Histology Cheat Sheet PDF

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Summary of the contents taught in Histology ...

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Histology Cheat Sheet Generic Title Macrophage 1. Liver —> Kupffer Cell 2. Brain —> Microglial Cell 3. Skin —> Langerhans Cell 4. Bone —> Osteoclast 5. Cartilage —> Chondroclast 6. Lungs —> Dust Cell

Gall bladder Small non vital organ that concentrates bile excreted by the liver by removing the 90% of the water. It is capable of storing 30-50ml of bile and bile is discharged in response to hormones (CCK) secreted by the endocrine cells of the duodenal mucosa. Layers of the Gall bladder Mucosa – Epithelium, lamina propria, Rokitansky–Aschoff Sinuses Muscularis Externa – Smooth muscle, cholecystokinin, elastin fibres Adventitia – Loose connective tissue, adipocytes. Serosa – Mesothelium Mucosa: Contains a simple columnar epithelium that have basally located nuclei. Beneath the epithelium is connective tissue known as laminar propria. Impeding luminal structures within the mucosa contain simple cuboidal epithelium, indicating they are large bile ducts. These are known as Rokitansky Aschoff sinuses. Muscularis Externa: Loaded with smooth muscle cells that respond to autonomic nervous system signals and to the peptide cholecystokinin released by the duodenum when food is actively being absorbed. These cells contract to release stored concentrated bile. Adventitia: Loose connective tissue alongside fat cells. Impeding arteries, veins and nerves. Also contain an extensive network of lymphatic vessels. Serosa: Contains the mesothelium (most outer connective tissue lining). This epithelium is composed of a simple squamous epithelium that actively secretes mucous. Concentration of Bile Salt from the cytoplasm are actively secreted into the intercellular space (between cells). An increase in the [electrolytes] forms a osmotic gradient. Water and salt travel from the lumen within the gall bladder to the intercellular spaces leaving concentrated bile. Finally, the fluid moves from the intercellular space into the laminar propria where it moves away to the blood.

Trachea The trachea is a highly structured airway with cartilage and forms the initial passage way for air to enter. The trachea then proceeds to form the bronchial tree by forming the two bronchi that continuously divide throughout the lung. From here bronchi are named as such given that they contain cartilage rings. In the state where there are absent, we will term these bronchioles. Finally, bronchioles form their terminal end where they become alveoli. This is where air exchange occurs. Pseudostratified Epithelium lines the mucosa of the trachea all the way down to the bronchioles. These cells contain cilia that allow them to “beat” to push mucous along the respiratory tract. Goblet cells also exist to continuously form mucous to collect debris and become a substance to move along the respiratory tract.

The trachea is a short, flexible, air tube about 2.5cm in diameter and about 10cm long. It serves as a conduit for air, additionally, its wall assists in conditioning inspired air. The trachea extends from the larynx to about the middle of the thorax, where it divides into the 2 main primary bronchi. The lumen of the trachea consists of; - Mucosa; composed of ciliated, pseudostratified epithelium and an elastic fiber-rich lamina propria. - Submucosa; composed of slightly denser connective tissue than the lamina propria - Cartilaginous layer; composed of C-shaped hyaline cartilages - Adventitia; composed of connective tissue that binds the trachea to adjacent surfaces. C shaped Hyaline Cartilage is found within the lamina propria and provides structural integrity to the trachea. The trachea runs down the body until it divides into bronchi.

Skin The skin or the integument system, is the largest organ within the body. It contains an epithelium at its surface and underlying connective tissue imbedded with fibroblasts and melanocytes towards the bottom. It also contains a spread of blood vessels to supply nutrients to the surrounding and upper avascular epithelium alongside sensory nerves to detect changes in pressure and temperature. Function 1. Barrier against physical, chemical and biological damage 2. Immunological information 3. Participates in homeostatis 4. Conveys sensory information 5. Part of endocrine system 6. Has some excretory functions Layers of the skin 1. Epidermis 2. Dermis 3. Hypodermis Epidermis A stratified squamous epithelium + Keratin that contains melanocytes and keratinocytes. The epidermis can be broken up into more layers, that we will see further on. Consist of 5 layers – Stratum corneum – Stratum lucida (*only in thick skin) – Stratum granulosum – Stratum spinosum – Stratum basale (germinativum) Site of mitosis possesses melatonin in their cytoplasm and rest on the basement membrane. 1. Stratum Corneum Cells have lost their nucleus and contents and have produced a keratin layer. In thick skin the keratin layer is very thick (hands, feet) whilst in thin hairy skin it is relatively thin. 2. Stratum Lucida The Stratum Lucidum is an intermediate layer between the stratum Corneum and stratum Granulosum. It is typically a clear/refractive layer where keratinocytes are within an intermediate phase of transitioning their cell bodies over to keratin. 3. Stratum Granulosum These cells are superficial within epidermis. They contain their nucleus and numerous granules that contain keratohyalin. Thus, they are referred to as keratohyalin granules. 4. Stratum Spinosum This layer contains prickle cells as they appear to have a prickle appearance on their membranes. Although, these are actually stretched and expanded desmosomes between cells. The cells shrink in preparation for keratinization, giving them a spiny or prickly look to them. 5. Stratum Basale This layer contains the precursor cells (stem) that are mitotically active and begin the differentiation by moving upwards. They are connected to the basement membrane of the epithelium. Moreover, these are where melanocytes reside. Surface Structure of the skin

