Development of the giraffe horn and its blood supply.

The giraffe horn is an unusual cranial exostosis that lacks clear delineation and categorization as either horn or antler. The distinction between the two is made by contrasting their methods of development and basic composition. This process of development has not been detailed in the giraffe, a factor contributing to the difficulty in distinguishing the classification of these horns. In a chronological series of giraffe horns from prenatal and postnatal animals, we have observed unique morphologies that define their proper location on the skull, the blood supply to them, and the transitions involved in their histological development. While our observations have facilitated the classification of the giraffe horn, our interpretations were not always in accord with previous reports.

Developing antler, a model for endocrine regulation of bone growth. Concentration gradient of T3, T4, and alkaline phosphatase in the antler, jugular, and the saphenous veins.

Thyroxine (T4), triiodothyronine (T3), and alkaline phosphatase (AP) were assayed monthly in white-tailed deer plasma obtained from the antler (A), jugular (J), and the saphenous (S) veins during the period of antler growth and the period of mineralization. The concentrations of T3, T4, and AP in the J and S were not significantly different in any month. During the most intense antler growth period (June and July) AP levels were significantly higher and T3 significantly lower in A as compared to J and S, but no differences were found during the mineralization phase in August. T3 values from all three veins were significantly lower in the mineralization period than J and S values during the growing phase. Variations of T4 levels between A, J, and S as well as differences between months were only negligible. The data indicate that AP is produced in large quantity in the antler tissue during the growth period, but the amount is sharply reduced during the mineralization phase. T3 is utilized in the growing antler; the rate of utilization correlates with the intensity of antler growth. T4 is probably not utilized in the antler bone. Developing antler might be used as a model to study growth and mineralization processes of bony tissues.

Do the blood vessels of the antler velvet of the red deer have an adrenergic innervation?

Samples of antler velvet and pedicle skin of Red deer were snap-frozen, freeze-dried and treated with hot formaldehyde vapour. Sections (4 microns thick) were epi-illuminated with ultra-violet light. No blue-green fluorescence characteristic of adrenergic nerves was seen in sections from the antler velvet except in one castrate, although the blood vessels of the pedicle skin were well innervated. Yellow fluorescence characteristic of 5-hydroxytryptamine was present in mast cells in the velvet. The lack of a discrete adrenergic vasomotor innervation of the antler blood vessels makes it unlikely that antlers in velvet control heat loss by functioning as heat radiators.

Blood flow, calcium deposition and heat loss in reindeer antlers.

Antler blood flow was studied in a 2 year old male reindeer during the last half of the antler growth period using an electromagnetic flow probe chronically implanted around the superficial temporal artery. Arteriovenous (a-v) differences of calcium were measured on antler blood. The blood flow increased from 60--90 ml/min when the antler was half-grown to 100--120 ml/min when fully developed. Subsequently a reduction was observed towards shedding. Positive a-v plasma calcium differences (on average 0.2 mM) were recorded during the period of active growth. Two bulls maintained positive a-v calcium differences after a 48 hour starvation period, in spite of reduced arterial calcium concentrations. Exercise to near exhaustion caused a 2 degrees C rise in the rectal temperature. Antler blood flow was decreased immediately after exercise and returned to pre-exercise values usually within 5--10 min. Since no overshoot in antler blood flow was recorded during the hyperthermia it is concluded that variations in blood perfusion of the antlers are without importance in the defence against hyperthermia during and after exercise.