering epithelium, glandular epithelium and special epithelium. By "epithelium" we usually mean covering epithelium. Covering epithelium is distributed on the body surface or lines the inner surface of various organs, cavities and sacs. Following the principle that structure suits function, covering epithelium is divided into two major categories: simple epithelium and stratified epithelium. Simple epithelium is further divided into simple squamous epithelium, simple cuboidal epithelium, simple columnar epithelium and pseudostratified ciliated columnar epithelium.
Simple squamous epithelium consists of a single layer of flat, fish-scale-like cells, hence also called simple squamous epithelium. The simple squamous epithelium lining the chambers of the heart and the lumina of blood and lymphatic vessels is called endothelium; endothelium is very thin and smooth, reducing resistance to blood and lymph flow and facilitating exchange of substances across the epithelium. The simple squamous epithelium covering the pleura, peritoneum and pericardium is called mesothelium; the mesothelial surface is moist and smooth, reducing friction during visceral movement.
Simple cuboidal epithelium consists of a single layer of neatly arranged cuboidal cells. Seen from the surface, these cells are roughly hexagonal or polygonal; seen from the side, they are roughly cube-shaped with a spherical nucleus located centrally. Such cells have absorptive and secretory functions and are mainly distributed in the thyroid follicles and renal tubules.
Simple columnar epithelium consists of a single layer of regularly arranged tall prismatic cells. From the surface, the cells are hexagonal or polygonal; from a vertical section, they are columnar with an oval nucleus near the basal part of the cell. Simple columnar epithelial cells have absorptive and secretory functions and are mainly distributed on the inner surface of organs such as the stomach, intestine, uterus and uterine tube.
Pseudostratified ciliated columnar epithelium consists of columnar cells, spindle-shaped cells, goblet cells and pyramidal cells. In vertical section, the bases of all these cells attach to the basement membrane, but only the upper ends of the columnar and goblet cells reach the free surface of the epithelium; pyramidal cells only approach the basement membrane, and spindle-shaped cells are sandwiched between the above. The nuclei of these cells lie at different levels, giving the appearance of several layers. The free surface of the columnar cells also has cilia that beat directionally, hence the name pseudostratified ciliated columnar epithelium. It is mainly distributed on the inner surface of the respiratory tract, filtering out foreign matter and protecting the respiratory tract. Nicotine in cigarettes causes cilia to disappear, so smokers lose the natural protective barrier of the respiratory tract, leaving them vulnerable to foreign invaders and leading to various diseases, including cancer.
Stratified columnar epithelium is composed of multiple layers of cells. Its deep part consists of one to several layers of polygonal cells, and its superficial part of a layer of relatively neatly arranged short columnar cells. Stratified columnar epithelium is mainly distributed at the conjunctiva of the eyelid, the male urethra, and the large ducts of certain glands.
Transitional epithelium, also called urothelium, consists of multiple layers of cells whose shape and number of layers change according to the distended or contracted state of the organ, hence its name. It is mainly distributed on the luminal surface of the renal pelvis, renal calyces, ureter and bladder. When the bladder is empty, the epithelium thickens to 5-6 layers; the superficial cells are cuboidal, relatively large, and some contain two nuclei - these are called umbrella cells; the intermediate cells are polygonal; the basal cells are short columnar or cuboidal. When the bladder is full, the epithelium thins to only 2-3 layers, and the superficial cells become flattened.
Glandular epithelium is composed of glandular cells and is mainly secretory in function. Organs formed by glandular epithelium are called glands; they play important roles in the body and secrete a wide variety of substances.
There are also special epithelia, such as sensory epithelium, formed during the differentiation of epithelial cells; this type can receive special sensory stimuli. For example, gustatory epithelium perceives taste, and olfactory epithelium perceives smell.
Tumors arising in epithelial tissue are called epithelial tumors; they fall into two major categories: malignant and benign. Adenomas arising in the thyroid, ovary, breast and intestine are benign tumors originating from glandular epithelium, with a clear boundary from surrounding normal tissue. Papillary epithelial tumors of the external auditory canal, penis, bladder and colon are relatively prone to malignant transformation into papillary carcinoma, with ill-defined borders from surrounding normal tissue. Nodules or masses with clear borders seen on checkup reports are mostly benign nodules or benign tumors; nodules or masses with blurred borders, spiculation or other disordered features are mostly malignant tumors.
Common types of malignant epithelial tumors include the following:
First, squamous cell carcinoma, or simply squamous carcinoma. This cancer usually occurs at sites originally covered by squamous epithelium, such as the skin, oral cavity, lips, uterine cervix, vagina, esophagus, larynx and penis. Under the microscope, layered keratin can appear in the center of the cancer nests in well-differentiated squamous cell carcinoma.
Second, basal cell carcinoma. This cancer is most common on the face of the elderly, such as the eyelids, cheeks and alae nasi; it arises from the epidermal epithelial buds or basal cells at these sites. The cancer nests are mainly composed of deeply stained basal-like cancer cells. This type grows slowly, often forms ulcers on the surface, and may infiltrate and destroy local deep tissue, but rarely metastasizes. It is highly radiosensitive, has a relatively low degree of malignancy, and a good prognosis.
Third, transitional cell carcinoma. It arises from the transitional epithelium of the bladder or renal pelvis; it is often papillary and multiple, may ulcerate or widely infiltrate the bladder wall. Under the microscope, the cancer cells resemble transitional epithelium, arranged in multiple layers with marked atypia.
Fourth, glandular epithelial carcinoma, usually simply called adenocarcinoma. It is a malignant tumor arising in glandular epithelium. By morphological structure and degree of differentiation, it is divided into well-differentiated adenocarcinoma with glandular structure and poorly differentiated solid carcinoma forming solid cancer nests. Well-differentiated adenocarcinoma is lower in malignancy, poorly differentiated adenocarcinoma is higher; the prognosis of the former is better than the latter.