Preparation of human spermatozoa for in vitro fertilization by isopycnic centrifugation on self-generating density gradients

An Update on Semen Physiology, Technologies, and Selection Techniques for the Advancement of In Vitro Equine Embryo Production: Section II

Simple Summary

In order to improve fertilization and pregnancy rates within artificial insemination or in vitro fertilization techniques in horses, producers may choose to select the best sperm within an ejaculate. In this paper, we review conventional and novel methods of sperm selection.

Abstract

As the use of assisted reproductive technologies (ART) and in vitro embryo production (IVP) expand in the equine industry, it has become necessary to further our understanding of available semen selection techniques. This segment of our two-section review will focus on the selection of spermatozoa based on quality and sex for equine intracytoplasmic sperm injection (ICSI), as well as current and future developments in sperm sorting technologies. Ultimately, novel methods of semen selection will be assessed based on their efficacy in other species and their relevance and future application towards ARTs in the horse.

Reproduction biotechnologies in germplasm banking of livestock species: a review

Many biotechnologies are currently used in livestock breeding with the aim of improving reproductive efficiency and increasing the rate of genetic progress in production animals. Semen cryopreservation is the most widely used cryobiotechnology, although vitrification techniques now allow embryos and oocytes to be banked in ever-increasing numbers. Cryopreservation of other types of germplasm (reproductive tissue in general) is also possible, although the techniques are still in the early stages of development for use in livestock species. Although still in their infancy, these techniques are increasingly being used in aquaculture. Germplasm conservation enables reproductive tissues from both animals and fish to be preserved to generate offspring in the future without having to maintain large numbers of living populations of these species. However, such measures need careful planning and coordination. This review explains why the preservation of genetic diversity is needed for livestock and fish, and describes some of the issues involved in germplasm banking. Furthermore, some recent developments in semen handling leading to improved semen cryopreservation and biosecurity measures are also discussed.

Recovery, Preparation, Storage and Utilization of Spermatozoa for Fertility Preservation in Cancer Patients and Sub-Fertile Men

Sperm cryopreservation is an important part of an infertility program for patients undergoing infertility treatments, fertility assurance for vasectomy cases, and for fertility preservation due to cancer or other medical conditions. With recent developments in reproductive technology, even men with severely impaired sperm parameters can benefit from cryopreservation as procedures such as intra-cytoplasmic sperm injection (ICSI) require only a few sperm to achieve fertilization and pregnancy. The increasing success of cancer treatment and concerted efforts to ensure quality of life after successful treatment have placed great emphasis on the need to preserve the reproductive capability of young men. It is a highly effective method of protecting male fertility potential, and involves collection, freezing, and long-term storage of sperm. Based on the etiological condition of the patients, sperm can be collected by ejaculation or by surgical retrieval from epididymis or testes. The option to bank sperm should be offered systematically to all patients who may benefit. However, this is not a standard of practice yet; it may be overlooked due to lack of physician awareness regarding the need for fertility preservation and the effectiveness of this option, and/or overestimating the limitations of poor baseline sperm quality leading physician to view cryopreservation as futile. Failure to offer cryopreservation ignores the only possible reproductive option available to certain patients.

Density gradients for the isolation of germ cells from embryoid bodies

In previous reports, the isolation of embryonic stem cell (ESC)-derived germ cells utilized fluorescent protein knock-in cell lines that could be sorted by flow cytometry. The present study aimed to isolate putative germ stem cells from embryoid bodies (EB) using Percoll and Nycodenz density gradients. The optimal ESC concentration to establish EB was identified as 15,000 cells per 30 mul drop and the optimal culture time to obtain the highest number of germ cells within EB was identified as 120 h, with over 25% of cells confirmed as germ cells for the specific cell line used. Germ cells were isolated from 120-hourold EB by Percoll and Nycodenz density gradients, while isolation of primordial germ cells from genital ridges of embryonic day 12.5 fetuses was used as a control. Putative germ cells were isolated from EB at proportions of 80.3 +/- 3.0% (SEM) and 75.8 +/- 0.9% for Percoll and Nycodenz respectively. Primordial germ cells were isolated from genital ridges at rates of 89.7 +/- 2.7% and 89.5 +/- 0.9% respectively. Although the purity of the isolated germ cells was similar between the two gradients, more germ cells with higher viability were obtained with the Percoll gradient.

