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Lampbrush Chromosomes” in INTRODUCTION to ANIMAL CYTOGENETICS, by H.C.Macgregor, Chapman & Hall, 1993

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Lampbrush Chromosomes” in INTRODUCTION to ANIMAL CYTOGENETICS, by H.C.Macgregor, Chapman & Hall, 1993 Revised and updated (2011) edition of Chapter 9 “Lampbrush Chromosomes” in INTRODUCTION TO ANIMAL CYTOGENETICS, by H.C.Macgregor, Chapman & Hall, 1993. History and technology Lampbrush chromosomes are something special for the cytologist. On the one hand they are big and they look quite different from any other form of chromosome. On the other hand they have offered a successful medium through which it has been possible to draw valid conclusions at the molecular level on the basis of observations and experiments carried out at the level of the light microscope. People have not only looked at lampbrush chromosomes very carefully indeed, they have also been able to do things to them and actually watch the consequences as they are happening. Of course, the total volume of published research that has been carried out on polytene chromosomes from things like Drosophila and Chironomus far exceeds that on lampbrush chromosomes. Nevertheless, lampbrush chromosomes have proved uniquely valuable in two respects. First, they are transitory structures that exist during an extended diplotene of the first meiotic division. The chromosomes go from a compact telophase form at the end of the last oogonial mitosis, become lampbrushy and then contract again to form perfectly normal first meiotic metaphase bivalents. Secondly, their most conspicuous feature is widespread RNA transcription from hundreds, and in some cases thousands, of transcription units that are arranged at short intervals along the lengths of all the chromosomes. In these senses it has been possible to exploit lampbrush chromosomes in the study of chromosome organisation and gene expression during meiotic prophase and in studies of the molecular and supramolecular morphology of RNA transcription. Lampbrush chromosomes were first seen in sections of salamander oocytes by Flemming in 130 years ago. Ten years later they were described in the oocytes of a dogfish by Ruckert. The name lampbrush comes from Ruckert, who likened the objects to a 19th Century lampbrush, equivalent to the 20th Century test-tube brush. The lampbrush type of chromosome is now known to be characteristic of growing oocytes in the ovaries of most animals, vertebrates and invertebrates, with the exception of mammals and certain insects. The chromosomes are greatly elongated diplotene bivalents, sometimes reaching lengths of a millimeter or more. Lampbrushes are exceedingly delicate structures and no further progress beyond the pioneer studies of Flemming and Ruckert was possible until a technique could be devised for dissecting them out of their nuclei and examining them in a life-like condition, separated from the remainder of the nuclear contents. That's not as difficult as it might sound. The largest lampbrush chromosomes are to be found in growing oocytes of newts and salamanders. These urodeles have big genomes, big chromosomes and big cells, so it is scarcely surprising that they have good lampbrushes. The best oocytes for lampbrush studies are the ones that make up the bulk of the ovary of a healthy adult female at the time of year when the eggs are actively growing in preparation for ovulation in the following Spring. They are about 1 mm in diameter. They have nuclei that are between a third and a half a millimeter in diameter, big enough to see with the naked eye. These nuclei are really not hard to isolate by hand and it is not much more difficult to remove their nuclear envelopes and spill out their chromosomes. Such a technique was introduced by Dr. Joseph Gall in 1954 (Figure 1), working in the University of Minnesota. Figure 1. Joe Gall in 2001 The oocyte is submerged in a suitable saline solution, it is punctured with a needle, the nucleus is gently squeezed out of the hole, picked up in a Pasteur pipette and transferred to fresh saline in a chamber constructed by boring a hole through a microscope slide and then sealing a coverslip across the bottom of the hole with wax. The nuclear envelope is then removed and the nuclear contents, including the lampbrush chromosomes, come to lie flat, and hopefully unbroken and well displayed, on the bottom of the chamber (Figures 2 and 3). Then, by using a phase contrast microscope with an inverted optical system, the chromosomes can be examined in a fresh and unfixed condition with the highest resolution and magnification obtainable with a light microscope. The reason for using an inverted optical system is simple. If the chromosomes are dissected onto a slide in a drop of saline and then covered with a coverslip and looked at with a normal microscope, the very act of placing a coverslip on top of them, a combination of movement, turbulence and surface tension, completely destroys them. So we have to dissect them into a flat bottomed chamber. When they are lying