Monitoring Blood Glucose in Mouse Offspring after Intracytoplasmic sperm injection - Embryo Transfer

Abstract

Human lifespan is considerably long, while mouse models can simulate the entire human lifespan in a relatively short period, with one year of mouse life roughly equivalent to 40 human years. Intracytoplasmic sperm injection (ICSI) is a commonly used assisted reproductive technology in clinical practice. However, given its relatively recent emergence about 30 years ago, the long-term effects of this technique on human development remain unclear. In this study, we established the ICSI combined with embryo transfer (ET) method using a mouse model. The results demonstrated that normal mouse sperm, after undergoing in vitro culture and subsequent ICSI, exhibited a fertilization rate of 89.57% and a two-cell rate of 87.38%. Following ET, the birth rate of offspring was approximately 42.50%. Furthermore, as the mice aged, fluctuations in glucose metabolism levels were observed, which may be associated with the application of the ICSI technique. These findings signify that the mouse ICSI-ET technique provides a valuable platform for evaluating the impact of sperm abnormalities on embryo development and their long-term effects on offspring health, particularly concerning glucose metabolism. This study provides important insights for further research on the potential effects of the ICSI technique on human development, emphasizing the necessity for in-depth investigation into the long-term implications of this technology.

Introduction

Fertility issues have emerged as a major focus of medical and sociological concern, especially in modern society where declining fertility rates and the increasing severity of infertility prevalence and severity have risen to prominence. Assisted Reproductive Technology (ART) provides a wide array of possibilities to tackle these challenges, with Intracytoplasmic Sperm Injection (ICSI) commonly utilized as a therapeutic intervention.

Since Palermo reported the first successful pregnancy achieved through Intracytoplasmic Sperm Injection (ICSI) in 1992, ICSI has become a pivotal technique in Assisted Reproductive Technologies (ART)¹. However, considering that ICSI has been used in clinical settings for only 30 years, a relatively brief period compared to the human lifespan, the long-term effects of ICSI, especially on offspring development, have not been extensively investigated and elucidated. At present, mice characterized by their uniform genetic background and shorter lifespan, have emerged as a widely utilized alternative model in medical research. Moreover, the mouse model can recapitulate the entire human lifespan within a compressed time frame, where one year in mice roughly corresponds to 40 years in humans².

Over the past decade, several small-scale studies have reported that individuals conceived through ICSI may be at an increased risk of developing metabolic syndromes, such as abnormal blood sugar levels, later in life^3,4. Although the evidence is not definitive, this finding has nonetheless raised serious concerns within the scientific community regarding the long-term health implications of ICSI. This situation underscores the pressing need for more rigorous assessments of ART and its long-term health consequences. Particularly in light of the limitations and ethical considerations of human studies, developing animal models that can precisely recapitulate the development of human offspring following ICSI has become increasingly crucial. In this context, the mouse ICSI-ET (Intracytoplasmic Sperm Injection-Embryo Transfer) model, owing to its capacity to mimic the human ICSI and facilitate long-term monitoring of offspring health outcomes, has become an effective tool for assessing the potential health risks of ICSI technology to offspring⁵.

This study aims to investigate the impact of ICSI-ET technology on a prevalent metabolic phenotype, namely offspring glucose metabolic health, by employing random blood glucose monitoring, fasting blood glucose testing, and glucose tolerance tests to assess the glucose metabolic state of mice. Random blood glucose monitoring is utilized to capture the natural fluctuations in glucose metabolism during normal physiological activities, whereas fasting blood glucose and glucose tolerance tests are employed to assess potential prediabetic states.

Protocol

The protocol of ICSI-ET described below follows the guidelines and has been approved by the Animal Ethical Review of the Shanghai Institute of Planned Parenthood research. Safety Procedures: Always wear appropriate personal protective equipment (PPE) when handling chemicals or biological materials. Use of Hoods: Perform all procedures involving volatile chemicals or aerosol generation within a certified fume hood or biosafety cabinet. Use female mice (6-8 weeks old B6D2F1 strain) for the superovulation procedure.

