Outcome 1 Guide PDF

Outcome 1 Describe the Appearance of and Identify Specific Cell and Tissue Types Using Brightfield Microscopy and/or Digital Imagery 1 Table of Contents Section 1 - Introduction to Microanatomy.....................................................................................

Outcome 1 Describe the Appearance of and Identify Specific Cell and Tissue Types Using Brightfield Microscopy and/or Digital Imagery 1 Table of Contents Section 1 - Introduction to Microanatomy....................................................................................... 3 1.1 Importance of microanatomy........................................................................................... 4 1.2 Define the terms cell, tissue, simple tissue, compound tissue, organ, and system............. 4 1.3 Classification of cells......................................................................................................... 5 1.4 Three-dimensional interpretation of histological sections................................................. 5 Section 2 – Epithelial Cells................................................................................................................ 6 2.1 Structural features of epithelial cells................................................................................. 7 2.2 Epithelial classification...................................................................................................... 8 2.3 Secretory cells.................................................................................................................. 9 2.4 Arrangements of secretory tissues.................................................................................. 10 Section 3 – Support Cells and Extracellular Matrix.......................................................................... 12 3.1 Support cells................................................................................................................... 13 3.2 Main components of the extracellular matrix................................................................. 13 3.3 Fibrillar proteins............................................................................................................. 14 3.4 Basement membrane..................................................................................................... 16 3.5 Other support cells......................................................................................................... 16 Section 4 – Contractile Cells........................................................................................................... 19 4.1 Contractile cells.............................................................................................................. 20 4.2 Muscle cells.................................................................................................................... 20 4.3 Growth and regeneration............................................................................................... 22 Section 5 – Lymphoid Tissue.......................................................................................................... 23 5.1 Immune System.............................................................................................................. 24 5.2 Thymus, lymph nodes, spleen, and MALT....................................................................... 24 Section 6 – Blood and Lymphatic Systems and Heart...................................................................... 27 6.1 Heart wall....................................................................................................................... 28 6.2 Blood circulatory system................................................................................................ 29 6.3 Lymphatic circulatory system......................................................................................... 32 References................................................................................................................................. 33 2 Section 1 - Introduction to Microanatomy Upon completion of this section you will be able to describe the importance of microanatomy in the histology lab and describe the functional classifications of cells and tissue arrangements. Learning Objectives To successfully complete this section, you should be able to 1.1 Describe the importance of microanatomy knowledge to the MLT 1.2 Define the terms cell, tissue, organ, system, simple tissue, and compound tissue. 1.3 Classify cells into traditional and modern groups according to their function 1.4 Apply the concept of three-dimensional interpretation regarding tissue sectioning 3 1.1 Importance of microanatomy Human histology is the microscopic study of biological material and focuses on the relationship between structure and function. The Anatomical Pathology department receives human tissues removed from the body during surgical procedures or autopsies and are processed by the medical laboratory technologist for examination by a pathologist. For microscopic study to take place, the specimen is processed in such a manner that thin slices (~5 µm) of tissue are prepared onto glass slides. Various tissue components may be visualized using a variety of procedures including traditional staining and immunohistochemical techniques. As MLT’s are involved in selecting appropriate samples to send to the pathologist, a basic understanding of tissue structure is vital. Knowledge of the histology of layered organs for example is particularly important when orienting the tissue into a paraffin block, a process known as embedding. When evaluating the quality of a prepared section, one factor the MLT must consider is whether the specimen is properly represented in the section. This requires basic knowledge of tissue microanatomy. When troubleshooting potential errors in the histology lab, knowledge of microanatomy is a useful tool in problem solving this situation. When performing special stains, it is also important that the MLT has a good understanding of microanatomy. For example, if elastic fibers are to be demonstrated, the technologist views these structures in the known positive control slide to establish staining time and to assess the success of the method. In immunohistochemical staining, normal tissues are frequently used as controls, and in order to determine if the method is a success, the technologist is required to know where to locate the particular cell or tissue type being demonstrated. 