Called Hairs and Hair follicles, they are responsible for thermoregulation and the production of hair, respectfully. They are made from keratin and are present all around our body with the exceptions of the palms on hands and soles of feet, lips and region around uro-genital regions. Sebacous Glands A groups of Acinus (“berry”) cells that stain poorly due to containing and secreting the fatty substance sebum. They are found in the dermis, connected to hair follicles, therefore not found within the thick skin. Sebacous glands secrete sebum which functions as 1. Antibacterial 2. Oily barrier on skin surface 3. Emollient (moisturiser) Sweat Glands Sweat glands, within the skin are typically simple/stratified cuboidal epithelium that are parts of a spiral staircase structure that coils up to the surface of the skin where it opens up to a pore. Through the pore sweat (or sebum) is secreted Simple/Stratified Cuboidal tubular gland. These can be compounded with multiple compound branched or compound tubular where there are many structures leading up to the luminal surface. (secretory - simple | ducts - stratified)

Appocrine sweat glands • - Limited to axilla, areola & nipple, around anus & genital regions • - Secrete into hair follicles • - Large lumen • - Myoepithelial cells • - Cuboidal or columnar epithelium • - Human – Secretes ordourless fluid with proteins – but due to bacteria on the skin surface it develops a acrid odour = BO • - Mammals – Secret pheromones – Involved in marking territory, courFng maFng etc Arrector Pili muscle Adjacent to the hair follicle, this smooth muscle Erect follicle/raise hair, for thermoregulation. Dermo-Epithelial Junction - Dermal papillae (up) & rete ridges (down) • - Increased & longer in places with high mechanical stress • - Hold the epidermis layer in place • - “fingerprints” Cells of the epidermis 1. Keratinocytes 2. Melanocytes (Halo cells) 3. Langerhan’s cells 4. Merkel cells

Keratocytes The principle cell of the skin, originating In the basal layer, they change shape as cell move towards the surface. Keratocytes contain keratin. Melanocytes (Halo Cells) Melanocytes are specialised cells that secrete melanin. These cells are located within the stratum basale and are “nerve-like” cells with dendrites. They actively synthesize and secrete the peptide melanin which causes the brown pigmentation within the skin. This peptide it knows to dissipate up to 99.9% of UV radiation from the sun, therefore it is believed to protect cellular structures (DNA) from UV radiation that cause mutations. Albinos contain a genetic defect where no melanocytes exist. Langerhan’s Cell Found in the immune system, possessing an antigen presenting cell and attached to the spinosum & basale Merkel Cells Found in the Basale, merkel cells is a modified epithelial cells that functions as a mechanoreceptors / sensory reception and is found in the nervous system.

Nerve Ending In Skin Free nerve endings - fine touch, heat, cold - Also surround hair follicles • - Meissner’s Corpusle, mechanoreceptor (touch) Located in dermal papillae • - Pacinian Corpusle, mechanoreceptor (touch/vibration) Located in deep dermis & hypodermis