Update on semen technologies for animal breeding

Despite the scale of the livestock breeding industry, where many millions of insemination doses are prepared each year, sperm preparation techniques are used infrequently in animal assisted reproduction compared with its human counterpart. However, some of the techniques used for human sperm preparation, for example, density gradient centrifugation, improve the quality of sperm preparations which is, in turn, reflected by an increased conception rate. The preparation technique separates motile spermatozoa with normal morphology and intact DNA from the total sperm population, leaving behind immature or senescent spermatozoa, morphologically abnormal ones and those with damaged DNA. Furthermore, the motile spermatozoa are removed from the seminal plasma which carries cells, cellular debris and reactive oxygen species, as well as pathogens. Gradient-prepared spermatozoa survive longer, either in liquid storage or when cryopreserved, and are free of bacteria and viral infectivity if prepared carefully. Preparation techniques such as density gradient centrifugation, or the simplified single layer centrifugation technique, have considerable potential for aiding sperm preparation from poor quality semen samples, such as may be obtained from unselected semen donors in captive breeding programmes, or from performance horses. Moreover, the removal of pathogens has important implications, both for disease control and for avoiding the use of antibiotics in semen extenders, which can be detrimental to sperm survival.

Sperm preparation for ART

The onset of clinical assisted reproduction, a quarter of a century ago, required the isolation of motile spermatozoa. As the indication of assisted reproduction shifted from mere gynaecological indications to andrological indications during the years, this urged andrological research to understand the physiology of male germ cell better and develop more sophisticated techniques to separate functional spermatozoa from those that are immotile, have poor morphology or are not capable to fertilize oocytes. Initially, starting from simple washing of spermatozoa, separation techniques, based on different principles like migration, filtration or density gradient centrifugation evolved. The most simple and cheapest is the conventional swim-up procedure. A more sophisticated and most gentle migration method is migration-sedimentation. However, its yield is relatively small and the technique is therefore normally only limited to ejaculates with a high number of motile spermatozoa. Recently, however, the method was also successfully used to isolate spermatozoa for intracytoplasmic sperm injection (ICSI). Sperm separation methods that yield a higher number of motile spermatozoa are glass wool filtration or density gradient centrifugation with different media. Since Percoll as a density medium was removed from the market in 1996 for clinical use in the human because of its risk of contamination with endotoxins, other media like IxaPrep, Nycodenz, SilSelect, PureSperm or Isolate were developed in order to replace Percoll. Today, an array of different methods is available and the selection depends on the quality of the ejaculates, which also includes production of reactive oxygen species (ROS) by spermatozoa and leukocytes. Ejaculates with ROS production should not be separated by means of conventional swim-up, as this can severely damage the spermatozoa. In order to protect the male germ cells from the influence of ROS and to stimulate their motility to increase the yield, a number of substances can be added to the ejaculate or the separation medium. Caffeine, pentoxifylline and 2-deoxyadenosine are substances that were used to stimulate motility. Recent approaches to stimulate spermatozoa include bicarbonate, metal chelators or platelet-activating factor (PAF). While the use of PAF already resulted in pregnancies in intrauterine insemination, the suitability of the other substances for the clinical use still needs to be tested. Finally, the isolation of functional spermatozoa from highly viscous ejaculates is a special challenge and can be performed enzymatically to liquefy the ejaculate. The older method, by which the ejaculate is forcefully aspirated through a narrow-gauge needle, should be abandoned as it can severely damage spermatozoa, thus resulting in immotile sperm.

Bacterial contamination and sperm recovery after semen preparation by density gradient centrifugation using silane-coated silica particles at different g forces

The effects of density gradient centrifugation through silane-coated silica particles (PureSperm) using 100, 200, 300 and 500 g on bacterial contamination of sperm samples and recovery of motile spermatozoa from sperm samples were investigated with conventional culturing techniques and microscopic visual assessment. The recovery of motile spermatozoa was variable and was not improved using 500 g compared to the recommended 300 g. The bacterial contamination was highly decreased by gradient centrifugation through PureSperm and was almost abolished when strict aseptic techniques were used, with changes to sterile Pasteur pipettes and tubes prior to washing procedures.

