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Transcript
Animal Histology: Form and Function This presentation includes representatives of the major animal tissue types. It also includes a description of the form and function of each tissue or organ. The photographs are taken from Carolina’s best microscope slides. The images used in the following slides are also included in a separate folder on this CD. You may use them in your classroom or lab to create your own presentations or student assessments. Tissues Tissues are groups of specialized cells that work together for a particular function. In humans, combinations of different types of tissues make up organs, and groups of organs work together to form organ systems. Four Types of Tissue 1. Epithelial 2. Connective 3. Nervous 4. Muscle Epithelial Tissue Epithelial tissues cover body surfaces. Skin and the lining of organs are epithelial tissues. Epithelial tissues play roles in absorption, secretion, and protection against foreign substances. Simple Squamous Epithelium SS Simple squamous (SS) tissue is composed of flat, scale-like cells. It lines the walls of blood vessels, pulmonary alveoli (shown here), and the lining of the heart, lung, and peritoneal cavities. Simple Cuboidal Epithelium This tissue is composed of a single layer of boxy cells (arrows). It lines the walls of kidney tubules. In describing epithelium, the term “simple” means one cell-layer thick. Simple Columnar Epithelium The cells of simple columnar epithelium (arrows) are taller, as their name suggests. This tissue is usually associated with secretion or absorption and is most often found lining the digestive tract. The section shown here is from an amphibian. Stratified Squamous Epithelium The term “stratified” refers to the layered arrangement of cells. The outer layers of cells appear flat, but the inner cells vary in shape from cuboidal to columnar. Stratified squamous epithelium serves as a barrier to the outside environment. Stratified Cuboidal Epithelium SC Stratified cuboidal epithelium (SC) is found in the ducts of sweat glands and surrounds Graafian follicles of ovaries (shown above). The outer layer of cells appears smaller than the inner layer. Stratified Columnar Epithelium Stratified columnar epithelium occurs only in parts of the male urethra and certain excretory ducts. Ciliated Epithelium Some epithelial membranes are made up of cells with cilia, tiny projections that beat in unison to move mucus along the surface. Ciliated epithelia in the trachea, for example, sweep debris out of the respiratory tract. Pseudostratified Columnar Epithelium BM Although this tissue appears stratified, it is actually composed of a single layer of cells of different types. Although their nuclei are found at different levels, each cell adjoins the basal membrane (BM). This tissue lines the larger respiratory passageways. It is often ciliated (arrows). Transitional Epithelium Transitional epithelium is found in the lining of the bladder. When the bladder is full, the epithelium is stretched and the cells appear flat. Cells appear rounded when the bladder is empty. Connective Tissue Connective tissue differs from other tissues in that it contains large amounts of intercellular matrix. Connective tissues function to bind other tissues together, provide support, provide nourishment, store wastes, or repair damaged tissues. Bone Bone is a type of connective tissue that secretes a matrix of mineral salts, such as calcium phosphate, and the protein collagen. The minerals give bones their hardness, while the protein gives them strength and resiliency. Bone supports the body, protects internal organs, provides for muscle attachment, and serves as a calcium reservoir. Many bones contain a marrow cavity where red blood cells and white blood cells are formed. Compact Bone Compact bone consists of repeating units called osteons. Each osteon has concentric layers of the mineralized matrix deposited around a central canal. The canal contains blood vessels and nerves that serve the bone cells. Boneforming cells, called osteoblasts, deposit the matrix around the central canal and ultimately surround themselves with the mineralized material, forming a pocket called a lacuna. Once osteoblasts are surrounded by matrix, the cells are called osteocytes. Narrow connections called canaliculi extend from lacunae and connect osteocytes to each other and to the central canal. Compact Bone cc Compact bone is made up of concentric layers of bone matrix surrounding central canals (CC). A canal and its associated layers make up an osteon. Osteocytes are located within cavities called lacunae. Osteocytes