Changes in the fine structure of the human testicular interstitial cells after treatment with human gonadotrophins

Summary Study of the fine structure of the human interstitial cells after prolonged stimulation with human gonadotrophin reveals a striking increase in the quantity of the agranular endoplasmic reticulum. This is accompanied by an increase in the number of mitochondria which exhibit more extensive cristae, collections of intramitochondrial lipid and aggregations of electron-dense granular deposits. A rise is also evident in the number of lipofuscin pigment deposits and granular membrane-bounded bodies, both of which exhibit acid phosphatase activity. These changes after gonadotrophic stimulation are discussed in relation to steroid biosynthesis.In the pretreatment biopsies of these patients aged between 25–35 years, some interstitial cells contain intranuclear crystals which exhibit a hexagonal structure. The relationship of these intranuclear crystals to the cytoplasmic crystals of Reinke is discussed.The author is indebted to Dr. J. W. Johnstone and Dr. A. Long for the human material used in this study. Thanks are also due to Dr. H. P. Taft for helpful suggestions in the management of these patients, to Professor B. Hudson for the estimations of plasma testosterone and to Dr. J. B. Brown for the supply of human pituitary gonadotrophin and the estimations of urinary oestrogens. The technical help of Mr. T. Mezciems and the photographic assistance of Mr. J. S. Simmons F. R. P. S. and Miss S. Flett is gratefully acknowledged.     

Studies on the receptors in Ciona intestinalis

Summary A photoreceptor type structure not previously described has been found in the dorsal wall of the cerebral vesicle of the tadpole larva of Ciona intestinalis. The membranes of this receptor are organised as tubules some 60–100 nm in diameter and up to 1.5 m long. The tubules are confined in bundles about 1.5 m in diameter, which extend from the cell surface into the cavity of the cerebral vesicle. These tubules are similar to those in the rhabdomeric type of photoreceptor. However, in the cells from which the tubule processes arise are structures typical of the bases of cilia, and found in ciliary type photoreceptors.I should like to thank Professor J. Z. Young, F. R. S. for his continuing encouragement and help, and Dr. R. Bellairs for the use of electron microscope facilities. Mr. R. Moss and Mrs. J. Hamilton gave excellent technical assistance.     

Biogenesis of the crystalloid endoplasmic reticulum in UT-1 cells: evidence that newly formed endoplasmic reticulum emerges from the nuclear envelope

The crystalloid endoplasmic reticulum (ER), a specialized smooth ER of the compactin-resistant UT-1 cell, is composed of multiple membrane tubules packed together in a hexagonal pattern. This membrane contains large amounts of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase, an integral membrane protein that enzymatically regulates endogenous cholesterol biosynthesis. Using morphological and immunocytochemical techniques, we have traced the sequence of events in the biogenesis of this ER when compactin-withdrawn UT-1 cells, which do not have a crystalloid ER, are incubated in the presence of compactin. After 15 h of incubation in the presence of compactin, many cells had profiles of ER cisternae that were juxtaposed to the nuclear envelope and studded with ribosomes on their outer membrane. Both the outer nuclear membrane and the ER membrane contained HMG CoA reductase; however, there was little or no detectable enzyme in rough ER that was free in the cytoplasm. With longer times of incubation in the presence of compactin, these cells had lamellar stacks of smooth ER next to the nuclear envelope that contained HMG CoA reductase. Coordinate with the appearance of the smooth ER, crystalloid ER appeared in the same cell. Often regions of continuity were found between the membrane of the smooth ER and the membrane of the crystalloid ER tubules. These studies suggest that HMG CoA reductase is synthesized along the outer nuclear membrane and in response to increased enzyme synthesis, a membrane emerges from the outer nuclear membrane as smooth ER cisternae, which then transforms into crystalloid ER tubules.     