A comparison of clomiphene citrate and human menopausal gonadotropin for use in conjunction with intrauterine insemination

OBJECTIVE

To compare the outcome of superovulation using clomiphene citrate (CC) versus hMG in conjunction with IUI.

DESIGN

Sequentially assigned, observational study. Couples initially were assigned to receive either CC or hMG for three cycles.

SETTING

The Clinical Outpatient Department of the Dartmouth-Hitchcock Medical Center.

PATIENT(S)

Eighty-three infertile couples.

INTERVENTION(S)

IUI with hMG use.

MAIN OUTCOME MEASURE(S)

Conception rate, term pregnancy rate (PR), and pregnancy complications, such as spontaneous miscarriage and multiple gestation.

RESULT(S)

Of 83 couples who underwent at least one treatment cycle, 29 (35%) conceived during the study period. The relative rate of conception for hMG versus CC was 2.08 (95% confidence interval [CI], 0.93 to 4.68). The relative term PR was 2.10 (95% CI, 0.77 to 5.73) for hMG versus CC. There was no difference in the miscarriage rate for hMG versus CC.

CONCLUSION(S)

Both the conception rate and the term PR were higher using hMG, compared with CC, in combination with IUI, and showed a trend toward statistical significance.

Profiles of sperm morphology and motility after discontinuous multiple-step Percoll density gradient centrifugation

Morphology and motility are important parameters for assessing the fertilizing capacity of spermatozoa. This investigation reports a systematic study on the profiles of these parameters after percoll gradient centrifugation. Spermatozoa from normal human semen were fractionated by discontinuous Percoll gradients (30%, 45%, 75%, 90%). Spermatozoa washed with Earle's balanced salt solution were used as a control. After centrifugation, sperm morphology was evaluated according to strict criteria; motility was assessed by a computer-assisted semen analysis system. The results showed that the percentage of spermatozoa with normal morphology increased, while those with severe morphological defects decreased, as the density of Percoll increased. The percentages of motile spermatozoa and hyperactivated spermatozoa, and the velocity and amplitude of lateral head displacement of the spermatozoa were significantly higher in the 75% and 90% Percoll fractions than that in the 30% and 45% ones, and in the control. These results demonstrated that Percoll density gradient centrifugation enriched spermatozoa in terms of morphology and motility.

Sperm function and its manipulation for microassisted fertilization

Comprehension of the intricate complexities of sperm function is clearly crucial to the success of attempts to manipulate it for the purposes of assisted conception. This is particularly important when considering various procedures for microassisted fertilization since these bypass critical physiological events that are mandatory for normal fertilization, to varying degrees. Methylxanthine derivatives such as pentoxifylline are useful agents for the management of oligoasthenozoospermic patients. This is particularly so for procedures such as SUZI where adequate motility of spermatozoa injected into the perivitelline space is crucial for fusion with the vitelline membrane to achieve fertilization. The generation of minute concentrations of reactive oxygen species in vitro may prove to be a valuable technique in this respect, in the light of recent evidence for their involvement in capacitation and hyperactivation. Induction of the acrosome reaction by non-invasive, non-toxic agents should markedly improve success rates for microassisted fertilization. Acrosin appears to play a central role in this and, therefore, it would seem prudent to monitor levels of acrosin activity in samples of spermatozoa used in assisted conception procedures. With respect to microassisted fertilization, the potential to select recently acrosome-reacted spermatozoa coated by activated acrosin promises to be a major improvement. Current methods employed for determination of the fertilization potential of spermatozoa are clearly inadequate (Polansky and Lamb, 1988; Aitken, 1990). In fact, the prevailing evidence suggests that no single parameter of sperm function reflects this potential (Zaneveld and Jeyendran, 1988). Therefore, we have both a scientific and a moral responsibility to investigate these processes further. Subsequently, we should be in a position to identify individual gametes with the potential for fertilization and so utilize procedures that result in maximal fertilization rates with minimal risk of polyploidy or abnormality.

Factors affecting sperm yield during swim-up

PURPOSE

The migration of sperm (expressed as the percentage of the total motile cells in the ejaculate aliquot) from seminal plasma into overlying medium (swim-up) was increased with time, temperature, and increasing the surface area of contact between the two phases by increasing the angle of tube inclination.

