Book Lung
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A book lung is a type of respiration organ used for atmospheric gas exchange that is present in many arachnids, such as scorpions and spiders. Each of these organs is located inside an open ventral abdominal, air-filled cavity (atrium) and connects with the surroundings through a small opening for the purpose of respiration.
Book lungs are not related to the lungs of modern land-dwelling vertebrates. Their name describes their structure and purpose. Stacks of alternating air pockets and tissue filled with hemolymph[a] give them an appearance similar to a \"folded\" book.[1]
Their number varies from just one pair in most spiders to four pairs in scorpions. The unfolded \"pages\" (plates) of the book lung are filled with hemolymph. The folds maximize the surface exposed to air, and thereby maximize the amount of gas exchanged with the environment. In most species, no motion of the plates is needed to facilitate this kind of respiration.
Sometimes, book lungs can be absent, and gas exchange is performed by the thin walls inside the cavity instead, with their surface area increased by branching into the body as thin tubes called tracheae. These tracheae may possibly have evolved directly from the book lungs because the tracheae in some spiders have a small number of greatly elongated chambers. Many arachnids, such as mites and harvestmen, have no traces of book lungs and breathe through tracheae or through their body-surfaces only.
Tetrapulmonata have two pairs of book lungs found on the second and third abdominal segments (Schizomida have lost a pair, and most advanced spiders have replaced at least one of the pairs with trachea). Scorpions have four pairs of book lungs, found on abdominal segments number three, four, five and six.[2]
One of the long-running controversies in arachnid evolution is whether the book lung evolved from book gills just once in a common arachnid ancestor,[4] or whether book lungs evolved separately in several groups of arachnids as they came onto land. While the third abdominal segment in Tetrapulmonata have book lungs, the scorpions have a pair of sensory organs called pectines instead.
The oldest book lungs have been recovered from extinct trigonotarbid arachnids preserved in the 410 million-year-old Rhynie chert of Scotland. These Devonian fossil lungs are almost indistinguishable from the lungs of modern arachnids, fully adapted to a terrestrial existence.[5]
It is believed that book lungs evolved from book gills. Although they have a similar book-like structure, book gills are external, while book lungs are internal.[6] Both are considered appendages because book lungs develop from limb buds before the buds flatten into segmented lamellae. [7]
Book gills are still present in the marine arthropod Limulus (horseshoe crabs) which have five pairs of them, the flap in front of them being the genital operculum which lacks gills. Book gills are flap-like appendages that effect gas exchange within water and seem to have their origin as modified legs. On the inside of each appendage, over 100 thin page-like membranes, lamellae, appearing as pages in a book, are where gas exchange takes place. These appendages move rhythmically to drive blood in and out of the lamellae and to circulate water over them. Respiration being their main purpose, they can also be used for swimming in young individuals. If they are kept moist, the horseshoe crab can live on land for many hours.
Book lungs are the main respiratory organ in most arachnids (spiders and scorpions). Book lungs are within small openings in the abdomen of the arachnid. The book lungs themselves consist of a series of haemolymph filled plate-like structures. Between the plates there is an air space and this allows air to circulate around the plates. Gaseous exchange then occurs through the surface of the plates.
Like most spiders, members of the orb-weaving family Uloboridae have a dual respiratory system. Book lungs oxygenate the hemolymph and tracheae carry oxygen directly to tissues. Most members of the family are characterized by an extensive tracheal system that extends into the prosoma, where branches enter the legs. A comparison of both absolute and size-specific indices of these two respiratory components in six uloborid species using the independent contrast method shows that their development is inversely related and indicates that these two systems are complementary. Species that more actively monitor reduced webs have tracheae with greater cross sectional areas and book lungs with smaller areas than do orb-weaving species that less aggressively manipulate their webs. Thus, the acuteness of a spider's oxygen demands appears to influence the development of its respiratory components. As the tracheae assume more responsibility for providing oxygen the book lungs become less well developed and vice versa. J. Morphol. 236:57-64, 1998. 1998 Wiley-Liss, Inc.
