Division of Labor in Tissues (Histology)
Histology, or the science that deals with tissues, includes Cytology which was considered in the preceding chapter. The cells formed by successive mitoses from the fertilized egg differentiate into various tissues that constitute the body. A tissue is an association of similar cells which have undergone specialization for some particular purpose, and intercellular material, the amount of which varies greatly in different tissues. Thus, bone tissue includes bone cells, that are very much alike, and a considerable amount of hard intercellular material, while epithelial tissue is an association of epithelial cells, that resemble each other, but contain a minimum amount of intercellular substance.
The similar cells constituting a tissue may be connected with each other either by delicate strands of cytoplasm that penetrate the enclosing cell walls, or by an intercellular ground substance of some sort either secreted by the cells themselves in the form of exaggerated cell walls, or formed out of intruded interstitial material arising extraneously, like mortar between bricks.
Combinations of tissues make organs in much the same way that different textiles are combined into garments, and in turn organs make systems, just as different garments together make costumes. For example, the stomach is an organ that is assembled out of muscle tissue, blood tissue, gland tissue, nerve tissue, and the like, which together with other organs like the teeth, intestine, and pancreas, forms the digestive system.
For purposes of general description tissues may be classified as follows:
I. Fluid tissues
II. Stationary tissues
The fluid tissues are blood and lymph. Their cellular components are disconnected and are, therefore, constantly rearranging themselves with reference to each other, unlike the cells of other tissues which maintain a comparatively stable spatial relationship with each other.
In the lower invertebrates, such as the coelenterates and flatworms, the body fluid has no cells. Many invertebrates have only amoeboid white blood cells, but the blood of vertebrates generally is characterized by the presence of both white and red blood cells, and is consequently an elaborated fluid tissue.
Fluid tissues permeate the spaces which separate other tissues, and even the interstices between the cells of these tissues. They also occupy larger spaces, like the cavities of the joints, for example, and particularly circulate through special channels, called blood vessels and lymph spaces, that extend to almost every part of the body. In a later chapter, further consideration of the blood will be given.
Epithelial tissues (Fig. 97) are the most primitive of all tissues. They come into contact with other stationary tissues on one surface only, since they clothe the outer surfaces of the body and line various cavities and passage-ways, including the blood vessels. They produce both cells that receive stimuli from the outside world and those that secrete and excrete different substances, as well as giving rise to the highly important sex cells.
There is usually a minimum amount of intercellular material in epithelial tissues. The cells composing them may assume a squamous, cubical, or columnar form, and may be arranged in a single layer (simple epithelium) or a succession of layers (stratified epithelium).
Glandular epithelial tissues have a great variety of functions in the economy of the organism, namely, digestive in the salivary, gastric, and pancreatic glands; defensive in poison glands of snakes, and stink glands of skunks and other carnivores; protective in the mucous glands of fishes and amphibians, the shell-producing glands of mollusks, and the ink glands of squids; lubncative in the oil glands of the hair, and in mucous glands generally; nutritive in the mammary glands and in the albuminous glands of birds; constructive in the spinning glands of spiders and cocoon-forming insects; cleansing in the lacrimal glands of the eyeball; and temperature-regulating in the sweat glands of the mammalian skin.
When substances produced by gland cells are utilized for the benefit of the organism as a whole, they are defined as secreting glands, but if the substances produced are waste products that are cast out of the body, they are termed excreting glands.
If a gland is supplied with a duct whereby its products may reach the outside or be poured into some internal cavity or passage-way, it is termed an exocrine gland, but if no duct is present and the products of glandular activity must be transferred to the blood in order to be distributed, then it is known as an endocrine gland, and the substance which it produces, as a hormone.
The morphology and behavior of these various glands will receive more attention later. It is sufficient here merely to assign them to their proper place among the epithelial tissues.
Connective and Supporting Tissues
Connective and supporting tissues of vertebrates lie usually inside of the integument that clothes the body. The component cells of these tissues do not form layers, as epithelial tissues tend to do, but are massed together with more irregularity. Their intercellular substances are usually much more in evidence, particularly in cartilage and bone.
Included among connective tissues, whose mission is filling space between organs and parts of organs, are at least five different sorts that may in some instances merge into each other, namely, (1) gelatinous; (2) notochordal; (3) reticular; (4) adipose; and (5) fibrillar.