1.2 Define the terms cell, tissue, simple tissue, compound tissue, organ, and system Cells – the basic unit of structure of most living organisms. Classified according to their main function. Tissues – discrete, organized collections of cells having similar morphological characteristics. Functional arrangements of cells. Simple tissue – cells forming a tissue are all of the same structure. Compound tissue – a tissue containing a mixture of cells with different functions. Organ – anatomically distinct groups of tissues, usually of several types, which performs specific functions. System – cells with a similar function, but widely distributed in several anatomic sites, or, a group of organs that have similar or related functional roles. 4 1.3 Classification of cells Cells are classified according to their main function through. The development of structural attributes which provide the cell with a specific function is known as differentiation. Cells can be classified into the following groups: Epithelial cells Support cells Contractile cells Nerve cells Germ cells Blood cells Immune cells Hormone-secreting cells A cell however may have more than one function and be classified in more than one group. Figure 1.1 Functional Cell Classification1 1.4 Three-dimensional interpretation of histological sections The MLT must develop the skill of visualizing three dimensional structures while viewing a tissue section under the microscope. Thin slices cut through objects that are thicker than the 5 µm section can be very deceiving. A single cell can take on numerous appearances. Hollow and solid tubular structures can appear in many ways depending on the plane of cut. 5 Section 2 – Epithelial Cells Upon completion of this section, you will be able to describe and recognize epithelial cells in tissues using the light microscope. Learning Objectives To successfully complete this section, you should be able to 2.1 Describe the structural features of epithelial tissue 2.2 Classify epithelium according to traditional nomenclature 2.3 Recognize epithelia which are coverings and linings 2.4 Describe and recognize the various types of secretory cells 2.5 Describe and recognize the various arrangements of secretory tissues 6 2.1 Structural features of epithelial cells Epithelial cells are found in sheets (epithelia) covering the inside and outside of body surfaces. They have may functions such as: Secretion Absorption Protection Their function is related to specialized surface and internal modifications. Epithelial cells are attached to each other through three types of junctions. Occluding junctions – bind cells together and maintain the integrity of the epithelial barrier Anchoring junctions – link the cytoskeleton of cells to each other and the underlying tissues Communication junctions – allow direct communication between cells The cytoskeleton of one epithelial cell is anchored to the cytoskeleton of the adjacent epithelial cell. The epithelial layer is also anchored to the basement membrane, which is the layer of extracellular matrix between the epithelia and the support tissue. The basement membrane is composed of a special matrix protein called Type IV collagen and a structural glycoprotein called laminin. The basement membrane is difficult to demonstrate with routine tissue staining and requires specialized stains to identify. As epithelial cells are arranged very tightly to one another, the epithelium appears to be composed of only cells. Epithelium lacks a vascular supply and is nourished by diffusion from the underlying capillary beds. Epithelial cells are subject to physical and chemical change. The loss of surface cells by this change is overcome by continual cell regeneration. When an epithelial tissue consists of more than one layer of cells, regeneration occurs in the deepest layer, and mitotic figures are frequently observed here. As time passes, these young cells migrate towards the surface, and are eventually shed. The surface of epithelial cells can be specialized or adapted for certain functions. Surface adaptations allow cells to: Increase surface area – accomplished by microvilli and basolateral folds Move substances over the surface of the cell – accomplished by cilia Provide a protective barrier – accomplished by keratin 7  Microvilli are finger-like projections on the apical cell surface. They enhance the function of absorption and are found in the kidney tubules or intestinal tract. Stereocilia are extremely long microvilli and are not true cilia. They are found on epithelial cells lining the epididymis, which is a part of the male reproductive system. Basolateral folds can be found on the basal or lateral surface of cells. They are involved in fluid or ion transport and can be seen in renal tubular cells and the ducts of secretory glands. Cilia are motile projections on the surface of the cell and are involved in transport of fluid over the cell surface. They are found on cells lining the respiratory tract and in tissues of the female reproductive system. Keratinization is a process where the cytoskeleton of the cells in the outer layer of the epithelium become condensed to other proteins to form a protective mass. This layer of dead cells is impervious and acts as a protective layer called keratin. It is mechanically strong, but also flexible, which is an ideal adaptation for skin epithelium. 2.2 Epithelial classification Epithelial cells are classified based on their shape as well as how the cells are arranged or stacked together. Traditionally, cells are classified into three different shapes: Squamous – flat, plate-like Cuboidal – same height of cell as width Columnar – cells are 2-5 times taller than width Epithelial cells can be arranged in a single layer, where all the cells contact the underlying extracellular matrix. This is called simple epithelium. There are four types of simple epithelium: Simple squamous – very thin, flattened cells with irregular outlines that fit close together in a continuous sheet. Found in places of diffusion or filtration (alveoli in lung, loop of Henle in kidney). Endothelium is simple squamous epithelium lining the heart, blood vessels, and lymphatics. Mesothelium is simple squamous epithelium lining several body cavities. Simple cuboidal – cells are as tall as they are wide, central nucleus. Found in ducts, some kidney tubules, thyroid gland, and covering of the ovaries. Simple columnar – tall, column shaped cells. Free surface may contain cilia or microvilli. Nuclei may not be visible depending on the plane of cut. Found in places of secretion or absorption such as stomach, gallbladder, intestine or cervix. Pseudostratified columnar - crowded columnar cells that appear to be arranged in layers, but upon closer inspection all the cells are in contact with the extracellular matrix. Not all the cells reach the free surface of the epithelium. The nucleus of the cell is usually located 8  at the widest part of the cell. This type of epithelium can be ciliated. It is found in the pharynx, trachea, bronchi, and urethra. If the epithelial cells are arranged in several layers, and only the bottom layer is contacting the extracellular matrix, the term