OBJECTIVETo identify variables that may enhance medical student's preparedness for computer-based administration of the United States Medical Licensing Examination (USMLE).DESIGNA cross-sectional survey of 301 medical students who completed a self-administered questionnaire.MEASUREMENTSThe questionnaire was designed to obtain information about students' computer resources, personal experience with computers, computer expertise, opinions about computers, experience with computer-based testing, perceived preparedness for the computer-based USMLE, and demographic variables. Variables related to students' perceived preparedness for the computer-based USMLE were identified by ordinal logistic regression.RESULTSA significant regression model yielded four significant predictors: perceived preparedness for USMLE content (P: < 0.0001), opinions about computers (P: < 0.0012), gender (P: < 0.0001), and a gender by computer-based testing experience interaction (P: < 0. 0004). Computer resources, personal experience with computers, computer expertise, age, race, and year of medical school were not significant predictors.CONCLUSIONStudents' perceived preparedness for computer-based administration of high-stakes examinations may be facilitated by preparing them for examination content, by enhancing their opinions about computers, and by increasing their computer-based testing experiences.

The possible functional principle behind the existence of endocrine islets has led to the postulate that their intrapancreatic dissemination is a parallel of the distribution of the Leydig cells in the testis (Ferner 1957), and that the islet vessels form an insulo-acinar portal system.

As shown in Table 1.1, it is generally held that the cyclostomes have at least two types of islet cells, while the gnathostomes have at least four. However, additional and obviously independent cell types have been found in some species, particularly among the elasmobranchs. Thus, we must expect in these species further, perhaps totally new, islet hormones. On the other hand, it is difficult, if not impossible, to decide if islet cells, originally present, have disappeared secondarily. This question arises with the new cell types in Petromyzon marinus (Brinn and Epple 1976), and with the large number of islet cells in the elasmobranchs (see below). Currently, perhaps the most critical issue is the F-cell, the identification of which depends strongly on the particular batch of anti-PP antiserum used (see e. g., Sect. 4.2, Reptilia). The importance of careful evaluation of immunocytochemical data, which has been emphasized many times (see e. g., Falkmer and Van Noorden 1983; and below Sect. 4.2), need not be discussed here in detail. Possible pitfalls are illustrated by (1) staining of neuropeptide Y neurons with anti-PP (Stjernquist et al. 1983), (2) unspecific binding of immunoglobulins to somatostatin and glucagon cells through a non-antigen-antibody mechanism, mediated by the Clq fraction of complement (Buffa et al. 1979; see also Grube and Aebert 1981), and (3) interference by intracellular amines with the reaction between antibodies and peptide hormones (Polak and Buchan 1980). An equally critical issue is the possible existence of intermediate exocrine-endocrine cells (Gerlovin 1976; Yaglov 1976; Eusebi et al. 1981).

Before the discussion of the controversial topic of insulo-acinar portal systems (Chapter 6), we examine briefly the structure and organization of the exocrine pancreas.

Many endocrine glands do not simply function by release of hormones into the circulation. Instead, they deploy a host of “functional strategies”, which range from mechanisms at the molecular level to topographic associations (Epple 1982). Probably no other endocrine gland surpasses the islet organ in the number and diversity of such strategies. As a consequence, probably no other endocrine organ presents investigators with more difficulties. The following discussion should make the point.

As far as we are aware, this is the first attempt to cover the com parative physiology of the pancreatic islets in a monograph. The topics discussed would probably have sufficed to fill about half a dozen monographs, a matter that becomes obvious from a look at the Contents. Hence, we have tried to present the ma terial more in the form of a digest, to emphasize evolutionary perspectives, to point out critical issues, and to identify challenging topics for future research. This approach required an arbitrary reduction of the num ber of references, and we therefore join the chorus of recent authors who beg their colleagues for understanding if some of their publications do not appear in the bibliography. Keeping up with the current literature was like fighting one of those monsters that grow a couple of new heads for each one that is cut off. Nevertheless, we hope that we have covered most of the key publications up to the autumn of 1986. We gratefully acknowledge the advice of many colleagues, and in particular the invaluable criticisms of Robert L. Hazelwood and Erika Plisetskaya. Special thanks are due to the series editor, Donald S. Farner, for his patience and guidance, both of which were fresh proof of his legendary diplomatic skills. Finally, we wish to thank Dr. D. Czeschlik and his staff at the Springer Verlag for their patience and support. Philadelphia, PA AUGUST EpPLE Greenville, NC JACK E. BRINN September 1987 v Contents Chapter 1. Introduction .