In early studies with the light microscope and histological sections, the air sacs (air channels, lamellae, saccules) of developing spider and scorpion book lungs were suggested to be infoldings of the hypodermis from the spiracular invagination (primordial atrium) posterior to opisthosomal limb buds. This process was thought to be similar to the small amount of invagination that may occur along with outgrowth folds for book gill development at the posterior surface of branchial appendages in horseshoe crabs [2-10,18-22]. Slight widening of the air sac entrance at the atrial wall was interpreted as indications of hypodermal infolding. The presumed infoldings were thought to result in the parallel rows of lamellar precursor cells anterior to the atrium. In the spider species they examined, Montgomery [23] and Janeck [24] reported that the initial widenings of the air sac entrance are transitory, and the air sacs are formed from aligned cells in a cluster derived from the hypodermis.
In his diagram of histological sections of scorpion embryos, Brauer [19] showed some small folds in the atrial wall. This was considered as evidence of hypodermal invagination like that proposed for book gills [17,22,26] although the presumptive folds were not actually shown to be related to the formation of book lung lamellae.
As pointed out earlier [1], lamellate respiratory organs are important for our understanding of evolutionary history and taxonomic relationships, but modern procedures are needed for a more detailed comparison of cell activity during book gill and book lung development. The main objective herein is to use transmission electron microscopy (TEM) to examine cell ultrastructure during formation of scorpion book lungs. The results can then be used where relevant and helpful for evolutionary hypotheses and further comparative studies.
The scanning electron microscope (SEM) was used in recent developmental investigations of the respiratory organs in the scorpion [27] and horseshoe crab [1]; the present investigation is a continuation of that effort. The SEM study of book lung development in scorpions [27] provides an overview of the process, but the SEM is limited in the resolution of cell detail. Also, tissue preparation requires dissection and/or fracturing to expose components for viewing. This has potential for cell damage and/or loss, with emphasis on the surface features of the tissue or organ. In the present study, whole book lungs were removed, and sections were cut at successive stages of development in embryos and first and second instars.
Book lung formation in scorpions is a slow and gradual process [27]. It begins in the embryo with the appearance of a spiracle and a sac-like invagination (primordial atrium) just inside the spiracle. Lamellar development continues through birth and the first molt that occurs 1-2 weeks after the newborn first instars (pronymphs) climb up on the mother's back. The book lung gradually becomes a functional respiratory organ with about 50 lamellae in the active and foraging second instar. The book lung cuticle is replaced in subsequent molts [28], and lamellae are increased in size and number so there are more than 150 in the adult. The earlier [27] and present investigations show a complex developmental process of cell proliferation, migration, alignment and secretion of cuticular materials. The result is a stable and highly ordered series of page-like air and hemolymph channels.
Proliferation and migration of precursor book lung cells from the invaginated epithelium (E) of the atrium (At). Light micrographs (LM), ventral views, semi-thin sections. Centruroides gracilis. A. Newborn first instar. The air sacs (AS) in this book lung are at an early stage and barely discernible. Little or no widening is evident at the atrial origin of the air sacs. The cuticular wall (Cu) of the atrium is absent or very thin at the sites where air sacs are forming. The epithelial cells of the atrial wall are in a distinct layer where lamellae are not being formed; a basement membrane (BM) is present at the basal surface of these cells. The cells in the epithelial layer toward the left in the photo are more numerous as though proliferating near the site where air sacs are forming. On either side of the region of developing air sacs, the basement membrane is absent or disrupted and the precursor cells appear to be migrating inward (asterisks). B. Embryo book lung with some air sacs more advanced than those in Figure 2A. Inside the wall of the atrium, epithelial cells (E) of the hypodermis form a layer with a basement membrane (BM) at their basal surface. The cuticular wall (Cu) of the atrium is much thinner at the site where air sacs are forming. The primordial air sacs (AS) separate aligned precursor cells into double rows. Some cells (asterisks), not yet aligned into rows, appear to be dispersing inward from the atrial epithelium, and at these sites the basement membrane is disrupted or absent. Some widening (W) of the air sac entrance is evident at the atrial origin of two air sacs. In the lateral region (right) of the book lung, the developing air sacs are barely evident among cells not yet aligned. Scales, 20 μm. 59ce067264
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