Stages of the rat thymic medulla development in foetal period

Development of thymic medulla was examined on consecutive gestational days (GD) in Wistar rats. Medullary thymic epithelial cells (TEC) were identified by immunocytochemical localisation of neuron-specific enolase (NSE). Organisation of thymic medullary architecture was determined by interaction of thymocytes with NSE-positive TEC, that led to formation of lymphoepithelial complexes (GD 19), in which the cells exhibited proliferative activity or traits of apoptosis. The studies indicated that differentiation events and organisation of thymic medulla require stage-specific interactions between TEC and thymocytes.     

Histogenesis of the taste buds of the vallate papilla in the in the rat in the postnatal stages of development

The developing taste buds of vallate papillae were studied with electron microscope in rats during the first 7 days after birth. Two types of cells--light and dark--are identified in the taste buds of a one day old animal. The apical parts of dark cells are characterized by numerous dark granules. A distinguishing feature of light cells is the presence of synaptic contacts with afferent intragemmal nerves. On the 4th day of development on the top of the apical parts of the cell, a microvillar apparatus is seen to form, which does not yet communicate with the oral cavity. On the 7th day, basal cells appear in the taste buds. Some of these cells are seen mitotically dividing. The differentiated microvillar apparatus now communicates with oral cavity. The structure of the taste buds is getting similar to that in the adults. The structural and functional peculiarities of the developing taste buds are discussed in association with the period of ontogenesis under consideration.     

Ultrastructural localisation of NADPH-diaphorase in the chick thymic medulla

Nicotinamide-adenine-dinucleotide phosphate-diaphorase positive cells in the chick thymus were studied at the electron-microscopic level. The formazan, a marker for the enzyme nitric oxide synthase, labelled cystic, undifferentiated, endocrine-like and myoid cells in the medulla. Some lymphoid and reticulo-epithelial cells were also lightly labelled. The reaction product was predominantly bound to the membranes of the endoplasmic reticulum in all the cells labelled and also to the nuclear envelope and outer membrane of mitochondria. The Golgi apparatus and the plasma membrane were free of the reaction product.     

Heat-induced expression of albumin during early stages of rat embryo development.

An examination of heat-induced expression of proteins in tissues from adult and embryonic liver in rats shows that albumin, which is constitutively expressed in adult liver and is not synthesized in embryos before 16 days of gestation, appears in liver cells at earlier stages of development upon heat shock. On the basis of available evidence for the expression of heat shock proteins at distinct stages of development and on the basis of our findings, it may be argued that there could be common molecular events taking place during development and as a result of heat shock. We suggest also that one of the consequences of heat shock could be an internal change of pH within the cell which, in turn, might trigger alterations in gene expression.     

Human T cell antigen expression during the early stages of fetal thymic maturation

Human thymus tissue was examined from 7 wk of gestation through birth for the expression of antigens reacting with a panel of anti-T cell monoclonal antibodies. Additionally, the reactivities of reagents against the transferrin receptor, against leukocytes, against low m. w. keratins, and against major histocompatibility complex antigens were studied on human fetal thymic tissue. Frozen tissue sections were evaluated by using indirect immunofluorescence assays. At 7 wk of gestation, no lymphoid cells were identified within the epithelial thymic rudiment; however, lymphoid cells reacting with both antibody 3A1, a pan T cell marker, and antibody T200, a pan leukocyte reagent, were identified in perithymic mesenchyme. After lymphoid colonization of the thymic rudiment at 10 wk of fetal gestation, fetal thymic tissue reacted with antibodies T1, T4, and T8. At 12 wk of gestation, antibodies T3, T6, A1G3 (anti-p80, a marker of mature thymocytes), and 35.1 (anti-E rosette receptor) all reacted with thymic tissue. Our findings indicate that T cell antigens were acquired sequentially on thymocytes at discrete stages during the first trimester of human fetal development. The 3A1 antigen was present on fetal lymphocytes before lymphoid cell colonization of thymic epithelium, suggesting that passage through the thymus was not required for the expression of the 3A1 antigen by T cell precursors. The appearance of mature T cell antigens, T3 and p80, on thymocytes by 12 wk of gestation implies that the T cell antigen repertoire may be established in the thymus during the first trimester. Thus, a critical period of T cell maturation appears to occur between 7 and 12 wk of human fetal gestation.     