Despite a recent surge in interest, the neural regulation of the endocrine pancreas is poorly understood. Possibly, islet innervation is important for the pulsatile release of islet hormones. Pulsatile release of hormones, as opposed to tonic delivery, may have advantages such as increased efficiency at target organs (thus “saving” hormone), or prevention of receptor desensitization (for literature see Weigle et al. 1984; Samols et al. 1986). Another function of islet innervation may be an overriding control in the integration of islet responses. The vast majority of new data on islet innervation, which comes from studies in mammals, already reveals considerable inter- and intraspecific differences (Gerich and Lorenzi 1978; Woods and Porte 1978; R. E. Miller 1981; Smith and Madson 1981; Palmer and Porte 1983; Porte and Woods 1983; Rohner-Jeanrenaud et al. 1983; Smith and Davis 1983; Steffens and Strubbe 1983; Luiten et al. 1984; Jeanrenaud 1985). Although comparative studies are in their infancy, the available information shows that it is beforehand impossible to identify phylogenetic or functional patterns of islet innervation (Epple et al. 1980; Buchan 1984). Perhaps the extremes are best illustrated by the cyclostomes which totally lack an islet innervation and the teleosts in which islet cells show particularly intimate contacts with neurons (Epple and Brinn 1975).

Whereas the islet organ of the cyclostomes develops directly from the epithelium of the intestine or bile duct, the ontogenies of both components of the pancreas of gnathostomes are closely linked. Therefore, we consider here the embryonic development of the islet organ, now the “endocrine pancreas”, together with that of the exocrine pancreas. The older literature on this topic has been sumerized by Bargmann (1939); more recent reviews have been written by Falkmer and Patent (1972), Pictet and Rutter (1972), Epple et al. (1980), Rutter (1980), and Epple and Brinn (1986). It is generally held that the gnathostome pancreas develops from dorsal and ventral outgrowths (anlagen) of the small intestine. According to Siwe (1926), the ventral material disappears in the elasmobranchs very early, leaving the dorsal anlage to form all of the adult pancreas. In the “higher” gnathostomes there seems to be a basic pattern, consisting of one dorsal and two ventral anlagen. Whereas in all cases the dorsal anlage probably persists, the ventral anlagen may (a) fuse, (b) totally disappear or (c) develop only unilaterally as, for example, in the human. In mammals, the derivatives of the dorsal and ventral anlagen fuse almost indistinguishably, although the duct system and differences in cell populations of the adult pancreas reflect the respective origins (see Chap. 4.3). The chicken pancreas develops from three anlagen, a dorsal one appearing at 72 h of incubation and two ventral ones at 96 h (Przybylski 1967). The dorsal anlage gives rise to the third and splenic lobes while the dorsal and ventral lobes are derived from the two ventral anlagen (Dieterlen-Lièvre 1970).

A hormonal control of the pancreatic islets has been suspected for many decades, and a number of mechanisms have been implicated. In particular, the association of diabetes mellitus with pituitary- and adrenal-related aberrations (acromegaly, Cushing’s syndrome), the alleviation of diabetic symptoms by removal of pituitary and adrenal glands, and the induction of diabetic symptoms or islet alterations by injections of pituitary and adrenal hormones, raises the question of specific insulotropic hormonal effects (see e. g., Bratusch-Marrain 1983; Ganda and Soeldner 1983; Pek and Spangler 1983; Schade and Eaton 1983; Volk and Wellmann 1985e). However, no specific adenohypophysial or pituitary-dependent insulotropic hormones have been identified with certainty. Rather, the effects of these hormones (with the possible exception of the ovarian and thyroid secretions) seem to have been due to indirect, mainly metabolic, interactions. On the other hand, direct insulotropic effects of adrenomedullary hormones have been identified, and new insulotropic candidates have been ascribed to two additional endocrine systems: (1) hypothalamus; (2) gastrointestinal mucosa. The hypothalamic insulotropic secretions require further studies; the gastrointestinal “incretins” are poorly characterized even in mammals. Since we discussed the hypothalamic insulotropic factors in Chap. 7, we will deal in the following only with the adrenomedullary secretions and the incretins.

