Histology 1 - Lining Epithelia PDF

Overview of tissues Epithelial tissue —> covers body surfaces (epidermis), lines cavities (mucosae, intestine) and forms glands (pancreas, liver) Connective tissue —> supports the other 3 basic tissues (epithelia, muscle and nerve) (it includes specialised connective tissues such as cartilage, bone...

Overview of tissues Epithelial tissue —> covers body surfaces (epidermis), lines cavities (mucosae, intestine) and forms glands (pancreas, liver) Connective tissue —> supports the other 3 basic tissues (epithelia, muscle and nerve) (it includes specialised connective tissues such as cartilage, bones, blood and lymphoid organs) Muscle tissue —> made up by contractile cells, responsible for movement Nerve tissue —> allows reception, integration (very important) and transmission of information thus controlling all the activities of the body Three embryonic germ layers —> ectoderm, mesoderm and endoderm Epithelia comes from ALL THREE embryonic germ layers Differentiations from the ectoderm —> epidermis, cornea and lens epithelia of the eye, nasal mucosae, glands of the skin and mammary glands Differentiations from the endoderm —> lining epithelium of the respiratory and gastrointestinal tract (excepts the oral and anal epithelia), liver and pancreas Differentiations from the mesoderm —> kidney tubules, lining of urinary and reproductive tracts, epithelium enveloping the ovary, serosa (part of mesothelium, which lines inner organs), blood and lymphatic vessels epithelium (endothelium) Nervous tissue —> ectoderm Muscle tissue —> mesoderm Epithelial tissues —> 1. Lining epithelia: cells tightly apposed forming sheets (epidermis, mucosae…) 2. Glands or secreting epithelia: cells tightly apposed forming aggregates with different morphology General features of epithelia: Adjacent cells are closely apposed to one another They are polarised —> apical, basal and lateral domains have different features and functions Apical specialisation may include villi and cilia They present baso-lateral intracellular junctions They are avascular (no blood vessels nor nerves) Lining epithelia Functions: It covers the external surface of the body and the internal cavities (respiratory tracts and mucosa) Physical protection of organs from physical and chemical injuries (es: heat) It allows metabolic exchanges (secretion, absorption and gas exchanges) It receives sensitive stimulation (sensitive receptors located at the periphery of the tissue) Classi cation —> each epithelial tissue is given two names —> 1. First name indicates the number of layers present (simple: one, strati ed: more than one) 2. Second name describes the shape of the cell (squamous, cuboidal or columnar). The shape of the APICAL surface dictates the name of the tissue Gimportant Strati ed squamous tissue ultimately divides into keratinised and non-keratinised Exceptions —> pseudostrati ed (simple) (columnar) epithelium: all the cells lay in the same basal membrane but not all the cells reach the apical surface of the tissue, thus giving the false impression of a strati cation. Transitional (strati ed): depending on the organ it can have different morphological features (if the bladder is lled with urine the epithelium is attened while when it is empty the cells are higher) Simple Stratified p p stratified Pseudo The function of a lining epithelia is strictly related to its morphology Simple epithelia are mostly involved in absorption, secretion and ltration —> if you need to ltrate or absorb or secrete something it is better to have just one layer rather than multiple Strati ed epithelia are mostly related to the protection of a tissue/organ Lungs have a squamous single epithelium to allow the exchange of blood and Oxygen. The cells may be elongated and attened air Endothelium of a blood vessel —> single squamous —> not as tightly apposed as other lining epithelia as they need to exchange many particles Kidney glomerulus (also called Malpighi corpusculus) —> it is closed in a capsule called Bowman capsule —> the epithelium lining the capsule is simple squamous while kidney tubules have a cuboidal shape Simple squamous epithelium —> allows the passage of material by diffusion and ltration and may secrete lubricating substances. Es: lung alveoli, lumen of blood and lymph vessels, serosa and Bowman capsule Simple cuboidal epithelium —> covers the ovary surface, the kidney tubules, gland ducts, terminal bronchioles, follicles of the thyroid gland and retina  crosssectionofa follicle Colloid —> precursor of thyroid hormones nudeO Simple columnar epithelium —> uterine tube, small bronchioles bronchi, small intestine, stomach, gallbladder and excretory ducts. The nucleus is usually close to the base and round Intestinal