at the bottom of the chamber they remain undisturbed when we place a coverslip over the top of the chamber. If we then try to look at them from the top with an ordinary microscope, we are looking through the entire depth of the chamber, which will be a distance equal to the combined thickness of the microscope slide and the top coverslip. That distance is greater than the working distance of most high power microscope objective lenses. It will therefore be impossible to bring the chromosomes into focus. If, on the other hand we look at them through the bottom of the chamber then we are looking through a distance equal only to the thickness of the coverslip that forms the floor of the chamber, well within the working distance of all objectives Figure 2. An egg oocyte (growing ovarian egg) showing the relative dimensions of the egg, its nucleus and its lampbrush chromosomes, and the system for visualising the chromoisomes using an inverted microscope and a chamber constructed from a slide with a hole bored through it. Figure 3. A composite of 15 phase-contrast micrographs covering the entire area of over 1 square millimeter occupied by a full set of lampbrush bivalents freshly isolated from an oocyte of a plethodontid salamander, Plethodon cinereus. The numerous small bright dots and rings are the many nucleioli that are the products of ribosomal gene amplification in the early stages of oogenesis. Note how each lampbrush bivalent consists of two half-bivalents stitched together at several points along their lengths by chiasmata. Basic organisation The most important factor to keep in mind in relation to the structure of a lampbrush chromosome is that it is a meiotic half-bivalent. This means that it must consist of two chromatids. The entire lampbrush bivalent, of course, will have a total of 4 chromatids. The chromosome appears as a row of granules of deoxyribonucleoprotein (DNP), the chromomeres, connected by an exceedingly thin thread of the same material. Before we even start to think about the structure and functional organisation of a lampbrush chromosome, however, just take a careful look at the following micrograph (Figure 4). Figure 4. This photograph shows a medium sized lampbrush bivalent freshly isolated from an axolotl (Ambystoma mexicanum) oocyte and viewed in its life-like form with phase contrast microscopy. As a comparison, the inset shows AT THE SAME MAGNIFICATION a pair of medium sized mitotic metaphase chromosomes from an axolotl larva. It shows a medium sized lampbrush bivalent freshly isolated from an axolotl (Ambystoma mexicanum) oocyte and viewed in its life-like form with phase contrast microscopy. As a comparison, the inset shows AT THE SAME MAGNIFICATION a pair of medium sized mitotic metaphase chromosomes from an axolotl larva. That the relatively minute volume of chromatin represented by the metaphase chromosome should be so transformed and rearranged as to constitute the DNA axis of a lampbrush chromosome that is nearly a millimetre long and all its lateral loops signifies that something very special must be happening here: first a 100x extension in length, then the formation of discrete chromomeres – tiny packages of condensed chromatin - separated from one another by an almost invisible thread, then the growth of loops and then the population of each loop with all the enzymatic machinery and accessories needed for transcription and the packaging of transcribed RNA. This has to be the most spectacular genomic sartorial transformation in the entire living world. Chromomeres are 1/4 to 2µm in diameter and spaced 1 - 2 µm centre to centre along the chromosome. Each chromomere has 2 or some multiple of 2 loops associated with it. The loops have a thin axis of DNP surrounded by a loose matrix of ribonucleoprotein (RNP). The loops are variable in length, and during the period of oogenesis when they are maximally developed, they extend from 5 to 50 µm laterally from the chromosome axis, which means that the longest loops in such a case would be 100 µm long. The loops are also variable in appearance (Figure 5). Loops of the same appearance always occur at the same locus on the same chromosome, from one animal to another within a species. Accordingly, some loops with particularly distinctive appearances can be used reliably for chromosome identification. Very importantly, "sister" loops, arising from the same chromomere, have the same appearance and usually, but not always, are of the same length. Figure 5. A region of a lampbrush chromosome showing the following features (1) interchromomeric axial fibre (cf) connecting small compact chromomeres (c); (2) chromomeres bearing pairs (L) or multiples of pairs (LL) of lateral loops; (3) loops of widely different morphologies; (4) sister loops of the same or different lengths (Ll); (5) chromomeres without loops; (6) polarization of thickness along loops (P); loops consisting of a single unit of polarization (P); (8) loops consisting of several units of polarization with the same or different directions of polarities (ppp).
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