stratified epithelium is used. When stratified epithelium is present, the shape of the top layer is used to describe the epithelia, ex. stratified squamous epithelium. The bottom layer is known as the basal layer, and the cells are usually cuboidal in shape. There are four types of stratified epithelium: Stratified squamous – outer layer of cells are flat and scale like. May contain keratin in the outermost layer (skin). Found in areas where protection is needed from friction, mechanical insult or drying (mouth, esophagus, vagina) Stratified cuboidal – not a common type of epithelium but can be found in the ducts of sweat glands or larger ducts of the pancreas and salivary glands. Stratified columnar – rare type of epithelium, outer layer may be ciliated. Difficult to distinguish from pseudostratified epithelium. Found in the pharynx, larynx, and urethra, and wider ducts of some glands. Transitional epithelium or urothelium - specialized type of stratified epithelium and is found only in the urinary tract. The top layer is a cross between cuboidal and squamous, depending on the amount of stretching present. Outer layer can also appear dome shaped. 2.3 Secretory cells Types of secretion Epithelial cells may be specialized to synthesize and secrete macromolecules such as enzymes, mucin, and hormones. They may also be adapted for the secretion and transport of ions. Epithelial cells secreting enzymes include the pancreatic acinar cell. The anterior pituitary gland contains several cell types which synthesize and secrete peptide hormones. Cells producing steroid hormones are found in the adrenal glands, ovaries and testes. The classic example of the mucin secreting epithelial cell is the goblet cell located in the intestinal tract and in the respiratory tract. Epithelial cells are also active ion pumping cells. These include cells in gall bladder, kidney and sweat gland ducts and ducts of the salivary gland. Mechanisms of secretion Mechanism of secretion can be broadly divided into two groups. Exocrine secretion which passes to a free surface either directly or via a duct system. Endocrine secretion passes directly to the blood stream. There are three distinctive ways in which exocrine secretions occur. These are merocrine, apocrine and holocrine. Merocrine – exocytosis from cell apex into a lumen, ex. salivary glands, sweat glands, pancreas Apocrine – pinching off apical cell cytoplasm, ex. mammary glands 9  Holocrine – shedding of the whole cell, ex. sebaceous glands Figure 2.1 – Types of Cell Secretions2 2.4 Arrangements of secretory tissues Epithelial cells can be arranged into glands, which allows for the focused production of a secretion. These secreting cells can be scattered amount other non-secretory cells (ex. unicellular goblet cells). If increased production of secretions is required, the surface of the secretory epithelium can then be invaginated to form a straight tubular gland, or a more complex coiled or branched gland. In highly organized glands, the secretory cells are gathered in clusters called acini. Secretions from these cells travel through ducts lined with columnar epithelium. Figure 2.2 – Secretory Cells and Glands3 10  Table 2.1 – Arrangements of Secretory Tissues Gland Description Example Simple tubular Single, straight tubular lumen Large intestine glands Simple coiled tubular Single tube tightly coiled in three Sweat glands dimensions (often seen in various planes of section) Simple branched Several tubular secretory portions, Stomach tubular converging into a single, unbranched wider duct Simple acinar Pocket in the epithelial surface Urethra Simple branched acinar Several secretory acini that empty into a Sebaceous glands single excretory duct Compound branched Branched duct system with tubular Brunner’s glands of the tubular secretory structure duodenum Compound acinar Secretory units are acinar and drain into a Pancreas branched duct system Compound tubulo- Three types of secretory units; branched Submandibular salivary acinar tubular, branched acinar, and branched gland tubular with acinar end pieces 11 Section 3 – Support Cells and Extracellular Matrix Upon completion of this section, you will be able to describe and recognize support cells, extracellular matrix and fibrillar proteins in organs using the light microscope. Learning Objectives To successfully complete this section, you should be able to 3.1 Name the support cells 3.2 Describe the following terms: fibroblast, fibrocyte, amorphous ground substance, chondroblast, chondrocyte, osteoblast, and osteocyte 3.3 Describe and recognize the following elements: collagen, elastic fibers, basement membrane, fibrocollagenous tissue, adipocytes, adipose tissue, and cartilage 12 3.1 Support cells In any given tissue or organ, the cellular component consists of two types. Parenchymal cells are usually specific to that tissue and perform the major functions of that tissue, for example, hepatocytes are the parenchyma of liver. The second cell type is the support cell which is necessary to provide the structural scaffolding. These support cells produce the extracellular matrix. Support cells and the extracellular matrix is commonly called connective tissue. Support cells have the following common characteristics: derived from embryonic mesenchyme produce extracellular materials form sparsely cellular tissues predominately composed of extracellular matrix cell adhesion mechanisms to interact with other extracellular matrix materials There are five classes of support cells: Fibroblasts – secrete the extracellular matrix components in most tissues, usually collagen and elastin Chondrocytes – secrete the extracellular matrix components of cartilage Osteoblasts – secrete the extracellular matrix components of bone Myofibroblasts – secrete extracellular matrix components and have a contractile function Adipocytes – lipid-storing support cells that assist in energy storage, endocrine function, and physical protection 3.2 Main components of the extracellular matrix The extracellular matrix (ECM) is the scaffolding for tissues and is composed of fibrillar proteins, surrounded by glycosaminoglycans (GAG). The ECM provides structural and biological support to cells within the tissue and is produced by most support cells. The fibrillar proteins are arranged in a hydrated gel of GAG, creating and organized extracellular network. GAG are large polysaccharides that provide support and regulate the diffusion of substances through the extracellular matrix. They can be linked to proteins to form proteoglycans. Different types of GAGs can confer special attributes to the extracellular matrix, such as diffusion or binding of other substances (ex. retaining sodium ions and water). 