Abstract

The similarities and differences within the epidermis of five species of Cetecea were examined via tight microscopy. Noted in the study were such structural differences as cell stratification, cell size, and infracellular characteristics, as well as the general character of the epidermis as a whole. Connections were drawn between epidermal structure and function as related to the modes of life of the individual organisms involved.

Introduction

The epidermis of the Order Cetacea, as in all mammals, must perform a variety of functions vital to the survival of the organism. These range from protection from the elements and the need for efficient thermoregulation, to the need for the animals to be able to receive and respond to the stimuli from the environment around them. For the cetacean, as an aquatic organism, the epidermis must also have the ability to provide an efficient hydrodynamic advantage for the animal's mode of life.

This research has been concerned primarily with the characteristics of the structural (and therefore functional) components of the epidermis of the blue whate, (Balaenoptera musculus), the gray whale (Eschrichtius gibbosus), the beluga whale (Delphinapterus leucas), the killer whale (Orcinus orca), and the Dall's porpoise (Phocoenoides dalli). Epidermal modifications are discussed and related to each organism's mode of life.

Materials and Methods

Samples of skin from the five species of Cetacea were shipped to John Carroll University in 10% formalin fixative. Location of tissue sample sites can be found in Table 1.

Table 1. Epidermal Skin Sample Measurements

Each sample was cut into approximately 12 mm x 5 mm rectangular sections, and each section was then submitted to washing in tap water for 24-48 hrs. The samples were then placed through a dehydration series to absolute alcohol (35%, 50%, 75%, 95%, and 100% for 45 min each, respectively). The samples were then removed and introduced into a 1:1 solution of xylene and melted paraffin for 15 min, and then to 100% paraffin for 30 min to allow for maximum infiltration of oaraffin into the tissue. After infiltration, the samples were situated in paraffin molds, being oriented for both horizontal and vertical sectioning. Five and ten micron sections for study were cut utilizing an American Optical rotary microtome. The sections were then affixed to glass slides with Meyer's albumen and allowed to dry overnight on a warming table.

Affixed sections were submitted to either of the two staining methods utilized; the Friedlander hematoxylin-eosin method (1), or Mallory's Triple Stain Method (1). The first method was used for general outlining of the epidermis as a whole; the second was used for site specific staining. After staining was completed, sections were again dehydrated following the procedure previously mentioned, and then the sections were washed in two xylene baths for 15 min each. While still wet with xylene, coverslips were mounted over the sections using canada balsam and the prepared slides allowed 48 hrs to dry before study. Photomicrographs of the stained sections for histological study were obtained using an experimental Model 63264 Zeiss Photomicroscope with a built-in 35 mm camera.

Color slides of the sections were obtained with Kodachrome-25 slide film. All measurements found in Table I were made utilizing the Zeiss Photomucroscope in conjunction with an ocular micrometer.

Results

The first part of the analysis of the epidermal samples included the study of the general characteristics of all the samples, as well as the measurement of the different cell strata and the cells contained within them (see Table 1). As a rule, the epidermis of the Cetacea contains only three of the usual five layers found in terrestrial mammals: namely, the active mitotic layer, or stratum basale; the prickle-cell layer, or stratum spinosum; and the flattened outer layer, or stratum externum. All samples studied appeared to fit the general pattern of the epidermis seen in Diagram 1.

The regrettably small portion of dermis which accompanied the skin samples showeda remarkably extensive collagen fiber network running parallel or tangential to the epidermis above. Small pockets of adipose tissue imbedded within portions of this network were easily visible in all the species studied. Both elastic and dense fibers were seen in this network, but the ratio of these in the dermis varied from species to species. The highest amount of elastic fibers were found in the Dall's porpoise and the killer whale. A distinct separation between the epidermis and dermis is not as noticeable in the Cetacen as in terrestrial mammals. With the exception of the Dail's porpoise, no distinct basement membrane was noticed in any of the samples obtained. Another common characteristic within all samples studied was the complete lack of hair follicles, sebaceous glands, and accessory glands as are found in most terrestrial mammals. Instead, only dermal papillae extended up past the border of the epidermis. Within these coursed numerous dense and elastic fibers which branched upward from the collagenous network within the. dermis, as well as an anastomosed system of arteriole and venuole branches from the dermal blood vessels. This vascular network within the papillae was most striking in the tail fluke plug of the killer whale. A coronary-type blood vessel arrangement (2) was seen, with its characteristically large, central arteriole surrounded by a circle of smaller venuoles.

