Histology of Epithelia and Glands Lecture PDF

Histology of Epithelia and Glands Learning Objectives Explain the ways in which epithelia are classified Distinguish between, simple, stratified, and pseudostratified epithelia List characteristics of squamous, cuboidal, columnar, and transitional epithelia Identify...

Histology of Epithelia and Glands Learning Objectives Explain the ways in which epithelia are classified Distinguish between, simple, stratified, and pseudostratified epithelia List characteristics of squamous, cuboidal, columnar, and transitional epithelia Identify some common surface specializations of epithelia Discuss how the organization, thickness, surface specializations, and turnover of an epithelial layer might reflect its function What is a Tissue? A tissue is a group of cells  Common embryonic origin  Function together to carry out specialized activities Hard (bone), semisolid (fat), or liquid (blood) Histology is the science that deals with the study of tissues. Pathologist specialized in laboratory studies of cells and tissue for diagnoses 4 Types of Tissues  Epithelial Covers body surfaces and lines hollow organs, body cavities, duct, and forms glands  Connective tissues Protects, supports, and binds organs. Stores energy as fat, provides immunity, etc….  Muscular Generates the physical force needed to make body structures move and generate body heat  Nervous Detect changes in body and responds by generating nerve impulses Development of Tissues Tissues of the body develop from three primary germ layers: Ectoderm, Endoderm, and Mesoderm  Epithelial tissues develop from all three germ layers  All connective tissue and most muscle tissues derive from mesoderm  Nervous tissue develops from ectoderm Development of Tissues In all vertebrates, the tissues of the body develop from three primary germ layers. These are called the Ectoderm Mesoderm Endoderm Following fertilization and the formation of a zygote, repeated mitotic cell divisions convert the zygote into a tiny ball of cells, called a blastula. Germ Layers During the process of gastrulation, the morphology of animal embryos is reorganized to form the three germ layers These 3 germ layers are the forerunners of all adult tissues. Germ Layers Body tissues 4 Types of body tissues arise from the 3 germ layers 1. Epithelia - arise from all 3 germ layers 2. Connective Tissue – mesodermal origin 3. Muscle Tissue – mesodermal origin 4. Nervous tissue – ectodermal origin Cell Junctions Contact points between the plasma membranes of tissue cells  5 common types: Tight junctions Adherens junctions Desmosomes Hemidesmosomes Gap junctions Tight Junctions Web-like strands of transmembrane proteins  Fuse cells together  Seal off passageways between adjacent cells Common in epithelial tissues of the stomach, intestines, and urinary bladder Impair the passage of substances between cells and leaking into the blood or surrounding tissues Adherens Junctions Dense layer of proteins called plaque  Resist separation of cells during contractile activities  Located inside of the plasma membrane attached to both membrane proteins and microfilaments of the cytoskeleton  Transmembrane glycoproteins called cadherins insert into the plaque and join cells  In epithelial cells, adhesion belts encircle the cell Desmosomes Contain plaque and cadherins that extends into the intercellular space to attach adjacent cells together  Desmosome plaque attaches to intermediate filaments that contain protein keratin  Prevent epidermal cells from separating under tension and cardiac muscles cells from pulling apart during contraction Hemidesmosomes Resemble half of a desmosome  Do not link adjacent cells but anchor cells to the basement membrane  Contains transmembrane glycoprotein integrin  Integrins attach to intermediate filaments and the protein laminin present in the basement membrane Gap Junctions Connect neighboring cells via tiny fluid-filled tunnels called connexons  Contain membrane proteins called connexins  Plasma membranes of gap junctions are separated by a very narrow intercellular gap (space) Communication of cells within a tissue Ions, nutrients, waste, chemical and electrical signals travel through the connexons from one cell to another Epithelial Tissues Epithelial tissue consists of cells arranged in continuous sheets, in either single or multiple layers  Closely packed and held tightly together  Covering and lining of the body  Free surface 3 major functions:  Selective barrier that regulates the movement of materials in and out of the body  Secretory surfaces that release products onto the