Changes in gonadal and adrenal steroid levels in the leatherback sea turtle (Dermochelys coriacea) during the nesting cycle.

The reproductive endocrinology of nesting leatherback turtles (Dermochelys coriacea) was studied during the 1996-1997 and 1997-1998 nesting seasons at Los Baulas National Park, Playa Grande, Costa Rica. Blood samples were collected from nesting females throughout the season. Females were observed to nest up to 10 times during the nesting season. Plasma steroids were measured by radioimmunoassay and total plasma calcium was measured by flame atomic absorption spectrophotometry. Plasma testosterone and plasma estradiol levels declined throughout the nesting cycle of the female. Testosterone declined from a mean of 10.18 +/- 0.77 ng/ml at the beginning of the nesting cycle to 1.73 +/- 0.34 ng/ml at the end of the nesting cycle. Estradiol declined in a similar manner, ranging from a mean of 190.95 +/- 16.80 pg/ml at the beginning of the nesting cycle to 76.52 +/- 12.66 pg/ml at the end of the nesting cycle. Plasma progesterone and total calcium levels were relatively constant throughout the nesting cycle. Lack of fluctuation of total calcium levels, ranging from a mean high of 97.46 +/- 11.37 microg/ml to a mean low of 64.85 +/- 11.20 microg/ml, further suggests that vitellogenesis is complete prior to the arrival of the female at the nesting beach. Clutch size (both yolked and yolkless eggs) did not vary over the course of the nesting cycle.

Sex differences in plasma corticosterone in desert tortoises, Gopherus agassizii, during the reproductive cycle.

Blood samples from 30 female and 20 male adult desert tortoises, Gopherus agassizii, were collected at monthly intervals during the annual reproductive cycle (April to October). Plasma corticosterone and the sex steroids in each of the samples were analyzed by radioimmunoassay. Mean corticosterone levels in males were significantly higher than in females (P < 0.001) in every month. Male tortoises showed a marked seasonal pattern in plasma corticosterone with a highly significant peak in July, August, September, and October that corresponded with a similar peak in plasma testosterone. Testosterone and corticosterone in the male showed a highly significant correlation (P < 0.0001). The pattern of corticosterone in the female was less marked, with a significant peak in May during the mating and nesting season, but no association with the peak in estradiol in late summer was apparent. The highest levels of corticosterone in the males were associated with the peak in spermatogenesis and intense male-male combat. These results support similar data from other reptiles that suggest increased glucocorticoid secretion during periods of increased activity and metabolism.

Seasonality in plasma thyroxine in the desert tortoise, Gopherus agassizii.

To characterize seasonal changes in thyroid function in a terrestrial reptile, thyroid hormones were measured over a period of 2 years in desert tortoises, Gopherus agassizii, maintained at the Desert Tortoise Conservation Center in Las Vegas, Nevada. In all samples, triiodothyronine was nondetectable (less than 0.1 ng/ml). Thyroxine (T(4)) exhibited distinct cycles in both sexes, being lowest during hibernation and rising toward the time of emergence. Females exhibited only one peak in T(4), during the early spring. In males, T(4) levels peaked in early spring and again in late summer. The desert tortoise has distinct activity patterns that include increased feeding, mating, and locomotor activity in the early spring and increased mating and combat in the late summer. In an experiment to determine whether food intake influences T(4), food was withheld for 2 weeks. Compared to continuously fed controls, T(4) declined significantly in unfed tortoises, but increased significantly within 36 h of refeeding, indicating that thyroid activity is responsive to nutrient intake. The second seasonal peak of T(4) only in males suggests that male reproductive activity in late summer is associated with thyroid activation. To evaluate this possibility, adult, subadult, and juvenile males were sampled during the months of the second seasonal peak in T(4). Although all three age groups showed similar foraging and thermoregulatory behaviors, T(4) peaked in July only in the reproductively active adults, which also exhibited significantly higher testosterone levels. Elevated T(4) in desert tortoises is thus associated with periods of increased feeding and reproductive activity, supporting a role for thyroid hormones in these energy-demanding activities.