13 3.3 Fibrillar proteins Fibrillar proteins provide tensile strength and support to tissues. There are four major proteins that can form fibrils in the ECM Collagen Elastin Fibrillin Fibronectin Collagen Collagen proteins are the most important fibrillar components of the ECM and is synthesized by fibroblasts and other support cells. The collagen proteins can form filaments, fibrils, or meshwork, which interacts with other proteins to provide support. There are at least 28 different types of collagen polypeptide chains, which can combine to form various forms of collagen. Under the microscope, collagen fibers appear pink when stained with a routine histology stain such as the Hematoxylin and Eosin stain. Special stains can be used to identify collagen as it is difficult to differentiate from other structures that also appear pink in routine staining. Figure 3.1 - Types of Collagen4 Collagen types I, II, and III are the main forms of fibrillar collagen and form rope-like fibrils. When these fibrils are arranged into fibres, they provide protection from tensile stresses. Reticular fibres, or reticulin, are composed of type III collagen. These are very thin fibrils that can form a lose mesh in the extracellular matrix of hematopoietic and lymphoid tissues. In organs such as the liver the kidney, reticulin supports specialized epithelial cell. Reticular fibers are not easily demonstrated with routine histology staining, but special stains can be used to see them. 14 Collagen formation can be summarized as following: Three precursor protein chain wind together to form rigid linear triple helix structures known as procollagen Fibroblast secretes procollagen Proteolytic cleavage of amino and carboxyl ends of procollagen forms tropocollagen Tropocollagen molecules align into long, linear filaments, or microfibrils Microfibrils are arranged into fibres Fibres are arranged into larger bundles Figure 3.1 – Fibrillar Collagen5 Elastin and Fibrillin Elastin is a protein that forms into stretchable and resilient sheets or fibres. It is the main component of elastic fibres. Elastin is produced by fibroblasts. Elastic fibres are composed of elastin and fibrillin. Microfibrils of fibrillin surround and organize a core of elastin for form an elastic fibre. These fibres are found in many support tissues and confer elasticity and recoil to a tissue. In routine histology staining, elastic fibres appear as glassy, bright pink structures, usually appearing more prominent than collagen fibers. They can also be demonstrated with special staining techniques. Figure 3.2 – Elastic Fibre6 Fibronectin Fibronectin acts as mediator glycoprotein between cells and extracellular matrix components, such as collagen. 15 3.4 Basement membrane Basement membrane is composed of specialized sheets of extracellular matrix that are found between the parenchymal cells and support tissues. It is associated with epithelial cells, muscle cells, and Schwann cells. It also forms a limiting membrane around the central nervous system. Basement membrane has four major components: type IV collagen laminins perlecan nidogens (entactins) The basement membrane in different tissues can be specialized to the function of that tissue, however there is an overall general structure to basement membrane. In routine histology staining, the basement membrane is difficult to visualize as it is only 0.05mm thick and stains poorly. It can be demonstrated though using immunohistochemical or special chemical stains. Viewed with electron microscopy, the basement membrane is composed of three layers, or laminae, starting from the interface of the cell membrane down towards the extracellular matrix: Lamina lucida Lamina densa Fibroreticular lamina The basic functions of basement membrane are cell adhesion, diffusion barrier, and regulation of cell growth. When an adhesion interface occurs in non-epithelial cells, like in muscle cells, the term external lamina is used instead of basement membrane. 3.5 Other support cells Many support cells are derived from embryonic mesenchyme and develop into fibroblasts, myofibroblasts, lipoblasts, osteoblasts, and chondroblasts. The suffix ‘blast’ denotes that the cell is actively growing or secreting extracellular matrix material. The suffix ‘cyte’ represents that the cell is in a quiescent phase (ex. fibrocyte, osteocyte, chondrocyte). Fibroblasts and fibrocytes Fibroblasts are the support cell responsible for producing fibrocollagenous tissue, which is fibrous tissue composed mainly of collagen fibres. Along with collagen, fibrocollagenous tissue also contains GAG, elastic fibres, and reticular fibres. Fibroblasts are very resilient cells and have a role in tissue repair. Fibroblasts morphologically have a large oval nucleus, large nucleolus, and basophilic cytoplasm. As the fibroblast becomes less active, it shrinks in size and is now called a fibrocyte. Fibrocollagenous tissue can be described as loose connective tissue, or dense connective tissue. Loose connective tissue is composed of thin, haphazardly arranged collagen fibres which are widely spaced. Dense connective tissue is composed of broad collagen fibres which form a 16 confluent arrangement. The arrangement of collagen fibres in a tissue is based upon the local stress of that tissue. For example, very dense fibrocollagenous tissue is the main component of tendons and ligaments. Fibrocollagenous tissue is the main support tissue in most organs, and has the following functions: Supports nerves, blood vessels, and lymphatics Separates functional layers Supports immune cells, both transient and resident Formation of a fibrous capsule Formation of fibroadipose tissue Myofibroblasts These cells are not predominant in support tissues but resemble fibroblasts when viewed using light microscopy. Ultrastructurally, they have characteristics that resemble smooth muscle cells. Chondroblasts and chondrocytes Chondroblasts and chondrocytes are responsible for the formation of cartilage. Cartilage is composed of GAG and collagen fibres. Chondroblasts appear as vacuolated cells with a round nuclei, prominent nucleoli, and pale, vacuolated cytoplasm. When embedded in paraffin, an artefactual space is formed around the chondroblast, known as a ‘lacuna’. This is due to the high glycogen and lipid content in the cell which draws away from the extracellular matrix. Cartilage is formed due to the proliferation of chondroblasts which deposit cartilage. As the chondroblast slows down its metabolic activity, it becomes paler in appearance with a smaller nuclei and scant cytoplasm. This cell is now called a chondrocyte. There are three types of cartilage: hyaline fibrocartilage elastic The different types of cartilage contain different fibrous proteins. TYPE FIBROUS PROTEIN DISTRIBUTION Hyaline Type II collagen temporary skeleton in fetus, growth plates in long cartilage bones, articular surfaces of joints, support tissue in respiratory passages Fibro Type I & II collagen intervertebral