Histology and Embryology 1941 - Histology 1
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Nonidez JF. Histology and Embryology. (1941) Oxford University Press, London.
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|Histology and Embryology 1941: Histology - 1 The Cell | 2 The Tissues | 3 The Organs Embryology - 1 General Development | 2 Organogenesis | Bibliography|
Part One - The Cell
Cells can be studied in the living condition or after they have been killed with reagents and stained. Because of the difficulties encountered in the study of most vertebrate living cells the second procedure is the one generally followed in mammalian histology.
Most animal cells consist of a mass of cytoplasm enclosing a nucleus. Binucleate cells and cells with several nuclei may occur normally in mammals.
A. Cytoplasm. In fixed cells the cytoplasm appears as a slightly granular substance. This, however, is the result of coagulation. In the living condition it is supposed to be an aqueous colloidal solution of proteins, fats, carbohydrates and inorganic salts. It is bounded externally by a very thin membrane. Besides the nucleus, cytocentrum and organoids the cytoplasm may have inclusions which vary within the same cell at various times and differ in diverse cells.
1. Proteins. Usually appear in granular form.
2. Carbohydrates. Occur chiefly as granules of glycogen, regarded by some as due to precipitation.
3. Fats and lipoids. Fat can be seen as droplets of various sizes in living cells; in fixed and stained sections it has usually been dissolved, leaving empty spaces (vacuoles).
a. It can be stained with certain dyes (Sudan III, scarlet red).
4. Pigment. Occurs in many cells. It may be yellowish and rather soluble (lipochrome) or black (melanin). The latter is very resistant to the action of reagents.
a. Some cells contain pigment throughout life and are called chromatophores (or melanophores if they have melanin).
5. Crystals and crystaloids. They are known to occur in some mammalian cells, but their presence is by no means constant, a. Their chemical nature and function are unknown.
B. Nucleus. The nucleus is usually a spherical vesicle enclosed within a membrane and containing diverse substances which differ in appearance in stained cells.
1. Nuclear sap, a fluid ground substance which fills the nucleus.
2. Linin threads. They usually form a delicate meshwork which occupies the whole nucleus. Linin does not stain.
3. Chromatin. Appears as granules of various sizes scattered along the linin meshwork or congregated at the point of intersection of the mesh, where they form irregular knots. Chromatin stains deeply with basic dyes.
a. Its distribution within the nucleus is sometimes a valuable diagnostic feature.
4. Nucleolus. This is a spherical mass of acidophil substance floating in the nuclear sap or attached to the linin meshwork. More than one may be present.
C. Cytocentrum (cell center). In practically every cell there is a condensed portion of cytoplasm, termed the cytocentrum. In most mammalian cells it occurs near the nucleus.
1. It contains a small sphere (centrosome) which in turn has two or more granules or rods, the centrioles.
2. It plays an important part during mitotic division.
D. Organoids. They are structures found in all cells, comparable with the organs of the multicellular organism.
1. Golgi network (reticular apparatus). It consists typically of a variable number of anastomosed strands which can be impregnated with osmic acid and also with silver.
a. The network may occupy a restricted area of the cell or be more scattered, and in some instances the strands are independent from each other (dispersed state).
b. It has been regarded by some as an organoid concerned with secretory phenomena of the cell.
2. Mitochondria. Granular or rod-like structures which stain supravitally with Janus green. Their numbers vary considerably; they divide and when they appear as granules they can arrange themselves into filaments.
a. They have also been supposed to be concerned with secretion. 3. Fibrils. Fibrils are observed in certain cells, and they have been regarded by some as some sort of intracellular skeleton. This may be true in the case of the tonofibrils present in epithelial cells; in other cells, however, their function is probably more specific (neurofibrils of the nerve cell; myofibrils of the muscle fibers).
Cells have the properties of all living matter, namely, metabolism, irritability, contractility and reproduction. The study of the first three falls within the domain of general physiology. The phenomena of reproduction, on the other hand, are of direct interest to the histologist because they can be recognized in sections of tissues. Cell reproduction takes place through a process of division.
A. Amitotic (direct) division. This is of rare occurrence and is thought to lead to degeneration of the cells. The nucleus elongates, constricts in the middle and finally separates into halves. Fission of the cytoplasm takes place soon afterwards.
