Effects of an antiandrogen treatment on the antler cycle of male fallow deer (Dama dama L.)

The consequences of living longer-Effects of an experimentally extended velvet antler phase on the histomorphology of antler bone in fallow deer (Dama dama)

Antlers are periodically regenerated paired cranial appendages of male deer (both sexes in reindeer) that constitute the fastest‐growing bones in the animal kingdom. The annual antler cycle of male deer is linked to testicular activity and largely controlled by seasonal fluctuations of testosterone concentrations in their blood. We studied the effects of an experimental doubling (to eight months) of the velvet antler phase, during which the antlers are covered by skin (velvet), on the histomorphology of antler bone in three adult fallow bucks. Extension of the velvet antler phase in the experimental animals had been caused by administration of the antiandrogen cyproterone acetate (CPA). The distal portions of the antlers from two of the CPA‐treated bucks exhibited partial sequestration of the antler cortex, with the separation plane typically located along the border between cortex and spongiosa. It is hypothesized that this was caused by cortical necrosis due to severe ischemia during later stages of the extended velvet antler phase. In places, new cancellous bone had been deposited on the resorption surface of the spongiosa, indicating a regeneration process. Normal fallow deer antlers (“controls”) from this and a previous study, that is, antlers with a timespan of about four months between onset of new antler growth and velvet shedding, exhibited no or only minor bone remodeling and still contained remnants of calcified cartilage in their distal portions. In contrast, the antlers of the three CPA‐treated bucks showed evidence (secondary osteons and resorption cavities) of marked bone remodeling along their entire length and lacked remnants of calcified cartilage. Our results underscore that the typical histological features of antler bone reflect its short‐lived nature. Antlers are not mechanically loaded during the velvet stage, and it is presently unclear what triggered remodeling activity in the antlers whose lifespan had been experimentally extended.

Bilateral antler sequestration above the coronet in a red deer (Cervus elaphus) stag-Insights into the process of antler casting

This paper reports a case of delayed velvet shedding and bilateral premature antler casting above the coronets in a young adult red deer stag from Germany. Based on the established role of testosterone in the control of the antler cycle, the antler abnormality is considered to have been the result of a (temporary) androgen deficiency. The basal surfaces (separation planes or seals) of the cast antlers were markedly concave. Scanning electron microscopy revealed that the separation plane was densely covered with Howship's lacunae, denoting intense osteoclastic activity along the border between the proximal (living) and distal (dead) antler portions. Our observations and those of previous studies indicate that antler casting does not occur at a pre-determined separation plane, but along the border between living and dead bone, regardless of the position of this border within the cranial appendages. This is a major difference to autotomy of (living) appendages at fixed breakage planes, as it occurs for instance in lizard tails.

Antlers - Evolution, development, structure, composition, and biomechanics of an outstanding type of bone

Antlers are bony appendages of deer that undergo periodic regeneration from the top of permanent outgrowths (the pedicles) of the frontal bones. Of the "less familiar" bone types whose study was advocated by John Currey to gain a better understanding of structure-function relationships of mineralized tissues and organs, antlers were of special interest to him. The present review summarizes our current knowledge about the evolution, development, structure, mineralization, and biomechanics of antlers and how their formation is affected by environmental factors like nutrition. Furthermore, the potential role of antlers as a model in bone biology and several fields of biomedicine as well as their use as a monitoring tool in environmental studies are discussed.

The structure of pedicle and hard antler bone in the European roe deer (Capreolus capreolus): a light microscope and backscattered electron imaging study

Deer antlers are deciduous bony structures that develop from permanent frontal outgrowths, the pedicles. While growth and bone architecture of antlers have been studied in greater detail, information on pedicle formation and structure is scarce. The present study provides information on the structure of pedicle and hard antler bone in the European roe deer. A pronounced seasonal variation in pedicle architecture was observed, with high porosity around antler casting and a very compact bone structure during the hard antler stage. These observations suggest a corresponding marked variation also in the biomechanical properties of the pedicles. The seasonally alternating extensive resorption and formation processes make the pedicles of older deer heavily remodeled structures. Pedicles increase in thickness by apposition of primary bone that subsequently becomes replaced by secondary osteons. The antler cortex of roe deer is largely composed of a tubular framework of woven bone trabeculae with some remnants of mineralized cartilage, and primary osteons that have filled in the intertrabecular spaces. Secondary osteons are scarce, denoting little remodeling in antlers, which can be related to their short lifespan. The occurrence of cement lines around primary osteons indicates resorption on the trabecular scaffold prior to infilling of the intertrabecular spaces. The outer cortex showed a higher autofluorescence and a more immature structure than the main cortex, suggesting that it was secondarily formed by periosteal activity. Pedicles and antlers constitute a functional entity, and future histological and/or biomechanical studies should therefore consider both components of the cranial appendages.