RESULTS

The proportion of sperm migrating into the medium was relatively constant with varying sperm count. Stimulation of the progressive motility of spermatozoa in vitro using a motility enhancing drug (2-deoxyadenosine) increased sperm migration initially but no more than the maximum reached by controls after extended incubation.

CONCLUSION

Sperm yield from normozoospermic men and men with borderline asthenoteratozoospermia represented approximately 40 and 15% of the total motile cells in the ejaculate aliquots, respectively. In general, in both groups spermatozoa in the swim-up fraction exhibited superior motility, vitality, and morphology compared to the original ejaculates. Tail abnormalities were reduced more than head abnormalities and subjects with borderline sperm qualities exhibited less dramatic improvements in quality than the normozoospermic individuals.

Movement characteristics of human spermatozoa collected from different layers of a discontinuous Percoll gradient

In order to evaluate the functional state of human spermatozoa separated on discontinuous Percoll gradient, sperm provided by 15 healthy volunteers were recovered from different layers (60, 70, 80, 90, 100%) of the gradient, then submitted to a complete analysis of sperm movement characteristics. This one was performed using a computer assisted system (Cellsoft, Cryo Resources, Ltd., NY, USA). In all fractions sperm motility was highly maintained during 24 h and all movement characteristics, with the exception of ALH, were different from that observed in seminal plasma. Curvilinear velocity (VCL), linearity (LIN), and percentage of hyper-activated sperm (HA) increased in relation to the Percoll concentration. Nevertheless there was no dramatic difference, concerning motion parameters, between 100%, 90%, and 80% Percoll fractions, which can thereby be pooled in case of insufficient sperm recovering in 100% fraction. On the contrary there is some doubt about the fertilizing ability of spermatozoa recovered in 70% and 60% Percoll layers, since VCL, LIN, and especially HA are severely decreased.

Human spermatozoa selection in improved discontinuous Percoll gradients

OBJECTIVE

Improve the technique of human spermatozoa separation using Percoll density gradients.

DESIGN

We compared the spermatozoa separation obtained after density gradients from a physiological stock Percoll solution and a hyperosmotic stock Percoll solution.

SETTING

Sperm samples were obtained from the Laboratory of the Biology of Reproduction at Brest, France, a public hospital.

PATIENTS, PARTICIPANTS

One hundred thirty-eight healthy donors.

INTERVENTIONS

None.

MAIN OUTCOME MEASURE

The resolution of the separation was assessed by the number of motile spermatozoa recovered after separation using density gradients.

RESULTS

The improvement of the resolution of separation is at least twofold greater with the use of hyperosmotic gradients than with that obtained by physiological gradients. Moreover, the best resolutions of separation were obtained from pathological sperm. Finally, the influence of buoyant density spermatozoa on the resolution of separation by this technique is clearly demonstrated.

CONCLUSIONS

With this modification, the technique of Percoll density gradients, used for the separation of human spermatozoa, is strongly advised especially in pathological cases.

Comparison of characteristics of human spermatozoa selected by the multiple-tube swim-up and simple discontinuous Percoll gradient centrifugation

The multiple-tube swim-up (MT-SU) and simple, discontinuous Percoll gradient centrifugation are two recent methods for the processing of human spermatozoa for assisted reproductive technologies. The present study was undertaken to compare the effects of these two methods on various parameters of spermatozoal qualities of normospermic semen samples. Both methods were found to select spermatozoa of better percentage motility, percentage normal morphology, viability, functional integrity of plasma membrane, movement characteristics including the curvilinear velocity, linearity of movement and head beat frequency, and in vitro fertilizing capacity with comparable degree of efficacy. It is concluded that both methods are effective for the separation of spermatozoa from normospermic semen samples for use in infertility treatment.

The accuracy of sperm concentration determination by the automated CellSoft semen analyzer before and after discontinuous Percoll gradient centrifugation

The automated CellSoft semen analyzer identifies human spermatozoa on the basis of user-defined values for cell size and luminosity. Previous studies have shown that the non-sperm particles usually present in seminal plasma would interfere with the computerized determination of sperm concentration by the CellSoft system. Therefore, an effective method to separate the sperm from non-sperm particles would be desirable to obtain accurate determination of sperm concentration. In the present study, sperm concentrations in 72 semen samples before and after discontinuous Percoll gradient centrifugation were determined by the automated CellSoft system and the results compared with those obtained with the routine procedure using the haemocytomer according to the World Health Organization laboratory manual. The computerized measurement caused a significant overestimation when the sperm concentration in semen was less than 80 x 10(6)/ml. Processing of human semen sample by the simple two-layer discontinuous Percoll gradient centrifugation removed the majority of non-sperm particles and the overestimation by the automated CellSoft system became significant only when the sperm concentration in the final Percoll sperm preparation was less than 10 x 10(6)/ml. These findings indicate that the automated CellSoft semen analyzer has to be improved to allow for the correction of non-sperm particles in seminal plasma or processed sperm samples before it can be used to provide sufficiently accurate sperm concentration results for routine laboratory service or research purposes.