Gelatinous tissue reaches its most characteristic expression in sponges, and semi-transparent pelagic animals, such as medusae and ctenophores, in which the jelly-like bulk of the body is composed of secreted inter-cellular material throughout which are scattered a few cells, frequently joined together in a very open meshwork by delicate cytoplasmic bridges. This type of tissue does not commonly appear in the bodies of adult vertebrates, although during embryonic development the so-called mesenchyme passes through a gelatinous tissue phase.
In notochordal tissue, on the contrary, there is a great reduction of intercellular material, so that the thin-walled cells lie closely pressed together (Figs. 98 and 211). Whatever rigidity is attained by this tissue is due largely to the fact that the cells are so tightly packed within a tough sheath that a certain turgor results like that when sausage meat is crowded into a casing.
Reticular connective tissues (Fig. 99) form the meshlike supports that characterize many of the softer organs, like the liver, spleen, and lymph nodes, which are ordinarily thought of as being without internal skeletal devices of any kind.
Adipose tissue is somewhat similar to the ordinary reticular tissue that forms the skeletal matrix of soft parts, for in this tissue groups of cells that specialize in fat storage lie enmeshed in a loose reticulum. When the fat cells are melted out of a piece of fat pork by frying, for example, there is left behind a residual network which is the skeletal, or reticular, part of the adipose tissue.
Like other connective tissues, fibrillar tissue (Fig. 100) consists of cells but it is distinguished principally by fibers that interlace among the cells. These fibers, themselves the product of cellular activity, are of two sorts, white non-elastic fibers, and yellow elastic fibers. The yellow fibers are peculiar to vertebrates. They occur in such parts of the body as the walls of the blood vessels, valves of the heart, the lining of the alveolae of the lungs, and in intervertebral ligaments. Both yellow and white fibers may be densely compacted together, as in fascia and sheaths of muscles, in perichondrium and periosteum, around cartilage and bone respectively, or they may be arranged in the form of looser texture, such as is found in the walls of blood vessels and the dermal part of the mammalian skin where they form the substance that is manufactured into leather.
In the sclera of the eyeball and in tendons, between muscles and bones, the fibers are mostly white. Fibrillar tissue plays an indispensable part in holding things together and is probably the most widespread tissue in the vertebrate body.
Cartilage, or “gristle,” is a nerveless, bloodless, relatively flexible tissue that enters into the skeleton of vertebrates. Its texture is not as firm and unyielding as bone, and consequently it is better adapted as scaffolding for water animals, such as fishes, where the surrounding medium helps to support the body, than for such use in land animals whose weight is held up in thin air.
There may be distinguished at least five kinds of cartilage, namely, (1) precartilage; (2) hyaline; (3) fibrous; (4) elastic; and (5) calcified cartilage.
Precartilage is a temporary embryonic type that precedes the formation of other kinds, but may sometimes endure in the adult organism, as in the fin rays of certain fishes. It consists of cells (chondroblasts) that have the power to secrete a thickened cell wall, or an intercellular matrix, at the expense of their own cytoplasm.
When this process has continued until the diminishing cells have isolated themselves from their neighbors in a seat of surrounding matrix, which is somewhat firm and translucent, the hyaline stage has been reached (Fig. 101). Hyaline cartilage is found in the bendable and projecting part of the human nose, at the ends of the ribs joining the breastbone, in the stiff incomplete rings that keep the tracheal and bronchial tubes from collapsing, and in other parts of the bodily structure.
The matrix between the cartilage cells may be interwoven with fibers, either white or yellow, as in fibrillar connective tissue, in which case either fibrous or elastic cartilage is the result. Fibrous cartilage is typically represented by the padlike intervertebral discs separating the centra of the vertebrae in the backbone, while elastic cartilage is found in such places as the epiglottis, and the pinna of the external ear which fortunately springs readily back into its original shape when distorted.
Sometimes the intercellular matrix of hyaline cartilage becomes infiltrated with limy salts, when it is designated as calcified cartilage. In adult cartilaginous fishes much of the cartilage is of the calcified type.
Bone is the best known of the skeletal tissues of vertebrates. As contrasted with cartilage it is supplied with nerves and blood vessels, and is considerably more rigid. It includes at least two kinds of cells, osteoblasts and osteoclasts. The first of these are bone-forming cells which produce the limy intercellular matrix that characterizes bone. The osteoclasts, on the other hand, are bone-wrecking cells, that tear down bone tissue and make possible the rearrangement of material necessary to the accomplishment of growth among such unyielding building materials as bony plates.