Role of testosterone and photoperiod on seasonal changes in horn growth and sperm variables in the Iberian ibex: a model for polygynous wild bovids

This work examines the effect of testosterone secretion and photoperiod on seasonal changes in horn growth and sperm variables in the Iberian ibex (Capra pyrenaica), here used as a model for polygynous wild bovids. The hypothesis that high levels of testosterone provide an endocrine signal that inhibits horn growth in autumn was tested by assessing the effect of cyproterone acetate (CA), an anti-androgen, administered in October - coinciding with the period of natural increases in plasma testosterone concentrations - under different photoperiodic conditions (natural photoperiod and artificial long days). The persistence of horn growth during autumn in all ibexes held under the long-day photoperiodic conditions clearly shows that horn growth regulation in the mating season is primarily modulated by day length and not by a fall in testosterone concentration. A retrospectively designed second experiment involving testosterone propionate (TP) administration in April (when horns are growing) was then undertaken to confirm that high levels of testosterone do not inhibit horn growth. Overall, the results strongly suggest that the rise in testosterone secretion during the autumn mating season does not act as an endocrine signal for the arrest of horn growth, although the rate of horn growth before the mating season may be related to springtime testosterone levels. A direct relationship was seen between the rate of horn growth and the incidence of sperm abnormalities. Neither CA treatment in October nor TP administration in April affected the studied sperm variables. By contrast, CA treatment plus artificial long days in autumn had a negative effect on sperm motility and sperm morphology.

Deer antler regeneration: Cells, concepts, and controversies

The periodic replacement of antlers is an exceptional regenerative process in mammals, which in general are unable to regenerate complete body appendages. Antler regeneration has traditionally been viewed as an epimorphic process closely resembling limb regeneration in urodele amphibians, and the terminology of the latter process has also been applied to antler regeneration. More recent studies, however, showed that, unlike urodele limb regeneration, antler regeneration does not involve cell dedifferentiation and the formation of a blastema from these dedifferentiated cells. Rather, these studies suggest that antler regeneration is a stem-cell-based process that depends on the periodic activation of, presumably neural-crest-derived, periosteal stem cells of the distal pedicle. The evidence for this hypothesis is reviewed and as a result, a new concept of antler regeneration as a process of stem-cell-based epimorphic regeneration is proposed that does not involve cell dedifferentiation or transdifferentiation. Antler regeneration illustrates that extensive appendage regeneration in a postnatal mammal can be achieved by a developmental process that differs in several fundamental aspects from limb regeneration in urodeles.

Deer antlers: a zoological curiosity or the key to understanding organ regeneration in mammals?

Many organisms are able to regenerate lost or damaged body parts that are structural and functional replicates of the original. Eventually these become fully integrated into pre-existing tissues. However, with the exception of deer, mammals have lost this ability. Each spring deer shed antlers that were used for fighting and display during the previous mating season. Their loss is triggered by a fall in circulating testosterone levels, a hormonal change that is linked to an increase in day length. A complex 'blastema-like' structure or 'antler-bud' then forms; however, unlike the regenerative process in the newt, most evidence (albeit indirect) suggests that this does not involve reversal of the differentiated state but is stem cell based. The subsequent re-growth of antlers during the spring and summer months is spectacular and represents one of the fastest rates of organogenesis in the animal kingdom. Longitudinal growth involves endochondral ossification in the tip of each antler branch and bone growth around the antler shaft is by intramembranous ossification. As androgen concentrations rise in late summer, longitudinal growth stops, the skin (velvet) covering the antler is lost and antlers are 'polished' in preparation for the mating season. Although the timing of the antler growth cycle is clearly closely linked to circulating testosterone, oestrogen may be a key cellular regulator, as it is in the skeleton of other male mammals. We still know very little about the molecular machinery required for antler regeneration, although there is evidence that developmental signalling pathways with pleiotropic functions are important and that novel 'antler-specific' molecules may not exist. Identifying these pathways and factors, deciphering their interactions and how they are regulated by environmental cues could have an important impact on human health if this knowledge is applied to the engineering of new human tissues and organs.