Human sperm selection by glass wool filtration and two-layer, discontinuous Percoll gradient centrifugation

Glass wool filtration and two-layer, discontinuous Percoll (Pharmacia, Uppsala, Sweden) density gradient centrifugation resulted in an average recovery of 50% to 70% of the progressively motile and about 50% of the hypoosmotic swelling (HOS)-positive spermatozoa. Glass wool filtration tended to be more successful than Percoll centrifugation when the ejaculates were asthenozoospermic or produced a suspect/abnormal HOS test. After selection, the acrosin activity increased approximately two- to threefold, but no significant improvement in the percentage of normal sperm forms occurred. Experiments with mixtures of untreated and frozen-thawed ejaculates confirmed that glass wool filtration is more effective in removing nonmotile and HOS-negative spermatozoa than the two-layer Percoll centrifugation technique when the percentage of these types of spermatozoa in the ejaculate is high. The simplicity of these techniques and the good recovery of apparently viable spermatozoa makes these methods more desirable than other, more complicated techniques or procedures that yield a lower recovery of motile spermatozoa.

Artificial insemination with husband's sperm (AIH): techniques for sperm selection

Artificial insemination with husband's sperm (AIH) attempts to treat infertile couples by solving their particular infertility problems. This review is concerned with detailing and evaluating the experiences of different authors with AIH in infertile couples, types of insemination, and indications for insemination, with emphasis on techniques for sperm selection and separation: glass-wool filtration, separation on albumin columns, washed sperm, swim-up method, and spermatozoa selection in Percoll gradients.

Apparent densities of spermatozoa of various mammalian species

The apparent densities of sperm cells in several mammalian species were determined by means of Percoll density-gradient centrifugation. Semen samples were obtained from (1) ejaculates from the bull, human, and rabbit, and (2) from the cauda epididymis of the goat, golden hamster, house musk shrew, mastomys, and mouse. The profiles of sperm distribution showed a single peak after the centrifugation in the first group and goat, whereas two separate main peaks in the second group (except goat). This disparity in sperm distribution profiles may be due mainly to differences in the degree of maturity of the sperm. Highly motile and mature sperm were obtained at higher densities, whereas immotile or immature sperm were found at the apparent densities. Thus, in mammals, the profiles of sperm distribution in the Percoll density gradient are classified into two types, those with a single peak and those with two separate peaks. The apparent density of sperm cells may be of importance in sperm physiology.

Evaluation of Nycodenz and Percoll density gradients for the selection of motile human spermatozoa

Two high-density media, Percoll (Pharmacia, Uppsala, Sweden) and Nycodenz (Nyegaard & Co., Oslo, Norway), were assessed for efficacy of selection of motile spermatozoa from oligozoospermic and asthenozoospermic semen samples. The results indicated initially that the media were equivalent for motile sperm selection. However, only the spermatozoa selected from discontinuous Nycodenz gradients retained their motility for at least 21 hours (60% motility compared with 5% motility). A significantly higher number of motile spermatozoa were harvested from discontinuous Nycodenz gradients than from matched control samples using the swim-up procedure (12.0 +/- 3.0 standard error of the mean [SEM] X 10(6)/ml versus 1.9 +/- 0.7 SEM X 10(6)/ml; 27 experiments, P less than 0.001). Discontinuous Nycodenz gradients may be useful for selection of motile spermatozoa from poor-quality semen for in vitro fertilization.

Male-factor infertility and in vitro fertilization

In vitro fertilization (IVF) was developed primarily as a treatment for female and idiopathic infertility. However, with the discovery that relatively few sperm are required to achieve fertilization in vitro, it was proposed that IVF could be used also as an effective treatment for male-factor infertility. This review deals with the work that has been carried out by various groups in this area of male-factor infertility. As the standards of classification and the presentation of results vary from group to group, this also shows that there is a need for some standardization of how patient selection and the presentation of results are carried out in the area of male-factor infertility.

