1. Epithelial Tissue
1.1. Features - high cellularity, special sensory receptors, forms a barrier, avascular, innervated Surfaces - basal (hemidesmosomes, basement membrane), apical (lumen, microvilli, cilia, stereocilia), lateral (adhering junctions, tight junctions, desmosomes, gap junctions) Types - squamous, cuboidal, columnar, simple, stratified, pseudostratified, specialized
1.1.1. Epithelial tissue Epithelial tissue is a highly cellular tissue that overlies body surfaces, lines cavities, and forms glands. In addition, specialized epithelial cells function as receptors for special senses (smell, taste, hearing, and vision). Epithelial cells are numerous, exist in close apposition to each other, and form specialized junctions to create a barrier between connective tissues and free surfaces. Free surfaces of the body include the outer surface of internal organs, lining of body cavities, exterior surface of the body, tubes and ducts. The extracellular matrix of epithelial tissue is minimal and lacks additional structures. Although epithelial tissue is avascular, it is innervated.
1.1.2. Cell surfaces The cells of epithelial tissue have three types of surfaces differentiated by their location and functional specializations: basal, apical, and lateral.
1.1.2.1. Basal surface The basal surface is nearest to the basement membrane. The basement membrane itself creates a thin barrier between connective tissues and the most basal layer of epithelial cells. Specialized junctions called hemidesmosomes secure the epithelial cells on the basement membrane.
1.1.2.1.1. Apical surface The apical surface of an epithelial cell is nearest to the lumen or free space. Apical cell surfaces may display specialized extensions. Microvilli are small processes projecting from the apical surface to increase surface area. They are heavily involved in diffusion in the proximal convoluted tubule of the nephron and in the lumen of the small intestines. Cilia are small processes found in the respiratory tract and female reproductive tract. Their complex structure facilitates movement that brushes small structures through the lumen of either the trachea or Fallopian tubes. Stereocilia are similar to cilia in size and shape, however they are immotile and more frequently found in the epithelium of the male reproductive tract, specifically in the ductus deferens and the epididymis.
2. Connective Tissue
2.1. Structure - cells (structural, immunological, defense, energy reservoirs), extracellular matrix (ground substance, protein fibers) Types of fibers - collagen, reticular, elastic Classification - proper (dense, loose), embryonic (mesenchyme, mucous), specialized (cartilage, adipose, bone, blood)
2.1.1. Connective tissue Connective tissue is the most abundant tissue type in the body. In general, connective tissue consists of cells and an extracellular matrix. The extracellular matrix is made up of a ground substance and protein fibers. So, in a more detailed way, all connective tissue apart from blood and lymph consists of three main components: cells, ground substance and fibers.
2.1.2. Connective tissue cells The cells originate from mesenchyme, a loosely organized embryonic tissue featuring elongated cells in a viscous ground substance. Connective tissue cells do not oppose each other but rather are separated by a large extracellular matrix.
2.1.2.1. Cell Types: Structural – fibroblasts, chondroblasts, osteoblasts, odontoblasts Immunological – plasma cell, leukocytes, eosinophils Defense – neutrophils, mast cells, basophils, macrophages Energy reservoir – adipose cells
2.1.2.1.1. Connective tissue fibers The ground substance of connective tissue contains structural proteins called fibers. There are three types of connective tissue fibers: Collagen fibers are the most abundant fiber type. They have a high tensile strength but are also flexible. Collagen fibers are made up of many subunits, called collagen fibrils, that appear striated under electron microscopy. There are many types of collagen and the collagen types present in a tissue give it unique characteristics. For example, type I collagen provides resistance to stretch in bone tissue, while type IV collagen makes up the suprastructure of the basement membrane. Reticular fibers are thinner than collagen fibers. They are found in extensive networks and provide structural support and framework. Reticular fibers do not stain with regular H&E stain and a silver stain is needed to stain fibers black, making them visible. Elastic fibers are also thinner than collagen. They are strong but can be stretched up to 150% of their original length without breaking. When tension is released they are able to return to their original shape. Elastic fibers are found in skin, blood vessels and lung tissue.