Bone consists of two essential substances: (1), an organic base of living cells, and (2), in the excessively developed matrix surrounding these cells, an infiltrated mass of inorganic limy salts. These two components are so intimately joined that there is no visual way of separating them, yet each alone is sufficient to give characteristic contour to a bone, for when the organic part is burnt out by fire, or the inorganic component is dissolved away by acid, the part remaining in each instance preserves the original form of the bone.
In relative weight the inorganic, or mineral, part of bone is about three fourths of the whole, although the ratio of inorganic to organic varies with age, ordinarily becoming greater the longer the bone lives. According to Heintz an analysis of the mineral constituents of a human femur resulted as follows:
The embryonically active osteoblasts are responsible for the formation of bone tissue. By their rapid multiplication living bone cells are formed which in turn secrete the hard parts, or lamellae. As bone grows, however, it becomes necessary not only to add new tissue but also to remove that which has already been formed. Comparing the lower jaw of an infant with that of an adult (Fig. 102), it is evident that no single cell of the former structure can persist unchanged throughout the process of growth. The jaw of an infant is not simply added to as it becomes larger, but all the building material composing it must be broken down bit by bit and reassembled, and supplemented many times before the adult bone is fashioned. It is as if a stone building were enlarged not simply by adding to the outside of it as it stands, but by tearing it down and reassembling it with additional stones in order to enclose a larger area.
This wrecking of bone tissue already formed in order to make way for rearrangement and enlargement is accomplished by the rather large definitely identified cells called osteoclasts. The destructive work of these cells is not always followed by equally constructive reorganization, however, for when the work of the osteoclasts exceeds that of the osteoblasts, a bone decreases in size. Thus, in toothless old age (Fig. 103), the lower jaw not only becomes smaller through the loss of teeth, but the bony sockets in which the teeth were set also decrease in size through the removal of tissue by osteoclasts, with the result that the chin and nose tend to hobnob together (Fig. 104).
When a thin slice of bone is taken from a cross section through the shaft of the femur, for instance, and ground down to translucent thinness, if examined under the miscroscope, it is seen to be made up of innumerable small bony plates, or lamellae (Fig. 105). Even to the naked eye there are two kinds of bone, namely, spongy and compact. In spongy bone the lamellae are arranged in an open framework leaving many spaces filled with bone marrow. Compact bone appears solid but actually includes many minute spaces, in which cells lie or through which blood vessels and nerves run. The lamellae are arranged in at least three different ways, concentrically, interstitially, and circumferentially.
Concentric lamellae, somewhat like rings of growth around the pith of a woody stem, envelop small tubelike branching passage-ways, the Haversian canals, which permeate the bone lengthwise, forming the conduits for the passage throughout the bony tissue of capillaries, lymphatics, and nerves. The Haversian canals, surrounded by concentric lamellae, are best seen in the dense tissue of the cylindrical shafts of the long bones in the appendages, where they communicate both with the periosteal coverings of the bone on the outside and, through the entire bony tissue, with the marrow cavity inside. They constitute the subways for organic traffic throughout the bone tissue, making this part of the skeleton a living adjustable structure.
Interstitial lamellae are necessarily irregular since they fill in spaces between neighboring Haversian systems.
Finally, circumferential lamellae are arranged either around the outside margin of the whole cross section of the bone shaft, just beneath the periosteum, or as an internal layer grading over into the spongy tissue that borders the marrow cavity within the bone.
The Haversian canals are not the only spaces between the lamellae that help to make the living bone porous. Between the hard lamellae separating them from each other are tiny spaces, called lacunae, or “little lakes,” in which lie imprisoned the living bone cells (Fig. 106). Lacunal spaces communicate with each other through canaliculi, microscopic passageways through the walls of the limy lamellae.
Most hard parts of the skeleton are laid down first as connective tissue, followed by cartilage which is later slowly replaced by bone. These are replacing or cartilage bones. A few parts, notably the flat bones of the skull, are laid down directly in connective tissue membranes without a cartilage intermediary. To these the name investing or membrane bones is applied. In either case the hard parts are produced by connective tissue cells which acquire the ability to produce cartilage or bone. Although at first all of the bony tissue is of the spongy type, a thick region on each surface is later converted into compact bone, leaving inside a filler of spongy tissue in which the spaces are occupied by bone marrow. The connective tissue membrane which remains as a covering of the bones is called the periosteum.
