May is the season of the
Maikäfer in Germany, and moles become most active. Hence, in this month we celebrate Gustav Heinrich Theodor Eimer's arguably greatest discovery. Even
Frau von Welt discussed it, though in fleeting. Theodor Eimer was born in Zürich, Switzerland, on Feb. 22, 1843, and passed away on May 29, 1898, in Tübingen, Southern Germany. His biography remains scant. After an extended search, I gleaned his birthday from the simple plaque on his grave pictured in this
tour of Tübingen's Bergfriedhof. Plaque is a misnomer here. It consists of a rectangular steel plate with black lettering painted on gray rust-proofing. The simplicity does not come close to reflect his discovery.
Theodor Eimer aspired a career in comparative zoology and anatomy. He spent his junior faculty years as prosector at
Julius-Maximillian's University, Würzburg, Germany, and moved on to hold a chair in zoology at
Eberhard-Karl's University, Tübingen, where he led a productive life as an evolutionary biologist, struggling with Darwin's theory until his untimely death at the age of 55. His greatest scientific contribution, however, may have been as an early neuroethologist. Perhaps, it is this contribution that helped Tübingen's university and its three affiliated
Max Planck Institutes develop into a hotbed of neuroscience today.
During his tenure in Würzburg, Theodor Eimer became the first to describe the discrete microscopic organ of touch that densely populates the tip of the nose of the European mole
Talpia europaea. The organ is named in his honor. In his original publication (
Eimer, 1871), he examined in great detail the structure of the nose, the distribution of the touch organs on the nasal skin, and the relationship of their density with the nose's use for palpation. Eimer sought to establish a connection between structure and function.
Eimer's organ fascinated me, ever since I saw one under the microscope in histological preparations Kenneth Catania showed me. Kenny is studying the North American
star-nosed mole Condylura cristata. He counted roughly 25,000 Eimer's organs on these moles' nose appendages called rays. The organ consists of a minute skin papilla between a tenth and a fifth of a millimeter in diameter. At the papilla's core, a beautifully geometric constellation of nerve fibers with free endings is embedded with great symmetry in a conspicuous column of epithelial cells.
Eimer (1871) saw two to three single nerve fibers, rising strait in the middle of the column and ending in the fifth layer under the stratum corneum that forms the hard top of the epidermis. The fibers extend short protrusions perpendicularly into each epithelial layer they traverse, where the protrusions end in 'buttons'. They are ringed by a circle of roughly 19 evenly-spaced nerve fibers, known as satellite fibers, whose protrusions point inwards. In addition,
Eimer (1871) distinguished a separate set of nerve fibers with free nerve endings. By contrast to the fibers in the papilla's core, these travel obliquely toward the surface at the papilla's perimeter. Both sets are illustrated in his Fig. 2 of his 1871 publication, partially reproduced below:
His legend to this figure reads: Longitudinal section through the frontal surface of the mole's snout treated with the Gold-method. T touch cone, N nerve bundle, E epithelium, C corneum, magnification 400/1.
With improved histological techniques, a second touch receptor type,
Merkel cell-neurite complexes, was found in the stratum germinativum at the bottom of the epidermis (darkest part in Eimer's figure), and a third,
lamellated corpuscles of Vater and Pacini, was discovered in the stratum papillare of the dermis (white in Eimer's figure) underneath the Merkel cells (
Halata, 1975).
Today, we still do not understand precisely how these receptors transduce touch into the electrical signals that the nerve fibers transmit to the brain. But we have learned much about the properties of touch, e.g. frequency and force, to which the receptors respond and how their responsiveness changes with prolonged stimulation. The receptors can be functionally distinguished based on these features. The nerve fibers with free nerve endings and the nerve fibers ending on Merkel cells adapt their responses to touch rapidly, whereas the nerve fibers ending in the lamellated corpuscles are considered slowly adapting.
Marasco and others (2006) were able to attribute different functions to Eimer's two sets of free-ending nerve fibers in the star-nosed mole and the
coast mole Scapanus orarius. The authors furnish outstanding micrographs of the organ and its innervation, depicting Eimer's free-ending fibers as well as the Merkel cell-neurite complexes and the Vater-Pacini corpuscles. Using a histochemical marker for a protein known to be involved in the processing of pain, they were able to label the nerve fibers at the perimeter of the papilla, suggesting that they are nociceptive. That is, they respond to pain. By contrast, the fibers in papilla's core did not stain for the protein, suggesting that they are mechanoreceptive. These nerve fibers as well as the Merkel cell-neurite complexes are known to respond to local touches with great sensitivity, whereas the Vater-Pacini corpuscles are highly tuned to the frequencies of dispersed vibrations. Eimer's organ, therefore, forms a receptor complex, integrating pain receptors as well as three fundamentally different types of touch receptors which preferentially respond to either skin indentations or vibrations. The follicles of
whiskers, also known as vibrissae or sinus hairs, and the push rods in monotremes (
Proske and others, 1998) represent the only other known discrete structures in the skin that combine three mechanoreceptor types.
The Eimer's organs on the nose may be the mole's main tool with which the animal can capture a refined picture of its underworld.
Catania and Kaas (1995) have shown that the nose of the star-nosed mole is mapped in multiple topographic representations on an extraordinarily large swath of cerebral cortex that processes touch. Discrete morphological modules of nerve cells that are clearly discernible in histologically stained sections represent each ray in the same order as they surround the nose. This topographic morphological representation of the sensory periphery is similar to that of the facial whiskers by cytoarchitectonic modules called barrels in the rodent cerebral cortex. I touched on whiskers and barrels in my posts dated
Oct. 12, 2009, and
Dec. 24, 2008.
To date, two complete cortical maps of the nose with its rays have been found in the brain of the star-nosed mole. There may be more. The nose's disproportionate representation in cerebral cortex is suggestive of a fovea for nose touch in the mole's somatic sensory system (
Catania, 1995).
Already Professor Eimer recognized the importance of the mole's nose to the behavior of the species. He begins the introduction to his 1871 publication with the statement: "The mole's snout must be the seat of an extraordinarily well developed sense of touch, because it replaces almost entirely the animal's sense of face, constituting its only guide on its paths underground." He estimated that the nose of the European mole was covered with more than 5,000 Eimer's organs which were invested with 105,000 nerve fibers. He took the abundance of sensory innervation to affirm his contention that the nose's touch must represent the moles dominant facial sense. Professor Eimer ends his report with the assertion that his interpretation is entirely consistent with the common knowledge of his time. His last sentence reads: “This enormous richness of innervation easily explains the well-known fact that already a light blow to the snout kills the mole instantly.”
Roughly 130 years after Professor Eimer's discovery, Catania and colleagues accomplished to record striking behavioral evidence in favor of his conclusion, using a
high-speed camera that the eminent professor could hardly have imagined (
Catania and Remple, 2004). Professor Eimer would have been fascinated, watching this footage. Moles with the help of their Eimer's organs may be perfectly poised to detect seismic vibrations.
References
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Addendum
Impressively shown by the two Reuters news clips below, high-speed video cameras have developed into invaluable tools in the study of animal behavior. Their application range from the examination of bat flight
to feline drinking, providing new insights propelling engineering design (11/12/10).
