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January 15, 2018 | Author: Anonymous | Category: , Science, Health Science, Histology
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Dr. Ahmed M. Almosaui

Histology Humans are composed of billions of cells. A tissue is a collection of cells with similar structural characteristics. Organs are groups of tissues. Most organs are complex groupings of different tissue types. An organism is composed of organs that are grouped together and functionally integrated. The four types of human tissue 1. Epithelium. Epithelial cells form sheets of cells that are tightly joined to one another. These cells cover surfaces, line cavities, and form the secretory portions of many glandular structures, which are complex imaginations of the body surface. 2. Connective tissues join epithelial structures to other parts of the body. They exist under all epithelial layers and have a cellular component and an extracellular matrix. Bone, cartilage, and blood are specialized connective tissues. 3. Muscle tissue a- Skeletal muscle tissue is found in the gross muscles that cause skeletal movement. b- Cardiac muscle tissue is found in the heart wall and proximal portions of the aorta. c- Smooth muscle tissue is a prominent component of blood vessel walls and visceral organs such as those of the gastrointestinal, urinary, and reproductive systems. 4. Nerve tissue. The brain, spinal cord, autonomic ganglia, peripheral nerves, and portions of sensory organs are composed of nerve tissue.

Tissue derivation. The three primary germ layers in embryology are ectoderm (the outer germ layer), mesoderm (the middle germ layer), and endoderm (the inner germ layer). 1. Epithelium is derived from each of these layers. A. Ectoderm forms epidermis. B. Mesoderm forms mesothelium. 1

Dr. Ahmed M. Almosaui C. Endoderm forms the lining of the gastrointestinal tract. 2. Most connective tissue and muscle tissue is derived from mesoderm. 3. Most nerve tissue is derived from ectoderm.

Epithelial Tissue Epithelia are specialized layers that line the internal and cover the external surfaces of the body. An epithelium consists of a sheet of cells lying close together with little intercellular space. These cells have strong adhesion and form cellular sheets that cover the surface of the body and line its cavities. General classifications 1. Epithelia are classified by the number of cell layers they contain. a- Simple epithelia have one cell layer. All cells rest on the basement membrane and reach the apical surface. b- Stratified epithelia have more than one cell layer, consequently not all cells rest on the basement membrane or reach the apical surface. c- Pseudostratified epithelia are simple epithelia that appear to be stratified. In these epithelia, all cells rest on the basement membrane; however, not all cells reach the apical surface. The stratified appearance occurs because nuclei lie at different levels in the epithelia. 2. Epithelia are further classified by cell shape. a-Squamous cells are flat. b-Cuboidal cells have approximately equal height and width. c-Columnar cells are taller than they are wide.

Epithelial Tissue Types : * Simple squamous epithelium is a single layer of flat cells.

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Dr. Ahmed M. Almosaui 1- Simple squamous epithelium coats, or partially coats, the stomach, liver, gallbladder, and other visceral organs. It also is found in respiratory system alveoli and in the thin limbs of the loop of Henle in the kidney. 2- Mesothelium is the simple squamous epithelium that lines serous cavities (peritoneal, pleural, and pericardial cavities) and coats many of the organs in these cavities. 3- Endothelium is the simple squamous epithelium that lines the lumen of the cardiovascular system. * Simple cuboidal epithelium is a single layer of cells that are equal in height and width. It typically exists in areas where ion transport occurs (e.g., in kidney tubules, sweat glands, and some glandular ducts). It also covers the choroid plexus the four clusters of capillaries in the walls of the ventricles of the brain that help produce cerebrospinal fluid (CSF). Simple cuboidal epithelium is abundant in the kidney, in the proximal and distal tubules and parts of the collecting-duct system. Often, cuboidal epithelial cells have many apical microvilli and mitochondria, which facilitate ion pumping and fluid transport. * Simple columnar epithelium is a single layer of tall cells and is present in areas where absorption occurs. 1-Simple columnar epithelium composed of absorptive cells and goblet cells covers the lining of the small and large intestines . 2- It is found in some glandular ducts and the gallbladder, and it lines papillary collecting ducts in the urinary system. 3- Uterine epithelium is a simple columnar epithelium. * Pseudostratified columnar epithelium is a layer of cells, in which all of the cells rest on the basement membrane but only some extend to the apical surface of the epithelium. 1- Cell height varies in pseudostratified epithelium, as does the position of the nuclei with respect to the apical and basal limits of the epithelium. The apparent layering of nuclei makes the epithelium appear stratified.

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Dr. Ahmed M. Almosaui 2- Pseudostratified columnar epithelium exists in many sections of the respiratory system, such as the nasal cavities, nasopharynx, trachea, and bronchi . 3. Pseudostratified columnar epithelium is abundant in the male reproductive system. It is the luminal epithelium of the epididymis, vas deferens, prostate gland, seminal vesicles, and the prostatic urethra. * Stratified squamous epithelium contains many layers of cells, including an apical layer of flat cells. These epithelia are classified according to the characteristics of the apical layer. 1- Stratified squamous epithelium exists in locations subjected to chronic abrasion, The apical layers are continuously sloughed and then replaced by cell division in the basal layer. 2- The body is covered by a stratified squamous epithelium called epidermis. This epithelium is keratinized because cells in the outer layer contain the protein keratin. 3- The digestive system contains some stratified squamous epithelium. Stratified squamous epithelium on the tongue and lining the esophagus resists the abrasion of mastication, swallowing, and the passage of food from the oral cavity to the stomach. Stratified squamous epithelium in the anal canal resists the abrasion of passing semi-solid feces. 4- In the female reproductive system, stratified squamous epithelium is abundant in the vagina and covering the cervix * Transitional epithelium is a stratified epithelium found exclusively in the urinary passages of the urinary system. It contains many layers of polyhedral cells and an outer apical layer of round pillow-shaped cells. It is found in the minor calyces, major calyces, renal pelvis, ureters, urinary bladder, and p roximal urethra. Transitional epithelium in the bladder undergoes a reversible morphologic change during bladder distension and evacuation .