oc Osteocytes (OC) are connected by cellular extensions called canaliculi. These extensions allow nutrients to pass from cell to cell. Adipose Tissue Adipose tissue is a specialized form of loose connective tissue that stores fat in adipose cells distributed throughout a non-cellular matrix. Adipose tissue protects internal organs, insulates the body, and stores energy as fat molecules. Each adipose cell contains a large lipid-filled vesicle. Adipose cells do not reproduce but are long lived, functioning for many years. Adipose Tissue Adipose cells are bundled together by connective tissue. The connective tissue creates lobules of adipose cells. Each cell appears as a clear space, representing the site of the large drop of lipid (fat) before it dissolved during preparation of the microscope slide. The nuclei appear as small disks on the periphery of cells. Loose Connective Tissue Loose connective tissue has few fibers, a number of cell types, and a large amount of matrix. It functions to bind epithelia to underlying tissues. Dense Connective Tissue Dense connective tissue contains a large number of fibers with only a few cells. Fibers shown here are all running parallel to each other, and no cells are present. Tendons are composed of dense connective tissue. Hyaline Cartilage Hyaline cartilage serves in support and acts as a lubricating surface for some joints. Hyaline cartilage appears as a translucent mass with chondrocytes embedded in the matrix within cavities called lacunae. Fibrous Cartilage Fibrous cartilage is composed of bundles of dense collagenous connective tissue with small groups of lacunae in a hyaline cartilage matrix. This cartilage is found in the intervertebral discs. Blood Blood is considered a connective tissue because it has an extracellular matrix called plasma. This liquid matrix is made up of water, salts, nutrients and an assortment of dissolved proteins. Suspended in the matrix are two types of cells, erythrocytes (red blood cells) and leukocytes (white blood cells), as well as specialized cell fragments called platelets that function in clotting blood. Erythrocytes RBCs Red blood cells (RBCs) in humans are flattened disks because the cells lack a nucleus. The pigment hemoglobin, which gives blood its red color, binds to oxygen. Red blood cells are responsible for transporting oxygen to all the cells of the body. Leukocytes White blood cells function mostly in fighting diseases. Some of them move through the walls of blood vessels and enter body tissues to engulf bacteria. There are five types of white blood cells: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Neutrophils Neutrophils are granular with segmented nuclei with 2 to 5 lobes. They are the most numerous leukocytes and are the first line of defense, actively phagocytizing foreign substances. Lymphocytes Lymphocytes typically have large, round nuclei and lack granules in the cytoplasm. There are two types of lymphocyte: B cells produce antibodies, while T cells fight virally infected cells and cancer cells. Monocytes A monocyte is nongranular and has a large nucleus that tends to be oval or kidney shaped. Monocytes function outside the bloodstream where they transform into macrophages. Eosinophils An eosinophil has a two-lobed nucleus and granules that stain a deep red. Eosinophils phagocytize antigen-antibody complexes outside the bloodstream. They also attack parasitic worms. Basophils Basophils contain granules that appear blue or purple when stained. The granules contain histamine and heparin. Basophils function in body-wide allergic responses by releasing histamine. Nervous Tissue Nervous tissue, which occurs throughout the body, receives and transmits stimuli. It converts a stimulus, whether chemical or physical in nature, into an electrical impulse that is conducted by neurons. Neurons, also called nerve cells, are the functional unit of nervous tissues. Neuron Axon Cell Body Dendrites A neuron consists of a cell body, dendrites, and axons. Dendrites carry impulses to the cell body, whereas axons transmit impulses toward another cell body or an effector, a muscle or organ that responds to the impulse. Axons Axons are usually much longer than dendrites, sometimes reaching a meter in length. Many axons are enclosed in an insulating, lipid layer called a myelin sheath. This sheath is produced by Schwann cells, a type of supporting cell for the nervous system. The sheath not only protects the axon but also allows impulses to move more quickly along the axon. Axons arranged in ropelike bundles wrapped in connective tissue make up nerves. Nerves carry sensory impulses to the brain and