Identification of early stages of T lymphocyte development in the thymus cortex and medulla

Thymocyte subpopulations with a phenotype suggesting they are early stages of T cell development in the adult mouse thymus were characterized and isolated by using multiparameter flow cytometry and sorting, in conjunction with selective killing with antibody and complement (C). The intrathymic localization of these subpopulations was assessed by dipping the thymus in fluorescent dyes to selectively label outer-cortical cells. The main phenotypic markers used were sensitivity to C-mediated lysis by the monoclonal antibody B2A2 (which spares most prothymocytes but kills most thymocytes), the expression of the T cell lineage specific markers Ly-2 and L3T4, and the levels of the common T cell antigens Ly-1 and Thy-1. A preliminary selection for cells lacking Ly-2 and L3T4, or resistant to B2A2 and C, produced a population of large cells, only 5% of all thymocytes and distinct from the typical cortical blast cells. This population of putative early thymocytes was itself heterogeneous, consisting of eight subpopulations separable by phenotype and intrathymic localization. One group of two subpopulations (B2A2-, Ly-1++, Thy-1+ and either Ly-2+ L3T4- or Ly-2- L3T4+) appeared to be of medullary location, and their phenotype suggested they could have been early members of the medullary lineages. Another group of two subpopulations (B2A2-, Ly-1++, Thy-1-, Ly-2-, L3T4- and B2A2-, Ly-1++, Thy-1+, Ly-2- L3T4-) did not show a clear localization pattern and may have represented cells in an earlier stage of transition to medullary phenotype and location. A quite different group of three subpopulations (B2A2++, Ly-1-, Thy-1-, Ly-2- L3T4-; B2A2++, Ly-1-, Thy-1+, Ly-2-, L3T4-; and B2A2++, Ly-1+, Thy-1++, Ly-2- L3T4-) was concentrated in the outer cortex and seemed to represent a series of stages of a cortical pathway, before the typical cortical blast cells. Finally, a very minor subset (0.2% of thymocytes), lacking all these markers, was concentrated in the outer cortex; this fifth group had the phenotype expected of the earliest intrathymic precursor cells. The results suggest that the separate developmental streams of cortical and medullary thymocytes may be traced back, via these minor early blast subpopulations, to common precursor cells in the outer cortex.     

Changes in the adrenal medulla of the rat during immobilization stress

A combined histofluorescent, light optic and electron microscopic investigation has been performed to study the medulla of the adrenals in intact rats (August strain) and immediately after immobilization for 30 h and 24 h after. In the intact animals heteromorphism of the medulla is demonstrated; this depends on the fact that adrenocytes are at different stages of the secretory cycle. Immobilization for 30 h results in synchronization of secretion; this is determined by adaptation of the adrenal system to immobilization. One day after immobilization restoration of heteromorphism in chromaffinocytes is demonstrated. The changes described are compensatory-adaptive reactions of the adrenal medulla to the stress in the animals survived.     

CCR7 directs the migration of thymocytes into the thymic medulla

Developing thymocytes migrate from the cortex to the medulla of the thymus as a consequence of positive selection. This migration is likely to be essential for tolerance because it allows the developing cells to move into an environment that is optimal for negative selection. Guidance mechanisms that draw positively selected thymocytes into the medulla have not been clarified, but several studies have implicated chemokines in the process. CCR7, the receptor for the medullary chemokines CCL19 and CCL21, is induced on thymocytes during their positive selection. In this study we show that premature expression of CCR7 repositions CD4(+)CD8(+) double-positive cells into the medulla of transgenic mice. This repositioning of the thymocytes is accompanied by impairment of their development. The data show the involvement of CCR7 in medullary migration and emphasize the importance of proper thymocyte positioning for efficient T cell development.     