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Dr. Ahmed M. Almosaui Most epithelia rest on connective tissue. In the case of epithelia lining the cavity of internal organs (especially in the digestive, respiratory, and urinary systems) this layer of connective tissue is often called the lamina propria. The lamina propria not only serves to support the epithelium but also provides nutrition and binds it to underlying structures.

Function of Epithelial Tissue 1. Transcellular transport of molecules from one epithelial surface to another 2. Absorption occurs via endocytosis or pinocytosis in various organs (e.g., the proximal convoluted tubule of the kidney). 5

Dr. Ahmed M. Almosaui 3. Secretion of various molecules (e.g., hormones, mucus, proteins) occurs by exocytosis. 4. Protection from abrasion and injury is provided by the epidermis, the epithelial layer of the skin. Apical protrusions : The epithelial apex often contains protrusions from the cell surface. The protrusions may be scattered, as in mesothelium and endothelium, and may take the form of microvilli. a. Microvilli are cylindrical, cell-surface projections, 80 nm wide and 1-2 .m long, which increase the cell surface area for absorbing materials from the lumen. b. Stereocilia are long microvilli present in the male reproductive tract and in the membranous labyrinth of the inner ear. They are similar to microvilli except that they are longer and constricted at the point where they join the cell apex. little is known about their movements in vivo. Sensory stereocilia contain many actin containing microfilaments and large amounts of myosin. c. Cilia. Many epithelia apices are ciliated in some locations (see Chapter 5 III A for information about ciliary ultrastructure). 1- In the trachea, certain columnar epithelial cells have many apical cilia that move mucus along the apical surface of the tracheal lumen (airway). 2- Epithelial cells in the uterine tubes have apical cilia that help move the fertilized ova into the reproductive tract.

1. Basement membranes a- Most epithelia have a basement membrane (lymphatic capillaries are a notable exception), which ranges from scant to thick, depending on the epithelium. b- Some basement membranes have an amorphous basement lamina adjacent to the epithelium and a fibrous reticular lamina below the basement lamina. c- The fine structure of the basement membrane varies considerably with the location and function of the epithelia.

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Dr. Ahmed M. Almosaui Basement membrane functions 1-The basement membrane is the basal limit of an epithelium. In many luminal epithelia, the basement membrane is the boundary between the epithelium and subjacent connective tissue. 2-The basement membrane anchors the epithelium and may be a substratum for epithelial cell and connective tissue cell attachment. 3-The basement membrane maintains the shape of acini and branched ducts and tubules. 4-In some instances, the basement membrane is a selective barrier .

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Dr. Ahmed M. Almosaui

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Dr. Ahmed M. Almosaui

Glandular Tissue Two types of glands exist: endocrine glands and exocrine glands. Both types of glands are composed of and properly classified as epithelial tissues. A. Endocrine glands 1.

Endocrine glands secrete products into the bloodstream. They are derived from surface epithelium during development; however, they soon lose their connection to the body surface.

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Endocrine tissue retains many epithelial characteristics. For example, thyroid follicular epithelial cells form sheets, are surrounded by a basement membrane, secrete hormone precursors apically, exhibit apical-basal polarity, and contain junctional complexes.

B. Exocrine glands 1.

Exocrine glands secrete products onto the body surface through ducts. The skin and digestive tract, which are continuous with each other, receive exocrine secretions from glands such as sweat glands (skin) and the liver and pancreas (digestive tract).

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Exocrine gland classification. Exocrine glands are classified in several ways according to their mode of cellular secretion and arrangement of cells and ducts. They can be simple glands or compound glands.

a) Simple tubular glands consist of a simple straight tubule and exist in the small intestine. b) Simple Coiled tubular glands consist of a coiled tubule. Sweat glands are one example of coiled tubular glands. c) Simple Branched tubular glands have branches deep in the gland. This type of gland exists in the stomach and endometrium. d) Simple tubuloalveolar glands (Simple acinar ) have a single duct leading to a cluster of alveoli, or acini. Examples of this type of gland include the small salivary glands in the oral cavity and Brunner's glands in the duodenum. 9

Dr. Ahmed M. Almosaui e) Simple alveolar glands (Simple Branched acinar ) have several acini attached to a single duct. The sebaceous glands of the skin are this type of gland. f) Compound tubular glands have numerous tubules connected to multiple ducts. These glands exist in the testes. g) Compound tubuloalveolar glands (Compound tubuloacinar ) have numerous secretory acini that drain into numerous efferent ducts. These ducts typically merge into a smaller number of main ducts. The parotid salivary glands and the pancreas are examples of this type of gland. h) Compound alveolar glands (Compound acinar ) are compound glands because they have numerous draining ducts. However, they terminate in acini that have flat squamous cells rather than acini with cuboidal or pyramidal cells. The lungs are compound alveolar glands. Modes of secretion 1- Merocrine secretion involves the release of membrane-bound packets of secretion product. The packets are formed as membranes derived from the endoplasmic reticulum and Golgi apparatus surround the secretion product. The thyroid gland and pancreas exhibit merocrine secretion. 2- Apocrine secretion products include a portion of the apical cytoplasm from the secretory cell. Mammary glands exhibit apocrine secretion. 3- Holocrine secretion is characterized by whole cells bursting open to become the secretory product. Epidermal sebaceous glands exhibit holocrine secretion.

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Dr. Ahmed M. Almosaui

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