Along with their unique vascularization, the dermal papillae themselves extended upwards to over half the width of the epidermis itself (in the Dall's porpoise, over 80% of the total thickness of the epidermis) bringing the blood vessels in them to within a few tenths of a millimeter from the outer surface of the epidermis see Table 2).

Table 2. Comparison of Dermal Papilla Height to Epidermal Thickness

The epidermis above was anchored to the dermis by a series of vertical rete pegs which interdigitated with the dermal papillae. The number of these pegs per average distance varied slightly from sample to sample, with the highest aggregation being found in the blue and gray whales. The cells of the stratum basale lined the innermost portions of these pegs, lying adjacent to the dermal papillae, and thus being provided the necessary blood flow to sustain their high mitotic activity. It is interesting to note that the increased convolutions caused by the rete pegs also increased the apparent surface area of basal cells, providing, perhaps, more efficient replacement of cells in the outer layers of the epidermis. In all species studied, these cells were the most heavily coated with pigment granules of all the cells of the epidermis. In cross-section, these cells, as well as those of the spinosum and externum, were surrounded by longitudinal and transverse septa (2) bordering the long and short axis of the cells, respectively. These septa were seen in the blue and gray whales to protrude into the underlying dermis, as well as binding adjacent cells to each other.

In addition to this, in all specimens studied, basal cells along various portions of the lower two-thirds of the dermal papillae intruded into the papillae; in the blue and gray whales, over one-third of the cell body could be seen extending into the papillae. In all species involved, pigment-coated, cytoplasmic extensions from the basal cells were visible within the papillae.

The stratum spinosum, the thickest layer of the epidermis, contrasted markedly between species. Within this region, cell shape varied from polygonal-like conformations in the cases of the blue and beluga whales to more ovoid shapes in the cases of the Dall's porpoise, killer whale, and gray whale. In all species examined, the long axis of these cells was directed vertically. The cross-sectional diameter of cells in this region ranged from 0.009 mm to 0.021 mm (see Table 1). Irregularly shaped nuclei were seen in cells of this region, as well as a characteristically clear infranuclear cytoplasm. The general pigmentation of this region within all species remained fairly consistent, with greater concentrations of pigment granules coating the membranes of cells in closer proximity to a papillae, and lesser amounts on cells further from the papillae.

In the killer whale, it was also noticed that bands of pigment, beginning in the papillary region and extending upward toward the surface, followed a light to dark to light pattern, with the heaviest pigmentation occuring in cells lying above the apex of a papilla, and progressively lighter pigmentation as one moved laterally from the papilla.

The thin stratum externum was characterized by a change in cell orientation. The long axis of the cells in this region was oriented in a more horizontal plane than the cells on the spinosum. As one moved up into the higher layers of the externum, a pronounced flattening of the epidermal cells was obvious. Interestingly, the nuclei as well as the cytoplasmic constituents of these cells were still fairly intact; an abnormal condition in terestrial mammals, named parakeratosis, for the incomplete enzymatic digestion of intracellular constituents in the outer layers of the epidermis, similar to psoriasis in man (3). However, this condition appears to be physiologically normal in all the Cetacea studied, and agrees with the findings of Sokolov in other cetarean species (2). In this study, the condition was most pronounced in the Dall's porpoise and killer whale, where healthy, normal-looking nuclei could be found in cells making up the outermost layer of the stratum externum.