discs, tendon attachment, cartilage junctions in flat bones of pelvis Elastic Type II collagen & external ear, external auditory canal, elastic fibres auditory tube, epiglottis 17 Osteoblasts and osteocytes These cells are responsible for the formation of bone, which is composed of an extracellular matrix material called osteoid. Osteoblasts secrete the osteoid, which calcifies to form bone. Osteoid is mostly type I collagen. Osteoblasts are roughly cuboidal shaped cells and occur on the free surface of bone. Osteocytes are inactive osteoblasts trapped in mineralized osteoid. Osteocytes lie in lacuna which are connected to each other by minute channels called canaliculi. Adipocytes Adipocytes have a function of storing fat and are responsible for the formation of adipose tissue. There are two types of adipose tissue, unilocular, or ‘white fat’, and multilocular, or ‘brown fat’. Unilocular adipose tissue is composed of adipocytes which store fat as a source of energy for other body tissues. The adipocytes are 50-150 μm in size and polyhedral in shape. In routine histology staining they appear as strings of cytoplasm surrounding an empty space. As most routine specimens are processed using lipid solvents, all fat components from the cells is removed, leaving behind an empty vacuole. Along the edges of cytoplasm small, flattened nuclei can be seen. In large numbers, these adipocytes are pushed together, and the shape is more polygonal. The overall appearance resembles chicken wire. Between the cells is an extremely rich capillary blood supply as well as nerve endings. Multilocular adipose tissue is most often found in the newborn and functions to produce heat through the metabolism of fat. Morphologically, multilocular adipose tissue appears more eosinophilic microscopically, and brown macroscopically. It is not usually found in adult tissue. 18 Section 4 – Contractile Cells Upon completion of this section, you will be able to describe recognize the contractile cells in organs using the light microscope. Learning Objectives To successfully complete the section, you should be able to 4.1 Name and describe the types of contractile cells 4.2 Describe and recognize the following tissue: Skeletal muscle Cardiac muscle Smooth muscle 4.3 Compare and contrast the histological features of the various muscle cells 4.4 Briefly compare the growth and regeneration of the various muscle cells 19 4.1 Contractile cells Specialized cells can generate motile forces through contraction, specifically due to the interaction of the proteins actin and myosin. There are four types of contractile cells: muscle cells o main type of contractile cell o striated voluntary muscle o cardiac muscle o smooth involuntary muscle myofibroblasts o can secrete collagen pericytes o muscle-like cells that surround blood vessels myoepithelial cells o component of some secretory glands 4.2 Muscle cells Skeletal muscle Muscles are formed from skeletal muscle cells, which carry out voluntary movement under influence from the nervous system. Each skeletal muscle cell is formed when hundreds of myoblasts fuse together. This forms a syncytium which contains hundreds of nuclei. Each skeletal muscle cell is a long, thin, cylinder, usually 50-60 μm in diameter and up to 10 cm long. Each muscle cell is also surrounded by an external lamina (a type of basement membrane). Skeletal muscle cell components can be described using alternate terminology: Sarcolemma – cell membrane Sarcoplasm – cell cytoplasm Sarcoplasmic reticulum – endoplasmic reticulum Contraction of skeletal muscle is due to the interaction of actin and myosin. The thick myosin filaments and thin actin filaments overlap in a repeating pattern to form the contractile elements of skeletal muscle cells, or myofibrils. One muscle fibre has hundreds of myofibrils running parallel along the length of the fibre, with the alternating thick and thin filaments creating a striated pattern, hence the term striated muscle. Accessory proteins hold the thick and thin filaments in place, but these cannot be visualized with routine examination. 20 Individual muscle fibres or cells are surrounded by endomysium, which is composed of sheets of external lamina. These muscle fibres are then bundled together to form muscle fascicles, which are surround by fine sheets of fibrocollagenous tissue known as perimysium. Muscle fascicles are then grouped together to form an anatomical muscle, and these groups of fascicles are surrounded by a thick layer of fibrocollagenous support tissue known as epimysium. Figure 4.1 – Organization of Muscle Fibres into Muscle7 In cross section, skeletal muscle cells have a hexagonal shape, and are grouped together. Nuclei can bee seen beneath the cell membrane. In longitudinal section, skeletal muscle appears as parallel, non-branching fibres with evident cross striations. The nuclei appear flattened and are peripherally located. Cardiac muscle Cardiac muscle is another form of striated muscle but does have differences from skeletal muscle. It has a similar arrangement of actin and myosin; however, cardiac muscle cells have a central nucleus and are shorter cells than skeletal muscle cells. Cardiac muscle cells can also be arranged end to end to create longer muscle fibres through specialized cell junctions. These intercellular junctions can be seen as faint transverse lines, known as intercalated discs. Cardiac muscle also lacks a population of resident stem cells, which means damage to cardiac muscle tissue is permanent, and dead cells are replaced by collagenous scar tissue. Smooth muscle Smooth muscle cells contain a less organized arrangement of contractile proteins compared to skeletal and cardiac muscle. Smooth muscle cells are the main contractile cell in the walls of organs such as the gut, urinary bladder, and uterus. They are also found in the walls of blood vessels and gland ducts. They are related to functions that require slow, rhythmic, involuntary contractions. 21  Smooth muscle cells can be bound together by basement membrane material. Each cell is spindle shaped, with a centrally located elongated nucleus. In cross section, smooth muscle cells are polygonal in shape, but in longitudinal section they appear as linear bundles. The contraction of smooth muscle is coordinated by bundles of contractile proteins that crisscross the cell and are inserted into anchoring points. Tension generated in one cell is passed to the surrounding cells, which allows a mass of cells to function as one unit. 4.3 Growth and regeneration Skeletal muscle cells are highly specialized and cannot divide. Skeletal muscle contains a population of reserve cells called satellite cells, which are a potential source of myoblasts that can fuse to form a new skeletal muscle fibre. This regenerative ability is very limited and unable to compensate for major muscle degeneration. Skeletal muscle can increase in size in response to exercise. This is a result of the increase in size of the individual cells and not as an increase in the number of cells. This is called hypertrophy. Cardiac muscle cells cannot divide but are also capable of hypertrophy. Unlike skeletal muscle, cardiac muscle does not have the equivalent of the satellite cell and so there is no muscle cell regeneration. Smooth muscle can respond to increased demand by hypertrophy, but it also retains the capability for mitosis and so can increase in numbers. This is called hyperplasia. Smooth muscle can also be generated from a cell called the pericyte which is seen in association with small blood vessels. These factors give smooth muscle a substantive regenerative capability. 