Tissues are composed of cells embedded in nonliving matter called the [1] matrix, or extracellular material . This nonliving matter is further subdivided into ground substance and [2] fibers . All permanent tissues arise from three primary germ layers of the embryo. From the most superficial to the deepest layer, these are the [3-5] ectoderm mesoderm endoderm .

Tissues are typically preserved for study in chemicals called [6] fixatives , sliced into thin histological [7] sections , then mounted on a slide and colored with [8] stain to bring out contrast. A [9] longitudinal section is a slice taken along the longest dimension of an organ, whereas a [10] cross (transverse) section is taken perpendicular to this, across the shortest dimension.

Epithelium is classified as [11] simple if it has only one layer of cells, all resting on the basement membrane. If these cells are thin and flat, it is called a [12] simple squamous epithelium, which is the type best suited to rapid diffusion of solutes. One epithelium in class 11, called [13] pseudostratified , looks multilayered because some of the cells are too short to reach the free surface, but all of them touch the basement membrane. When an epithelium truly consists of multiple layers of cells, and some cells rest on top of others instead of on the basement membrane, the epithelium is said to be [14] stratified . The epidermis and vaginal surface have a type called [15] stratified squamous epithelium, but the epidermis differs from the vaginal lining because it is [16] keratinized to retard water loss and invasion by microorganisms. Multiple layers of rounded cells, seen in the ovaries and testes, form a [17] stratified cuboidal epithelium. Much of the urinary tract is lined by [18] transitional epithelium, which is well adapted to stretching.

The most common cells of fibrous connective tissue are the large, flat [19] fibroblasts , which produce the extracellular material. Large phagocytic cells called [20] macrophages develop from monocytes and also populate the connective tissues. White blood cells called [21] leukocytes are also found in connective tissue. The fibers of connective tissue are of three kinds: [22-24] collagenous elastic reticular . Two of these types are made of varieties of [25] collagen , the most abundant protein in the body. The ground substance of connective tissue often has a gelatinous to stiff texture due to [26] proteoglycan molecules, which are polysaccharide chains called GAGs covalently bound to a core protein.

[27] Areolar tissue is a form of loose fibroconnective tissue found in nearly all tissue sections and having an abundance of clear space filled with ground substance. Lymphatic organs and bone marrow have a spongy framework of [28] reticular tissue, made of branching fibers and cells. Fat cells are the dominant cell type in [29] adipose . These fat cells, called [30] adipocytes , can also be found to a lesser degree in 27 tissue. Tendons and ligaments are made of [31] dense regular connective tissue, so called because it consists mainly of densely packed, parallel collagen fibers. [32] Dense irregular connective tissue also contains densely packed collagen, but with fibers running in random directions. This type of tissue often forms a protective capsule around organs such as the spleen and testes.

[33] Cartilage is a rubbery tissue with chondrocytes occupying cavities called [34] lacunae amid a stiff matrix. If the fibers in this matrix are too fine to be seen in ordinary preparations and the matrix looks clear, the tissue is called [35] hyaline cartilage . Another type called [36] fibrocartilage has an abundance of coarse, visible collagen fibers and is usually found where 35 overlaps with a tendon or ligament. A fibrous sheath called the [37] perichondrium surrounds most cartilage, but is never found in 36. Bone is also called [38] osseous tissue. The type called [39] compact bone is arranged in concentric lamellae (layers) and forms the external surfaces of all bones. The bone cells found in these lamellae are called [40] osteocytes . Bones are surrounded by a fibrous sheath called the [41] periosteum .

[42] blood is a fluid connective tissue. It consists of a liquid ground substance called [43] plasma and membrane-bounded components (cells and cell fragments) collectively called [44] formed element .

Nerve cells, also called [45] neurons , have a large number of supportive cells called [46] neuroglia, or glial cells . Processes that transmit information toward their cell bodies are called [47] dendrites , and a single process that transmits information away from the cell body is called the [48] axon, or nerve fiber .

Skeletal muscle cells are often called [49] muscle fibers because they are so long and thin. They exhibit transverse light and dark bands called [50] striations . [51] Cardiac muscle also has these bands, but differs from skeletal muscle in that it is not under voluntary control. It also exhibits [52] intercalated discs where cells meet end to end. These discs form mechanical and electrical linkages between cells. The third muscle type, [53] smooth muscle, lacks the aforementioned bands. It is found in the walls of many internal organs. [54] Skeletal muscle has multiple nuclei, while the other two types have only one nucleus per cell.

When it is important that matter not be allowed to seep between epithelial cells, the cells may be held together by a type of intercellular junction called a [55] tight junction . If two cells need to be attached to each other to withstand stress but do not need to be sealed all the way around, they are joined by [56] desmosomes , somewhat like rivets or spot welds between two pieces of metal. [57] Gap (communicating) junctions form channels between one cell and another that allow ions or other solutes to pass directly from the cytoplasm of one cell to the next.

[58] exocrine glands usually have [59] ducts that convey their products to the body surface or the lumen of another organ. However, [60] endocrine glands secrete their products directly into the bloodstream; these products are chemical messengers called [61] hormones . Many epithelia have [62] unicellular glands, formed by mucus-secreting goblet cells. Most glands, however, are multicellular. The connective tissue framework of these glands is called the [63] stroma , and the secretory tissue is called the [64] parenchyma . Glands are called [65] simple if they have unbranched ducts and [66] compound if their ducts branch. In many cases, the secretory cells form a dilated sac called a/an [67] acinus (alveolus) at the end of a duct. Mucous glands produce mucus, while [68] serous glands produce thinner, more fluid secretions. [69] merocrine, or eccrine glands release their products by exocytosis, while in [70] holocrine glands, the secretion consists of entire cells that have decomposed.

Body passages that are open to the exterior are lined by [71] mucous membranes, which secrete mucus and protect the body against microbial invasion. They typically consist of an epithelium overlying an areolar tissue layer called the [72] lamina propria . The body cavities and external surfaces of many viscera are lined with layers of areolar connective tissue called [73] serous membranes. Blood vessels are lined with a membrane called the [74] endothelium .

Transformation from unspecialized cells to cells specialized for certain functions is called [75] differentiation . When a tissue transforms from one type of tissue to another, as from simple cuboidal to stratified squamous epithelium, it is said to undergo [76] metaplasia . Tissue growth is called [77] hyperplasia if it results from multiplication of cells and [78] hypertrophy if it results from enlargement of preexisting cells. Shrinkage of a tissue from lack of use is called [79] disuse atrophy . The abnormal death of tissue is called [80] necrosis . [81] Gas gangrene is a type of tissue death caused by certain bacteria. Tissue death due to a lack of blood flow is called [82] infarction and is the usual cause of heart attacks. If a damaged tissue is replaced by the same kind of tissue, it is said to exhibit [83] regeneration , but if it is replaced by scar tissue, it is said to show [84] fibrosis .