A. Kent Christensen

Shape of large bound polysomes in cultured fibroblasts and thyroid epithelial cells.

Large bound polysomes were observed by conventional electron microscopy in surface or en face views of the rough endoplasmic reticulum (RER) in two cultured cell types. Cultured thyroid follicular epithelial cells and dermal fibroblasts, both from rats, were prepared for electron microscopy. Ultrathin sections were cut in the plane of the flattened cells to maximize the incidence of RER surface views. Some observations were also made on tissue sections of rat thyroid. Most of the large, RER‐bound polysomes in both cell types appeared as two parallel rows of ribosome, thus resembling the shape of long hairpins, although probably closed at both ends. The two parallel rows of ribosomes were about 14 nm apart, and the center‐to‐center distance between ribosomes in the strands averaged 25 nm. Most of the large bound polysomes in thyroid epithelial cells were presumably making thyroglobulin subunits (330 kDa), while a majority of those in the fibroblasts were probably making prepro‐α chains of collagen I (150 kDa). It was not possible in this material to see complete large polysomes, because their size usually caused them to extend out of the plane of section. In addition to the hairpin polysomes, there were smaller numbers of other forms. A characteristic large spiral polysome was seen occasionally in both cell types and contained as many as 31 ribosomes. One or two dense particles were sometimes seen in the center of spiral or circular polysomes. The consistent hairpin shape of most large bound polysomes observed in this study suggests that their shape is quite stable. Anat Rec 255:116–129, 1999.

LH RECEPTORS AND STEROIDOGENESIS IN DISTINCT POPULATIONS OF LEYDIG CELLS

Luteinizing hormone (LH) has been shown to be the only hormone essential for maintenance of testicular testosterone pr0duction.l LH mediates its effect on testicular steroidogenesis by binding to high affinity receptors in the plasma membrane of Leydig cells. In recent years, numerous investigators have reported that in vivo administration of LH or human chorionic gonadotropin (hCG) results in the reduction of the number of testicular LH receptors1-’ accompanied by a decrease in testosterone production 4, 6+ in response to subsequent stimulation by LH or hCG. Studies from our laboratory, however, have demonstrated that changes in LH receptor number may be dissociated from changes in LH-stimulated testosterone production following in vivo treatment with gonadotropins.1 We observed that only a single high dose of LH, 100 pg or greater, caused a decrease in testicular responsiveness, whereas twice daily injections of LH for six days or longer were accompanied by a time-related increase in in vivo testicular testosterone secretion in response to a subsequent stimulatory dose of LH. The LH-induced loss of testicular LH receptors was similar in rats that received a single high dose of LH and in rats that received twice daily injections. These observations indicate that different modes of in vivo LH treatment have markedly different effects on testicular responsiveness to subsequent LH/hCG stimulation. Interpretation of these results, however, is complicated by the possibility that chronic pituitary hormone treatment may change ( 1 ) sensitivity of the Leydig cells to subsequent LH stimulation, ( 2 ) secretion or clearance rate of testosterone, and/or ( 3 ) total number of Leydig cells. To distinguish among these possibilities, changes in Leydig cell LH receptor concentration and responsiveness to LH were assessed in the same cells after different modes of in vivo hormone treatment. Using Metrizamide gradient centrifugation to separate dispersed testicular cells, we found evidence for two distinct populations of Leydig cells.* Leydig cells of these two populations contain similar numbers of LH receptor sites but exhibit marked differences in testosterone production in response to in vitro

Fine Structure of Testicular Interstitial Cells in Humans

It is now well established that the interstitial or Leydig cells are the principal site of androgen production in the mammalian testis. In rats, the interstitial tissue can be separated from the seminiferous tubules and shows considerably more activity than the tubules in synthesizing androgen from cholesterol in vitro (1, 2). Proof that the interstitial cells are indeed the source of this activity within the interstitial tissue comes from histochemical studies in which the reaction product of 3s -hydroxysteroid dehydrogenase appears mainly in these cells. This approach has been applied to a variety of vertebrate species with consistent results, although it appears somewhat more difficult to demonstrate in the human testis (3).

