Martin Macklin

Transepithelial Potentials in Hydra

There is a maintained electrical potential of 15 to 40 millivolts across the two epithelial layers forming the body wall of Hydra, the inside of the animal being positive. Negativegoing (depolarizing) spikes are recorded spontaneously and sometimes in response to depolarizing current pulses. These spikes usually overshoot the zero potential level. The large size of the spikes and the orientation of the potential difference across the body wall indicate that this electrical activity is epithelial rather than nervous in origin.

THE EFFECT OF URETHAN ON HYDRA

1. The relaxed length of hydra is increased in the presence of urethan. This suggests a direct effect of urethan on the inner circular myotome layer.2. Urethan causes the normal transepithelial resting potential to reverse such that the gut becomes negative relative to the external medium.3. Urethan also causes contraction pulses to become reduced in magnitude.4. It is suggested that urethan has both a direct effect on the hydra myotomes and on the sodium transport mechanism.

Processing the Abdominal Fetal ECG

A system capable of recording fetal electrocardiograms and fetal vectorcardiograms is described. This system has enabled us to record signals of higher quality than any that have been published to date. It is also possible to study some of the diagnostic potentials of the fetal electrocardiogram signal during the course of gestation. Records with this system have been obtained during the period of 20 weeks to 40 weeks gestation using active surface electrodes placed on the mothers abdomen. The system has the following features: 1. low noise recording of the fetal signal; 2. signal averaging to improve signal to noise ratio; 3. removal of the maternal electrocardiogram component; 4. coordinate axis rotation for the display of the fetal vectorcardiogram and/or electrocardiogram in any convenient frame of reference. This system is currently being used to study the characteristics of the human fetal electrocardiogram in vivo.

Water excretion by hydra.

Hydra were cut so that regenerates consisting only of the central gastric region were formed. This region, which has no natural opening to the environment, is capable of osmoregulation and of removing excess fluid from the gut. The fluid is excreted through a break in the body wall created as a result of a strong contraction when the gut is distended with fluid. A normal hydra, therefore, must remove excess fluid by contracting and expelling it through its mouth.

REVERSAL OF CELL LAYERS IN HYDRA: A CRITICAL RE-APPRAISAL

1. Hydra were everted by pulling their basal disc through their mouth and were then observed to determine the mechanism of regeneration. Animals that were not seen to re-turn to their original body layer orientation were fixed at time intervals from one-fourth to 144 hours and examined histologically. No evidence was found to support Roudabushs hypothesis that cell migration across the mesoglea could re-establish the normal body layer orientation. But areas damaged during everting were found which correspond with the regions of cell migration described by Roudabush.2. The regeneration of everted hydra occurs solely as a result of re-turning by two methods: (a) slow re-turning resembling the turning of a sock when it is being removed; and (b) fast re-turning accomplished by a rapid contraction by the everted animal followed by relaxation to the re-turned or normal body layer orientation.

Ionic Requirements of Transepithelial Potentials in Isolated Cell Layers of Hydra

Because hydra is one of the few fresh water coelenterates, it requires a unique system to maintain ionic and osmotic equilibrium. Several studies in the past have begun to elucidate the mechanism of ionic and osmotic regulation in hydra (Josephson and Macklin, 1969; Macklin, 1967; Macklin and Josephson, 1971; Marshall, 1969). These studies have demonstrated that there is a maintained electrical potential across the hydra epithelium with the gut positive relative to the external medium and it was shown by Macklin and Josephson (1971) that this sustained positive potential was related to an active sodium transport mechanism. Further, superimposed on this sustained positive potential, there are negative going spikes termed “contraction pulses” (CPs) which had been shown to relate to contraction of the body column (Josephson, 1967). Various studies of the osmotic properties of hydra cells and tissue (Benos and Prusch, 1972; Koblick and Yu-Tu, 1967; Lilley, 1955; Marshall, 1969; Steinbach, 1963), have demonstrated that hydra tissue is isosmotic to a solution with an osmotic strength of 40 to 60 milliosmol, and that the gut of the animal is approximately isosmotic to the hydra tissue. These experimental results have led to the conclusion that ionic and osmotic regulation of the intact hydra is maintained by an active transport of sodium with passive movement of water and an anion and with isosmotic flow from the tissue into the gut.