Seasonal reproductive cycle of the Kemp's ridley sea turtle (Lepidochelys kempi).

The seasonal reproductive cycle of the Kemp's ridley sea turtle (Lepidochelys kempi) was studied under seminatural conditions at the Cayman Turtle Farm, Grand Cayman, British West Indies, from June 1987 to July 1988. Male L. kempi displayed a prenuptial rise in serum testosterone 4 to 5 months prior to the mating period (March). Male testosterone then declined sharply during the mating period. Female L. kempi also displayed a prenuptial rise in serum testosterone, estradiol, and total calcium 4 to 6 months prior to the mating period (March). Female testosterone and estradiol declined during the nesting period (April to July) immediately following the mating period (March). Elevated levels in female estradiol and total calcium corresponded with the period of vitellogenesis as determined from gel electrophoresis and ultrasonography. Serum thyroxine also fluctuated seasonally with elevated levels observed in females associated with the period of vitellogenesis. L. kempi displayed a distinct seasonal reproductive cycle in captivity. Nesting in the captive study group corresponded with nesting in the wild population at Rancho Nuevo, Mexico (April to July). Female endocrine cycles during the nesting period were similar to those observed in the wild population.

Leydig cell cytoplasmic content is related to daily sperm production in men.

The relationship between the abundance of specific Leydig cell organelles and daily sperm production (DSP) was determined. Testes from 10 men (26-53 years of age) were obtained at autopsy within 10 h of traumatic death or heart failure and fixed by vascular perfusion. Testicular tissue was processed for light and electron microscopy. DSP/testis and Leydig cell cytoplasmic volume/testis were determined by stereology of histologic sections. The Leydig cell organelle content was determined by point counting electron micrographs for smooth endoplasmic reticulum (SER), rough endoplasmic reticulum, mitochondria, lipofuscin pigment, lipid, Golgi bodies, and Reinke crystals. Men were divided equally into two groups based on DSP/testis. Men with low DSP/testis had less SER volume density (P less than 0.01) and lower SER volume per testis (P less than 0.05) than men with high DSP. Other organelles were unrelated to DSP. When all men were combined, the volume density of SER (r = 0.80; P less than 0.01), the volume SER per testis (r = 0.69; P less than 0.05), and the volume SER per Leydig cell (r = 0.84; P less than 0.01) were significantly related to DSP. Hence, there appears to be a significant relationship between Leydig cell SER and the level of spermatogenesis in men.

Increased germ cell degeneration during postprophase of meiosis is related to increased serum follicle-stimulating hormone concentrations and reduced daily sperm production in aged men.

Aged men, known to have high serum gonadotropin levels and reduced spermatogenic potential, were used to study the relationship between serum follicle-stimulating hormone (FSH) and germ cell degeneration. Serum hormones were measured from blood obtained at autopsy. Phase-contrast cytometry was used to enumerate germ cells in homogenates of fixed testes from 13 younger (24-51 yr) and 14 aged (69-90 yr) men. The developmental steps of spermatogenesis during which germ cells degenerate were determined by comparing potential daily sperm production based on primary spermatocytes with daily sperm production based on two different types of spermatids. During spermiogenesis, there was no significant degeneration in the younger or aged men. During postprophase of meiosis, aged men had more (p less than 0.01) germ cell degeneration, significantly lower (p less than 0.05) serum testosterone, and greater (p less than 0.01) serum FSH than did younger men. Germ cell degeneration during postprophase of meiosis was negatively correlated (p less than 0.01) to daily sperm production and significantly (p less than 0.01) related to serum concentrations of FSH. As revealed in these aged men, meiotic germ cell degeneration has a direct effect on daily sperm production and is significantly related to serum FSH concentrations.