Cornification of the cells in the stratum externum was slight, and only in the beluga whale did the Mallory's stain show any sizable amount of keratohyalin. Keratocytes were not apparent in the sections obtained, but probably exist in small quantities in most cetaceans (2).

Discussion

It was obvious from the study of the structure of the epidermis of these cetaceans that this organ is quite modified to provide the necessary adaptations to life in the sea.

Various additions to and deletions from the terrestrial mammal's epidermal scheme were necessary for the aquatic mammal to carry on both the physiological and behavioral activities for survival in the Ocean.

For a mode of life in an environment with such a high coefficient of friction, the ability of the body surface to reduce this friction, and thus cut down on the amount of energy utilized for such activities as predation and migration, was obviated. In all specimens studied, the most apparent example of modifications to this end were the complete absence of hair an the body surfaces studied, and the lack of a true, horny layer in the outer portions of the epidermis. Both of these deletions left the body surface smooth and free from structures which would otherwise add to the problems of reducing drag in the water.

Another problern unique to life in the sea is the need for adequate thermoregulation. In an environment which extracts heat from objects at a much higher rate than air, the cetacean skin must provide efficient insulation to maintain homeothermy. An obvious adaptation to this end is the relatively great thickness of the epidermis as opposed to that of terrestrial mammals. Furthermore, the extremely thick subcutaneous fat layer, and the large pockets of adipose in the upper portions of the dermis of the blue and gray whales studied, exemplify the adequate insulation which the cetacean epidermis provides.

Along the same lines, the need for adequate body heat dissipation is no less important for proper homeothermy. In terrestrial mammal skin, cutaneous sweat glands perform such a function. But in the non-evaporative environment of the ocean, such an adaptation would be fruitless, thus explaining the lack of such structures in the cetaceans.

Instead, the. closer proximity of blood vessels to the body surface perform this function. In more benthic, slower-moving cetaceans such as the blue whale, this study found that the dermal papillae did not extend as far up into the epidermis as they did in the agile, faster swimming killer whale and the Dall's porpoise. The answer to this discrepancy lies in the amount of heat generated by an organism in its particular mode of life, and therefore the amount of heat which must be dissipated. In forms such as the porpoise which actively pursue prey, the amount of heat generated would be much higher than in forms such as the blue whale which is primarily a filter-feeder. These relationships are most explicit in Table 11, where the percentage of dermal papillae height to total epidermal thickness are listed for each of the five species studied.

Another important epidermal structural/functional relationship for the Cetacea lies in the variable elasticity of the skin itself. From the samples studied, a definite relationship between the elasticity of the skin and the behavior each species exhibits in the wild was clear. In the diving forms, such as the gray and beluga whales, the ratio of dense to elastic fibers in the the papillae and the dermis was extremely high. This provides the epidermis with the rigidity necessary to sustain the pressures on the body during a dive. In the cases of the Dail's porpoise and killer whale, which are more pelagic and fast-swimming forms, this ratio tended to the opposite extreme. Again, this modification gives the body surface more elasticity to allow the skin to conform in such a way as to reduce drag and provide a better laminar flow over the entire body.

While there is still much study to be done in this area, such as research into the effectsofvarious aquaria an the healthy functioning of cetacean skin, studies such as this allow the examination of the intimate relationship evolution has created between the structure of the epidermis and the mode of life in which it has placed each of these cetaceans. Further studies in this area can only provide additional clues into the physiology of these organisms, and make veterinary diagnosis and treatment of skin diseases more effective, and at the same time, less deleterious to the proper functioning of the epidermis as a whole.

References

1. Gray, P. The Microtomist's Formulary and Guide. Blakiston Company, Inc., New York, 1954, p. 365.
2. Sokolov, V.E. Mammal Skin. University of California Press, Berkeley, Los Angeles, 1962, pp. 284-289.
3. Ling, J.K. The integument of marine mammals. - In: Functional Anatomy of Marine Mammals, R.J. Harrison, Ed. Academic Press, New York, 1974, pp. 1-44.