L2 Other forms of Epithelium & Epithelial Cells Polarity PDF

2 Other forms of epithelium & epithelial cells polarity ILOs By the end of this lecture, students will be able to 1. Interpret structural adaptation of special epithelium to its relative function. 2. Correlate cell membrane specialization to its function. 3. Interpret some cli...

2 Other forms of epithelium & epithelial cells polarity ILOs By the end of this lecture, students will be able to 1. Interpret structural adaptation of special epithelium to its relative function. 2. Correlate cell membrane specialization to its function. 3. Interpret some clinical disorders related to structural abnormalities of cell membrane specialization. Specialized forms of covering epithelium These are types of epithelia that have an additional special function. 1. Germinal epithelium: cells that differentiate into sperms in male or ovum in females. It is present in the seminiferous tubules of testis in male and in ovaries in female. 2. Sensory epithelium: cells have a sensory nerve supply and function as sensory receptors, such as taste buds, hair cells of the ear and retina of the eye. Epithelial cell polarity Most epithelial cells demonstrate structural changes in one or more of its cell membranes that represent cell adaptation to perform a special function. Subsequently, cell membranes with special structure have a polarity; directed towards a certain surface in relation to function. Figure 1. Types of cell surface specializations & polarity 1 Types of polarity I- Apical domain The apical domain, the region of the epithelial cell facing the lumen. Functional specialization: It functions in transport of ions, molecules and water. It is rich in ion channels, carrier proteins, H+-ATPase (adenosine triphosphatase), glycoproteins, and hydrolytic enzymes, as well as aquaporins, channel-forming proteins that function in regulation of water balance. Structural modification in the apical domain: These include microvilli with associated glycocalyx and, in some cases, stereocilia, cilia, and flagella in sperms. a. Microvilli Microvilli are small finger-like cytoplasmic projections, covered by the cell membrane, emanating from the free surface of the cell into the lumen. They increase surface area of upper cell membrane involved in high rate of absorption. They contain a bundle of microfilaments in the core to maintain rigidity. (Fig. 2) Microvilli represent the striated border of the intestinal absorptive cells and the brush border of the kidney proximal tubule cells observed by light microscopy. Figure 2. Structure of microvilli Scanning EM b. Stereocilia (not be confused with cilia) are long microvilli found only in the epididymis. These nonmotile structures are unusually rigid because of their core of actin filaments. In the epididymis, they probably function in increasing the surface area. 2  Figure 3. Structure of stereocilia Figure 4. Structure of a cilium Scanning EM c. Cilia; are long motile apical cell processes that have a regular rhythmic bending movement. The core of the cilium contains a complex of uniformly arranged microtubules called the axoneme. The axoneme is composed of a constant number of longitudinal microtubules arranged in a consistent 9 + 2 organization (two central single microtubules and outer nine doublets). The microtubule-associated protein dynein, also active in flagella, which has ATPase activity to release energy. In the ciliated epithelia of the respiratory system (e.g., trachea and bronchi) and in the oviduct, there may be hundreds of cilia in orderly arrays on the luminal surface of the cells. Clinical Hint: Kartagener's syndrome results from hereditary defects in the ciliary dynein that would normally provide the energy for ciliary bending. Persons having this syndrome are susceptible to lung infection (why?). Additionally, males with this syndrome are sterile (why?). II- Basolateral domain The basolateral domain may be subdivided into two regions: the lateral plasma membrane and the basal plasma membrane. Each region possesses its own junctional specializations and receptors for hormones and neurotransmitters. In addition, these regions are rich in Na +,K+- ATPase and ion channels and are sites for constitutive secretion. The basal domain specializations Three important features mark the basal surface of epithelia: the basal lamina, plasma membrane enfolding, and hemidesmosomes, which anchor the basal plasma membrane to the basal lamina. 3  a. Basal enfolding The basal surface of some epithelia, especially those involved in ion transport, possesses multiple finger- like enfolding of the basal plasma membranes that increase the surface area of the plasmalemma and partition the mitochondria-rich basal cytoplasm. b. Hemidesmosomes They resemble half desmosomes and serve to attach Figure 5. Structure of hemidesmosome the basal cell membrane to the basal lamina. Keratin tonofilaments insert into the plaques. The cytoplasmic aspects of transmembrane linker proteins are attached to the plaque, whereas their extracellular moieties bind to laminin and type IV collagen of the basal lamina. The transmembrane linker proteins of hemidesmosomes are integrins, a family of extracellular matrix receptors. c. Basal lamina It is located at the plane of junction between the epithelium & the underlying connective tissue It is formed of adhesive glycoproteins as laminin and collagen type IV. Clinical Hint Metaplasia; is transformation of one type of epithelium into another with loss of function. Malignant tumors arising from epithelia are called carcinomas. If the malignant cells cross the basal lamina, the tumour classification is upgraded to a more dangerous level with a worse outcome. Figure 6. Basal domain specializations Basal enfolding b. Basal lamina 4 Glandular epithelium It is the type of epithelium found in different types of glands. Glands are classified according to different characters as number of cells, presence, or absence of the duct, branching of the duct (simple or compound), mode &type of secretion. (Will be discussed later) According to duct presence, it could be classified into exocrine (have a duct that opens into a cavity) or endocrine that secrete hormones directly into the circulation. Epithelium form the secretory units of glands, as well as, lining the ducts of exocrine glands. 5