Dermis The dermis is the underlying layers of the epidermis and consists of; 1. Papillary Dermis - The closest portion to the epidermis. It contains papillae finger projections that intertwine with the epidermal rete ridges 2. Reticular Dermis - The reticular dermis contains the accessory glands such as the sebaceous, sweat glands and hair follicle structures. It is within a dense collagenous connective tissue alongside impeding blood vessels. 3. Hypodermis - The hypodermis is the second underlying layer from the reticular dermis and consists of mostly loose connective fatty tissue and blood vessels. Papillary Dermis The dermal papillae layer are finger like projections that protrude up from the dermis and into the epidermis. The dermal papillae are the more lightly stained finger like projections into the epidermis, whilst in contrast, darker stained finger-like projections protrude down and connect with the dermis. These complementing structures are known as rete ridges or epidermal ridges. Reticular Dermis This layer sits below the epidermis. It is packed with collagen and elastin fibres. A normal H&E stain will not show the elastin fibres. Sensory Nerves and blood vessels are spread throughout the space. It’s important to reinforce that epitheliums are avascular. Thus, they need to receive nutrients via diffusion. Typically, ground substance that is lost through staining is a dynamic fluid that acts as a solvent within the ECM to deliver nutrients from blood vessels to avascular tissues. Hypodermis This layer is fatty-fibrous connective tissue. It consists of largely adipocytes and intertwining loose collagenous connective tissue.

Lungs Mesothelium (Simple Squamous) lines both the inner and outer surface of the lungs. This supplies a lubricating fluid to reduce friction between tissues. - Visceral Pleura (Endothelium): Lines the inner side of the lung - Parietal Pleura (Mesothelium): Lings the outer the side of the lung (within the pleural cavity). Sections of lung tissue have the appearance of fine lace because most of the lung is composed of thin-walled alveoli. The alveoli are composed of a single layer of squamous epithelium. Between the alveoli you may see a thin layer of connective tissue and numerous capillaries also lined with simple squamous epithelium. This slide also contains sections across many bronchioles of various sized. Bronchioles can be recognized by the fact that they are lined by ciliated columnar epithelium (larger bronchioles) or by cuboidal epithelium (smaller bronchioles leading to alveoli). Remember that bronchioles are tubes and may be sectioned either transversely (across) or longitudinally. What will you see in each case? You may also see sections across small aeterioles and venules.

The conduction portion of the lung begins at the trachea and extends to the terminal bronchioles. Outside the lungs, the conduction system consists of the nasal cavities, nasopharynx, larynx, and trachea. Entering the lungs, the conducting portion spits into paired main bronchi. The bronchi begin a branching pattern, splitting next into lobar (secondary) bronchial branches and then again into segmental (tertiary) bronchi. The tertiary bronchi continue to divide into small bronchioles where the first change in histology takes place as cartilage is no longer present in the bronchioles. The end of the conduction portion of the lungs is at the final segment called the terminal bronchioles. The terminal bronchioles open into the respiratory bronchioles. This is the start of the respiration function of the lung.

