Light and electron microscopy were used to study the morphology of adipose cells in newborn Merino lambs (Ovis aries). Postnatal changes in morphology were also examined in naturally fed Merino lambs held at 26°C or at 3°C for up to 32 days from the time of birth, and in lambs fasted for up to three days at 26°C or at 3°C since birth. All adipose cells examined in the newborn lambs showed the morphological characteristics of brown adipose cells; no white adipose cells could be found. The brown adipose tissue in normally fed lambs was replaced progressively by white adipose tissue during the first two or three weeks of life, and this replacement was retarded in the lambs held at 3°C. During replacement a continuous spectrum of cells with morphological characteristics between those of brown and white adipose cells were seen. No degenerating brown adipose cells were observed; and, apart from brown adipose cells, no cells were identified that could have been precursors of white adipose cells. This evidence suggests that in various body regions of lambs white adipose cells are derived from brown adipose cells.Lambs fasted at 3°C or under conditions that approached thennoneutrality (26°C) showed rapid depletion of lipid from brown adipose cells; hence brown adipose tissue of the lamb differs from that in the newborn rabbit in which this tissue is not readily depleted of fat during starvation under thermoneutral conditions.
SUMMARY1. Electron microscopic examination revealed that most of the adipose tissue of new-born calves had the cellular morphology of brown adipose tissue; only subcutaneous tissue had the cellular morphology of white adipose tissue.2. The cellular morphology of the brown adipose tissue changed progressively to that of white adipose tissue as the age of the calves increased.3. Infusion of noradrenaline (i.v.) at rates of 1 and 5 #ug/kg. min into new-born calves exposed to a thermoneutral environment increased metabolic rate two-to threefold, and also increased rectal temperature and respiration rate. The responses declined as age of calf increased.4. Approximately 2 % of the body weight of new-born calves appears to be brown adipose tissue.
The development of subcutaneous and perirenal adipose tissue in foetal sheep of 40--150 days conceptual age was examined using electromicroscopy. Adipose tissue from both sites developed from pre-adipose cells of similar appearance, with few mitochondria, and commenced accumulating lipid on about day 70 of gestation. In the perirenal fat there was marked proliferation of mitochondria between days 80 and 90 and the tissue developed into brown fat, cell profiles being characterized by many mitochondria with numerous distinct cristae, whereas in the subcutaneous cells which developed into white adipose tissue few mitochondria with fewer cristae were observed. Multilocular and unilocular cells occurred in both tissues near term. Nerves were first seen between brown adipose cells in a 130-day-old foetus and were frequently seen in older foetuses, but very few nerves were seen in the white adipose tissue. In hypophysectomized foetuses, either full- or post-term, the subcutaneous fat and internal fat were shown to be white and brown adipose tissue respectively, as in normal foetuses, although in one 164-day-old foetus the perirenal tissue appeared to be a mixture of brown and white fat.
The lungs of newborn northern native cats, Dasyurus hallucatus and newborn brushtail possums, Trichosurus vulpecula were examined by both light and electron microscopy. The native cat has a birth weight of 18 mg after a gestation of about 21 days, whereas the brushtail possum weights 200 mg at birth and has a gestation period of 17.5 days. The lungs of the native cat are two large respiratory sacs, with a respiratory lining of squamous cells and surfactant-secreting cells. The capillaries are located within the connective tissue just below this respiratory epithelium. The visceral covering of the lung is formed by squamous cells. The lungs of the possum are composed of numerous large respiratory sacs which are separated by connective tissue septa in which the capillaries are located. The sacs, as in other species, are lined with squamous cells and surfactant-secreting cells. It is proposed that the structure of the lung of the newborn marsupial is related more to the size of the newborn rather than to the length of the gestation period.
The numbers and distribution of T and B cells in the thoracic thymus, spleen and intestinal tissue and the proliferation of T lymphocytes were examined during pouch life and in the adult to determine when the developing brushtail possum reaches immunological maturity. CD3-positive cells were observed in the thoracic thymus at day 2 post-partum indicating that the thymus produces T lymphocytes at or soon after birth. By day 25 the thymus was fully populated with CD3-positive T lymphocytes and they were observed in distinct regions of the cortex and medulla. By day 48 post-partum, B and T lymphocytes were identified in the follicles and parafollicular areas of the spleen. Although the numbers of T and B cells in the spleen increased significantly from day 25 to day 100 post-partum (P < 0.005), fewer cells were present at day 150 post-partum than in the adult (P < 0.05). Peyer's patches were not observed in the intestines up to day 73 post-partum. However, both T and B cells were observed in the intestinal lymph nodes. Although the T lymphocytes at weaning showed a proliferative response, the response was not as great as that observed in the adult possum. Thus, the immune system of the possum is not fully developed at weaning but continues its development after pouch life.
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