1. ICSI in mice

1. Superovulation of mice
   1. Administer an intraperitoneal injection of pregnant mare serum gonadotropin (PMSG) at a dose of 7.5 IU per mouse at around 20:00 on the first day.
   2. Administer an intraperitoneal injection of human chorionic gonadotropin (HCG) at a dose of 7.5 IU per mouse at around 20:00 (48 hours after PMSG injection) on the third day.
2. Prepare HTF medium into 1 mL aliquots and leave them uncovered for overnight equilibration at 37 °C in a 5% CO₂ incubator on the same day of HCG injection in female mice.
3. Preparation of ICSI fertilization droplets
   1. On the same day of HCG injection in female mice, divide a 50 x 9 mm cell culture dish lid into upper and lower halves. Use the upper half to create 4-6 droplets of 5 µL each, using 10% polyvinylpyrrolidone (PVP) for sperm placement. Use the lower half to create 8-10 droplets of 5 µL each, using M2 medium for the ICSI procedure.
   2. Cover the PVP and M2 medium droplets with approximately 3 mL of mineral oil (Figure 1).
4. Preparation of culture dishes
   1. On the same day of HCG injection in female mice, dispense 12 drops of KSOM medium (MR-020P-D) of 20 µL evenly in a 35 x 10 mm cell culture dish. Cover the drops with approximately 3 mL of mineral oil.
5. Sperm harvesting:
   1. Euthanize male mice by cervical dislocation at approximately 9:00 am on the fourth day (13 h after HCG injection).
   2. Isolate the epididymal cauda and thoroughly clean it to eliminate any adipose tissue and blood remnants by gently blotting with sterile gauze and rinsing in prewarmed HTF medium.
   3. Cut open the cauda epididymis to allow the sperm to freely swim out into the HTF medium. Gently squeeze the cauda using forceps to release the sperm, collect them in preequilibrated HTF tubes, and keep them uncovered in a 37 °C, 5% CO₂ incubator for sperm capacitation for further use.
6. Oocyte retrieval
   1. Euthanize female mice stimulated for oocyte retrieval by cervical dislocation at around 9:30 am on the fourth day. Carefully make an incision along the midline of the abdominal skin and muscle layer to expose the abdominal cavity.
   2. Use fine forceps to gently isolate tissue from both sides of the oviducts, especially the swollen ampulla regions, and transfer it into M2 medium in a sterilized culture dish. Repeat this procedure until the oviducts from all the designated mice are collected.
   3. Under a dissecting microscope, use a 1 mL syringe needle to puncture the transparent section of the swollen ampulla of the oviduct gently, facilitating the release of cumulus-oocyte complexes (COCs) into the medium.
   4. Using the same syringe, carefully pick and transfer the COCs into a new dish containing 100 µL of fresh M2 medium. Repeat this step until all COCs are successfully transferred from the original oviduct segments.
   5. Wipe away blood and tissue fluids, and under a dissecting microscope, tear open the ampulla of the oviduct using a 25 G needle, resulting in the release of numerous COCs.
   6. Remove the oviduct and tissue fragments, transfer the COCs into droplets of M2 medium containing hyaluronidase (HY), and prewarm them to 37 °C.
   7. After incubation for approximately 5 min, carry out gentle pipetting to facilitate the separation of cumulus cells and oocytes. Transfer the metaphase II (MII) oocytes from the HY-containing solution to fresh M2 medium; then, perform the sequential transfer into multiple droplets of M2 medium.
   8. After 2-3 rounds of washes, transfer the oocytes to droplets of KSOM culture medium and keep them in a 37 °C, 5% CO₂ incubator for further use.
