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The Gross Morphology of the Golgi Apparatus
The earlier workers such as Perroncito (references in Wilson's “Cell”) considered the Golgi apparatus of Invertebrata such as the pulmonate Mollusca, to be short rods, often crescentic in appearance, which he named “dittosomi” (dictyosomes) from the Greek, meaning a stick-like body. More recently, light microscopists came to believe that, in germ cells, the dictyosome is a sphere, one side of which is thickened to give a crescentic appearance. These opinions were based on observation of these very small objects with the inadequate resolution of the glass lens of the optical microscope. The electron microscope micrographs of Golgi bodies have added a new interpretation. In micrographs studied at Argonne National Laboratory by Gatenby and Roth it has been shown that the pulmonate molluscan dictyosome is formed of slightly bent rectangular plates, and is not a spherical body as more recently assumed by light microscopists Thus the older drawing of the dictyosome as a short rod was more nearly exact than that of a sphere. The lamellar and proacrosome parts shown in Text-fig. 12 would with the poorer magnification and resolution of the optical microscope appear spherical In the case of the oligochaete Golgi apparatus shown in Pl. 7, figs. 6–9, the appearance seen with the light microscope is spherical or more usually helmet-shaped. In the Lepidoptera the dictyosomes of the male germ cells have for many years (Gatenby, 1921) been known to be ovoid or spherical. Recently Roth (unpublished) has produced micrographs of such spherules, which show that the wall is multilamellar. These dictyosomes are true spheres just as they appear under phase contrast.
Spindle Fibres
The older light microscopists universally described spindle fibres during mitosis and meiosis. The best preparations of these fibres were obtained by fixatives which were strong protein coagulents. As often happens in such cases, the presence of spindle fibres intra vitam was denied by subsequent workers until the centromere attachment of the chromosome was discovered, when a reversal of view took place. The first electron micrographs of dividing cells have been disappointing in this respect In fact, in mammalian mitosis spindle fibres in dividing cells have only recently been shown by the electron microscopist. Dr. Bernard Nebel, working on mouse cells Dr. Nebel's cells were, it is understood, purposely fixed in a moribund state. The present micrographs of lumbricid spermatocyte divisions clearly show both spindle fibres and centromeres.
Acrosome Formation
In Mammalia, Insecta, and Mollusca, the acrosome, after its appearance, is placed upon the nucleus in the manner shown in Text-figs. 1–3. The applies to Homo, Cavia, Mus, and Oryctolagus, and probably to all other vertebrates. After the second maturation division the spermatid becomes oriented, possibly by the position of the centriole, or possibly by the position of the whole cell with reference to the spermatic tube lumen, the latter possibility is the more likely. The position of the Golgi apparatus or acroblast with its pro-acrosome or acrosome is variable. If it lies away from the now anterior end of the cell, it migrates up to the anterior end, and deposits the centriole usually in the correct place. Slight deviations from this do take place, but, either by movement of the nucleus or of the acrosome, the latter comes to he in the long axis of the cell, as in Text-fig. 3. In Insecta. which have been much studied, there are sometimes several Golgi bodies—e g, in Lepidoptera, and these may secrete and deposit a number of acrosomes which tend to lie in the correct position and later fuse and become exactly oriented in the long axis of the nucleus, probably in some cases by migration over the nuclear membrane.