22 Section 5 – Lymphoid Tissue Upon completion of this section, you will be able to describe and recognize tissues of the immune system using the light microscope. Learning Objectives To successfully complete the section, you should be able to 5.1 Name the lymphoid organs and the types of lymphoid tissue 5.2 Describe the structure of the thymus, lymph node, spleen, and mucosa associated lymphoid tissue 5.3 Recognize the structure of the lymph node, spleen, and mucosa associated lymphoid tissue. 23 5.1 Immune System The immune system is the body’s defence system to protect against invading pathogens. The main cells involved in the immune system are white blood cells, particularly lymphocytes, granulocytes, macrophages, and dendritic cells. Lymphocytes are produced in the bone marrow and thymus gland. Specific immune responses occur in secondary immune organs, which are lymph nodes and the spleen, as well as lymphoid tissue located in epithelial surfaces. This specific type of lymphoid tissue is called mucosa-associated lymphoid tissue, or MALT. There are three types of lymphocytes in the immune system: B cells T cells Natural killer (NK) cells B lymphocytes originate in the bone marrow and then take residence within the specialized lymphoid organs (lymph nodes, spleen, tonsils, gut mucosa). They can also be found circulating in the peripheral blood and can migrate through tissues. B cells can be stimulated by antigens, which triggers proliferation and conversion to a plasma cell. Plasma cells are then able to secrete immunoglobulins, or antibodies, which are a part of the humoral immune response. T lymphocytes also originate in the bone marrow but mature in the thymus gland. They can also be found in specialized lymphoid organs (lymph nodes, spleen, and gut mucosa) and in peripheral blood. T cells can be stimulated by antigens and the specific antigen presenting cell to undergo proliferation and recruitment of other immune cells. They can also directly attack diseased cells, which is a part of the cell-mediated immune response. Natural killer cells can also be activated to become cytotoxic lymphocytes. They have a specific role in eliminated virus infected cells, as well as the elimination of cancer cells. The mononuclear phagocytic system is also an important part of the immune system. This system is composed of phagocytic cells that are dispersed in tissues as macrophages and dendritic cells. Macrophages have the role of removing dead cells in normal cell turnover. In diseased tissue, macrophage activation is increased. Tissue macrophages can be specialized in their role and morphology based on their location of residence. Specialized macrophages are found in the lung as alveolar macrophages, in the liver as Küppfer cells, in the brain as microglial cells, and in the spleen as sinusoidal lining cells. 5.2 Thymus, lymph nodes, spleen, and MALT Thymus The thymus is the location of T-cell development. During maturation, T cells can recognize the difference between self and foreign antigens. The thymus is an endocrine organ, meaning it can secrete hormones which control T cell development and function in peripheral tissues. It is a lobulated organ, located in the mediastinum. During maturation the thymus becomes infiltrated with fatty tissue. The outer cortex of the thymus is more cellular than the inner medulla. The cortex contains densely 24 packed lymphocytes, with scattered macrophages. The medulla contains a large epithelial component, which from Hassall’s corpuscles. These appear as concentric lamella around a central hyalinizing mass. They thymus also contains a rich vascular supply which allows lymphoid cells to migrate. Lymph Nodes Lymph nodes are important in the generate of the immune response. They are small organs that are found in groups or chains where lymphatic vessels converge their drainage into a larger lymphatic vessel. They have two functions: Contain non-specific phagocytic cells which remove particulate matter such as microorganisms Allow for interaction of lymphocytes with antigens and antigen presenting cells. As lymph nodes become more active, they greatly enlarge in size. When inactive, they are only a few millimeters in length. Three types of cells are found in the lymph nodes: Lymphoid cells – lymphocytes of all types Immunological accessory cells – macrophages Non-immunological active stromal cells – lymphatic and vascular endothelial cells, fibroblasts Lymph nodes are bean shaped, with a fibrocollagenous capsule. Fibrous trabeculae extend into the node from the capsule. Lymph is transported to the node by the afferent lymphatic vessels. These vessels enter the subcapsular sinus. The lymph passes to the cortical sinus and may percolate through the cortical tissue before collecting into medullary sinuses. Medullary sinuses join to form one efferent lymphatic vessel. This leaves the node at the hilus. Lymphatic vessels are lined by endothelium. A small artery enters, and a small vein leaves at the hilus. This vascular supply forms a capillary network and is lined by endothelium. The blood supply of lymph nodes is the main route of entry of lymphocytes. The lymph node is divided into an outer portion known as the cortex and the inner medulla. The cortex also can be subdivided into the superficial cortex and the paracortex which is adjacent to the medulla. The superficial cortex contains densely staining aggregates of lymphocytes known as lymphoid follicles. Primary follicles appear as uniform in their staining intensity, but follicles responding to antigens have a less densely staining germinal centre and are known as secondary follicles. Surrounding the germinal centre is an area known as the mantel zone, which is a ring of small lymphocytes. Follicles are comprised mostly of B cells. The paracortex is composed of mostly T cells, which can undergo proliferation and activation and be disseminated to peripheral tissues via the circulatory system. The medulla consists of wide medullary sinuses and cords of cells. Larger blood vessels and their supporting trabecula are also found. The predominant cell in the medulla is the plasma cell. These cells are responsible for secreting antibody into efferent lymph. 