7. Around 10:00 am on the fourth day, sonicate 100 µL of the capacitated sperm (see step 1.5.2.) for 5 s to separate the heads from the tails at a frequency of 46 kHz, with a power setting of 50 watts.
   NOTE: When sonicating sperm to truncate the tail, be cautious to control the time, which should not exceed 5 s. A few seconds after sonication, sperm can be observed under an inverted microscope; stop sonicating when approximately half of the sperm show truncated tails.
8. ICSI procedure
   1. Use a commercially available micromanipulation needle with a flat tip and an inner diameter of 6 µm, set at a 25° angle, for the injection. Fill the needle with the operating fluid with the filled portion measuring a length of 0.5 cm inside the needle. Ensure that the level of the fluid reaches ~0.5 cm.
      NOTE: Maintain room temperature at 19 °C during injection to avoid high temperatures that may reduce the survival rate of fertilized oocytes. The precision in ensuring the fluid level reaches 0.5 cm is crucial for maintaining the integrity of the injection process and ensuring the viability of the oocytes.
   2. Install the injection needle on the right arm of the micromanipulator, securely fixed by tightening the holding cap. Install a commercially available flat-tipped micromanipulation holding pipette designed for mouse oocytes on the left arm of the micromanipulator, and position the injection needle and the holding pipette in the microscope's field of view.
   3. Add 1 µL of sonicated sperm to a droplet of PVP solution using a pipette.
   4. Place 20 MII oocytes in a droplet of M2 medium.
   5. Use the injection needle to aspirate 10-20 sperm heads in PVP solution. Then, move the injection needle into the droplet containing the oocytes.
   6. Directly determine the position of the metaphase spindle within the oocyte under a polarization microscope to ensure that the spindle apparatus is not on the side of injection, with the polar body typically located at either the 12 o'clock or 6 o'clock position relative to the oocyte's orientation.
   7. Keep the oocyte in contact with the bottom of the culture dish. Adjust the focal plane so that the zona pellucida of the oocyte and the injection needle are in the same plane.
   8. Upon contact of the injection needle with the zona pellucida, apply a slight negative pressure to the injection needle, drawing in a portion of the zona pellucida.
   9. Simultaneously activate piezo pulses with an intensity setting of 5 and a frequency of 1, effectively creating a perforation in the zona pellucida and allowing the injection needle to pass through.
   10. Advance the injection needle into the perivitelline space, pushing the sperm head to the tip of the needle. Insert the injection needle into the oocyte until it reaches the opposite side of the zona pellucida.
   11. Use piezo pulses set to an intensity of 1 and a frequency of 1 to create a small pore in the plasma membrane, ensuring the sperm head is injected into the ooplasm with minimal entry of PVP and optimal preservation of the oocyte's integrity.
   12. Repeat step 1.8.11 until all 20 oocytes are injected, followed by a 15 min incubation at 19 °C.
   13. Transfer the surviving oocytes to KSOM medium and place them in the incubator. Observe 2-cell, 4-cell, morula, and blastocyst formation rates.
       NOTE: Timing for intracytoplasmic single sperm injection (ICSI) into the oocyte should be controlled, aiming to complete the procedure within 14-18 h after the injection of HCG.