free surface  Protective surfaces against the environment General Features of Epithelial Cells Surfaces of epithelial cells differ in structure and have specialized functions  Apical (free) surface Faces the body surface, body cavity, lumen, or duct  Lateral surfaces Faces adjacent cells  Basal surface Opposite of apical layer and adhere to extracellular materials General Features of Epithelial Cells Basement membrane  Thin double extracellular layer that serves as the point of attachment and support for overlying epithelial tissue  Basal lamina Closer to and secreted by the epithelial cells Contains laminin, collagen, glycoproteins, and proteoglycans  Reticular lamina Closer to the underlying connective tissue Contains collagen secreted by the connective tissue cells Epithelial Cells Epithelial Tissues  Own nerve supply  Avascular or lacks its own blood supply  Blood vessels in the connective tissue bring in nutrients and eliminate waste  High rate of cell division for renew and repair  Numerous roles in the body (i.e. protection and filtration) Covering and lining epithelium Outer covering of skin and some internal organs Glandular epithelium Secreting portion of glands (thyroid, adrenal, and sweat glands) Covering and Lining Epithelium Normally classified according to:  Arrangement of cells into layers  Shapes of cells Covering and Lining Epithelium Arrangement of cells in layers  Consist of one or more layers depending on function  Simple epithelium Single layer of cells that function in diffusion, osmosis, filtration, secretion, or absorption  Pseudostratified epithelium Appear to have multiple layers because cell nuclei at different levels Not all cells reach the apical surface  Stratified epithelium Two or more layers of cells that protect underlying tissues in areas of wear and tear Different Types of Covering and Lining Epithelium Cells vary in shape depending on their function  Squamous Thin cells, arranged like floor tiles Allows for rapid passage of substances  Cuboidal As tall as they are wide, shaped like cubes or hexagons May have microvilli Function in secretion or absorption Different Types of Covering and Lining Epithelium  Columnar Much taller than they are wide, like columns May have cilia or microvilli Specialized function for secretion and absorption  Transitional Cells change shape, transition from flat to cuboidal Organs such as urinary bladder stretch to larger size and collapse to a smaller size Simple Epithelium  Simple squamous epithelium  Simple cuboidal epithelium  Simple columnar epithelium (nonciliated and ciliated)  Pseudostratified columnar epithelium (nonciliated and ciliated) Simple squamous epithelium  Single layer of cells that resembles a tiled floor on the surface Nucleus is centrally located and appears flattened oval or sphere  Found at sites for filtration or diffusion Covering and Lining Epithelium Endothelium  The type of simple squamous that lines the heart, blood vessels, and lymphatic vessels Mesothelium  The type of epithelial layer of serous membranes such as the pericardium, pleura, or peritoneum Both are derived from embryonic mesoderm Simple cuboidal epithelium  Cuboidal shaped cells  Cell nuclei round and centrally located  Found in thyroid gland and kidneys  Functions in secretion and absorption Simple columnar epithelium  Column shaped cells  Oval nuclei at /near base  Nonciliated and ciliated Nonciliated simple columnar epithelium  Contains columnar cells with microvilli at their apical surface and goblet cells  Secreted mucus serves as lubricant for the lining of digestive, respiratory, reproductive and urinary tracts  Also prevents the destruction of the stomach lining by acidic gastric juices Ciliated simple columnar epithelium  Columnar epithelial cells with cilia at the apical surface In respiratory tract, goblet cells are interspersed among ciliated columnar epithelia Secreted mucus on the surface traps inhaled foreign particles. Beating cilia moves particles to the throat for removal by coughing, swallowing, or sneezing Cilia also moves oocytes to the uterine Covering and Lining Epithelium Pseudostratified columnar epithelium  Appears to have several layers due to nuclei at various depths  All cells are attached to the basement membrane in a single layer but some do not extend to the apical surface  Ciliated cells secrete mucus and bear cilia  Nonciliated cells lack cilia and goblet cells Covering and Lining Epithelium  Stratified Epithelium Two or more layers of cells Specific kind of stratified epithelium depends on the shape of cells in the apical layer  Stratified squamous