25 Spleen The spleen has two main functions: Mount a primary immune response to antigens in the blood Filter out particulate matter and aged or abnormal red blood cells and platelets from circulation The spleen is composed of vascular sinusoids which are supported by a reticulin framework. A thin, fibrocollagenous capsule surrounds the organ and short septa extend into the spleen. The sinusoids are filled with blood and make up the red pulp of the spleen. A series of branching arteries are also found along with aggregates of lymphoid tissue known as white pulp. The vasculature of the spleen allows for blood to be filtered through the red pulp. The splenic red pulp consists of loose support tissue with reticulin fibres. There are three important components: Capillaries which terminate in a macrophage lined space known as a sheathed capillary. The parenchyma is formed of stellate reticular support cells which surround sponge like spaces. The blood percolates through these spaces from the sheathed capillaries Venus sinuses collect blood that has been filtered through the parenchyma. Mucosa Associated Lymphoid Tissue (MALT) Lymphoid cells can be found concentrated in various mucosal surfaces and solid glands throughout the body as an additional defense mechanism. These aggregates of lymphoid cells can be found in the walls of the gastrointestinal, respiratory, and urogenital tracts, conjunctiva, and skin. These tissues are known as mucosa associated lymphoid tissue, or MALT. This can be in the form of diffuse infiltrating cells, or a defined nodule. In large aggregates, the lymphoid tissue can be arranged in follicles, often containing a germinal center, similar to what is seen in the lymph nodes. MALT can be further classified into: gut associated lymphoid tissue (GALT) o tonsils o esophagus o Peyer’s patches in the small intestine o lymphoid aggregates in the large intestine and appendix o lymphocytes in the lamina propria of the small and large intestines bronchus associated lymphoid tissue (BALT) o beneath the mucosa of the bronchi nasal associated lymphoid tissue (NALT) o nasopharynx skin associated lymphoid tissue (SALT) o Langerhans’ cells and scattered lymphocytes in the dermis and epidermis 26 Section 6 – Blood and Lymphatic Systems and Heart Upon completion of this section, you will be able to describe and recognize the blood and lymphatic systems and the structure of the heart wall using the light microscope. Learning Objectives To successfully complete the module, you should be able to 6.1 Describe and recognize the structures of the heart wall, arteries, veins, microvasculature in tissue, and capillaries 6.2 Compare and contrast the structure of an artery and a vein 6.3 Describe the structure of the lymphatic vessels 27 6.1 Heart wall The heart is a muscular pump with four chambers, enclosed in the pericardial sac. The two atrial chambers received blood from the systemic and pulmonary venous circulation, and the ventricles pump blood into the systemic and pulmonary arterial circulation. In between the chambers and at points of blood outflow, heart valves are in place to prevent blood backflow. The heart wall is composed of three layers: Epicardium – thin outer layer composed of flat mesothelial cells Myocardium – middle layer composed of cardiac muscle cells Endocardium – thin inner layer composed of endothelial cells The pericardium is the sac that surrounds the heart and is also lined by mesothelial cells. The sac itself is composed of compact fibrocollagenous and elastic tissue. A small amount of serous fluid lies between the pericardium and the epicardium to prevent friction during heart muscle contractions. The epicardium is also composed of fibrocollagenous tissue and elastic fibres. Within this layer are large coronary arteries and veins which supply or carry away blood from the heart. These vessels can also be surrounded by adipose tissue. The myocardium contains mostly specialized striated muscle, or cardiac muscle. The size and amount of myocardium is related to the workload of the specific chamber, with the left ventricle having the thickest myocardium. Endocardium lines all four heart chambers, and is made up of three layers: outer layer in contact with the myocardium o composed of irregularly arranged collagen fibres, may contain Purkinje fibres that are a part of the conducting system middle layer o thickest layer regularly arranged collagen fibers and some elastic fibres inner layer o flat endothelial cells Heart valves are composed of fibroelastic tissue, covered with a layer of endothelial cells. Purkinje fibres are large muscle fibres with vacuolated cytoplasm and lie in bundles of about six fibres. They are paler staining and lie just under the endocardium. A layer of loose fibrocollagenous tissue separates them from the myocardium. As these fibres have a higher glycogen content, a special stain for glycogen can be used to visualize them, or by detecting specific muscle proteins using an immunohistochemical stain. The heart has a rich vascular supply to keep up with the large energy demands. Two coronary arteries supply the heart and are direct branches off the aorta. They are classified as medium sized muscular arteries. 28 6.2 Blood circulatory system The blood circulatory system is responsible for transporting oxygen, carbon dioxide, nutrients, waste products, immune cells, chemical messengers and other important molecules throughout the body. The transferring of substances between the blood and tissues occurs at the level of the capillaries. There are two types of large blood vessels: arteries – carry blood away from the heart towards the tissues, under higher pressure veins – carry blood back to the heart, under lower pressure The systemic arterial circulation has a structure that reflects the high pressure of the system. Large elastic arteries (aorta, carotid, subclavian, renal arteries) contain a high component of elastic tissue which smoots the high-pressure systolic wave. As the arteries move away from the heart, the diameter of the vessels become smaller and the walls become more muscular. These arteries are now termed muscular arteries. Eventually these transition into the smaller arterioles, finally leading into the system of fine vessels called capillaries. Blood then flows through the capillary network and makes its way into the venules, and then into veins. These vessels become larger as they move closer to the heart. As the veins are under low pressure, their structure contains less muscle compared to arteries. Figure 6.1 – Systemic and Pulmonary Blood Circulations8 29 Large blood vessels are composed of three layers; however, each vessel type has a different composition of