Dominique G. Homberger

Structure and functions of keratin proteins in simple, stratified, keratinized and cornified epithelia

Historically, the term ‘keratin’ stood for all of the proteins extracted from skin modifications, such as horns, claws and hooves. Subsequently, it was realized that this keratin is actually a mixture of keratins, keratin filament‐associated proteins and other proteins, such as enzymes. Keratins were then defined as certain filament‐forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament‐forming proteins that were extracted from the living layers of the epidermis were grouped as ‘prekeratins’ or ‘cytokeratins’. Currently, the term ‘keratin’ covers all intermediate filament‐forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized or non‐keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws and hooves is cornified, that the non‐modified epidermis is a keratinized stratified epithelium, and that all other stratified and non‐stratified epithelia are non‐keratinized epithelia. At this point in time, the concepts of keratins and of keratinized or cornified epithelia need clarification and revision concerning the structure and function of keratin and keratin filaments in various epithelia of different species, as well as of keratin genes and their modifications, in view of recent research, such as the sequencing of keratin proteins and their genes, cell culture, transfection of epithelial cells, immunohistochemistry and immunoblotting. Recently, new functions of keratins and keratin filaments in cell signaling and intracellular vesicle transport have been discovered. It is currently understood that all stratified epithelia are keratinized and that some of these keratinized stratified epithelia cornify by forming a Stratum corneum. The processes of keratinization and cornification in skin modifications are different especially with respect to the keratins that are produced. Future research in keratins will provide a better understanding of the processes of keratinization and cornification of stratified epithelia, including those of skin modifications, of the adaptability of epithelia in general, of skin diseases, and of the changes in structure and function of epithelia in the course of evolution. This review focuses on keratins and keratin filaments in mammalian tissue but keratins in the tissues of some other vertebrates are also considered.

Functional-morphological and biochemical correlations of the keratinized structures in the African Grey Parrot, Psittacus erithacus (Aves)

SummaryKeratinized structures from the African Grey Parrot (feather, down, claw, scale, rhamphotheca, soft lingual epithelium, and lingual nail) were compard by combining biochemical and functional-morphological approaches. At the molecular level, the keratinized structures of Psittacus erithacus are organized essentially like those of other avian species. Correlations were established (or verified) between the mechanical properties of the tissues and the molecular size of the keratin monomers, between the mechanical properties and the x-ray diffraction patterns of the tissues, and between the Polyacrylamide gel electrophoresis (PAGE) patterns of the keratins and certain aspects of growth patterns of the structures. The keratin proteins of the lingual nail, described here for the first time, resemble those of the claw and rhamphotheca. Morphological, biochemical and functional differences between the lingual nail and the rest of the lingual epithelium were established.

The structure of the cornified claw sheath in the domesticated cat ( Felis catus ): implications for the claw-shedding mechanism and the evolution of cornified digital end organs

The morphology of cornified structures is notoriously difficult to analyse because of the extreme range of hardness of their component tissues. Hence, a correlative approach using light microscopy, scanning electron microscopy, three‐dimensional reconstructions based on x‐ray computed tomography data, and graphic modeling was applied to study the morphology of the cornified claw sheath of the domesticated cat as a model for cornified digital end organs. The highly complex architecture of the cornified claw sheath is generated by the living epidermis that is supported by the dermis and distal phalanx. The latter is characterized by an ossified unguicular hood, which overhangs the bony articular base and unguicular process of the distal phalanx and creates an unguicular recess. The dermis covers the complex surface of the bony distal phalanx but also creates special structures, such as a dorsal dermal papilla that points distally and a curved ledge on the medial and lateral sides of the unguicular process. The hard‐cornified external coronary horn and proximal cone horn form the root of the cornified claw sheath within the unguicular recess, which is deeper on the dorsal side than on the medial and lateral sides. As a consequence, their rate of horn production is greater dorsally, which contributes to the overall palmo‐apical curvature of the cornified claw sheath. The external coronary and proximal cone horn is worn down through normal use as it is pushed apically. The hard‐cornified apical cone horn is generated by the living epidermis enveloping the base and free part of the dorsal dermal papilla. It forms nested horn cones that eventually form the core of the hardened tip of the cornified claw. The sides of the cornified claw sheath are formed by the newly described hard‐cornified blade horn, which originates from the living epidermis located on the slanted face of the curved ledge. As the blade horn is moved apically, it entrains and integrates the hard‐cornified parietal horn on its internal side. It is covered by the external coronary and proximal cone horn on its external side. The soft‐cornified terminal horn extends distally from the parietal horn and covers the dermal claw bed at the tip of the uniguicular process, thereby filling the space created by the converging apical cone and blade horn. The soft‐cornified sole horn fills the space between the cutting edges of blade horn on the palmar side of the cornified claw sheath. The superficial soft‐cornified perioplic horn is produced on the internal side of the unguicular pleat, which surrounds the root of the cornified claw sheath. The shedding of apical horn caps is made possible by the appearance of microcracks in the superficial layers of the external coronary and proximal cone horn in the course of deformations of the cornified claw sheath, which is subjected to tensile forces during climbing or prey catching. These microcracks propagate tangentially through the coronary horn and do not injure the underlying living epidermal and dermal tissues. This built‐in shedding mechanism maintains sharp claw tips and ensures the freeing of the claws from the substrate.