The conducting portion provides the pathway for the movement and conditioning of the air entering the lung. Specialized cells collaborate to warm, moisturize, and remove particles that enter. These cells are the respiratory epithelium and comprise the entire respiratory tree. Most the respiratory epithelium is ciliated pseudostratified columnar epithelium. The following 5 types of cells are in this region: 1. Ciliated cells 2. Goblet cells 3. Basal cells 4. Brush cells 5. Neuroendocrine cells The ciliated cells are the most abundant. They control the actions of the mucociliary escalator, [1], a major defense mechanism of the lungs that removes debris. While the mucus provided by the goblet cells traps inhaled particles, the cilia beat to move the material towards the pharynx to swallow or cough out. Goblet cells, so named for their goblet-shaped appearance, are filled with mucin granules at their apical surface with the nucleus remaining towards the basilar layer. Goblet cells decrease in number as the respiratory tree gets progressively smaller and are eventually replaced by club cells (previously Clara cells) when they reach the respiratory bronchioles. The basal cells connect to the basement membrane and provide the attachment layer of the ciliated cells and goblet cells. They may be thought of like the stem cells of the respiratory epithelium as they maintain the ability to potentiate ciliated cells and goblet cells.[2] Brush cells occasionally referred to as a type III pneumocyte cells are sparsely distributed in all areas of respiratory mucosa. Brush cells may be columnar, or flask-like and are identified by their squat microvilli covered apical layer–resembling a push broom or appropriately, a brush. No function has been officially assigned to the brush cells though there are many proposed mechanisms. One popular proposal suggests they have a chemoreceptor function, monitoring air quality, due to their associated with unmyelinated nerve endings. [3] The bronchial mucosa also contains a small cluster of neuroendocrine cells, also known as Kulchitsky cells. [4] They have neurosecretory type granules and can secrete several factors, including catecholamine and polypeptide hormones, such as serotonin, calcitonin, and gastrin-releasing factors (bombesin). Like brush cells, these neuroendocrine cells make up only a small portion of mucosal epithelium, around 3%. Within the bronchial submucosa are submucosal glands composed of a mixture of serous and mucinous cells, similar to salivary gland tissue. Secretions are emptied into ducts and then on the bronchial mucosa. Older individuals may show oncocytic metaplasia of these glands. Smooth muscle bundles are present at all levels of the airway to allow for regulation of airflow. There are progressively fewer smooth muscle fibers progressing from bronchi to alveoli. Function In the conducting zone, air is moistened, warmed, and filtered before it reaches the start of the respiratory zone at the respiratory bronchioles. The respiratory zone is where gas exchange occurs, and blood is oxygenated in exchange for carbon dioxide. As the respiratory tree transitions from the conducting zone at the terminal bronchioles, goblet cells diminish as club cells increase and the cartilage present in the conducting zone is absent once it reaches the respiratory bronchioles. The acinus is directly distal to the terminal bronchioles and where the respiratory zone begins. The acinus is composed of respiratory bronchioles, alveolar ducts, and alveolar sacs. It is roughly spherical, resembling a bunch of grapes. Each respiratory bronchiole gives rise to several alveolar ducts and alveolar sacs–giving it that characteristic grape bunch appearance. The alveolar sacs are the ends of the respiratory tree and the site of gas exchange. Alveolar epithelium is composed to type I pneumocytes, type II pneumocytes, and the occasional brush cell. Also present in the alveolar walls are the club cells and alveolar macrophages. The alveolar walls contain the pores of Kohn[5][6] which allow communication between adjacent alveoli. This allows air to flow from one alveolus to another which may be beneficial if there is any blockage preventing air from entering alveoli through a direct route. Type I pneumocytes make up roughly 90% to 95% of the alveoli. They are flat, squamous epithelia that resemble plate-like structures that allow gas exchange. Their thin membrane allows for easier gas permeability between the alveoli and the blood vessels. Despite being the primary method of respiration, they cannot replicate and are very susceptible to toxic injury. Type II pneumocytes make up much of the remaining cell type in the alveoli, accounting for nearly 5%. Despite their low number, they are vital as they secrete pulmonary surfactant. The surfactant is necessary to maintain an open airway. It lowers the surface tension and prevents the alveoli from collapsing upon themselves during exhalation. By histology, these cells have foamy cytoplasm which results from surfactant that is stored as lamellar bodies. Type II pneumocytes are also mitotically active and can replace the easily damaged type I

pneumocytes. Type II pneumocytes cells can be recognized on histology by their rounded shapes that bulge into the alveolar space. Alveolar macrophages (or dust cells) may be free within the alveolar space or connected to the alveolar wall. If particles make it down to the acinus, the macrophages are the last defense and janitors of the respiratory epithelium. The black staining seen in lungs of smokers results from macrophages cleaning and sequestering particles that make their way inside. The visceral pleura of the lung is lined by a mesothelial layer with underlying connective tissue and elastic fibers. An elastin stain may be used to identify the elastic layer.

Testes Tunica Vaganalis - A connective tissue containing a serous space that envelopes on itself around the testes. Tunica Albuginea - A dense connective tissue that surrounds the tissue (also seen in the ovaries) Seminiferous Tubules - The convoluted tubules in which spermatogenesis occurs. Spermatozoa - move from the seminiferous tubules into the rete testis. Rete Testis - Collects the spermatozoa Epididymis - Contains a head, body and tail. The body of the epididymis is where spermatozoa matures into sperm, whilst the tail is where the sperm is stored. Vas Deferens - A lumen surrounded by layers of smooth muscle that contract to push sperm along their path through the prostate and along the urethra. Spermatogenesis Spermatogonia —> Cells sits basally on the seminiferous tubules and are responsible for spermatogenesis Type A (Sa- Dark) —> Stem cells sit basally on the basement membrane and do not commit to spermatogenesis, instead they maintain sperm population Type A (Sa- Pale) —> Coming to differentiating into type B cells Type B (SB) —> These cells differentiate into spermatozoa Sertoli cells (ST) —> Supportive cells S1 - Primary spermatocyte = (Large open nucleus preparing for 1st meiotic division – takes about 22 days) S2 - Secondary spermatocyte = Results from first meiosis division. Hard to find since it is rapid S3 - Spermatids = Result of second meiosis division, Small dark nucleus cells, located towards the lumen S4 - Spermatozoa = Spindle nucleus, maturation stage of sperm (Still non-motile), proceeding towards spermiogenesis

Sertoli Cells (Nurse Cells) Columnar epithelial cells (non-replicating) that surround and support t...