In the case of a replacing bone like the tibia of the lower leg (shank), a cartilaginous rod is laid down in the connective tissue. Later this cartilage is replaced by bone in somewhat the following manner. Some of the perichondrial cells and others inside of the cartilage near the middle of the rod become bone builders (osteoblasts) and lay down a ring of bone at that point. Adjacent to this ring the cartilaginous matrix begins to dissolve, probably under the influence of certain cells which have been called chondroclasts. As fast as the cartilage is destroyed, spongy bone takes its place (Fig. 107). From the middle of the bar these changes spread toward the ends until the entire shaft (diaphysis) has ossified leaving only the ends (epiphyses) of the tibia as cartilage. This is the condition in man at about the time of birth. Soon after birth a center of ossification appears in each epiphysis and enlarges until the only cartilage left here is a cap on the articular surface and a plate between diaphysis and epiphysis (Fig. 108). The articular caps remain throughout life but the plate is a temporary device to permit elongation of the bone. The cartilage in the middle of the plate continues to grow and expand. At the same time the process of minute replacement goes on at the two surfaces where it is in contact with diaphysis and epiphysis. At an age of 17-25 years the cartilage of the plate ceases regeneration and the invasion of bone completely replaces the plate. The bone has then completed its elongation. During this developmental period there is also an increase in the diameter of the bone, by the addition of material on the surface. At about the time of birth osteoclasts commence to destroy the bony plates in the center of the shaft leaving only the soft marrow which, from the beginning, is in between the plates. A large marrow cavity, running the entire length of the shaft, thus develops. This cavity slowly enlarges as the bone increases in length and diameter. During this same period the rest of the spongy bone of the diaphysis, but not of the epiphysis, is reorganized into compact bone laid down in concentric rings around the numerous small blood vessels which run through the bony tissue, thus forming Haversian systems.
In this manner the adult condition of the bone is reached. In the long bones the diaphysis consists of compact bone surrounding a large elongate marrow cavity. The epiphyses remain as spongy bone except for a very thin, outer layer of compact bone, which may not completely cover these parts. The spaces in this spongy bone are continuous with the marrow cavity and, as previously mentioned, filled with marrow. The connective tissue covering, which was called perichondrium in the cartilage stage, becomes known as periosteum where bone is present beneath it.
One of the commonest manifestations of life is movement. Even in stationary plants living cytoplasm streams about within the cells, and fluids are passed from one part to another. Within the animal body certain members of the cell community, like blood cells, shift about with much freedom, while other kinds of cells, leucocytes for example, are liable to change their shapes. The well-nigh universal ability of living cells to move or change shape, culminates in muscle cells, whose conspicuous contractility not only causes internal movement but exercises an influence for motion also in more or less distant parts of the body to which they are directly or indirectly attached.
The cytoplasm of the elongated muscle cells is differentiated into sarcoplasm and myofibrils, as well as sheaths which clothe the sarcoplasm like a thin rubber glove. Myofibrils are embedded in the sarcoplasm and are the particular mechanism of contractility. They are peculiar in that they effect contraction in only one direction instead of in any direction, as in the case with ordinary contractile cytoplasm. Muscular movement is always brought about by the pull of muscles, while restorative movements are in turn effected by the pull of antagonistic muscles, and not by the relaxation that follows contraction. Muscle tissue is, therefore, simply specialized tissue in which the general function of contractility is carried out more effectually than elsewhere.
Dr. A. E. Shipley has vividly emphasized the power that may be stored in muscle tissue by citing the performance of a jumping flea. Some patient person who succeeded in weighing nine fleas found their average weight to be 0.38 milligrams. Fleas can leap from 8 to 13 inches. If a man who weighs 70 kilograms, or about 150 pounds, made a corresponding leap, he “could leap to the moon in about ten jumps.”
There are three kinds of muscle tissue that differ in the degree or manner of differentiation, namely, smooth, striated, and cardiac.
Smooth muscle cells have a single nucleus near the center of the cell, are usually spindle-shaped, and rarely forked at the ends. In man they vary in size from 15 micra (15/1000 of a millimeter) in blood vessels, to 200 micra in the digestive tube, while in the walls of the uterus during pregnancy they may reach 600 micra in length. In the pliant walls of the bladder they are more or less interlaced or felted together, lying in every direction, so that the bladder when it is emptied contracts like a toy balloon rather than collapsing like an empty hot-water bag.