The functional morphology of the pectoral fin girdle of the Spiny Dogfish (Squalus acanthias): Implications for the evolutionary history of the pectoral girdle of vertebrates

Fresh functional-morphological observations and theoretical considerations warrant a re-analysis of the traditional assumptions that the head skeleton of sharks consists of the chondrocranium and visceral arches and that the pectoral fin girdle is part of the appendicular skeleton. The scapulocoracoid cartilage of the Spiny Dogfish (Squalus acanthias) plays at least three major roles: (1) As a mechanical separator between the lateral undulations of the trunk and the vertical movements of the visceral arches; (2) as a structural basis for the anchoring of the pectoral fins: and (3) as the place of origin of the cucullaris and hypobranchial muscles which not only open the jaws and expand the branchial basket, but at the same time also affect the contractions of the heart and the timing of blood flow through the gills. Hence, the scapulocoracoid cartilage is an integral part of the head. It most likely evolved in connection with the evolution of the jaw apparatus and not simply as a part of the appendicular skeleton. Furthermore, the head is a mechanically coherent system, in which the central alimentary, circulatory, respiratory and nervous systems of the organism converge. These central feeding, respiratory and circulatory functions are not simply choreographed by the central nervous system, but are concatenated by the mechanical construction of the head itself. Such a conceptualization of the selachian head raises fresh questions concerning the constraints and potentialities of the evolutionary transformations of the various components of the head and pectoral appendages during the transition from aquatic pisciform to terrestrial tetrapod vertebrates.

Assessment of the mass, length, center of mass, and principal moment of inertia of body segments in adult males of the brown anole (Anolis sagrei) and green, or carolina, anole (Anolis carolinensis)

This study provides a morphometric data set of body segments that are biomechanically relevant for locomotion in two ecomorphs of adult male anoles, namely, the trunk‐ground Anolis sagrei and the trunk‐crown Anolis carolinensis. For each species, 10 segments were characterized, and for each segment, length, mass, location of the center of mass, and radius of gyration were measured or calculated, respectively. The radii of gyration were computed from the moments of inertia by using the double swing pendulum method. The trunk‐ground A. sagrei has relatively longer and stockier hindlimbs and forelimbs with smaller body than A. carolinensis. These differences between the two ecomorphs demonstrated a clear relationship between morphology and performance, particularly in the context of predator avoidance behavior, such as running or jumping in A. sagrei and crypsis in A. carolinensis. Our results provide new perspectives on the mechanism of adaptive radiation as the limbs of the two species appear to scale via linear factors and, therefore, may also provide explanations for the mechanism of evolutionary changes of structures within an ecological context. J. Morphol., 2012.

Imaging tissue structures: assessment of absorption and phase-contrast x-ray tomography imaging at 2nd and 3rd generation synchrotrons

Several methods have been proposed for imaging biological tissue structures at the near micron scale and with user-control of contrast mechanisms that differentiate among the tissue structures. On the one hand, treatment with high-Z contrast agents (Ba, Cs, I, etc.) by injection or soaking and absorption edge imaging distinguishes soft tissue from cornified or bony tissue. This experiment is most compatible with high-bandpass monochromators (ΔE/E between 0.01 - 0.03), such as recently installed at the LSU synchrotron (CAMD). On the other hand, phase contrast imaging does not require any pre-treatment except preservation in formalin, but places more demands upon the X-ray source. This experiment is more compatible with beam lines, such as 13 BM-D at APS, which operates with a narrow bandpass monochromator (ΔE/E ≈ 10-4). Here, we compare imaging results of soft, cornified and bony tissues across the 2x2 matrix of absorption edge versus phase contrast, and high versus narrow bandpass monochromators. In addition, we comment on new data acquisition strategies adapted to the fragile character of biological tissues: (a) a 100 % humidity chamber, and (b) a data acquisition strategy, based on the Greek golden ratio, that more quickly leads to image convergence. The latter incurs the minor cost of reprogramming, or relabeling, images with order and angle. Subsequently, tomography data sets can be acquired based on synchrotron performance and sample fragility.