transmit motor responses from the brain to effector organs. Nerves that make up the peripheral nervous system bring impulses from the entire body to the spinal cord, which then transmits them to the brain. The spinal cord and brain are the central nervous system. Axons The junction of two Schwann cells is called a Node of Ranvier (arrows). Electrical impulses must jump across the space between the cells, speeding up the rate of conduction. Brain The brain and the spinal cord make up the central nervous system. In the brain, incoming sensory information is received and interpreted. A response leaves the brain and travels down the spinal cord to peripheral nerves that take motor impulse to the designated effector organs. Cerebrum The cerebrum, the largest region of the brain, is composed of a right and left hemisphere. The cerebrum processes conscious thought, complex movement, sensations, intellect, and memory. A series of elevated ridges called gyri vastly increases the surface area of the cerebral cortex. This convoluted surface allows for the huge number of cortical neurons required for the complex functions of the cerebrum. Cerebrum Pyramidal cells, like the one shown above, are found at the surface of the precentral gyrus, in the frontal lobe of the cerebrum. This is part of the primary motor cortex, whose neurons control somatic motor neurons of the brainstem and spinal cord to direct voluntary movements. Cerebellum The cerebellum functions in the coordination of skeletal muscles. The cerebellum receives sensory information from proprioreceptors about the position of the joints and the length of muscles. It also receives information about motor responses sent by the cerebrum. The cerebellum uses this information to coordinate muscle movement and to control balance and posture. Cerebellum The surface of the cerebellum is highly fissured. The outer layer is the molecular layer, which contains some glial cells but consists mostly of dendrites and unmyelinated axons. The darkly stained area is the granular layer and is composed mostly of nerve cell bodies of the tiny granule cells (multipolar neurons that carry impulses to Purkinje cells) along with some larger Golgi cells. Between the molecular and granular layers lies the Purkinje layer. Spinal Cord WM GM CC The spinal cord is a large tract of nerve fibers and cell bodies. In the center of the cord is an H-shaped region known as gray matter (GM), which contains cell bodies. The anterior horns of the gray matter contain large motor neurons. White matter (WM) surrounds the gray matter and is composed of myelinated and nonmyelinated axons. A central canal (CC) runs the length of the spinal cord and contains cerebrospinal fluid. Muscle Tissue Muscle is a contractile tissue. There are three types of muscle: skeletal, cardiac and smooth. Skeletal Muscle Skeletal muscle is distinguishable by its striations and its long, unbranched, multinucleate cells or fibers. The striations are due to the arrangement of contractile units called sarcomeres, along the length of the fiber. Skeletal muscle is attached to bone and is responsible for voluntary movement. When signaled to contract, all the fibers in a particular bundle contract. The number and size of the bundles involved determines the strength of the contraction. Skeletal Muscle Nucleus Striations are visible. Each fiber runs the entire length of the muscle. Nuclei can be found around the cell periphery. Cardiac Muscle Cardiac muscle makes up the wall of the heart and is also striated. The cells are branched and usually contain only one nucleus. Cardiac muscle fibers are composed of a number of cardiac cells. Adjacent muscle cells are held together by intercalated disk. At each disk, the cell membranes of cardiac muscle cells are intertwined and held together by gap junctions. Because of these connections, impulses spread quickly through the muscular walls of each chamber. Cardiac Muscle Striations are evident as well as intercalated disks, the light bands between cells in each fiber. Smooth Muscle Smooth muscle is found in walls of internal organs and lacks striations. Depending on location, cells of smooth muscle may be slender and spindle-shaped (gastrointestinal tract), short and thick (walls of small arteries), or folded and twisted (walls of large arteries). Generally, smooth muscle cells are densely packed and are connected where cells’ projections fit into other cells’ depressions. Smooth muscle contracts more slowly than skeletal muscle but can contract for longer periods of time. It is responsible for involuntary movements. Smooth Muscle Arteries Arteries carry blood away from the heart and