Crystalloid inclusions in the Sertoli cell of the koala,Phascolarctos cinereus (Marsupialia)

Summary Crystalloid inclusions are a common feature in the basal region of Sertoli cells in the koala, Phascolarctos cinereus. Generally located near the nucleus, they are non membrane-bounded, slender rectangular structures composed of tubules which are orientated at right angles to the long axis of the crystalloid and regularly arranged in rows parallel to this long axis. The tubules in adjacent rows are offset from one another at definite angles and extensively interconnected by filaments. Neither the composition nor function of the crystalloids has been determined, but their association with tonofilaments and the presence of ribosomes in the vicinity suggests that they are most likely proteinaceous.The authors would like to thank Mr. D. Harbrow, Mr. S. Brown and Ms. B. Canty for technical assistance; the Queensland National Parks and Wildlife Service and the Victorian Ministry of Conservation (Fisheries and Wildlife Division) for providing permits to work on this protected species; the staff of the Lone Pine Koala Sanctuary, Brisbane, for their assistance in obtaining animals. This work was supported by a grant from the Australian Research Grants Committee, Number DI-77/15525     

Occurrence,composition, and structure of microbody crystalloids in charophycean gametangial sheath cells

Summary An ultrastructural study was made of the cellular sheaths surrounding the sexual organs of five species of algae in the three genera ofCharophyceae: Nitella flexilis, N. mirabilis, Chara brattnii, Tolypella boldii andT. intricata. Microbodies similar in appearance, with crystalline nucleoids, were present in the sheath cells of all five species. The microbodies resembled in size and topographical associations those of other green algae. The hexagonal-shaped crystalloids consisted of parallel arrays of fine tubules of about 15 nm in diameter arranged parallel to the long axis of the crystalloid. In cross sections of the crystalloid, the close packing of the tubules showed hexagonal arrays. The intertubular distance is about 7 nm. At higher magnification there is a suggestion that the walls of these tubules are themselves constructed of smaller tubules. Further electron microscopic observations of diaminobenzidine (DAB)-treated preparations revealed pronounced deposition of reaction product in the microbodies, particularly on the crystalloids. The reaction was completely blocked by the catalase inhibitor, aminotriazole. These results strongly suggest that catalase is involved in this reaction and that catalase is located in the crystalloids.     

Fine structural observations on untreated and ACTH treated adrenocortical cells of the zona reticularis of Syrian hamsters

Summary The fine structure of the zona reticularis of adult Syrian hamsters was studied and compared with that of hamsters given subcutaneous injections of ACTH (10 units daily) for three days. The general ultrastructural features of the cells of the reticularis were described. The major emphasis however, was placed upon the mitochondria which underwent alterations following ACTH injections. Untreated hamsters exhibited elongated mitochondria, attenuated in their middle parts, which formed wrappings around structures, frequently other mitochondria. The tubular cristae of the majority of the mitochondria projected only part of the way into the matrix so that the interior were relatively free of tubules. In many cases the tubules were arranged parallel with the surface so that the mitochondria appeared to be surrounded by numerous membranes. The animals treated with ACTH very rarely showed elongated, attenuated mitochondria: The mitochondria were more nearly equal in size compared with those of untreated animals and the tubular cristae occupied the majority of the interior of the mitochondria. The possible relationship between the mitochondrial changes and increased steroid synthesis was discussed.Research supported by USPHS Grant AM 08222-02, NB 05665-01 and Anatomy Training Grant 5 T 1 GM 459-05.Appreciation is extended to Doctor Donald Duncan whose research is supported by USPHS Grant NB 00690-10 S 1 for the use of the electron microscope and to Mr. Joe Mascorro for technical assistance.     

Histochemical demonstration of l-amino acid-tetrazolium reductase

Summary A method for the histochemical demonstration of l-amino acid-tetrazolium reductase is described. The diformazan deposition was obtained with l-leucine as substrate and no precipitate was present when l-serine and l-lisine were used. In the kidney the reaction was positive in the podocytes, the cells of proximal and distal convoluted tubules, in the ascending limbs of Henle and in the cells of the collecting tubules. In the liver the reaction was positive in the cytoplasm of the hepatocytes. The reaction pattern suggests that it is predominantly extramitochondrial. The specificity of this method is discussed.     

Fine structure of smooth muscle and neuromuscular junctions in the foot of Helix aspersa

Summary The smooth muscle cells in the foot of Helix aspersa are arranged in bundles which interweave to form a complex mesh. In the peripheral cytoplasm of the muscle cells there is a system of interconnected obliquely and longitudinally orientated tubules. The full extent of this system has not been determined; its possible function in relation to Ca⁺⁺ storage and excitation-contraction coupling is discussed. Longitudinal tubules are present among the myofilaments and in association with mitochondria. Distributed throughout the myofilaments are elliptically shaped dense bodies, the fine structure of which resembles an accumulation of thin filaments. Located on the plasma membrane of the muscle cells are dense areas; the fine structure and relationships of these cellular elements resemble desmosomes. They may serve as attachment points for thin, cytoplasmic filaments (not necessarily myofilaments). The muscle cells are innervated by axons which diverge from a coarse, neural plexus (the sole plexus). The axons initially come into close contact with the muscle cells and then pass over their surfaces for up to 35 before being gradually enveloped by flange-like protrusions of the muscle cells. These axons contain either, (i) agranular vesicles (600 Å in diameter), (ii) agranular and very dense granular vesicles (1000 Å in diameter) or (iii) agranular and less dense, granular vesicles (1000 Å in diameter). The possible role of these inclusions as sites of excitatory and inhibitory transmitters is discussed.I wish to thank Professor G. Burnstock for making laboratory facilities available. This work has been supported by the Australian Research Grants Committee.     