The Avian Lingual and Laryngeal Apparatus Within the Context of the Head and Jaw Apparatus, with Comparisons to the Mammalian Condition: Functional Morphology and Biomechanics of Evaporative Cooling, Feeding, Drinking, and Vocalization

The lingual and laryngeal apparatus are the mobile and active organs within the oral cavity, which serves as a gateway to the respiratory and alimentary systems in terrestrial vertebrates. Both organs play multiple roles in alimentation and vocalization besides respiration, but their structures and functions differ fundamentally in birds and mammals, just as the skull and jaws differ fundamentally in these two vertebrate classes. Furthermore, the movements of the lingual and laryngeal apparatus are interdependent with each other and with the movements of the jaw apparatus in complex and little-understood ways. Therefore, rather than updating the existing numerous reviews of the diversity in lingual morphology of birds, this chapter will concentrate on the functional-morphological interdependences and interactions of the lingual and laryngeal apparatus with each other and with the skull and jaw apparatus. It will: 1. Briefly review the salient features of the mammalian head as a baseline against which to understand the uniqueness of the avian head 2. Describe general morphological features of the lingual and laryngeal apparatus within the context of the skull and jaw apparatus 3. Contrast some fundamental functional-morphological differences that exist among the jaw, lingual and laryngeal apparatus of birds 4. Provide models of the movements of the various parts of the lingual and laryngeal apparatus based on biomechanical analyses 5. Integrate these models with behaviors in thermoregulation, feeding, drinking, and vocalization 6. Briefly demonstrate how detailed morphological and functional analyses can be tested and expanded by using 3D visualization and animation 7. Place the provided data in an evolutionary framework

The Human Shoulder Suspension Apparatus: A Causal Explanation for Bilateral Asymmetry and a Fresh Look at the Evolution of Human Bipedality.

The combination of large mastoid processes and clavicles is unique to humans, but the biomechanical and evolutionary significance of their special configuration is poorly understood. As part of the newly conceptualized shoulder suspension apparatus, the mastoid processes and clavicles are shaped by forces exerted by the musculo‐fascial components of the cleidomastoid and clavotrapezius muscles as they suspend the shoulders from the head. Because both skeletal elements develop during infancy in tandem with the attainment of an upright posture, increased manual dexterity, and the capacity for walking, we hypothesized that the same forces would have shaped them as the shoulder suspension apparatus evolved in ancestral humans in tandem with an upright posture, increased manual dexterity, and bipedality with swinging arms. Because the shoulder suspension apparatus is subjected to asymmetrical forces from handedness, we predicted that its skeletal features would grow asymmetrically. We used this prediction to test our hypothesis in a natural experiment to correlate the size of the skeletal features with the forces exerted on them. We (1) measured biomechanically relevant bony features within the shoulder suspension apparatus in 101 male human specimens (62 of known handedness); and (2) modeled and analyzed the forces within the shoulder suspension apparatus from X‐ray CT data. We identified eight right‐handed characters and demonstrated the causal relationship between these right‐handed characters and the magnitude and direction of forces acting on them. Our data suggest that the presence of the shoulder suspension apparatus in humans was a necessary precondition for human bipedality. Anat Rec, 298:1572–1588, 2015.

3DMRI Modeling of Thin and Spatially Complex Soft Tissue Structures without Shrinkage: Lamprey Myosepta as an Example: 3D MRI Modeling of Connective Tissue

3D imaging techniques enable the nondestructive analysis and modeling of complex structures. Among these, MRI exhibits good soft tissue contrast, but is currently less commonly used for nonclinical research than X‐ray CT, even though the latter requires contrast‐staining that shrinks and distorts soft tissues. When the objective is the creation of a realistic and complete 3D model of soft tissue structures, MRI data are more demanding to acquire and visualize and require extensive post‐processing because they comprise noncubic voxels with dimensions that represent a trade‐off between tissue contrast and image resolution. Therefore, thin soft tissue structures with complex spatial configurations are not always visible in a single MRI dataset, so that standard segmentation techniques are not sufficient for their complete visualization. By using the example of the thin and spatially complex connective tissue myosepta in lampreys, we developed a workflow protocol for the selection of the appropriate parameters for the acquisition of MRI data and for the visualization and 3D modeling of soft tissue structures. This protocol includes a novel recursive segmentation technique for supplementing missing data in one dataset with data from another dataset to produce realistic and complete 3D models. Such 3D models are needed for the modeling of dynamic processes, such as the biomechanics of fish locomotion. However, our methodology is applicable to the visualization of any thin soft tissue structures with complex spatial configurations, such as fasciae, aponeuroses, and small blood vessels and nerves, for clinical research and the further exploration of tensegrity. Anat Rec, 301:1745–1763, 2018.