villi O O Loostemaeen As epithelia don’t contain blood vessels they obtain nutrients from the connective tissues laying beneath them Excretory duct of a gland columnar glandular epithelium Pseudostrati ed epithelium —> nuclei are arrayed at variable heights so the epithelium seems strati ed. If the epithelium has cilia on top and it seems strati ed then it probably is pseudostrati ed (strati ed cells don’t have cilia). This type of epithelium is recognisable by the arrangements of nuclei, which is usually at the bottom of the cell Pseudostrati ed epithelium: Mucosa of the airways Urethra Epididymis Ear canal Big excretory ducts of glands Strati ed squamous non-keratinised (alive): Oral cavitiy (+ some tract of oral mucosa) Pharynx Larynx Esophagus Rectal canal Vagina Keratinised —> cells are dead and don’t present a nucleus anymore basallayer me In strati ed epithelia the basal layer is called GERMINAL LAYER and it’s the site of proliferation of cells (important for the renovation of tissues). It can be recognised by a more marked staining (due to a reduced cytoplasm and thus a more apparent nucleus) Strati ed squamous keratinised —> in the upper layer I cells don’t present nuclei and organelles —> they are empty sacks (dead cells) Strati ed squamous keratinised: Skin External part of gingival epithelium Tongue Nasal epithelium Strati ed cuboidal epithelium Big gland ducts (sweat glands, sebaceous glands and exocrine pancreas) collagenfibers Strati ed columnar epithelium Larynx mucosa 9 Male urethra Conjunctiva Big gland ducts need havetwo 8 Transitional epithelium: Urinary tract —> renal pelvis, urethra, bladder, urether In transitional epithelium cells have different shapes which depend on whether the organ is lled or empty (empty bladder: taller epithelium, lled: shorter) Dome-shaped cells are a typical sign of transitional epithelium (they don’t create a at surface when bound together but rather an irregular one) Summary Epidermis It is a strati ed squamous keratinised epithelium Epidermal ridges —> they project into the dermis (stratum laying underneath the epidermis) Dermal papillae —> nger like connective protrusions that indent the deeper layers Thickness —> 1.5-2 mm  ised keratin or ⬅ Layers of the epidermis Cells from the keratinised layer are dehydrated dead cells Basal layer —> it is the mitotic layer (continuously replaces epidermal cells, a complete turnover happens between 25-50 days) and it contains the only receptor of the epidermis (Merkel cell) important yo Spinous layer —> cells are pushed upward and attened out Stratum granulosum —> cells containing granules of keratin (keratin is present in all the epidermis cells but is granulated only in the stratum granulosum) Stratum lucidum —> only present in non-hairy or thick skin, it is made of several layers of dead cells with indistinct boundaries Stratum corneum —> 25 or more layers of dead squamous cells still joined by desmosomes. When desmosomes break cells are released and desquamation happens (we lose dead skin) CLINICAL DROP: Mutations of keratin produce a pathology (epidermolysis bullosa simplex) characterised by the formation of blisters induced my mechanical stress that breaks cells in the basal layer of the epidermis (breaks hemidesmosomes) Pemphigus foliaceus —> loss of desmoglein 1 damaged by anti-bodies (last lesson) Cells of the epidermis: Keratinocytes (95% of the cell) Melanocytes (melanin synthesis) Langerhan cells (defence) Merkel cells (tactile receptors) Melanocytes —> they have a heterogeneous shape and their role is that of synthesising and distributing melanin (pigment). Sunlight increases melanin production as it damages the epidermis (damage = increased production of melanin to protect itself). Melanosomes give color and photoprotection to the skin and they are the cellular site of storage and transport of melanin pigments. When they start to proliferate without stopping they give life to a melanoma. Melanosomes are produced as a response to the release of MSH from the pituitary gland Langerhans cells —> DIFFERENT from the Langerhans islets in the endocrine pancreas (they are just named after the same scientist). They are activated by the presence of pathogens and act starting the immuno response. They are also phagocytes (also called dendritic cells for their shape which resembles dendrites). Dendritic cells capture antigens in the environment, process them and then present them on their surface to show to T-cells (thus migrating to the nearby lymph node) Merkel cells —> single cells located at the base of the epidermis attached to the end somatosensory nerve bres (with which they have synaptic contacts). They are associated with the sense of light touch and the discrimination of shapes and textures