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Fig. 1.—Part of spermatid of Lumbricus, showing cristae (cr) in the mitochondrion (m), the centriole with the central filament (c) developed. The Golgi apparatus has been cut horizontally (refer to Text-fig. 17, D-E). Fig. 2.—Somewhat oblique longitudinal section of the nearly ripe spermatozoon. At the head (a) is the acrosome carrier, with the acrosome passing out to the head of the spermatozoon. At (nx) the nuclear fin is cut, and at (un) is the nucleus containing a space possibly arising from the nucleolus At (g) the Golgi apparatus at the posterior end of another sperm is cut. The separate parts or lamellae of the Golgi apparatus can be seen at (e). Fig. 3.—Part of a spermatocyte Golgi field (g) showing the supposed pro-acrosomic vesicles (x). The lamellae are less clear, the Golgi bodies being mainly formed of small vesicles as at (g) lower right Ergastoplasm (ep) is scanty. It conforms to the usual type Fig. 4.—A spermatid showing the acrosome carrier belonging to this cell, but not cut quite across its centre Compare with its position in Fig. 2. The acrosome carrier shows the acrosome (a), some granules (r) and the vacuole above Compare with the spent carrier in Pl. —, fig. 13 (ac). The mitochondrial middle-piece (m) and the lamellated Golgi apparatus (g) are well shown
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Fig. 5.—High power micrograph of Golgi apparatus (g) with about eight lamellae, at (o) are vesicles which may represent the dictyosome cap of the grvllid the acrosome (a). The mitochondrial middle-piece (m) is formed around the centriole adjunct material or post-nuclear region (pn). The centrioles are not properly cut across but lie at (C) where the cell wall is thickened. The nature of the vesicle (/) is not known but may be part of the forming acrosome carrier. Figs. 6–8—Parts of the post-nuclear region of the lengthening spermatid showing the hollow head centriole (C) tail flagellum with central fibre and in Fig. 7 spilled over lamellae of the Golgi apparatus reject (g). Fig. 9.—On the left a spermatocyte showing ovoid hollow centriole (c) probably preparatory to division On the right the Golgi apparatus (upper) is cut transverselv below horizontally
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Fig. 10.—A group of four cells in metaphase of second maturation division. At (h) upper, a centiomere. to the left of which is the oblique section of the spindle fibre tube. At (p) lower left the spindle fibre tube (p) is cut longitudinally. At (p) right a part of the astral fibres are cut across. The mitochondria (m) become scattered during meiotic divisions (Compare with Text-figs. 15 and 16.) Fig. 11—Neck of protoplasm (k) by which all sister spermatic cells are attached to the non-nucleated central mass, partly cut on left. This mass contains fat etc. and mitochondria (m). An unspent acrosome carrier on right containing the acrosome (a). Two flagella (f) cut across showing internal fibres nine peripheral and one central.
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Fig. 12.—Lower part of meiotic metaphase probably the first. (Compare with Text-figs. 15 and 16). This micrograph shows well the Golgi apparatus (g), spindle fibre tubes (p). At (ep) ergastoplasm. The crater-like attachment of the centromere (h) appears well in some of the chromosomes. At (px) is a hollow spindle fibre tube. At six o'clock is a part of the Golgi dictyosome. Figs. 13–14—Sections across the upper end of nearly ripe spermatozoa across the region (a) in Pl. — fig. 2 Full and spent acrosome carriers are cut across in Fig. 13 Acrosome (a). In Fig. 14, the nuclear fin at (n), and at (a) higher up across the acrosome.
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Text-figs. 12–18.—Fig. 12—Golgi apparatus (dictyosome) of Nemobius sp. (Gryllidae). Fig. 13—Suggested function of lamella. Fig. 14—Plan of association of fluid vacuoles with lamellae. Fig. 15—Semi-diagrammatic reconstruction of second maturation division in Lumbricus. Fig. 17—Semi-diagrammatic drawing of pile of lamellae in Lumbricus Golgi body (acroblast). Fig. 18—Golgi body of lepidopteran (Roth)
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In other insects, such as the Gryllidae, the acroblast or Golgi body is single, as in Mammalia, and the acrosome is usually deposited very little outside its definitive position. If it is out of position it appears to migrate over the nuclear membrane.
In all these cases the anterior centriole has already become fixed to the nucleus, and, so far as is known, does not move, and always constitutes the one fixed point in the long axis of the nucleus and cell.