epithelium  Stratified cuboidal epithelium  Stratified columnar epithelium  Transitional epithelium Stratified Squamous Several layers of cells that are flat in the apical layer  Epithelium New cells are pushed up toward apical layer As cells move further from the blood supply they dehydrate, harden, and die  Keratinized form contain the fibrous protein keratin Found in superficial layers of the skin  Nonkeratinized form does not contain keratin Found in mouth and oesophagus Stratified Cuboidal Epithelium  Fairly rare type of epithelium  Apical layers are cuboidal  Functions in protection Stratified columnar epithelium  Also very uncommon  Columnar cells in apical layer only  Basal layers have short, irregular shaped cells  Functions in protection and secretion Transitional Epithelium  Found only in the urinary system  Variable appearance  In relaxed state, cells appear cuboidal  Upon stretching, cells become flattened and appear squamous  Ideal for hollow structure subjected to expansion HISTOLOGY OF GLANDS Glands - Classification Classified into 2 depending on how their products are released Exocrine:  Secrete onto a body surface directly/through epithelial ducts connected to surface  Ducts may/may not alter secretion from gland by absorbing/secreting substances Endocrine:  Lack a duct  Secrete into circulation  hormones Glands Paracrine secretion refers to cellular secretions that do not reach the circulation but act locally on cells within the same tissue Autocrine secretion – has effect on cell by which it is secreted Exocrine glands have 3 release mechanisms :  Merocrine  Apocrine  Holocrine Release Mechanisms Merocrine  Release by exocytosis – fusion to apical cell membrane of filled vesicles  Commonest, e.g. pancreatic acinar cells Apocrine  Release from apical cell surface of contents surrounded by cytoplasm and an envelope of the cellular plasma membrane  Lactating mammary gland (releases lipid droplets into milk)  Ciliary glands (Moll’s) of eyelid,  Ceruminous glands of EAM Release Mechanisms Holocrine Secretion  Secretory products accumulate within a maturing cell  Cell undergoes apoptosis, thereby discharging both products and cell debris into gland lumen  Sebaceous glands of skin Glands Also classified as Unicellular or Multicellular Unicellular  Single secretory cells distributed amongst other nonsecretory cells  E.g. Goblet cells in columnar epithelium Simple columnar epithelium, ileal mucosa Multicellular Glands Multicellular  Exhibit varying degrees of complexity  Further sub classified according to the arrangement of the secretory cells and the presence or absence of branching duct elements  multicellular glands form tubular invaginations forming a secretory portion and a duct Multicellular Glands Ducts :  Simple glands  duct is not branched  Compound glands  duct is branched Secretory portion :  Tubular if shaped like a tube  Alveolar/acinar if shaped like a flask  Tubuloalveolar if duct terminates in a sac-like dilatation  Tubular portions can be straight/branched/coiled  Alveolar portions can be single or branched Structural Classification of Exocrine Glands Multicellular glands are categorized according to two criteria:  Ducts are branched or unbranched  Shape of the secretory portion of the gland Simple gland duct does not branch Compound gland duct branches Tubular glands have tubular secretory parts Acinar glands have rounded secretory parts Tubuloacinar glands have both tubular and rounded secretory parts Gland secretions Exocrine glands that empty into internal body surfaces (GIT, Respiratory and urogenital tracts) have mucous and/or serous secretions Mucous secretions are viscous  Goblet cells, cells of sublingual salivary glands, stomach goblet cells  Protein secretion that is extensively glycosylated with anionic oligosaccharides  PAS +ve mucinogenic granules that are lost in tissue preparation  Therefore in H&E sections, the cytoplasm of mucous cells appears empty with a basal nucleus GLAND SECRETIONS Serous secretions are watery  Contain non-glycosylated protein secretions  Cells have cytoplasm that stains intensely with eosin  E.g. parotid gland GLAND SECRETIONS The staining difference is important and reflects fundamental differences in cell function. Serous acini secrete a protein-rich product and stain fairly well due to the presence of rER and secretory granules (dense protein-filled vesicles) in the cytoplasm. Mucous acini, like goblet cells, secrete mucus, which contains less protein but much watery mucopolysaccharide. Mucous cells stain very poorly because they contain little rER and their mucin granules