Histological structure of antlers in castrated male fallow deer (Dama dama)

Antlers are periodically replaced cranial appendages that, except for the reindeer, are grown only by male deer. The annual antler cycle is controlled by seasonal fluctuations of sex steroid concentrations in the blood, and accordingly castration of male deer causes deviations from normal antler growth. The present study investigated antler histology of castrated fallow bucks (Dama dama). Castration in early spring was followed by casting of the hard antlers carried by the bucks and the growth of a new set of antlers, which remained in velvet permanently. In the following year, numerous bony protuberances developed from the original antler surface. Further growth of these protuberances, which were formed by subperiosteal intramembranous ossification, led to a marked increase in antler diameter in the affected areas. Compared to antlers of intact bucks, the antlers of the castrates showed histological signs of immaturity, suggestive of a reduced bone remodeling and an impairment of the mineralization process. These changes point to the dependence of the above processes on a stimulation by higher levels of sex steroids. Two years after castration, the antlers also developed integumental thickening and showed an initial formation of skin outgrowths. Cystic structures were present in the skin, which were often filled with a presumably sebaceous and/or keratinous material. Formation of intradermal bone or cartilage was not observed in the antlers of the castrated fallow bucks. The histological structure of the skin outgrowths suggested that they were caused by a hypertrophy of the dermal component of the velvet. Due to the localized bone overgrowth, resulting from the periosteal bone apposition onto the original antler surface, skin-lined infoldings originated, which reached deep into the newly formed bone. Our study revealed no indication of invasive/destructive bone growth in the antlers, i.e., of a penetration of the newly formed bone tissue into the pre-existing bone. The hypertrophic bone growth in the antlers of the castrates is compared with other forms of periosteally derived hypertrophic bone formation, including osteomas, in the mammalian skeleton. It is discussed whether the skin and bone outgrowths of the antlers of castrated fallow bucks may be classified as benign tumors.

Histological studies of bone formation during pedicle restoration and early antler regeneration in roe deer and fallow deer

The purpose of the present study was to examine the process of bone formation in the regenerating cranial appendages of roe deer (Capreolus capreolus) and fallow deer (Dama dama) during the early postcasting period. After the antlers are cast, osteoclastic and osteoblastic activities lead to a smoothing of the pedicle's separation surface, a strengthening of the pedicle bone, and a partial restoration of the distal pedicle portion that was lost along with the cast antler. Initially, bone formation occurs by intramembranous ossification, but early during the regeneration process cartilage is formed at the tips of the cranial appendages, and is subsequently replaced by bone in a process of endochodral ossification. Shortly after the antlers are cast, the cambium layer of the periosteum in the distal pedicle is markedly enlarged, which suggests that the periosteum serves as a cell source for the bone-forming tissue covering the exposed pedicle bone. The histological findings of our study are consistent with the view that the bony component of the regenerating cranial appendages of deer is largely derived from the pedicle periosteum. Based on findings in other bone systems, we speculate that stem cells that can undergo both osteogenic and chondrogenic differentiation are present in the pedicle periosteum. The early onset of chondrogenesis in the regeneration process is regarded as an adaptation to the necessity of producing a huge volume of bone within a short period. This parallels the situation in other cases of chondrogenesis in membrane bones.

Effect of antiandrogen cyproterone acetate on the development of the antler cycle in Southern pudu (Pudu puda)

The antler cycle of pudu is similar to other cervids, but unlike most boreal deer, male Southern pudu (Pudu puda) exhibits two seasonal peaks of LH and testosterone. In that respect, pudu is similar to roe deer. Whereas the antler cycle in some deer species, such as roe deer or white-tailed deer, is very sensitive to variation of testosterone, in other cervids, such as fallow deer or reindeer, a blockade of androgens with cyproterone acetate (CA) has little or no effect on the timing of the antler casting. In order to test the sensitivity of pudu antlers to variations of androgens, CA (administered 2x weekly at 50 mg/buck) was injected intramuscularly for 3 weeks in 5 adult male pudu, starting February 19 (late summer). Four other males of similar age served as controls. The experiment was performed at the University of Concepcion, Chile, latitude 36.6 degreeS. Blood samples were taken once a week between January 19 and April 3. In CA-treated bucks, the antlers were cast approximately 3 weeks after the initiation of CA treatment and a new antler growth began almost immediately. The antlers reached about 5 cm in length, before ceasing to grow at the end of April, when they became mineralized and were subsequently polished. CA had no effect on the already declining levels of LH. Plasma levels of testosterone in controls increased from February 15, whereas in CA-treated bucks remained depresses until March 21. It is concluded that similarly to white-tailed deer, the antler cycle of Southern pudu is very sensitive to manipulation of androgen levels.