In Text-figs. 1–11, from stages 1–3, this almost universal simple type of acrosome formation is given. At stage 2 slight rotation of the nucleus, or short migration of the acrosome to its definitive position, has been assumed to occur, but it is certain that in the majority of cases migration upwards of the Golgi apparatus (acroblast) with its acrosome in stage 1 occurs. It is at this period also that the hitherto scattered mitochondria (M) pass down to the back or post-nuclear region. How these differential movements occur is unknown.
In Text-figs. 4–7 is shown the curious case of Melanoplus differentialis discovered by M. Devine at the Argonne National Laboratory. In Text-fig. 4, the acrosome is deposited usually quite near the presumably fixed centriole (C), and therefore well away from its future position at the anterior end of the nuclear axis. Soon the Golgi apparatus parts from the acrosome, which now becomes detached from the nucleus, and is next found stuck to the cell wall opposite (Text-fig. 3). During its time on the cell wall, the acrosome causes the development of an extra-cellular cap (P), which later becomes part of the ripe spermatozoon acrosome. The subsequent stages which bring about the deposition of the acrosome in its definitive position on the nucleus are not fully understood, and there are several possible explanations. The important point that in all the cases known the anterior centriole (C) is fixed has been mentioned. This is assumed to be true, because once the centriole becomes adherent to the nuclear membrane, a reaction takes place in this region which produces the so-called centriole seat of Gatenby (1931). In other cases a pin or peg, is produced in this region, which would appear to anchor the centriole permanently. Thus some explanation of the ultimate arrangement of acrosome and centriole in the long axis of the nucleus must be advanced. In Text-fig. 6, there is, first, one explanation—namely, that the nucleus plus centriole and mitochondrial body all rotate to meet the acrosome. Re-orientation of the whole spermatid with reference to the testicular lumen presumably follows. A second possible explanation is that given in Text-fig. 7, where the acrosome and its outer cap (P) moves up to the anterior end of the cell: a small rectification of the position of the nucleus and acrosome or both would then bring the acrosome into its definitive position.
It is interesting to note that the gryllid Nemobius sp., and Gryllus (Acheta), have the type of acrosome formation shown in Text-figs. 1–3, whilst the locustid Melanoplus differentialis, also an orthopteran, has the quite different system given in Text-figs. 4–7.
Still another method of acrosome formation has now been found in the oligochaete Lumbricus by Gatenby and Dalton (1958), and is given in Text-figs. 8–11. In the spermatid, the usual Golgi apparatus plus acrosome (G, A) is found. In addition a small hollow sphere appears in this region (AL). This is the acrosome carrier anlage, which swells to form a bowl-like structure (Text-fig. 9) into the mouth of which passes the acrosome. The spermatid nucleus has become elongated (Text-fig. 10), marking a point of development at which in other spermatids, such as in Homo, Mus, Cavia, and Invertebrata in general, the acrosome has already become attached to the nucleus as in Text-fig. 3. It has been assumed by Gatenby and Dalton, that the bead of protoplasm containing the acrosome carrier now moves upwards and finally reaches a point where the acrosome can pass over to its correct position as in Text-fig. 11. Examination of stages 9–11 suggests that another explanation of the manner of arrival of the acrosome at the head of the cell might be put forward. If the anterior end of the cell elongated so as to leave the bead
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(A, AL) in its position this elongation might eventually produce the condition found in Text-fig. 11. It is not possible at present to decide this point, but an attempt to do so by light microscopy is being made.
The Position of the Centrioles
Reference to Text-fig. 11 shows the two centrioles lying behind the mitochondrial middle-piece (M). In all other spermatozoa of the flagellate type the head centriole is depicted by light microscopists as lying between the post-nuclear region and the upper end of the mitochondrial middle-piece, as in Text-fig. 3. It has been concluded by the present author and Dalton, that just before the stage shown in Text-fig. 8 the head centriole retreats behind the middle piece Convincing micrographs of this phenomenon have been given in the previous paper. It is not believed that there are three centrioles in the stage of Text-fig. 8 and in subsequent stages. The area between C1 and C2 in Text-fig. 11 is a post-middle-piece region.