An electron microscopical study of the ventral nerve cord of the leech

Summary Three types of fibrillar structure can be seen with the electron microscope in nerve cells of the vental nerve cord of the leech: the neurofibrillar bundles, the tubules and the tonofibrils. In neuroglial cells only the tonofibrils are present. The three types are structurally distinct, and, contrary to past suggestions, there is no evidence that neurofibrillar bundles may consist of tightly packed or badly fixed tubules.In vertebrates the electron microscope reveals bundles of discrete neurofilaments that form the basis for the argyrophilic neurofibrillae seen by light microscopy. Each neurofilamentous unit appears as a dot in cross section. In contrast, in the leech, the electron microscope shows compact fibrillar bundles that clearly correspond to the neurofibrils described by light microscopists. These bundles are made up of closely packed units rather than discrete filaments and where the units occur singly they are seen to have an angular or stellate outline in cross section. To make this distinction clear these have been termed neurofibrillar bundles rather than neurofilaments.Attachment plaques occur in both neurons and neuroglia. These plaques have tonofibrils attached, and the glial tonofibrils are far more numerous than the neuronal tonofibrils. The glial fibrils are identical with the tonofibrils in the glial cells.The attachment plaques are invariably related to an extracellular space that contains material identical with the basement membrane. This material is continuous, by a complex system of channels and diverticulae, with the outer basement membrane in the neuron packets, but forms isolated patches in the other parts of the nervous system.We are grateful to Prof. J. Z. Young, F. R. S., for his encouragement to Mrs. Astafiev for the drawings, to Miss B. Shirra and Mr. K. Watkins for technical assistance and to Mr. S. Waterman for photography.     

Cytodifferentiation of spermatozoa in Drosophila melanogaster: the effect of elevated temperature on spermiogenesis

Summary An electron microscope study of spermiogenesis in maleDrosophila melanogaster cultured at 18° C and 26° C has revealed that there is no apparent morphological basis for sterility observed at elevated temperatures. Temperature does not seem to affect significantly the processes of acrosome differentiation, centriole and manchette development and organization, and differentiation of the nucleus and mitochondrial crystalloid. Only a few grossly abnormal spermatozoan tails (mitochondrial crystalloid and flagellum) observed in some testicular cysts of males cultured at 26° C were interpreted as temperature-induced effects. However, the frequency of atypical spermatozoa is too small to correlate with the data ofPeacock andErickson (1965) showing that 50% of the spermatozoa are nonfunctional. Spermatozoa in the vas deferens of males cultured either at 18° C or 26° C possessed no structural aberrations. The ultrastructural features of normal spermiogenesis are also presented in this report.     

Endocytosis in the ureteric duct epithelium of the hagfish (Myxine glutinosa L.)

Summary Solutions containing ferritin or thorotrast particles were microperfused through the ureteric duct of the hagfish. The markers were taken up by the epithelial cells by way of endocytosis and were transported in bulk in apical vesicles. Newly formed apical vesicles containing marker showed bristle coating on the cytoplasmic side of their limiting membrane. This coating appeared to be lost during the movement of vesicles deeper into the cytoplasm.The findings indicate that the epithelial cells in the ureteric duct have capablity for extensive bulk uptake of macromolecules from the luminal fluid. The mechanisms involved in absorption appear to be similar to those in proximal convoluted tubules of mammals.The apical dense tubules observed with some fixation techniques appear to represent collapsed endocytotic vesicles.The authors are indebted to Finn Walwig, Cand. real., Marine Biology Station, University of Oslo, Dröbak, Norway for kindly supplying the hagfishes used in this study. The technical assistance of Miss Signe Fjeldsenden and Miss Britt-Marie Pettersson is gratefully acknowledged.This work has been supported by grants from the Karolinska Institutet Medical School, Stockholm, Sweden (Therese och Johan Anderssons Minne).     