the three layers. The layers (tunica) are: intima o lining layer of endothelium; specialized, flattened epithelial cells o sits on an internal elastic lamina supported by a thin layer of fibrocollagenous support tissue o support layer contains myointimal cells - contractile cells that can synthesize collagen and elastin and have phagocytic properties media o composed mostly of smooth muscle with layers of organized elastic tissue (elastic laminae) o more prominent in arteries compared to veins, indistinct in small vessels o vessels closer to the heart have a higher content of elastic tissue o in muscular arteries and arterioles, an external elastic lamina lies between the tunica media and the tunica adventitia adventitia o composed of fibroblasts and collagen o smooth muscle cells may be present in veins o most prominent layer in veins o thick walled vessels contain small blood vessels in the tunica adventitia (vasa vasorum) o contains autonomic nerves to activate the smooth muscle in the tunica media Figure 6.2 – General Structure of Blood Vessel Wall9 30 The endothelium is a very specialized layer of flattened endothelial cells with various roles. As the cells are so flat, the cytoplasm is not discernible, and the nucleus appears very flat. Each cell is anchored to the underlying basal lamina. Ultrastructurally, a dense cytoplasmic organelle called the Weibel-Palade body can be seen. This is a storage site for granules of various molecules that allow for the diameter of the vessel to be altered. The endothelial cells are also able to recognize changes in blood pressure, oxygen tension, and blood flow, and can secrete various molecules in response to these changes. Endothelial cells have an important role in in blood coagulation, and in normal conditions, the smooth layer prevents blood clots form forming. Elastic Arteries These are the largest arteries and are subject to a high pressure as they receive blood from the left ventricles of the heart. Their high component of elastic tissue provides them with resilience to handle the high pressure. The tunica media is highly developed with a large amount of elastic fibres arranged circumferentially in sheets between layers of smooth muscle. In the largest elastic arties, there can be 50 or more layers. Muscular Arteries Muscular arteries typically only have two layers of elastic fibers in the tunica media. They also have an internal and external elastic lamina, at the interface between the intima and the media and the adventitia and media respectively. The media is composed mostly of smooth muscle, which allows them to be highly contractile. Larger muscular arteries will have 30 or more layers of smooth muscle. Arterioles Arterioles have a wide range in their diameter, and are typically composed of a tunic intima, a tunica media with one or two layers of smooth muscle, and an insignificant tunica adventitia. Microvasculature The microvasculature is a system of small diameter blood vessels with thin walls that are partially permeable. This allows the transfer of some components between the blood and the tissues. This system starts at the level of arterioles and continues into the extensive capillary network. Capillaries These are the smallest vessels of the circulatory system and have the thinnest walls, allowing for the exchange of gases. The walls of the capillaries are composed of an endothelium and occasional contractile cells called pericytes. The endothelium can be one of two types: continuous – most common, endothelium is complete with no defects fenestrated – seen in the gastrointestinal mucosa, endocrine glands, and renal glomeruli; endothelial layer is pierced by pores or fenestrations to allow diffusion of small molecules 31  Figure 6.3 – Types of Capillary10 Venules Postcapillary venules are the smallest venules and resemble capillaries but contain more pericytes. These venules drain into large collecting venules, which have a continuous layer of pericytes as well as surrounding collagen fibres. As the venules enlarge, the pericytes are replaced with smooth muscle, becoming one to two layers thick. The tunic adventitia also becomes more apparent. These are termed muscular venules, which drain into the smallest veins. Veins Veins can vary in size and when compared to an artery of the same diameter, have a larger lumen and thinner wall. In histological sections, veins often appear as collapsed. The three layers are less recognizable than in arteries. Small veins have more defined muscle cells compared to muscular venules, and medium sized veins have a discontinuous internal elastic lamina. Large veins contain valves which assist in movement of blood towards the heart. 6.3 Lymphatic circulatory system The lymphatic system carries fluid that drains from the intercellular space of tissues. In the intercellular space are small, endothelial lined tubes, like blood capillaries but with blind ends. These are lymphatic capillaries. Like blood capillaries, the endothelium can be fenestrated to permit the passage of larger molecules. Surplus fluid known as lymph is drained from the tissue spaces into the capillaries, which eventually become larger lymphatic venules and medium sized veins. Like veins, lymphatic vessels also contain valves. Along the system are lymph nodes, which the lymph passes through. In histological sections, small lymphatic vessels appear collapsed. 32 References 1. Lowe JS, Anderson PG, Anderson SI. Stevens & Lowe’s Human Histology. 5th ed. Elsevier; 2020. Figure 1.3, Functional Cell Classification; p. 3. 2. Lowe JS, Anderson PG, Anderson SI. Stevens & Lowe’s Human Histology. 5th ed. Elsevier; 2020. Figure 3.15, Types of Cell Secretions; p. 52. 3. Lowe JS, Anderson PG, Anderson SI. Stevens & Lowe’s Human Histology. 5th ed. Elsevier; 2020. Figure 3.15, Secretory Cells and Glands; p. 52. 4. Lowe JS, Anderson PG, Anderson SI. Stevens & Lowe’s Human Histology. 5th ed. Elsevier; 2020. Figure 4.3 Important Molecular Forms of Collagen; p. 58. 5. Lowe JS, Anderson PG, Anderson SI. Stevens & Lowe’s Human Histology. 5th ed. Elsevier; 2020. Figure 4.6 Fibrillar Collagen; p. 59. 6. Lowe JS, Anderson PG, Anderson SI. Stevens & Lowe’s Human Histology. 5th ed. Elsevier; 2020. Figure 4.9 Elastic Fibre; p. 61. 7. Lowe JS, Anderson PG, Anderson SI. Stevens & Lowe’s Human Histology. 5th ed. Elsevier; 2020. Figure 13.3 Organization of Muscle Fibres Into Muscle; p. 224. 8. Lowe JS, Anderson PG, Anderson SI. Stevens & Lowe’s Human Histology. 5th ed. Elsevier; 2020. Figure 9.1 Systemic and Pulmonary Blood Circulations; p. 141. 9. Lowe JS, Anderson PG, Anderson SI. Stevens & Lowe’s Human Histology. 5th ed. Elsevier; 2020. Figure 9.2 General Structure of Blood Vessel Wall; p. 141. 10. Lowe JS, Anderson PG, Anderson SI. Stevens & Lowe’s Human Histology. 5th ed. Elsevier; 2020. Figure 9.10 Types of Capillary; p. 147. 33