2. Uterine transplantation of mouse blastocysts

1. Preparation of vasectomized male mice
   NOTE: Use ICR male mice aged 6-8 weeks for the study.
   1. Induce anesthesia by intraperitoneal injection of 1.25% 2,2,2-tribromoethanol at a dosage of 0.2 mL/10 g of body weight. The absence of any pain response indicates successful anesthetization.
      NOTE: While the current protocol does not include the application of vet ointment to the eyes of anesthetized animals, it is recommended as a best practice to prevent eye dryness and ensure the health and comfort of the animals during procedures. Throughout the surgical and recovery process, all surgical instruments were sterilized before use, and the surgical area was prepared using antiseptic solutions to ensure a clean operating environment.
   2. To safeguard against potential stress or injury, house the mice that have undergone surgery individually during the initial recovery phase.
   3. Administer buprenorphine at a dosage of 0.05 mg/kg of body weight to ensure optimal pain relief.
      NOTE: To ensure the well-being of the mice, closely monitor the mice during the recovery period and do not leave them unattended until they regain sufficient consciousness to maintain sternal recumbency.
   4. Prepare the surgical site at the lower abdominal region, superior to the testes. Remove the fur and disinfect the area with alcohol. Using sterile scissors, make a 1 cm longitudinal incision along the linea alba. Gently remove any blood with sterile, dust-free paper and proceed to bluntly dissect the abdominal muscles along the linea alba and access the abdominal cavity.
   5. Using curved forceps, gently grasp and expose the left testis and its associated fat pad. Identify the vas deferens and carefully isolate it using pointed forceps, creating a 10 mm segment for manipulation.
   6. Sterilize the curved forceps by holding them in the outer flame of an alcohol lamp for several seconds. Gently grasp the vas deferens, causing it to become flattened and whitened due to cauterization. Create a second cauterization point 3-5 mm distal to the first. Using sterile surgical scissors, excise the segment of the vas deferens between the two cauterization points.
   7. Carefully inspect the cauterized ends of the vas deferens, ensuring that a short, flattened, whitened segment remains. Using curved forceps, gently return the testis and its associated fat pad to the abdominal cavity.
   8. Repeat the procedure described in step 2.1.6 on the contralateral (right) side.
   9. Close the surgical site using sutures, typically placing one stitch in the muscle layer and three stitches in the skin. Ensure proper alignment of the surgical incision and disinfect the area with alcohol spray. Transfer the animal to a heating pad for observation, monitoring for 10-15 min until the animal regains consciousness. Once recovered, return the animal to its housing cage.
      NOTE: Following vasectomy surgery, mice require a 14-day recovery period, followed by a 1-week trial mating experiment.
   10. During this week, continuously mate the vasectomized males with females and observe the pregnancy status of all females presenting with copulatory plugs. Successful mating is indicated by the presence of a yellowish-white, blocky vaginal plug at the vaginal orifice of the female mouse.
       1. Pair the mice between 4:00 PM and 6:00 PM on the first day and check the vaginal plugs between 7:00 AM and 9:00 AM on the second day. Consider females with vaginal plugs to be in a pseudopregnant state at 0.5 days and use them for subsequent embryo transfer experiments. Return females without vaginal plugs to the recipient group.
          NOTE: These females without vaginal plugs can continue to be used for mating to prepare pseudopregnant females. Ensure that all plugged females do not become pregnant; only then can the vasectomized males be used for formal experiments.
       2. While vasectomized males can mate daily, allow for 1 day of rest between mating sessions.
          NOTE: Based on our experience, vasectomized males can maintain a high copulatory plug rate for over a year. However, vasectomized males that fail to produce copulatory plugs in mating females for 4-6 consecutive attempts should be removed from the study.
2. Preparation of pseudopregnant females
   1. Generate pseudopregnant female mice by mating ICR female mice (aged 7-10 weeks, weighing over 25 g) with vasectomized male mice (3-6 months old) at a 1:1 ratio.
      NOTE: The vasectomized males were used to stimulate the uterine cervix of the female mice, resulting in the activation of the corpora lutea.
3. Preparation of the pipette for transferring blastocysts
   1. Take the embryos to be transplanted out from the incubator and transfer them into a droplet of M2 medium at a temperature of 37 °C.
   2. Use a transfer pipette to aspirate a small volume of liquid, draw a small amount of air, and aspirate 6-10 embryos. Finally, aspirate another small volume of air to seal the pipette.
      NOTE: The length of the fluid in the transfer pipette should not exceed 0.5 cm.
4. Uterine transplantation (blastocysts)
   1. Weigh the pseudopregnant female mice (2.5 days post mating) and anesthetize them by intraperitoneal injection of 1.25% 2,2,2-tribromoethanol at a dose of 0.2 mL/10 g of body weight. Gently press the ear with forceps.
   2. After administering anesthesia to the mice and ensuring asepsis through alcohol disinfection, make a midline longitudinal incision on the ventral abdomen. Taking advantage of the loose subcutaneous tissue, shift the incision to the left or right side as required for the transplant. Use small scissors to perform blunt dissection of the abdominal wall muscle layer at ~0.5 cm lateral to the lumbar spine to access the peritoneal cavity.
   3. Using atraumatic forceps, carefully exteriorize the ovaries, oviducts, and uterus.
   4. Using the tip of a syringe needle, make a small, upward-angled incision on the uterine wall below the utero-tubal junction, avoiding blood vessels. This incision provides access to the uterine cavity; gently insert the oviduct 2-3 mm through the small hole.
   5. Carefully expel the small air bubble at the end of the transfer pipette while cautiously watching the transfer pipette during the slow injection of the embryos. Once the central portion of the culture medium is confirmed to be expelled, halt the procedure and gently remove the transfer pipette.
   6. Reposition the ovaries, oviducts, and uterus back into the abdominal cavity. Suture the muscle layer and skin separately. Place the mice on a heating pad until they recover from anesthesia. To ensure the well-being of the mice, closely monitor the mice during the recovery period and do not leave them unattended until they regain sufficient consciousness to maintain sternal recumbency.
5. Observation of pregnancy
   1. Following the surgery, observe the mice for 18-21 days to monitor the number of offspring born. In case of dystocia (difficult birth), perform a cesarean section. Allow the newborn mice to be fostered by lactating dams of the same age group.