3. Muscle Tissue
3.1. Features - elastic, extensible, contractile Types - skeletal, cardiac (gap junctions, intercalated discs), smooth, striated, nonstriated
3.1.1. Muscle tissue Muscle tissue is both extensible and elastic, in other words, it can be stretched and returned to its original size and shape. The cells of muscle tissue are unique in that they are contractile, or capable of contraction. This contraction is a result of sliding actin and myosin filaments. Muscle tissue is easily distinguishable by its highly organized bundles of cells. Although there are three types of muscle tissue with unique cell morphologies, the fiber bundles of each tissue type are arranged in parallel oriented on the long axis and are distinct from surrounding connective tissue. Muscle is classified according to the appearance of the contractile cells.
3.1.2. Skeletal muscle Skeletal muscle is responsible for the voluntary movement of the body. For example, movement of the limbs, skin of the face, and orbits. Contraction of skeletal muscle tissue is rapid and strong. Cells are large, cylindrical, and elongated. In embryonic development, myoblasts fuse together to form one larger muscle cell, resulting in syncytial, multinucleated cells. Nuclei of skeletal muscle cells are peripheral and ovoid. When viewed under a microscope, the arrangement of actin and myosin gives skeletal muscle a striated appearance.
3.1.2.1. Cardiac muscle Cardiac muscle is found in the heart wall also known as myocardium. Like skeletal muscle, actin and myosin also give cardiac muscle a striated appearance. The movement that cardiac muscle cells provide is involuntary and coordinated by gap junctions. A major defining characteristic of cardiac muscle tissue is the presence of intercalated disks. Cardiac muscle cells are elongated and branched. Intercalated disks are present at the junctions between two cells. Although gap junctions allow this tissue to function as a syncytium, each cell has one, centrally located nucleus.
3.1.2.1.1. Smooth muscle Smooth muscle tissue is associated with arteries and tubular organs such as the intestinal tract. This type of tissue provides weak, slow involuntary movements. Smooth muscle cells are spindle shaped with one central nucleus. The contractile fibers of smooth muscle cells are arranged perpendicular to each other rather than in parallel, therefore smooth muscle tissue does not appear striated.
4. Nervous Tissue
4.1. Neurons - soma, dendrites, axons, ganglia (PNS), nuclei (CNS)
4.1.1. Neurons Cells of the nervous system are highly specialized to transmit electrical impulses around the body. There are two main types of cells found in nervous tissue: neurons and glia. Neurons tend to have a large cell body, or soma, and long projections used in transmitting information. These projections are referred to as axons or dendrites. Axons send impulses away from the soma and dendrites carry incoming information. Neurons are most easily identified by their axons in either longitudinal or cross-sectional slide. Groups of neurons are referred to as ganglia in the peripheral nervous system and as nuclei in the central nervous system.
4.1.2. Glia are the supporting cells of nervous tissue and significantly outnumber neurons. These cells differ by region of the nervous system. Astrocytes support neurons, especially near synapses, and provide a protective barrier surrounding blood vessels. Oligodendrocytes are found in the white matter of the central nervous system. Large projections from these cells wrap around the axon of a neuron insulating it to allow for faster projection of impulses. In the peripheral nervous system, Schwann cells accomplish the same task. Oligodendrocytes and Schwann cells are useful in identifying nervous tissue because the sheathing they provide appears as a thick layer surrounding a tubular axon. Microglia are the macrophages of the nervous system. These cells constantly survey nervous tissue to destroy invaders and clear cell debris. Nervous tissue exhibits a fluid-filled extracellular space through which ions and neuromediators travel to transmit impulses. Because the generation of action potentials requires a specific concentration of ions, the extracellular environment is highly regulated by glia. Capillaries passing through nervous tissue are completely surrounded by glia to form the blood brain barrier.Glia are the supporting cells of nervous tissue and significantly outnumber neurons. These cells differ by region of the nervous system. Astrocytes support neurons, especially near synapses, and provide a protective barrier surrounding blood vessels. Oligodendrocytes are found in the white matter of the central nervous system. Large projections from these cells wrap around the axon of a neuron insulating it to allow for faster projection of impulses.In the peripheral nervous system, Schwann cells accomplish the same task. Oligodendrocytes and Schwann cells are useful in identifying nervous tissue because the sheathing they provide appears as a thick layer surrounding a tubular axon. Microglia are the macrophages of the nervous system. These cells constantly survey nervous tissue to destroy invaders and clear cell debris.Nervous tissue exhibits a fluid-filled extracellular space through which ions and neuromediators travel to transmit impulses. Because the generation of action potentials requires a specific concentration of ions, the extracellular environment is highly regulated by glia. Capillaries passing through nervous tissue are completely surrounded by glia to form the blood brain barrier.