Regulation of chromatin structure during thymic T cell development

Development is the process whereby a multipotent cell gives rise, through series of divisions, to progeny with successively restricted potentials. During T cell development, the process begins with a multipotent hematopoietic stem cell (HSC) in the bone marrow, moves to the thymus where early T cells or thymocytes pass through signal‐initiated developmental checkpoints, and ends in the periphery where mature T cells reside. At each step along this developmental pathway, T lymphocyte progenitors must be able to turn genes on and off, creating a specialized program of gene expression, to allow further development. How is gene expression coordinated? This review will summarize what has been learned about the function of chromatin structure in generating a “blueprint” of gene expression during T cell development. This will include discussion of mechanisms of chromatin remodeling, histone modification, and heritable gene silencing. In many cases, these processes are carried out by multi‐protein complexes whose components are largely ubiquitously expressed. The spatial and temporal specificity of these complexes is contributed by sequence specific DNA binding factors, some of which are cell type restricted in their expression. This review will summarize research underway to identify these key genetic “targeters.” Taken together, the research reviewed here provides a glimpse into the importance of regulation of chromatin structure in T cell development and the “players” involved. © 2005 Wiley‐Liss, Inc.     

Sexual behavior and sexual orientation of the female rat after hormonal treatment during various stages of development

The amount of circulating sex steroids during Postnatal Days 30-90 was varied in normally developed and in androgenized female rats. The influence of these manipulations on sexual behavior and sexual orientation was investigated. Normally developed or neonatally androgenized females were ovariectomized and implanted with estradiol through Postnatal Days 30-90 or sham-implanted. The remaining subjects were left intact during that period. The hormonal condition during Postnatal Days 30-90 significantly affected the behavior of normally developed females, but affected the behavior of neonatally androgenized females only to minor extent. Estrogen implants in normally developed females enhanced masculine sexual responses and induced a female-directed sexual orientation. Feminine sexual responses were unaffected by this treatment. Sham-implanted, normally developed females showed a male-directed sexual orientation and fewer masculine sexual responses than subjects which were left intact during Postnatal Days 30-90. Neonatal androgen treatment in general resulted in elevated levels of masculine Neonatal androgen treatment in general resulted in elevated levels of masculine sexual responses, inhibited feminine sexual behavior, and facilitated a female-directed sexual orientation.     

Hormone secretion by euthyroid and hypothyroid rat ovaries during the early stages of hCG-induced ovarian cyst development

This study was undertaken to examine ovarian steroid production during the early stages of hCG-induced ovarian cyst formation in the hypothyroid rat. Rats were placed into two groups with one group made hypothyroid by adding thiouracil to their diet. After 10 days, each group was divided into two subgroups with one subgroup receiving daily injections of hCG for 2 days and the other subgroup receiving saline. On the morning of Day 13, ovaries were removed and incubated for 2 hr. No significant difference in progesterone secretion was observed. However, ovaries from hypothyroid, hCG-treated rats secreted significantly more testosterone and estradiol than ovaries from vehicle-treated, hypothyroid rats and euthyroid, hCG-treated rats. In a second experiment, ovaries from euthyroid and hypothyroid rats treated with hCG were incubated in medium supplemented with 100 nM androstenedione and 0 or 100 ng FSH/ml. FSH failed to affect progesterone, testosterone, and estradiol secretions by ovaries from euthyroid, hCG-treated rats. In contrast, FSH significantly enhanced testosterone and estradiol secretion by ovaries from hypothyroid, hCG-treated rats. These results support the hypothesis that increased levels of testosterone and estradiol secretion have a central role in the induction of polycystic ovaries by hCG in the hypothyroid rat.