Correlates of electrical activity inXenopus laevis embryos

Summary1.The presence of neuroid conduction in the skin ofXenopus laevis embryos gives them a precocious capacity to respond to stimuli. This conduction system constitutes a precursor of the sensory system, in that interactive electrical coupling exists between the myotomes and the epithelium prior to the appearance of sensory nerves.2.Propagated electrical activity in the epithelium ofXenopus laevis embryos was recorded using extracellular electrodes for morphological stages in which myotomes are presumably not innervated (i.e., up to Nieuwkoop and Faber, stage 27). Relative to the external medium, transepithelial negativegoing spikes are superimposed on a positive transepithelial potential. Only epithelial activity was recorded; myotome activity was not recorded directly.3.Specific correlates of transepithelial electrical activity were identified for the characteristic maneuvers for four behavioral stages. These stages are designated nonmotile, premotile, head flexure and coil, and alternating flexure. Earliest epithelial electrical activity in the nonmotile period, is thought to correspond with cytoplasmic contractions in myotomes. During the premotile period, which corresponds with Nieuwkoop and Faber stages 22 to 24 at a rearing temperature of 22° C, movement is minimal and only to one side. Head flexure and coil involve most of the myotomes capable of contractions. This activity usually involves a single contraction. Next the animal spontaneously bends from side to side repetitively during the alternating flexure stage. This corresponds with stages 25 to 27. In all stages different behavioral maneuvers can be identified from the electrical records.4.Behavioral maneuvers were studied from film records to correlate movements with electrical recordings.5.Morphological and behavioral stages were correlated at three rearing temperatures. The appearance of behavioral stages are retarded more than the appearance of morphological stages at reduced temperatures.

Theoretical biology: A statement and defense

There is currently, in the science of biology, a controversy concerning the status of theoretical biology. This controversy takes the form of several different but related attacks on the fruitfulness - or even the possibility - of theoretical biology as a legitimate scientific activity. We take the following to be a proper characterization of theoretical biology: the application of propositions, techniques, and procedures from mathe matics and symbolic logic to biological phenomena at all levels (i.e., sub cellular, cellular, tissue, organ, organismal behavior, organismal group behavior, and evolution). We assume that this characterization of theo retical biology (hereafter referred to as TB) is uncontroversial and would be acceptable to disputants on both sides of the controversy. It should be noted that this conception of TB is not committed to any single view concerning either the reduction of biology to physics and chemistry or the nature of ultimate biological laws and explanations. That is, we wish to remain neutral with respect to questions about what the science of biology will eventually look like. Our account is compatible with a number of alternative views, including both the possibility of reductionism and the possibility of irreducible biological laws and theories. Moreover, at least at present, TB is unstructured, as compared with modern chemistry and physics. The conception of TB under discussion incorporates a number of different sorts of mathematical approaches to biological phenomena. Consequently, there is no specific and unique structure which characterizes the various approaches employed by theoretical biologists. In the account that follows, we shall attempt to defend the enterprise of theoretical biology against several lines of attack. Three representative objections to theoretical biology can be stated as follows:

PhD/MD TRAINING IN BIOMEDICAL ENGINEERING

Publisher Summary This chapter reviews PhD/MD training in biomedical engineering. The ultimate in biomedical engineering training that combines both engineering and the life sciences is one leading to the PhD in engineering and to the MD. Even before the identification of biomedical engineering as a distinct discipline, there were isolated examples of students achieving degrees in engineering or science and the MD. The medical training usually did not follow as a result of long-range planning but rather as a result of a change in vocational objective. The student entering the biomedical engineering PhD/MD program must have a B.S. degree and this usually is in some branch of engineering. The PhD portion of the training is generally completed first, ideally in the first four or five years. Regardless of educational level or specific goal, all biomedical engineering programs are characterized by the integration of training in engineering science and technology with the life sciences. What distinguishes the PhD/MD in biomedical engineering is the broad introduction to the basic life sciences coupled with training in clinical medicine. The PhD part of the training closely parallels that of the PhD bioengineer except that typically the research topic is one reflecting a clinical problem. Students are attracted to the PhD/MD program to further their interest in research that may involve human subjects and generally wish to combine some clinical practice with their conduct of research.

Determination of tissue movement patterns from mitotic counts in hydra

Abstract Mitotic counts in Hydra oligactus have been used to predict the movement of a hypothetical tissue marker along the body column. Analysis was performed using all mitotic figures counted and mitotic figures whose spindle was parallel to the long axis of the animal. This latter group of mitoses is assumed to account for most tissue migration. Tissue movement was determined for different assumed locations of the stationary plane to show the different pattern that would result if the stationary plane is located other than at the usually assumed position below the hypostome. These results suggest that cell cycle time in the hypostomal region of the animal is longer than in other body regions. Also when compared to experimental measurements of tissue marker movement, the results presented imply that counts of mitotic figures in the gastric region and peduncle of hydra may be interpreted as cell divisions/body segment/15 hr.