Smooth muscle cells are often isolated or in thin layers, or more rarely massed together into bulky tissues. They are widely distributed throughout the body, and are found for example in the skin where they act as hair-raisers, feather-fluffers, or around the openings of glands where they act as doorkeepers. They also form a large part of the contractile walls of various tubes and passage-ways, such as blood and lymph vessels, the digestive tube (except the upper part of the esophagus), the trachea and bronchi, the reproductive ducts, and the ureters.
Striated muscle tissue is “flesh,” and in man it constitutes approximately fifty per cent of the weight of the entire body. It is found not only in the bulky body wall, and the muscles of the limbs where it effects locomotion, but also, at least in the higher vertebrates, in the diaphragm, tongue, esophagus, pharynx, larynx, and the muscles of the eyeball.
The component fibers of straited muscle tissue, which are quite evident in cooked corned beef, are commonly large, elongated cells that are no longer able to be served, like smooth muscle cells, by a single nucleus. In consequence scattered along the fiber many nuclei are present, like substations.
The descriptive term “striated” refers to the fact that the embedded elastic myofibrils, which extend throughout the length of the fibers, are differentiated into alternate beadlike bands lying side by side across the bundles of fibers in such a fashion as to produce a striated effect. These beadlike parts of myofibrils are physically and chemically unlike the connecting parts between the “beads,” because they stain differentially with aniline dyes and refract light differently, the dark beads, or anisotropic bands, being doubly refractive in polarized light, while the isotropic bands, or parts between the beads, are singly refractive in polarized light.
Moreover there is a physiological difference, as well as physical and chemical, in these parts of the contractile fibrils within a muscle fiber, since anisotropic bands shorten more than isotropic bands during contraction.
In birds the “white meat” of the breast is characterized by an excess of myofibrils, while the “dark meat” has more sarcoplasm and less myofibrillar substance in its fibers.
In general the striated muscles effect quick movements of comparatively short duration and are voluntary, that is, under the control of the will, while smooth muscle tissue is involuntary and much slower in action. There are certain notable exceptions to this generalization among invertebrates, for the body muscles of some mollusks are smooth and voluntary, while the visceral muscles of insects and crustaceans are typically striated and involuntary.
The tissue of the muscular vertebrate heart is intermediate in character between smooth and striated muscle, in that the component cells are comparatively short, branching, and involuntary in action, although striated in appearance and multinuclear (Fig. 109).
The enormous dvnamic force exercised by any kind of muscle tissue is seldom realized. The tireless heart of man, for example, knows no rest, as one ordinarily thinks of rest, but throbs faith-fully day and night without skipping a beat throughout a long lifetime.
Nerve tissue is characteristic of animals rather than plants, although the nervous function of sensitivity is a fundamental property of cytoplasm, by no means absent from plant life. This tissue consists of specialized nerve cells, or neurons, accompanied by nutritive components of various sorts, connective tissue, and non-nervous supporting neuroglia cells of ectodermal origin. The cytoplasm of neurons is differentiated by the presence of neurofibrils, that differ from the rest of the cell in chemical composition, as shown by staining methods, and which are particularly fitted for reception of stimuli and the transmission of impulses, just as myofibrils are specialized instruments of contractility in muscle cells.
Neurons exhibit extreme modification from the characteristic spherical embryonic form, the cytoplasm being drawn out into extremely elongated processes or fibers of two kinds, called respectively dendrites and neurites. Dendrites are numerous and branch freely like a tree, as their name indicates, while there is only a single neurite to each cell.
When impulses travel through a neuron along the neurofibrils, they do not go at random in any direction but always enter through dendrites and pass out through the neurite. Impulses are relayed from cell to cell by chainlike contact (synapse) between the neurite of one neuron and the dendrite of the next.
“Nerves” are bundles of neurites and dendrites that extend like cables outside of the central nervous system. They are enclosed in a common sheath of connective tissue. Ganglia, also outside of the central nervous system, are aggregates of cell bodies of neurons.
Nervous tissue accomplishes a double mission: first, that of relating the organism to its environment through sense organs; and second, that of regulating and correlating the bodily activities by means of the central and autonomic nervous apparatus.