Retrograde ejaculation: a systematic approach to non-invasive recovery of spermatozoa from post-ejaculatory urine for artificial insemination

Retrograde ejaculation is an uncommon but treatable form of male infertility. Successful recovery of live spermatozoa from the post-ejaculatory urine for artificial insemination is dependent on careful regulation of pH and osmolarity of the urine into which ejaculation takes place, and separation of the motile spermatozoa from the debris and cells which are found in these samples. Three pregnancies established by artificial insemination of spermatozoa recovered by noninvasive means from the bladders of men suffering from retrograde ejaculation are described. The techniques for preparing the urine for spermatozoal survival, and for removal of cells and debris by sedimentation or buoyant density centrifugation are discussed.

Sialic acid in purified human sperm

Sialic acid levels of human sperm were estimated using newly developed sperm purification procedure (inner column method). This was used to estimate net sialic acid content of forward motile sperm of normal morphology without any contamination of the seminal plasma, other cellular components, and immature or abnormal sperm. The sialic acid content in purified sperm was approximately 40% of that in the washed (dilution-washing method) sperm. It is suggested that biochemical properties of human sperm should be reevaluated using the inner column purification procedure.

Effects of gradient centrifugation on human sperm

The quality of ejaculated human sperm was examined for 24 h at room temperature by Percoll continuous density gradient centrifugation. One hour after ejaculation, apparent densities of good motile sperm with normal morphology were above 1.10 g/ml, whereas those of immotile and/or immature sperm recovered from upper layers were 1.06-1.09 g/ml. As time passed, sperm became immotile and the number of sperm with higher densities was diminished, their density shifting to 1.06-1.09 g/ml. Morphological observations by transmission electron microscope showed that after 24 h the sperm with higher densities were morphologically normal, except that the plasma membrane and the outer acrosomal membrane were lost. On the other hand, the sperm with lower densities were abnormal, such as immature, without the outer and the inner acrosomal membrane, with the nuclear decondensation, and with membraneous structure in the nucleus. These results suggest that after ejaculation the apparent density of human sperm became lower, concomitant with the loss of motility.

Removal of bacterial contaminants from semen for in vitro fertilization or artificial insemination by the use of buoyant density centrifugation

Buoyant density centrifugation of semen produces the accumulation of populations of highly motile, morphologically normal spermatozoa in the lowermost 1 ml of Percoll (Pharmacia Fine Chemicals AB, Uppsala, Sweden) density gradients. In addition, the majority of bacteria present in semen are retained in the seminal plasma at the top of the gradients. Of 40 semen samples examined, 37 contained detectable bacteria, but after buoyant density centrifugation, the spermatozoal populations collected from the lowermost 1 ml of the Percoll columns were found to contain few or no bacteria. When preparations were collected using sterile technique (by boring a hole through the bottom of the centrifuge tube), 14 of the 20 preparations were found to be bacteria-free. When preparations were collected by passing a spinal needle from the surface through the seminal plasma to the bottom of the centrifuge tube, the sterility of the final spermatozoa preparations was not maintained, with only 5 of the 20 samples completely free of bacteria. The residual bacterial contamination of the remaining 15 samples was, however, very low (less than 5 colonies after a 48-hour culture period).

Characterization of rabbit sperm by equilibrium sedimentation in Percoll during frequent ejaculation

Qualitative and quantitative changes of rabbit sperm were observed during frequent ejaculation. Semen volume, sperm concentration, percentage of motility, and fertility index of Farris (FI) decreased with frequent ejaculation, and these values recovered to initial levels after a 24-day abstinence period. A remarkable change was found in the sperm distribution profile in Percoll density gradients after equilibrium sedimentation. The main peak of sperm was shifted from fraction number 4 on the first day of ejaculation to number 7 on the fifth day. Fraction numbers 1-4 showed a high percentage of motility, whereas the sperm motility from fraction numbers 7-10 was quite low. Motile and morphologically mature sperm had higher apparent density than immotile or immature sperm. This suggests that rabbit sperm motility might be related to the apparent density of the sperm cell.