B. Mitotic (indirect) division.
1. Somatic mitosis. Takes place in all cells of the body. Four periods are recognizable: prophase, metaphase, anaphase and telophase.
a. Prophase. The most characteristic feature is the transformation of the chromatin-linin reticulum into more or less elongated bodies (chromosomes) the number of which is characteristic for a given species (48 in man).
(1) Simultaneously with the nuclear changes the centrioles â€” usually surrounded by fine radiating filaments (astral filaments) â€” move away from each other and place themselves in opposite poles of the nucleus.
(2) Toward the end of the prophase the nuclear membrane is dissolved, releasing the chromosomes which become attached to some of the astral filaments; the threads attached to the chromosomes are now called spindle fibers.
b. Metaphase. The chromosomes arrange themselves into a plate at right angles to the axis determined by the centrioles (equatorial plate).
(1) Each chromosome splits lengthwise (if not already split during the prophase).
(2) The spindle is now fully constituted.
c. Anaphase. The astral fibers attached to the chromosomes seem to contract, pulling the halves of each chromosome toward the poles of the spindle.
(1) As a result of the longitudinal splitting of the chromosomes each daughter cell will receive the same number (48 in man).
d. Telophase. This is the final stage of the process during which the daughter groups of chromosomes (now in the poles of the spindle) gradually fade from view as they elongate and lose their compact appearance.
(1) A nuclear membrane appears around each group.
(2) Division of the cytoplasm takes place at this moment or earlier, in the anaphase.
2 . Maturation mitosis. This occurs in the course of the formation of the germ cells in the two sexes. It differs from the somatic in that it separates whole (homologous) chromosomes previously paired during synapsis (see pp. 77, 83).
3. Abnormal mitoses. Irregularities in the process of mitosis are by no means rare, particularly during the formation of the male germ cells. They usually lead to death of the cell.
a. Multipolar mitoses, i.e. mitoses in which several spindles are formed, are found in pathological growths such as cancer and tumors.
III. Degeneration and Death
Cells die in various numbers during the life of the individual without affecting his health. Widespread cell death caused by alterations of metabolism, poisons and bacterial toxins, prolonged anaemia due to thrombosis (infarction), inflammation, hemorrhage, etc., falls within the domain of pathology. Death due to senility of the cells or to physiological changes must be taken into account in histology because degenerating or dead cells can be found in sections of almost any organ.
A. Senility. The life span of the cells of the body varies greatly. Some elements as the red blood corpuscles and leucocytes are shortlived and perish daily in large numbers; others seem to live for many months or years. Degenerative changes are manifested in various ways:
1. Nuclear changes. The chromatin contracts into a dense mass (pyknosis), or breaks up into irregular, deeply stained masses (karyorhexis) or is gradually dissolved in the cytoplasm (karyolysis). These changes, however, normally take place during the formation of red blood corpuscles, which is not a degenerative process.
2. Cytoplasmic changes. The cytoplasm of degenerating cells may become homogeneous or glassy, or the whole cell changed into colloid, or the cytoplasm may develop large vacuoles, or else it may shrink and stain more deeply.
a. The changes mentioned above also occur when cells degenerate under the influence of pathological agents; they are also seen in cells that have been phagocytosed.
B. Physiological degeneration. Some cells are doomed as the result of the production of substances which, however, play a protective role for the organism.
1. The production of keratin in the cells of the superficial layers of the skin and of fat in the cells of the sebaceous glands may be mentioned as examples.
C. Physiological atrophy. This term is applied to the decrease in size which occurs normally in the cells of certain organs; it may be also caused by old age.
1. Involution atrophy. This occurs in organs which undergo regression after a period of physiological activity (thymus, corpus luteum of ovary).
2. Senile atrophy. Observed in many organs of subjects of advanced age. The cells become progressively smaller while retaining their normal characteristics.
3. Fat cell atrophy. Seen in lean but otherwise normal individuals. Cells undergoing atrophy closely resemble early stages of fat deposition in the fetus.
D. Desquamation. The cells drop out of alignment and are lost (skin, mucosa of mouth, oesophagus, bladder, etc.).
E. Postmortem changes. They are seen in organs fixed some time after death, especially in material collected at the autopsy room. The changes observed are to be distinguished from those mentioned above.
Cite this page: Hill, M.A. (2020, October 19) Embryology Histology and Embryology 1941 - Histology 1. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Histology_and_Embryology_1941_-_Histology_1
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