Doctoral Dissertations

Date of Award

12-1999

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Animal Science

Major Professor

Charmi Mendis-Handagama

Committee Members

J. Godkin, T. Chen, H. Eiler, R. Henry

Abstract

Changes in the rat testis interstitium from birth to adulthood were studied using Sprague Dawley rats of 1, 7, 14, 21, 28, 40, 60 and 90 days of age. The results revealed that fetal Leydig cells (FLC) are present in rat testes from birth to 90 days, and they were the only steroidogenic cells in the testis interstitium at day 1 and 7. Except for FLC, all other interstitial cell numbers and volumes increased from birth to 90 days. The average volume of a FLC and the absolute volume of FLC per testis were similar at all ages except at day 21, where lower values were observed for both parameters. FLC number per testis remained constant from birth through 90 days. These observations suggested that the significance of FLC in the neonatal-prepubertal rat testis is to produce testosterone to activate the hypothalamo-hypophyseal-testicular axis for the continued development of the male reproductive system. Adult Leydig cells (ALC) were the abundant Leydig cell type by number and absolute volume per testis from day 14 onwards. The absolute numbers of ALC and mesenchymal cells per testis increased linearly from birth to 90 days, with a slope ratio of 2:1, respectively, indicating that the rate of production of Leydig cells is two fold greater than that of mesenchymal cells in the postnatal rat testis through 90 days. In addition, it was discovered that the mesenchymal cells are an active cell population during testis development and their numbers do not decrease but increase with Leydig cell differentiation and testicular growth up to sexual maturity (90 days).

Leydig cells in the adult rat testis differentiate postnatally from spindle-shaped cells in the testis interstitium during the neonatal-prepubertal period. However, which spindle-shaped cell type(s) is the precursor for Leydig cells and the stimulus for the initiation of their differentiation are two unresolved issues that need to be addressed. The results of the second study of this dissertation revealed that out of all spindle-shaped cell types in the rat testis interstitium, only the peritubular mesenchymal cells differentiate into Leydig cells and the onset is on postnatal day 11. The three steroidogenic enzymes tested, i.e., 3β-hydroxysteroid dehydrogenase (3β-HSD), cytochrome P450 side-chain cleavage (P450scc) and cytochrome P450 17α-hydroxylase (P450cl7) were expressed simultaneously in these cells and their numbers increased significantly thereafter. LH receptors were gained in these cells later in the development, i.e., on postnatal day 12 that is after acquiring the steroidogenic enzyme activity. It was also discovered that daily injection of LH caused a significant increase in the abundance of fetal Leydig cells, however, the onset of mesenchymal cell differentiation was delayed until postnatal day 16. Hypothyroidism induced via addition of propylthiouracil (0.1%) to mother's drinking water resulted in an arrest of mesenchymal cell differentiation in the neonatal rat as previously observed. By contrast in triiodothyronine treated rats, the onset of mesenchymal cell differentiation was advanced to postnatal day 9. These findings suggest that thyroid hormone has a regulatory role on the onset of the differentiation of ALC.

The effect of hyperthyroidism on the testis interstitium during prepuberty was further investigated in the 3rd study using Sprague Dawley rats. The results demonstrated that hyperthyroidism stimulates premature hypotrophy of fetal Leydig cells and early differentiation of increased numbers of mesenchymal cells to adult Leydig cells in the prepubertal rat testis, further supporting the view that thyroid hormone has a regulatory role in initiating mesenchymal cell differentiation into adult Leydig cells in the prepubertal rat testis.

Leydig cell differentiation in the postnatal rat testis was further investigated in adult rats following ethane dimethane sulphonate (EDS) treatment which kills Leydig cells within 2 days of administration. The findings showed that the process of Leydig cell differentiation in the testicular interstitium following EDS treatment is very similar to what is observed in the prepubertal rat testis during adult Leydig cell differentiation, although it occurs in the absence of increase in testis weights associated with testicular growth. In addition, Leydig cell number per testis continued to increase beyond untreated control values through day 60 postEDS.

Effect of triiodothyronine on Leydig cell regeneration in the adult rat testis after ethane dimethane sulphonate (EDS) treatment was investigated next. It was discovered that hypothyroidism inhibits Leydig cell regeneration and hyperthyroidism resulted in more precursor cell differentiation into Leydig cells following EDS treatment. These observations suggest that the process of Leydig cell differentiation is possibly similar in the adult and prepubertal rat testes.

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