Effects of testosterone either alone or with IGF-I on growth of cells derived from the proliferation zone of regenerating antlers in vitro

Deer antlers are male secondary sexual characters and are the fastest growing mammalian tissue. As such, both androgens and growth factors play a major role in antler development. The timing of the antler cycle is controlled by the seasonal fluctuations of testosterone, and the actual growth of antlers is mainly stimulated by growth factors including insulin-like growth factor-1 (IGF-I). However, whether or not testosterone at low levels plays a growth-promoting role during antler formation is controversial. In the present study, we took an in vitro approach to investigate whether testosterone either alone or with IGF-I had mitogenic effects on mesenchymal or cartilaginous cells derived from the proliferation zone of regenerating antlers. In addition, a binding assay was carried out to determine whether the specific binding sites for testosterone were preserved after cell disaggregation. The results showed that testosterone either in physiological concentrations or at low levels did not exert direct mitogenic effects on antler cells derived from the proliferation zone in serum-free medium in vitro (P>0.05), even if the specific binding sites for testosterone in these cells were well preserved. Likewise, testosterone in a very wide range of concentrations not only failed to enhance (P>0.05), but at certain levels (0.1-5 nM) impaired the mitogenic effects of IGF-I on these antler cells in vitro (P<0.001). Therefore, these results support neither a conclusion that low level testosterone has growth-promoting effects on antler formation nor the hypothesis that testosterone effects may be achieved through sensitizing these antler cells to the mitogenic effects of IGF-I.

Fluoride concentrations in antler bone of roe deer (Capreolus capreolus) indicate decreasing fluoride pollution in an industrialized area of western Germany

In order to reconstruct temporal changes in ambient fluoride levels in the industrialized Ruhr area (western Germany), we analyzed the bone fluoride content of 167 antlers of roe deer (Capreolus capreolus) killed between 1951 and 1999 in the northern part of this region. Individual values ranged between 110 and 8,178 mg F-/kg ash, and there was an overall marked decrease over the sampling period. Average bone fluoride concentrations in antlers from the periods 1980 through 1989 (geometric mean [95% confidence interval]: 1,490 [1,193-1,861] mg/kg ash) and 1990 through 1999 (753 [644-882] mg/kg ash) differed significantly (p < 0.001) and were both significantly (p < 0.001) lower than those from the periods 1951 through 1969 (3,720 [3,227-4,288] mg/kg ash) and 1970 through 1979 (2,573 [2,203-3,006] mg/kg ash). The findings are seen as indicative of a progressively reduced atmospheric fluoride deposition into the study area, caused by effective emission-control measures in Germany and neighboring countries. Because antlers are replaced annually, grow during a fixed period of some months, and are regularly collected and kept as trophies, they are well suited as monitoring units for analyzing temporal trends in environmental pollution by fluoride and other bone-seeking pollutants.

Serum testosterone, 5-alpha-dihydrotestosterone and different sex characteristics in male fallow deer (Cervus dama): a long-term experiment with accelerated photoperiods