3. Random blood glucose, fasting blood glucose, and glucose tolerance test

1. Tracking random blood glucose levels during mouse growth
   1. Since the birth of the mice, monitor the random blood glucose level every 4 weeks at 9 am after a good night of feeding and fun.
      1. Puncture the mouse tail tip to obtain tail vein blood.
      2. The first drop of blood has a higher sugar measurement value due to the presence of the tissue fluid mixed with it. Therefore, wipe the first drop of blood using clean absorbent paper gently.
      3. Measure the blood glucose from the second drop (~4 µL) from the tail vein with a handheld glucometer.
2. Tracking changes in fasting blood glucose and glucose tolerance during mice development
   NOTE: Considering that the first blood sampling time point is 15 min, all mice need to be injected within 15 min. Since each mouse injection took 20-30 s, to ensure that the injections are completed within this timeframe, the maximum number of mice that can be used in each test cannot exceed 30.
   1. Perform the fasting blood glucose and glucose tolerance tests every 4 weeks after birth in overnight-fasted mice.
      1. On the first day, prepare fresh irradiated shavings bedding, clean cages, lids, and water bottles. Transfer the mice into a fresh cage without food. Add do not feed tags to the cages.
      2. Prepare sufficient 10% glucose injection (sterile and pyrogen-free) and 1 mL syringes.
      3. Ensure that the mice in each group fast for 16 h, from 5:00 pm on day 1 to 9:00 am on day 2, but allow free access to water. Measure the fasting blood glucose levels as described in steps 3.1.1.1-3.1.1.3 using a glucose meter.
   2. Weigh the mice and mark their tails to facilitate rapid identification. Prepare glucose syringes with 10 µL/g of mouse weight.
   3. At 30 s intervals, inject 10 µL/g glucose into the intraperitoneal cavity of the mouse.
      NOTE: Special attention needs to be paid to injection techniques here to avoid inaccurate liquid injection volume. The key points of operation are as follows:
      1. Immobilize the mouse and restrain the tail with one hand. Keep the head slightly lower than the buttocks so that the organs in the abdominal cavity will shift towards the head, leaving enough injection space.
      2. Position the needle between the midline and the hip bone, insert it through the skin at a 45-degree angle, then gently guide the needle deeper into the abdominal cavity while keeping it aligned parallel to the skin surface for ~0.5 cm.
         NOTE: Pay attention to any resistance during injection to determine whether the internal organs are accidentally punctured.
      3. Eject the syringe into the abdominal cavity.
      4. Record the time when the injection started and use a timer to control the injection rate. Set the timer to 90 s for three mice to avoid losing time in handling the timer.
   4. Take the blood glucose measurement from the tail vein at the desired intervals (0 prior to glucose administration, 15, 30, 60, 90, and 120 min after the glucose loading).
      NOTE: Subsequent blood glucose testing should also take 20-30 s for each mouse.
   5. Return the mice to their home cages equipped with food and water after the experiment.

Representative Results

In our laboratory, we have achieved a fertilization rate of 89.57% and a 2-cell rate of 87.38% using ICSI with epididymal caudal sperm in mice. The birth rate of offspring following ET is approximately 42.50%. Remarkably, all the fertilization rates, 2-cell rates, and offspring birth rates are comparable to the levels achieved in human ART, enabling a comprehensive simulation of different stages of human ART techniques in mice. Further details are provided in Table 1 and Table 2. There was no signification fluctuation in random blood glucose levels between mice born naturally and ICSI mice during the process of growth and development (Figure 2). However, male ICSI mice showed continuously high levels of fasting blood glucose throughout the experiment (Figure 3), indicating impaired fasting blood glucose homeostasis. Interestingly, female ICSI mice showed no changes compared with the control group. In GTT, the blood glucose level showed significant change between male ICSI mice and the control group (Figure 4), and there was no such difference in female mice. These results suggested that the offspring obtained using ICSI technology might suffer from diabetes, and the outcome was related to sex. Although the female mice did not show the diabetes phenotype, the overweight phenotype of both male and female ICSI mice was consistent (Figure 5).