tend to react poorly with most stains. THE SKIN The integumentary system Made up of the skin and its accessory structures Different tissue types constitute the largest organ in the body 3 principal components: 1. Epidermis 2. Dermis 3. hypodermis Functions of the skin Protection: an anatomical barrier from pathogens Sensation: contains a variety of nerve endings that react to heat and cold, touch, pressure, vibration, and tissue injury Thermoregulation: constricted blood vessels reduce cutaneous blood flow and conserve heat, while dilated blood vessels increase cutaneous blood flow and release heat. Sweat glands release sweat, which cools down the body. Erector pili muscles contract and cause the hair follicles to move upwards and retain heat (gooseflesh). The subcutaneous fat also serves as insulation. Control of evaporation: the skin provides a relatively dry and semi-impermeable barrier to fluid loss. Storage and synthesis: acts as a storage centre for lipids (subcutaneous fat) and water Absorption: Oxygen, nitrogen and carbon dioxide can diffuse into the epidermis in small amounts. Water resistance: The skin acts as a water resistant barrier so essential nutrients aren't washed out of the body. Epidermis A keratinized stratified squamous epithelium The predominant cell type is the keratinocyte Other cell types are melanocytes, Langerhans cells and Merkel cells Epidermis - Melanocytes Melanocytes are the melanin producing cells, the compound responsible for skin color Melanin absorbs UV light to protect from damaging UV radiation Epidermis – Langerhans cells These are dendritic cells of monocytic origin and contain intracellular Birbeck granules They are immune cells – antigen presenting cells. They function in the capture, processing and presentation of antigen Epidermis – Merkel Cells Located in the deep epidermis these cells contact the tactile disc of sensory neurons Function in sensation of touch (mechanotransduction) Epidermal Layers Stratum basale Deepest layer of cuboidal / columnar cells regenerative cells undergoing mitosis Includes scattered melanocytes and Langerhans cells Epidermal Layers The stratum spinosum is the layer above the basal layer Polyhedral keratinocytes adopt a gradually more flattened appearance Spine like appearance on histology sections where the intracellular intermediate filaments insert into desmosomes Epidermal Layers The middle layer of flattened keratinocytes undergoing apoptosis is called the stratum granulosum Named after the conspicuous keratohyalin granules that stain darkly Also contain lamellar granules, these are lipid rich intracellular organelles that secrete a lamellar layer between the epidermal cell layers Epidermal layers Stratum lucidum - present only in very thick skin (palms, soles) Pale-staining layer of cells between the stratum corneum and stratum granulosum in which the dying keratinocytes contain a lot of keratin but are not completely replaced by it Epidermal layers The stratum corneum Composed of dead flat (squame) keratinocytes and densely packed intermediate filaments and keratohyalin Continually shed off and replaced by cells from the deeper layers. The Dermis Composed of typical connective tissue containing collagen and elastin, fibroblasts and macrophages Divided into two regions 1. A superficial papillary dermis 2. A deeper reticular dermis The dermis The papillary dermis - consists of loose areolar connective tissue containing collagen and fine elastic fibers It forms finger like projections – the dermal papillae – that indent the adjacent epidermis and contain capillaries Also contains free nerve endings The dermis The reticular dermis contains dense, irregular and coarse collagen fibers and thick elastic fibers interspersed with fibroblasts and blood vessels and nerves Also contains sweat and sebaceous glands BASAL CELL CARCINOMA The most common type of skin cancer. Development of a shiny pearly nodule. Commonly seen on sun exposed areas of the body. Malignant but does not usually metastasize. SQUAMOUS CELL CARCINOMA Malignant tumour of squamous cell epithelium. Slow growing but rapidly spreads to neighbouring tissues. Often starts off as actinic keratosis (rough red bumps on skin) but may later be transformed to invasive squamous cell carcinoma. Commonly seen in fair skinned people. http://homemoleremovalguide.com MELANOMA A malignant tumour of melanocytes – cells responsible for the formation of melanin pigment of the skin. Early signs of melanoma are summarised by the mnemonic A – asymmetry B – border (irregular) C – colour (variegated) D – diameter E – evolving over time