To investigate the photoperiodic influence on androgen, testis volume, neck girth, and antler-cycles, five fallow bucks were exposed consecutively to different accelerated photoperiodic cycles (three 6-month, three 4-month, and one 3-month cycle). All parameters immediately followed artificial light cycles. Unlike in natural conditions, antler casting, regrowth, and velvet shedding occurred at decreasing day-lengths. The experiment provided further evidence that in fallow bucks the reproductive system is under strictly photoperiodic control and directly and/or indirectly dictates the course of the antler cycle via gonadal hormones. Depending on the test-system, the experiment revealed similar threshold values for the effect of testosterone (T) and/or 5-alpha-dihydrotestosterone (DHT) concentrations on the antler cycle as previously detected in fallow bucks living under natural conditions. Antlers were cast when T levels dropped below 3 ng/ml, and velvet shedding occurred after T and/or DHT values reached at least 5 to 7 ng/ml. In all cases, amplitudes of androgens remained lower than observed under natural conditions. Frequently during early antler growth, DHT values appeared higher than T levels. With increasing acceleration of the photoperiod, the antler cycle achieved limits of adaptation. The faster the photoperiods were accelerated, the smaller the antlers were developed. In addition, the antler tips were porous, blunt, and not completely mineralized. In all animals, accelerations of the photoperiods evoked phase displacements between measured parameters, pointing to an increasing internal desynchronisation. The present results indicate that the whole reproductive system of fallow deer is directly controlled by the photoperiod and that related morphological and physiological processes are indirectly determined by photoperiodic changes.

Effects of castration on antler growth in fallow deer (Dama dama L.)

Morphology and histological structure of antlers grown after castration (performed on March 25) were studied in six young fallow bucks. In the year after castration, antlerogenesis occurred during the species-specific time span, and the shape of the antlers, which remained permanently in velvet, was normal. During a cold period in December/January, the distal parts of the antlers suffered from frostbite and were subsequently detached. The process of sequestration was similar to that leading to normal antler casting. The sequestration sites were soon covered with skin, but (limited) regrowth of antler tissue from the stumps was not observed before late April/early May, i.e., the time of normal antler regeneration. Simultaneously, growth of knobby protuberances started on the surface of the antlers. Histological analysis of biopsies taken on December 20 in the year after castration revealed that the central parts of the antlers consisted of cancellous lamellar bone with mainly secondary osteons. Peripheral to this, the bone tissue (forming the protuberances) was of a more immature nature and exhibited larger intertrabecular spaces. The outermost layer consisted of woven bone formed by intramembranous ossification from the periosteum and was undergoing active growth and remodeling at the time of biopsy. Thus, bone formation at these sites occurred during a period when no antler growth is observed in normal fallow bucks. The velvet covering the bony protuberances was of normal appearance.

The macroscopic and microscopic structure of double-head antlers and pedicle bone of cervidae (Mammalia, Artiodactyla)

Ortho- and heterotopically formed double-head antlers were studied in red, fallow and roe deer. The malformation was the result of new antler growth without previous casting of the old antlers. Thus, two antler structures belonging to successive antler generations originated from one pedicle. These two structures were always separated by a horizontal groove. Histologically, signs of osteoclastic resorption were observed in the interior and at the outer circumference of the distal parts of the pedicles of the double heads. The resorptive process had, however, not been of an intensity necessary for subsequent antler casting. We also observed that the double-head's second antler generation had developed as a periosteal exostosis of the distal pedicle bone. Thus, we assume that in normogenesis formation of the bony component of subsequent antlers is also probably dependent on cells derived from pedicle periosteum. Finally, the process of antler regrowth in deer is compared with epimorphic regeneration occurring in other vertebrates.

Effects of an antiandrogen treatment on morphological characters and physiological functions of male fallow deer (Dama dama L.)

From mid-April to late November, i.e., during antler growth phase and main breeding season, a group of four (later three) male fallow deer was treated with increasing doses of the antiandrogen cyproterone acetate (CA). Throughout the treatment period, plasma testosterone levels of the bucks were either undetectable or only slightly above the detection limit of 0.1 ng/ml. In marked contrast to controls, testicular volumes and neck girths of the CA treated bucks were basal during the application period, proving the strong antigonadotropic effect of CA (testicular volume) and the absence of androgen action at the normal target organs (neck girths), respectively. Antler growth in the CA treated bucks occurred during the normal time span for fallow deer and the antlers produced were of species specific shape. From this it is concluded that in fallow deer, onset, duration, and termination of antler growth are independent of androgen action. Velvet shedding in the CA treated bucks however did not occur at the normal time (late August/early September), but was postponed until late December/early January, i.e., about 5 weeks after the termination of CA application. Activity of alkaline phosphatase in the experimental animals increased during the antler growth phase, but was markedly lower than in controls. Maximum body weight in the CA treated bucks was reached already in June, i.e., about 3 months prior to controls. Furthermore, the rut associated, drastic decline in body weight observed in controls did not occur in the experimental animals.