Postnatal Development of the Thyroid Gland in the Opossum (<i>Didelphis virginiana</i>)

Abstract

Thyroid lobes of the newborn opossum consist of small epithelial cords or plates, the constitutent cells of which are joined by tight junctions and surround a minute lumen that courses throughout the length of each cord. Occasional cells, not directly associated with the lumina of the cords, show central nuclei and clusters of small electron-dense granules: these may represent parafollicular cells at an early state of development. By the end of the 1st week after birth, parafollicular cells are well established and appear as groups between the epithelial cords and as scattered individual cells within the cords. Growth of the thyroid occurs as the cords continue to elongate and branch. 11 days after birth, thyroid development in the opossum corresponds morphologically to that of eutherians just prior to birth. Primary follicles are present near the periphery of the thyroid lobes, having formed from evaginations of the epithelial cords. There is evidence of synthetic activity by the epithelial cells and colloid is present. Secondary follicles form as a result of evaginations from the epithelium of the primary follicles. At the end of the 2nd postnatal week, epithelial cords are no longer a prominent feature of the opossum thyroid, and the organ is now characterized by numerous, small colloid-filled follicles. As development progresses, the follicles increase in size and number and, by 35 days after birth, the thyroid shows adult features.

Thyroid development in the marsupial bandicoot, Isoodon macrourus

“…In the tammar wallaby, Macropus eugenii, the initiation of thyroid activation was closely related to the development of thermoregulatory mechanisms and began about 140 days postparturn, which was in the latter third of pouch life (Setchell, 1974). These studies of development of the bandicoot, tammar wallaby, brushtailed possum, and opossum (Setchell, 1974;Thorburn and Hopkins, 1973;Krause and Cutts, 1983) suggest that thyroid activation in the marsupial occurs during the latter half of pouch life.…”

“…At birth the thyroid gland of the bandicoot, Zsoodon macrourus, like that of the opossum, Didelphis virginiana (Krause and Cutts, 1983) and the tammar wallaby, Macropus eugenii (Setchell, 19741, is at a stage of development comparable to the prefollicular stage of thyroid development described in several other mammalian species (Fisher and Klein, 1981).…”

“…This is indicative of a high degree of follicular cell endocytotic activity, since hyalinization and vacuolation of colloid occurs when the thyroid is stimulated (Leatherland and Ronald, 1979). Krause and Cutts (1983) observed that the ratio of thyroid weight (g) to body weight (100 g) was greatest in the opossum, Didelphis uirginiana, during pouch life, reducing to 0.0075 gI100 g in the adult. A similar ratio was obtained for the adult bandicoot.…”

“…Thyroid activation in the rat occurs postnatally (Childs, 1983) while in the mouse the thyroid is activat,ed shortly before birth (Van Heyningen, 1961). In most marsupials studied thus far thyroid function begins during the latter half of pouch life (Hopkins, 1972;Setchell, 1974;Krause and Cutts, 1983).The bandicoot, Zsoodon macrourus, is a small marsupial with a gestation length of 12.5 days and a lactation period of 59 days (Lyne, 1974;Gemmell, 1982;Gemmell and Johnston, 1985). During the period of lactation, the young increases in weight, becomes pigmented, and develops eyes and a covering of hair before finally leaving the pouch.…”

Opossum adrenal medulla: I. Postnatal development and normal anatomy

“…During the 60-to 70-day period following birth, the immature pouch-young undergo late embryonic and fetal development in the mother's marsupium. This has been shown for a number of organ systems, including endocrine organs such as the pituitary (Sherman and Dunkerley, 1982) and thyroid (Krause and Cutts, 1983). Thus, the neonatal opossum is accessible and potentially can be used in controlled, quantitative, experimental investigations of development.…”