Reproductive and endocrine organs of foetal, newborn and adult seals

Abstract

During late foetal life and at birth the gonads of both Grey and Common seals are enlarged, and there are indications of stimulation in the rest of the reproductive tract. The enlargement of the gonads is due mainly to the presence of very large numbers of cells resembling interstitial cells. The presence of groups of small round cells resembling lymphocytes and of cells intermediate between them and fully developed interstitial cells suggests that the small round cells may be the precursors of the latter. Whether these cells are lymphocytes or primitive mesenchymal cells cannot be stated with certainty. The ovaries of foetal Common and Grey seals contain no follicles beyond the stage where oocytes are ensheathed in a single layer of epithelial cells, but more marked follicular development was present in the ovaries of a still‐born Californian sea lion. No evidence of spermatogenesis was seen in the testes of any foetal or neonatal male seal. Regression occurs rapidly after birth and is nearly complete at the age of two weeks. The interstitial cells disappear by a process similar to fatty degeneration. Retrogression is accompanied by a remarkable obliteration of many blood vessels supplying the outer part of the medulla and the cortex of the ovary. The stimulus responsible for the precocious enlargement of the foetal gonads is removed with the birth of the young. The rapid decrease of endocrine influence after birth suggests that the placenta may be implicated. In the adult Common and Grey seal the corpus luteum persists until the end of pregnancy and begins to degenerate a few days after parturition. Follicular development occurs in the maternal ovaries towards the end of pregnancy and in the immediate post‐parturient period. The maternal ovaries are probably a source of oestrogens, both before, during, and after parturition, and such oestrogens could cross the placenta and affect foetal tissues. The prostate of all male foetuses of both species towards the end of pregnancy and immediately after birth is markedly enlarged. There is proliferation and hypertrophy of the prostatic alveoli, dilation of the tubules, and an increase in the size of the epithelial cells. A week after birth the weight of the gland is reduced by more than one‐half. Persistence and hyperplasia of the uterus masculinus occurs in some foetal and newborn pups of the Common seal, but no similar enlargement was seen in the Grey seal. The thyroid in the late foetal and neonatal Common and Grey seals showed wide differences in their histological appearances. The thyroid glands from an adult lactating seal of each species were widely different. In the Grey seal it was almost completely atropic but in the Common seal it appeared to be intensely active. The amount of medulla present in the adrenal of foetal seals is small by comparison with that of the cortex. A large number of cortical islands packed with lipid is present in the medulla, often containing more lipid than the cortex itself. The capsule of the adrenal varies considerably in thickness ...

Histological Structure of the Adrenal Gland of the Bottlenose Dolphin (Tursiops truncatus) and the Striped Dolphin (Stenella coeruleoalba) from the Adriatic Sea

“…The adrenal cortex and the adrenal medulla in the Adriatic bottlenose and striped dolphin species were separated by a clearly visible layer of connective tissue. The same was described by Clark et al (2005) for the Atlantic bottlenose dolphin; by Clark et al (2008) for the pan‐tropical spotted dolphin and the spinner dolphin and by Amoroso et al (1965) for a harbour seal male. However, a connective tissue layer between cortex and medulla of the adrenal gland for domestic mammals and humans was not described (Banks, 1993; Dellmann, 1993; Junqueira and Carneiro, 2005).…”

Growth and development of the ovary and small follicle pool from mid fetal life to pre-puberty in the African elephant (Loxodonta africana)

“…The tightly packed epithelioid interstitial cells secrete large quantities of 19-carbon androgen precursors, including androstenedione and dehydroepiandrosterone (DHEA), [ 34 , 35 ] and the very unusual 3-ß hydroxy-5,7 pregnanedien-20-one and 3-ß hydroxy-5,7 androstadien-17-one [ 36 ] which are then aromatised by the placenta to produce the relatively enormous (μg/ml) quantities of both phenolic (oestrone and oestradiol) and Ring B unsaturated (equilin and equilenin) oestrogens that are present in the blood and urine of pregnant mares during the second half of gestation [ 37 , 38 ] and which appear to be important for growth and development of the very precocious equine fetus at birth [ 39 ]. Similar interstitial tissue development results in ovarian hypertrophy in the Grey and Common seals, also without antral follicle formation [ 20 ]. In both horses and seals the interstitial tissue declines rapidly in late gestation so that the gonads have shrunk to their normal prepubertal size at birth [ 19 , 20 , 40 ].…”

“…Similar interstitial tissue development results in ovarian hypertrophy in the Grey and Common seals, also without antral follicle formation [ 20 ]. In both horses and seals the interstitial tissue declines rapidly in late gestation so that the gonads have shrunk to their normal prepubertal size at birth [ 19 , 20 , 40 ].…”

“…Interstitial cells are found in the fetal gonads of many mammalian species including the elephant [ 14 ]; human [ 15 ], spotted hyena [ 16 ], guinea pig [ 17 ], rock hyrax [ 18 ] and in some—for example the horse [ 19 ] and the Common and Grey seals [ 20 ]—they cause a significant increase in fetal gonadal volume. In the female fetus in most of these species, the interstitial cells develop independent of follicles within the presumptive ovarian medulla.…”

Stereology of the Thyroid Gland in Indo-Pacific Bottlenose Dolphin (Tursiops aduncus) in Comparison with Human (Homo sapiens): Quantitative and Functional Implications

“…From a morphological point of view, the volumetric fraction and activity of different histological components in the mammalian thyroid gland (follicular cells, C-cells, colloid, and interstitial tissue) changed considerably in the course of development [17]. Phocids [9],[18],[19] and cetaceans [9]–[11] have shown a marked variation in the apparent levels of activity of thyroid follicular cells at various times throughout the life history.…”