have walls much thicker than veins. Arteries are composed of three layers. Each layer is called a tunica. The tunica intima (TI) is the innermost layer and consists of endothelial cells that rest on a layer of connective tissue. The tunica media (TM) contains a large amount of smooth muscle and elastic material. The tunica adventitia (TA), the outermost part, consists mainly of connective tissue and contains blood vessels. Artery TA TM TI The three layers can be seen in the aorta, the largest artery in the body. The tunica intima is a relatively thin layer of endothelial tissue, whereas the tunica media is quite large and is made up of smooth muscle. Aorta The aorta is called an elastic artery because of the large amounts of elastic material in its walls. Elastic arteries can expand to hold an increase in the volume of blood, returning to their previous size by squeezing the blood out. This arterial elasticity helps ensure that the blood reaches every capillary. Veins TI TM TA Veins carry blood to the heart. The walls of veins are much thinner than those of arteries, mostly due to the reduction in smooth muscle in the tunica media. Tongue The tongue is a muscular organ containing skeletal muscle covered with a mucous membrane. The mucous membrane consists of stratified squamous epithelium, much of it containing keratin. Keratin is an insoluble protein that is also found in fingernails, hair, and skin. Keratinized cells resist dessication and provide a protective barrier. The tongue has numerous taste buds that allow us to perceive flavors. The taste buds are found in the papillae of the tongue. Fungiform and Filiform Papillae Fungiform Filiform Fungiform and filiform papillae are shown here. Filiform papillae occur over the tongue’s entire surface, while fungiform papillae are more numerous near the tip. Filiform papillae do not contain taste buds. Foliate Papillae with Taste Buds Taste Pore Gustatory Cell Taste Bud Taste buds are made of several different types of cells. One type, the gustatory cell, extends slender microvilli, referred to as taste hairs, through a taste pore, a narrow opening into surrounding fluids. Stomach The stomach is a J-shaped organ that stores food and regulates its entry into the small intestine. In addition, the stomach secretes digestive fluid called gastric juice that is mixed with the food by the churning action of the layers of smooth muscle in the stomach walls. A soupy mix called chyme results. Gastric juice includes hydrochloric acid and an enzyme called pepsin. Pepsin helps begin the breakdown of protein. The acid activates the pepsin enzyme and also kills bacteria that might have been in the food. The stomach defends itself against its own harsh internal environment by a coating of mucus, secreted by its lining of epithelial cells. Stomach The wall of the stomach consists of four layers; mucosa, submucosa, muscularis externa, and serosa. The mucosa is the innermost layer and contains gastric pits. The surface of the empty stomach creates folds called rugae. They are stretched out and disappear when the stomach is full. Stomach The surface of the mucosa is made up of simple columnar mucous cells. Small Intestine The small intestine is the longest portion of the digestive tract. Most of the enzymatic breakdown of macromolecules and most of the absorption of nutrients occurs here. The duodenum is the first segment of the small intestine. This portion receives chyme from the stomach as well as digestive secretions from the liver and pancreas. Most chemical digestion and absorption occurs in the second segment, called the jejunum. The third and final segment of the small intestine is the ileum, which is partially separated from the large intestine by a sphincter. Smooth muscles within the walls of the small intestine rhythmically contract to move chyme along the length of the intestine. This rhythmic contraction is called peristalsis. Small Intestine Villi The small intestine section shown here is from the mammalian jejunum. The villi are relatively large in the jejunum. Simple columnar epithelium lines the lumen. Colon The function of the colon is the processing of waste and absorption of water. The colon is a region with closely packed glands and no villi. The nucleus of these cells lies near the basal membrane, and the distal portion of the cell is filled with secretory vesicles. Liver The liver is the largest internal organ and has many diverse functions. It secretes bile, which functions in the emulsification of fats, into the gall bladder. It also functions as a storage site for vitamins, serves as a blood reservoir, and filters toxins from the