The liver of the slender salamander Batrachoseps attenuatus

SummaryThe structure of the crystalline inclusions found in Batrachoseps liver cells is described and it is shown that the most symmetric unit cell upon which the crystal lattice is built is a face-centered cube. Taking into consideration the physical properties of a face-centered cubic structure, an attempt is made to determine the nature of the macromolecules that comprise the crystal. It is concluded on the basis of available evidence that the macromolecules probably represent serum lipoproteins. The intracellular synthesis of the crystals and the possible functions they may subserve in the animal are discussed. A comparison is made between the crystals and granules in rat hepatocytes discussed by Bruni and Porter (1965).

A History of Leydig Cell Research

Franz Leydig first described the testicular cells in 1850 that now bear his name. For the next 50 yr after their discovery, Leydig cells were the subject of further studies by light microscopy, and diverse speculations were offered about their possible function. In 1903, Pol Bouin and Paul Ancel provided the first substantial evidence that Leydig cells constituted an endocrine gland controlling male secondary sexual characteristics. Their evidence seemed compelling at the time, but was necessarily circumstantial, because there was no direct proof that Leydig cells produced a male hormone. Over subsequent decades, workers found additional evidence that these cells had an endocrine function, but there were also other findings that cast doubt on the hypothesis, and increasing skepticism developed about the earlier evidence. By the late 1920s, many influential reproductive biologists suspected that the seminiferous tubules were the actual source of male hormone. During the 1930s, the male hormone was shown to be testosterone, its endocrine actions were studied extensively, and the role of the pituitary in regulating testicular function was demonstrated. From the 1930s through the 1950s, Leydig cells came back into favor as endocrine cells, although some uncertainty persisted and there was still no direct evidence that Leydig cells produced androgen. Finally, direct evidence came from histochemistry in 1958 and from biochemistry in 1965.

Negatively-stained polysomes on rough microsome vesicles viewed by electron microscopy : further evidence regarding the orientation of attached ribosomes

Rough microsomes, derived from rough endoplasmic reticulum of rat liver, were studied by electron microscopy after negative staining, to seek further information about the orientation of ribosomal small and large subunits in bound polysomes. Rough microsomal vesicles were fixed with 2% formaldehyde, centrifuged onto electron-microscopic grid membranes, and were then negatively-stained with 2% phosphotungstic acid. In these preparations, viewed with the electron microscope, flattened rough microsomal vesicles with bound polysomes were sometimes discernible, and the individual ribosomes in the polysomes occasionally showed small and large subunits. The small subunits were uniformly oriented toward the inside of the polysomal curve. The large and small subunits appeared to be alongside one another on the membrane, consistent with the orientation that has been described by Unwin and his co-workers. The boundary between the small and large subunits occurred at approximately the same level in the ribosome where inter-ribosomal strands have been described previously in surface views of bound polysomes in positively-stained electron-microscopic tissue sections. This further confirms the identity of the strands as messenger RNA.

Autoradiographic Localization of FSH-Binding Sites on Sertoli Cells and Spermatogonia in Testes from Hypophysectomized Rats

It is now well established that follicle-stimulating hormone (FSH) acts primarily on the seminiferous tubule of the mammalian testis, and that receptors for this action are located on tubule plasma membranes. Specific FSH-binding sites have been demonstrated in isolated seminiferous tubules from immature and mature rats, and it has been shown that most of the binding activity resides in the plasma membrane fraction (Means and Vaitukaitis, 1972; Means, 1973). The biochemical events that are initiated by the binding of FSH to its receptor are also well characterized. Incubation of FSH with either seminiferous tubules (Dorrington et al., 1972) or plasma membranes isolated from tubules (Means, 1975) results in an elevation of cyclic AMP (cAMP) levels that is hormone-and tissue-specific. The cAMP seems to act as an intracellular messenger, causing activation of cAMP-dependent protein kinase and subsequent transcriptional and translational events (Means, 1975).