FINE STRUCTURE OF SCIARA COPROPHILA SPERM

Though the fagellum of Sciara sperm arises from a blepharoplast and is characterized by doublet tubules with arms, it differs markedly from the familiar type of flagella in the number and arrangement of its tubules. The axial filament complex in sperm from the testis of Sciara consists of approximately 70 doublet tubules, each with an associated singlet tubule. Near the nucleus these tubules are displaced in an oval array. Posteriorly the oval breaks and coils from one free end so that the axial filament complex at posterior levels has the form of a spiral. The singlet tubules do not extend the full length of the sperm but terminate in order from inside the spiral. Farther posteriorly the axial filament complex reverses the direction of coiling, and the doublets terminate from outside the spiral. Four arms are specifically positioned on the singlet and doublet tubules. A single mitochondrial derivative extends most of the length of the sperm; it consists of a large mass of proteinacious material, a crystalloid located adjacent to the axial filament complex, and peripheral cristae. In the female genital tract, sperm undergo gross morphological changes which include sloughing of practically all the mitochondrial material except the crystalloid, repositioning of the crystalloid, and uncoiling and subsequent recoiling of the axial filament complex into a different configuration. From analysis of serial sections it was determined that the orientation of arms, when the axial filament is viewed from base to tip, is the same as in conventional flagella.     

Electron microscopic observations on the involution of the human corpus luteum of menstruation

Summary The involution of the granulosa lutein cell in the human corpus luteum is characterized by a dilatation of agranular endoplasmic reticulum vesicles and tubules. This process continues until the whole cell is filled with large vacuoles and the cytoplasm is reduced to thin strands between the vacuoles. The contents of the latter are of low electron density in contrast to the very electron dense lipid droplets in vascularization, bloom and early involution phase. Light microscopical evidence shows that the contents of the vacuoles must be lipid and the lower electron density might be explained by the relative decrease in phospholipids and increase in cholesterol and cholesterol esters during involution. Simultaneously processes of focal cytoplasmic degradation resulting in autophagic vacuoles occur in the cells. These lead in some cases to the formation of residual bodies which can be identified with lipofuscin granules. Finally, the degenerating cells disintegrate and part of the debris, including the lipofuscin granules are phagocytosed by macrophages, the so-called fluorocytes of Hamperl. During involution an amorphous substance with some protofilaments and collagen fibrils is deposited in the spaces between the shrinking lutein cells. This is the fibrohyalin material which will form the bulk of the corpus albicans.We are grateful for the assistance given by Dr. J. M. Moyes of the Women's Hospital, Crown Street, Sydney and the staff of King George V. Hospital at Sydney with the supply of the material.     

Development of P-protein in sieve elements ofMimosa pudica

Summary The P-protein in sieve elements of leaves ofMimosa pudica L. is first discernible as fine fibrous material which forms homogeneous aggregates. Ribosomes, rough endoplasmic reticulum, and dictyosomes with associated vesicles occur in the cytoplasm surrounding the aggregates. The plastids and mitochondria are in a parietal position in the parts of the cell where the nascent P-protein accumulates. In a later stage, the fibrillar material is organized into a three-dimensional system of five- and six-sided elongated compartments. The corners of the compartments appear solid at first, then they become electron lucent in the center and assume tubular form. Aggregates of mature P-protein tubules usually occur near the compartmentalized system. Tubules in pentagonal or hexagonal arrangements may be present in the aggregates and may be partly interconnected. The conclusion was drawn that the P-protein tubules are assembled at the corners of compartments within a continuous orderly system. The fully formed tubules occur first in aggregates, the P-protein bodies. Later the aggregates become loose and partly dispersed. Many of the dispersed tubules assume a loose, extended, helical form characteristic of P-protein in older sieve elements.This work was supported in part by National Science Foundation grant GB-5506. I am also grateful to MissHatsume Kosakai and Mr.Robert H.Gill for technical assistance.     