Phylogenetically, the islet hormones or closely related substances appeared long before the islet organ. Indeed, there is increasing evidence that all major types of messenger substances (steroids, catecholamines, peptides) are present in prokaryotes (bacteria) and/or unicellular karyotes (for literature see Coupland 1979; Hunt and Dayhoff 1979; Sandor and Mehdi 1979; LeRoith et al. 1983 a; Kolata 1984; LeRoith and Roth 1984). However, it has been questioned if this is a truly primitive condition, or the result of secondary gene transfer from higher organisms (LeRoith et al. 1983 b), a possibility supported by a recent report of fish to bacterium gene transfer (Bannister and Parker 1985). The functions of the messenger substances in unicellular organisms are largely unknown, although they may be involved in both intra- and extracellular actions. Csaba (1980, 1981) and Josefsson and Johansson (1979) have shown that there are specific receptors for vertebrate messenger substances (peptides and catecholamines) in unicellular organisms; there is no doubt that pheromone-type interactions involving these substances exist in at least some species (Bonner 1971; Kochert 1978; Dunny et al. 1979; Kaiser et al. 1979; O’Day and Horgen 1981). Among the islet hormones so far only insulin- and somatostatin-like material have been described for unicellular organisms. “Insulin” was identified in fungi (Aspergillus and Neurospora), the ciliate Tetrahymena pyriformis, and the bacterium Escherichia coli (cf. LeRoith et al. 1983 a).

Pancreatic polypeptide, a 36-amino acid peptide with a molecular weight of 4250, was first isolated from chicken pancreas (Kimmel et al. 1971) and named avian pancreatic polypeptide (aPP). Subsequent preparations of the homologous hormone from bovine and human pancreas were termed bPP and hPP, respectively. With the exception of two teleosts, Dicentrarchus labrax (Thorpe and Duve 1985), and Oncorhynchus kisutch (Kimmel et al, manuscript), PP has not been detected by RIA in islets below the amphibians (Greeley et al. 1984). Immunohistological findings suggest that it occurs in the pancreas of all vertebrates above the cyclostomes (see Chap. 4.2). In their report on the primary structure of rat PP, Kimmel et al. (1984) compiled the sequences of nine PPs, from mammals, birds, and reptiles. Of the 36 amino acids, 10 are invariant, the lowest percentage of invariant residues among the four major islet hormones (Table 12.1). Additional sequence changes might then be expected in amphibian and fish PPs. Indeed, in a recent study Kimmel and coworkers (manuscript) report that the salmon pancreas contains a 36-residue peptide that is more homologous with porcine NPY (83%) and PYY (75%) than any of the previously characterized pancreatic peptides; and they mention a similar peptide also for Lepisosteus spatula (see below). Another homolog of PP, peptide YG, was recently identified in the PP cells of the islets of the anglerfish by Noe et al. (1986). Compared with hPP, the sequence variations of the different PPs range from two amino acids in the dog and pig to 23 in the goose.

Fetal pancreatic islets (21.5 days old) were cultured in RPMI 1640 containing either 2.8 or 11.1 mM glucose for 7 days. After the 7-day culture period, islets cultured in 2.8 mM glucose demonstrated a minimal first phase of insulin secretion in response to acute glucose stimulation, whereas islets cultured in 11.1 mM glucose demonstrated a biphasic insulin secretory pattern. Islets cultured in 11.1 mM glucose initiated insulin secretion at 4.4 +/- 0.1 mM glucose and plateaued at 11.6 mM glucose when exposed to a linear gradient. In addition, culture in 11.1 mM glucose increased DNA content (P less than 0.01) and [3H]thymidine incorporation (P less than 0.05) in fetal islets. However, ultrastructural morphometric analysis indicated that the actual number of beta-cells within islets cultured in either 2.8 or 11.1 mM glucose did not increase. The insulin contents of islets cultured in 2.8 and 11.1 mM glucose were 0.46 +/- 0.06 and 1.14 +/- 0.10 mU/islet, respectively. During subsequent glucose stimulation, islets cultured in 2.8 and 11.1 mM glucose released 3% and 5.6% of their total insulin content, respectively. Ultrastructural morphometric analysis indicated that 11.1 mM glucose stimulated an increase in the volume of individual beta-cells, i.e. hypertrophy. The hypertrophy of beta-cells within islets cultured in 11.1 mM glucose resulted in a concomitant increase in islet volume. Finally, the hypertrophy of beta-cells within islets cultured in 11.1 mM glucose was a result of increased volumes of mitochondria, secretory granules, and, to the greatest extent, endoplasmic reticulum. These findings indicate that glucose is a potent factor in the maturation of cultured fetal rat islets.

