The mesencephalic V nucleus in anurans. II. The influence of thyroid hormone on cell size and cell number

The molecular basis of thyroid hormone-dependent central nervous system remodeling during amphibian metamorphosis

Tadpole metamorphosis involves a coordinated series of changes in virtually every tissue of the body. This developmental process is induced by the single morphogen, thyroid hormone (TH). The amphibian central nervous system (CNS) is a primary target for TH, and it undergoes dramatic morphological and cytoarchitectural changes in response to the hormone. TH acts by regulating gene expression and its actions in metamorphosis are thought to result from its ability to induce tissue-specific genetic programs. Receptors for TH are ligand-dependent transcription factors whose mRNA expression is upregulated by TH during metamorphosis (receptor autoinduction). Studies on the tadpole CNS have identified four general classes of early TH response genes. These genes code for: (1) transcription factors, that are likely to be required for the expression of downstream genes (i.e. secondary response genes), (2) cellular enzymes, which carry out hormone conversions, energy transformations and may possibly mediate extranuclear effects of TH on neural cells, (3) cytoskeletal elements required for axonal development, and (4) secreted signaling molecules that control the production of TH. Recent studies suggest a critical, evolutionarily conserved role for the TH-induced transcription factor genes in controling neural cell proliferation and differentiation.

Leg development in a frog without a tadpole (Eleutherodactylus coqui)

Direct developing frogs, like Eleutherodactylus coqui, have deleted the tadpole from their life history. Limb buds appear early and develop continuously through embryonic life. The capacity for autonomous development of E. coqui limb buds was tested by explanation and transplantation. When limb buds were explanted to in vitro culture, they progressed for a few stages and then arrested. When limb buds were transplanted to embryos of a typical tadpole species (Rana pipiens), the buds formed legs with knees, digits, and some cartilage, but they did not elongate. Since limb buds transplanted to various sites on E. coqui embryos tended to develop completely, the limited development of the explants in vitro and the transplants to R. pipiens suggests the presence of a systemic factor in the embryo involved in the normal continuous growth of E. coqui legs. Attempts to demonstrate a role for thyroid hormone have thus far been unsuccessful.

Mesencephalic fifth nucleus cell responses to thyroid hormone: one population or two?

Hypophysectomized Rana pipiens tadpoles 3-6 months old were placed in dl-thyroxine (T4) solutions of 4 to 200 micrograms/l for 1-18 days and fixed 1 day after removal from the hormone solution. Exposure times varied inversely with T4 concentration. Mesencephalic fifth nucleus (M-V) cells were counted on both sides, and cell and nuclear sizes were drawn and measured for each tadpole. Changes in M-V cell characteristics correlated well with T4 exposure times and concentrations, as did changes in external tadpole morphology. All T4 concentrations were effective. Cells and nuclei were distinctly larger in all T4-treated groups. The changes were greatest for cell sizes, less for nuclear dimensions, and still less for nucleo-cytoplasmic ratios. Significant changes were seen for minimum and maximum sizes as well as for the mean values. The greatest mean changes were seen at dosages of 50 micrograms/l for 7-9 days. Mean M-V cell numbers are significantly smaller in hypophysectomized tadpoles than in controls (about 420 vs. 650). Thyroid hormone treatment of the hypophysectomized animals abolishes much of the deficit, though M-V cell deaths at the higher concentrations and longer treatment times reduce the apparent increase in numbers. Are the additional cells obtained through cell division, or do they represent a preexisting subpopulation of prospective M-V cells that require stimulation by thyroid hormone for their full differentiation?

Disappearance of Rohon-Beard neurons from the spinal cord of larval Xenopus laevis

Rohon-Beard neurons are primary sensory cells located in the spinal cord of embryonic lower vertebrates. The kinetics of their normal, gradual, but complete disappearance in Xenopus tadpoles has been followed. Levels of acid phosphatase activity, a common histochemical correlate of cell death, were assayed and found to increase at the time of onset of disappearance of Rohon-Beard cells. Ultrastructural examination revealed the presence of numerous secondary lysosomes, swelling of endoplasmic reticulum and mitochondria, and a decrease in nuclear density. The disappearance of Rohon-Beard neurons may be attributed to autophagic cell death involving lysosomal acid hydrolases. This process begins only a few days after the maturation of voltage- and neurotransmitter-dependent membrane conductances and the electrical uncoupling of these neurons. The loss of Rohon-Beard neurons in embryos whose development was arrested by crowding was appropriate for the developmental stage of the animals rather than their chronological age.

Neuronal development in the trigeminal mesencephalic nucleus of the duck under normal and hypothyroid states: I. A light microscopic morphometric analysis

Light microscopic morphometric procedures were used in order to examine the effects of propylthiouracil (PTU) on the development of the mesencephalic nucleus of the trigeminal nerve in the duck. A single vascular injection of a 0.2% solution of PTU was administered at a dosage of 2 microliter/gm embryo weight on embryonic day nine (E9). Control embryos received a similar dose of Ringer's solution. The following parameters of cytodifferentiation of cells of the mesencephalic nucleus of V were studied: somal area profiles, nuclear area, and nuclear cytoplasmic ratios. In addition, the frequency of beak clapping was recorded from E16. Significant differences were observed in somal area profiles in the experimental group at E16 and E18 and in nuclear area profiles from E16 through hatching. Beak activity in the experimental embryos was drastically reduced. It is concluded that PTU induces a retardation in the differentiation of cells of the mesencephalic nucleus of V which may lead to behavior deficits as evidenced by reduction of beak activity. These observations provide a basis for the study of interactions between thyroid hormone and specific neuronal systems in the emergence of an adaptive function.

Effects of thyroid hormones on the hippocampal glial cells of fasting rats

The effects of the oral administration of thyroid hormones on the hippocampal glial cells in fasting or fed adult rats were studied. The hormone treatment and/or fasting were done during two periods of five days interpolated by ten days without any hormone treatment or fasting. Prominent histological changes and increased number of glial cells were observed in the hippocampus of rats when the thyroid hormone administration was associated with fasting.

Growth and death of cells of the mesencephalic fifth nucleus in Rana pipiens larvae

Positions, numbers, and nuclear sizes of mesencephalic fifth nucleus M-V) cells were determined in Rana pipiens larvae in stages XIII through the end of metamorphosis at stage XXV, in a few just postmetamorphic juveniles, and in adults. M-V cells are found throughout the tectum, with the greatest concentrations just anterior to each optic ventricle, and along the medioanterior aspect of each ventricle. Some cells lie below these levels, adjacent to the ependyma of the expanded aqueduct, the mesencephalic ventricle; a very few cells occur in the anterior medullary velum and in the adjacent anterior cerebellum. Up to 879 M-V cells were seen in single animals in stages XIV-XVIII, with mean values near 650 cells. Pyknotic M-V cells appear in all stages from XVII through XXV, and possibly in the youngest juveniles. Peak rates of cell loss occur at stage XXI, some 4-5 days after the forelimbs have emerged. Mean postmetamorphic cell numbers are about 350, suggesting an average M-V cell loss of 300 per animal. Nuclear cross-sectional areas are 72 microns2 at stage XIII, slightly above the size of such cells in hypophysectomized tadpoles. Nuclear growth is progressive to 102-111 microns2 at stages XXI-XXV. Adult sizes increase to a mean of 121 microns2. Hypophysectomized animals display pyknotic cells only after the tadpoles have been strongly stimulated by thyroid hormone, as demonstrated by significant growth of the legs and of the M-V cells themselves.