Synapses in the cerebral ganglion of adult Ciona intestinalis

Summary Electron microscopy of the synaptic morphology of synapses in the cerebral ganglion of the adult ascidian (sea squirt) Ciona intestinalis reveals that the synapses are restricted to the central neuropil of the ganglion. Many of the synapses show a polarity of structure such that pre and post synaptic parts can be identified. The vesicles in the presynaptic bag are of two main diameters 80 and 30 nm respectively. The large vesicles have electron dense contents that vary both in their capacity and dimensions.The pre and postsynaptic membranes are more electron dense than the surrounding membranes, but they are only slightly thicker. Both the pre and post synaptic membranes have electron dense dots some 10 nm in diameter associated with their cytoplasmic surfaces. Sometimes the presynaptic membrane has larger peg-like projections between the vesicles. Associated with the post synaptic membrane are tubules some 10 nm in diameter. These tubules may be the dots cut obliquely.The synaptic cleft material is more electron dense than the surrounding intercellular material, and in it there is a dense line made up of granules about 3–5 nm in diameter. This dense line is usually mid way between the pre and post synaptic membranes, but may be nearer the postsynaptic membrane.No tight junctions between adjacent nerve process profiles have been observed.I wish to thank Professors J. Z. Young, F. R. S. and E. G. Gray for much advice and encouragement, also Dr. R. Bellairs for the use of electron microscope facilities and Mr. R. Moss and Mrs. J. Hamilton for skillful technical assistance.     

Studies on the nephron of an elasmobranch fish Scyliorhinus stellaris (L.)

Summary The nephron of the elasmobranch Scyliorhinus stellaris was studied by macerating kidneys of newly hatched specimens in a solution of arsenic anhydride and by sectioning material of the same age and of late embryos. It was found that each nephron consists of (a) a dorsal conical portion containing a skein of canals, loosely intertwined and immersed in blood sinuses, (b) a Malpighian corpuscle, (c) a ventral portion formed by a tightly packed bundle of five tubules. It was not possible to disentangle the ventral tubules so as to recognize their sequence. It may be stated that the neck, originating from the corpuscle and coming back to it after a bend, forms two of such tubules; two more are similarly connected at the distal end by another loop; one forms the collecting duct. The dorsal part of the nephron contains two canals, greatly folded and interlaced. When completely dissociated they appeared to consist of (a) a larger and longer one, corresponding to the brush border portion of all vertebrates (it is thick for most its length, but becomes thin at one end), and (b) a smaller and shorter one, lined by low cells throughout, with no brush border. Although it is not possible to decide so far which of these canals precedes the other, it was ascertained that they are not continuous with each other inside the dorsal portion, but are connected by means of a loop situated in the ventral bundle. The renal tubule passes by, and adheres to the corpuscle four times. The small dorsal canal, although it does not correspond, either in topography or in structure, to the features of the alleged special segments, may be something peculiar to elasmobranchs.Dedicated to Professor Wolfgang Bargmann on his 60th birthday. — Research carried out under contracts with the U. S. Atomic Energy Commission (NYO-3355-3) and Euratom (043-65-1 BIOI), and supported by the Italian C. N. R. and Ministry of Education. — Thanks are due to Dr. P. Dohrn and the staff of the Zoological Station for their help in securing the material and to Dr. Elena Vivori who revised the English.     

The fine structure of differentiating interstitial cells in Hydra

Summary Interstitial cells of hydra are small undifferentiated cells containing an abundance of free ribosomes and few other cytoplasmic organelles. They are capable of differentiating into epitheliomuscular, digestive, glandular, nerve cells, and cnidoblasts. Developing epitheliomuscular and digestive cells acquire bundles of filaments, 50 Å in diameter, which later are incorporated into the muscular processes. Early gland cells develop an elaborate rough-surfaced endoplasmic reticulum and one or more Golgi apparatus. Secretory granules originate in the Golgi region eventually filling the apex of the cell. Neurons are recognized first by the presence of an elaborate Golgi apparatus, absence of a well-developed endoplasmic reticulum, and later the appearance of cytoplasmic processes. The most striking feature of nematocyst formation by cnidoblasts is the presence of a complex distribution system between protein synthesizing rough-surfaced endoplasmic reticulum and the nematocyst. This system consists of connections between cisternae of the endoplasmic reticulum with smooth Golgi vesicles which in turn are connected to minute tubules, 200 Å in diameter. The tubules extend from the Golgi region around the nematocyst finally entering the limiting membrane of the nematocyst. It is suggested that the interstitial cells of hydra represent a model system for the investigation of many aspects of cell differentiation.This work was supported by grants from the National Cancer Institute (TlCA-5055) and from the National Institute of Arthritis and Metabolic Diseases (AM-03688), National Institutes of Health, Department of Health, Education and Welfare.The author is indebted to Dr. Russell J. Barrnett for his guidance and interest throughout this investigation.     