Toascertain theusefulness ofMongolian gerbils as an inbred modelforotitis media, 52Mongolian gerbils (Meriones unguiculatus, strain MON/Tum)were compared with26chinchillas (Chinchilla laniger) forsusceptibility toStreptococ- cus pneumoniae type3,Haemophilus influenzae typeb,anda polymicrobic culture including anaerobes (Streptococcus intermedius, Propionibacterium ac- nes,Staphylococcus epidermidis, andCorynebacterium sp.). Organisms were inoculated percutaneously into thesuperior chamber ofthemiddle earbulla. The gerbils andchinchillas shared similar susceptibilities andresponses to the inoculated organisms asdetermined byX-ray, otoscopic, histopathological, and microbiological determinations at5 to7days. Koch's postulate studies proved the roleofS. pneumoniae andH.influenzae inthepathology foundinbothanimal models. Theanimals were alsosusceptible tothepolymicrobic culture, although therelative virulence oftheindividual membersofthismixture was low, suggesting that these species potentiated asapolymicrobic mixture. TheCoryne- bacterium sp.appeared toelicit thegreatest histopathological responseinchronic (8-week) studies ingerbils. Thegerbils were found tobeuseful as an alternative animal modelforthestudy ofotitis mediaofbacterial etiology.

The fetal rat pancreas, explanted at 18 days of gestation and cultured up to ten days, contains numerous acetylcholinesterase-positive neurons. These nerves usually appear in small ganglia although single nerve cells are encountered. The axons of these intrapancreatic nerves appear to terminate only in the islet tissue and not on any exocrine components of the expiant. It is concluded that the fetal rat pancreas contains an islet-specific group of cholinergic neurons.

The pancreas of the fetal rat was collected from the first appearance of the pancreatic bud at 11 days of gestation, and every day thereafter until birth. After birth the neonatal pancreas was collected every day for one week, and at intervals thereafter. Fetal B-cells were stained with Gomori’s aldehyde fuchsin at 16 µ days, and with the immunofluorescent technique for insulin at 14 days. The A-cells were stained as early as 13 days using the fluorescent antibody technique for glucagon. The D-cells first stained at 17 days with pseudoisocyanin. A 4th cell type was found which stained black with silver nitrate, using a method derived from the Grimelius technique for A-cells. This 4th cell type appeared at 15 days in the fetus, reaching its greatest abundance around 19 days, and then declined in numbers after birth until adulthood, when occasionally one or two cells were found.

Four types of acidophilic granular cells, in addition to B-cells, are identified in the islet organ of anadromous specimens of two subspecies of Petromyzon marinus by light and electron microscopy. Three of these acidophils (PI, pII and PIV-cells) occur in both the cranial and hepatic islets while a fourth type (PIII-cell) has only been found in the hepatic islet of some animals. The granules of the PI-cells stain with ponceau de xylidine, give a distinct tryptophan reaction and in ultrastructural examination show large, dense granules. The PII-cells contain unusual crystals and appear to be a non-secretory stage of the PI. The PIII-cells stain deep-red with acid fuchsin. They contain very large, dense granules and some lysosomes. PIV-cells stain selectively with phosphotungstic acid-hematoxylin and ultrastructurally, contain small, more or less dense granules. It appears that PI- and PIV-cells develop directly from B-cells, while the PIII-cells derive from PI-cells. Despite their direct or indirect origin from B-cells, the pI-, PIII- and PIV-cells show characteristic features of functionally independent endocrine cells. Petromyzon marinus may be an ideal model for the understanding of phylogenetic and pathological interrelationships between islet and gastrointestinal hormones. It is clear that the interpretation of the islet organ of the cyclostomes, which has been generally considered a source of insulin only, requires a revaluation.

With the bias of the mammalian situation slowly disappearing, the evolution of the structure and function of the islet organ has increasingly become a field of fruitful research activity. Recent morphological, physiological, and biochemical investigations have strongly modified the picture of the evolution of the islet organ. Studies in Petromyzon marinus suggest that the islet organ of the early vertebrates already had several functionally independent cell types besides β cells; the poorly developed islet organ of most of the living cyclostomes may be a neotenous phenomenon. In the gnathostomes, the identification of avian pancreatic polypeptide and somatostatin in endocrine pancreas cells raises new questions of islet–exocrine pancreas interactions. Findings in pancreatectomized eels indicate an important role of the islet organ in osmoregulation, mainly in the adaptation to sea water. The findings further more show that in pancreatectomized eels, insulin is much less important for the intermediary metabolism than in mammals.

The endocrine pancreas of the bullhead catfish, Ictalurus nebulosus, and the channel catfish, I. punctatas was studied by light and electron microscopy. In addition to the usual A, B and D cells, a fourth endocrine cell type was consistently observed in the electron microscope. All endocrine cell types were innervated. The vesicles of most of the nerve endings were ultrastructurally different from typical adrenergic and cholinergic vesicles, strongly suggesting the possibility of a third autonomic neurotransmitter serving as a regulator of catfish islet secretion.