body, among other numerous tasks. The liver is composed of lobules, clusters of cells arranged in layers radiating like spokes from central veins. Liver CV Here, in the center of a lobule, the central vein (CV) and the radiating spokes of cells are easily seen. The light areas between the rows of cells are sinusoids, vessels similar to capillaries. Lung The lung is the site where oxygen from outside air enters blood circulation and carbon dioxide, a by-product of cell metabolism, is taken from blood to be transported to the outside. Alveoli, tiny sacs in the lungs, are the functional units of respiration. Where the gas exchange takes place, the thin alveolar walls and the thin capillary walls bring the air and the blood into very close proximity. Lung Tissue Aveolus Sac Aveolus Sac Cap Aveolus Sac The alveoli themselves are spaces. The cells lining the alveolar sacs are squamous, but the sac walls contain abundant elastic fibers. Throughout the lungs are capillary beds, closely associated with the alveolar sacs. A cross section of a capillary (Cap) is seen here, surrounded by alveolar sacs. Spleen The spleen is a highly vascular organ whose primary function is to filter the blood. The spleen removes abnormal blood cells and other particles through phagocytosis by white blood cells found there. The spleen also stores iron, from the red blood cells decommissioned there. With the large number of lymphocytes present, the spleen is the site of immune responses by B and T cells activated by antigens circulating in blood. The spleen contains the largest collection of lymphoid tissue in the body. The mass of the spleen is of two types, the white pulp that forms the splenic nodules, and the red pulp that forms pulp cords and contains red blood cells. Spleen Ovary The ovaries are the female reproductive organs where eggs are produced. Each ovary is enclosed by a capsule of connective tissue and contains many follicles. A follicle consists of one egg cell, or oocyte, surrounded by layers of follicle cells that nourish and protect the egg cell. All of a female’s egg cells are formed before she is born-up to 400,000. Only a fraction of these will be released during the reproductive years. Starting at puberty and ending at menopause, usually one follicle matures and is released during each menstrual cycle. Release of the egg cell from the follicle is called ovulation. Ovary The section shown here is a human ovary section. A number of maturing follicles can be seen. Ovarian Follicle Nucleus Oocyte An oocyte can be seen in a mature follicle, also called a Graafian follicle. Testis The testes are the male reproductive organs in which sperm are produced. The testes are made up of seminiferous tubules, lined by cuboidal epithelium. Two types of cells are present within the tubules. Sertoli cells provide structure to the tubules and nutrition to the germ cells. Germ cells give rise to sperm. Testis An outer capsule protects the seminiferous tubules. Tubules are lined with a highly modified cuboidal epithelium. Kidney The kidneys, ureters, urinary bladder, and urethra comprise the urinary system. The kidneys have two main functions, excretory and endocrine. The excretory function is to remove from the blood the unused products of cellular metabolism and to help regulate the content, concentration, and volume of body fluid. The endocrine function is the production of two important enzymes: renin, which plays a key role in the regulation of blood pressure and sodium ion concentration, and erythropoietin, which stimulates production of red blood cells in bone marrow. Kidney The functional unit of the kidney is the nephron, a tubule closely associated with capillaries in a structure called the glomerulus. In the glomerulus, the liquid portion of blood first filters into the nephron. Along with this liquid are waste products and dissolved substances such as glucose and sodium. As liquid moves along the nephron, water, sodium and glucose are reabsorbed into the bloodstream, while waste products such as urea remain in the tubule. The tubule empties into a collecting duct that sends the waste ultimately to the ureter and out of the body. Kidney Cortex Medulla Pelvis The renal cortex is the outer region of the kidney and contains glomeruli. The renal medulla lies next to the cortex and contains loops of Henle and collecting ducts. The renal pelvis is continuous with the ureter. Nephron Glomerulus The glomerulus is a meshwork of capillaries surrounded by the Bowman’s capsule, made up of simple squamous epithelium. Outside the glomerulus are the proximal and distal convoluted tubules, both lined by simple cuboidal epithelium.