Starvation effects on the ultrastructure of amoeba mitochondria

Summary Mitochondria in non-starved giant amoebae, Pelomyxa carolinensis, contain tubules lying at random in the matrix. Many mitochondria in starved amoebae have enlarged tubules aligned in a zigzag pattern. Tubules within the zigzag region are separated by very little matrix material. Some of these altered mitochondria are found in 70% of amoebae starved for only 24 hours, and in nearly all P. carolinensis starved for 8 days or longer. The percentage of such altered mitochondria increases from zero in most well-fed amoebae, to about 60% after two weeks of continuous starvation. Most P. carolinensis starved at 25° C survive less than three weeks. Microfilament bundles are observed in the matrix of some mitochondria in amoebae starved for more than two days.Work supported by the U. S. Atomic Energy Commission.The authors acknowledge the assistance of Miss Doris Jean Buer and Miss Patricia Ann Sustarsic.     

Ultrastructural study of the regressing prothoracic glands of blattarian insects

Summary The prothoracic glands, source of the molting hormone ecdysone, regress within a few days after the final molt, a process which was analyzed with electron microscopic methods in the cockroaches Leucophaea and Blaberus. This strictly timed event is accompanied by drastic alterations in cellular fine structure. Early signs of breakdown appear in groups of nuclei whose substance becomes segregated into patches of contrasting electron density characteristic of pyknosis.The most conspicuous change in the cytoplasm of parenchymal cells concerns the appearance of large, heterogeneous inclusion bodies in which various cellular elements become segregated. These compartments seem to represent autophagic vacuoles within which the gradual degradation of much of their contents takes place, presumably under the influence of lysosomal enzymes. Undigested swirls of membranous character may remain sequestered within these packets for some time.At advanced stages of cellular atrophy, plasma membranes and nuclear envelopes have gradually disappeared, and masses of protoplasm undergoing autolysis become invaded by a greater number of hemocytes than are present in nymphal glands. These phagocytic elements appear to engulf debris of parenchymal cells as well as some degenerating connective tissue elements. After the completion of the regressive process, the axial band of musculature characteristic of the nymphal gland persists on its own. Whether or not some parenchymal cells (or possibly their precursors) capable of reactivation persist in the proximity of this muscle is unknown.The resorption of the prothoracic gland in the newly emerged insect is the result of physiological autolysis and seems to be aided by the activity of phagocytic hemocytes.Dedicated to Professor W. Bargmann on his 60th birthday in friendship and admiration.This study was supported by Research Grants AM-03984, NB-02145 and NB-05219 from the U.S.P.H.S.I wish to express my thanks to Mrs. S. Wurzelmann, Mrs. C. Jones, Mrs. C. Grubman, and Mr. S. Brown for their excellent technical assistance.     

THE NORMAL FINE STRUCTURE OF OPOSSUM TESTICULAR INTERSTITIAL CELLS

The interstitial tissue of the opossum testis includes interstitial or Leydig cells, macrophages, and small cells which morphologically resemble mesenchymal cells. The latter are thought to give rise to mature interstitial cells. The most prominent feature of the interstitial cell cytoplasm is an exceedingly abundant agranular endoplasmic reticulum. This reticulum is generally in the form of a meshwork of interconnected tubules about 300 to 450 A in diameter, but occasionally it assumes the form of flattened, fenestrated cisternae resembling those of pancreatic acinar cells, except for the lack of ribonucleoprotein particles on the surface of the membranes. The interstitial cells vary considerably in their cytoplasmic density. The majority are quite light, but some appear extremely dense, and in addition usually have a more irregular cell surface, with numerous small pseudopodia. These differences may well reflect variations in physiological state. Cytoplasmic structures previously interpreted as "crystalloids" consist of long bundles of minute parallel tubules, each about 180 A in diameter, which seem to be local differentiations of the endoplasmic reticulum. The mitochondria are rod-shaped, and contain a moderately complex internal membrane structure, and also occasional large inclusions that are spherical and homogeneous. The prominent juxtanuclear Golgi complex contains closely packed flattened sacs and small vesicles. The results of the present study, coupled with biochemical evidence from other laboratories, make it seem highly probable that the agranular endoplasmic reticulum is involved in the synthesis of the steroid hormones produced by the interstitial cell. This finding therefore constitutes one of the first functions of the agranular reticulum for which there is good morphological and biochemical evidence.