Using the intracytoplasmic sperm injection (ICSI) protocol described here, we achieved high rates of fertilization and embryo development in mice. In a representative experiment, 89.57% of oocytes were fertilized after ICSI, as evidenced by the formation of two pronuclei. At 24 h after ICSI, 87.38% of zygotes developed to the 2-cell embryo stage. After embryo transfer to recipient females, the birth rate of live pups was 42.50% (positive results). Using the optimized intracytoplasmic sperm injection (ICSI) protocol described here, we efficiently generated offspring from mouse oocytes and sperm. Importantly, a thorough characterization of ICSI-derived animals revealed metabolic perturbations in the absence of gross developmental defects.

Compared to naturally bred controls, male but not female ICSI mice displayed impaired glucose homeostasis, evidenced by elevated fasting blood glucose (Figure 3) and reduced glucose tolerance in intraperitoneal glucose tolerance tests (Figure 4). These alterations may stem from reprogramming deficiencies during ICSI-mediated fertilization or subtle genetic/epigenetic abnormalities incurred during sperm manipulation.

Additionally, both male and female ICSI mice showed increased body weight compared to controls (Figure 5), suggesting a propensity for obesity. While the mouse ICSI technique reliably models human ICSI outcomes, the observed metabolic phenotypes underscore the need to cautiously interpret results from ICSI animal models. Comprehensive evaluation of growth, health, and behavior is imperative to determine the suitability of ICSI mice for downstream applications.

Taken together, these data illustrate the utility of the mouse ICSI protocol to efficiently generate offspring for studies, while also highlighting the importance of thoroughly evaluating phenotypic outcomes in ICSI-derived animals. The sex-specific metabolic abnormalities observed in this example underscore the need for careful interpretation of results from ICSI mouse models.

Figure 1: Diagram of the dish preparation for intracytoplasmic sperm injection in mice. The cell culture dish lid was divided into upper and lower halves. The upper half contained droplets of 10% polyvinylpyrrolidone solution for sperm placement. The lower half contained droplets of M2 medium for conducting the ICSI procedure. All droplets were covered with mineral oil to prevent evaporation. This dish setup allowed the sequential transfer of sperm and oocytes between PVP and M2 droplets to enable the ICSI process. The separated areas prevented the mixing of media during the procedure. Abbreviations: ICSI = intracytoplasmic sperm injection; PVP = polyvinylpyrrolidone. Please click here to view a larger version of this figure.

Figure 2: Weekly random blood glucose levels in naturally bred control (n = 5) and ICSI (n = 5) mice during postnatal development. Blood glucose levels were measured weekly after overnight fasting from the tail vein of control and ICSI mice using a glucometer. Data are presented as mean ± SEM. Statistical significance was assessed using a two-way ANOVA, ns, P > 0.05. Abbreviations: SEM = Standard Error of the Mean; ICSI = intracytoplasmic sperm injection; ns = not significant; NM = natural mating. Please click here to view a larger version of this figure.

Figure 3: Weekly fasting blood glucose levels in naturally bred control (n =7 for males, n = 7 for females) and ICSI-treated mice (n = 7 for males, n = 7 for females) during postnatal development. Following overnight fasting, blood glucose levels were measured weekly from the tail vein of fasted control and ICSI-treated mice using a glucometer. It was observed that male ICSI-treated mice, but not female, exhibited significantly elevated fasting blood glucose levels compared to their sex-matched controls starting from 8 weeks of age. Data are presented as mean ± SEM. Statistical significance was assessed using a two-way ANOVA, ns, P > 0.05, ***P < 0.001,****P < 0.0001). Abbreviations: SEM = Standard Error of the Mean; ICSI = intracytoplasmic sperm injection; ns = not significant; NM = natural mating. Please click here to view a larger version of this figure.

Figure 4: Intraperitoneal glucose tolerance test in naturally bred control (n = 5) and ICSI-treated male mice (n = 5) at 16 weeks of age. Following an overnight fast, mice received an intraperitoneal glucose injection (2 g/kg of body weight). Blood glucose levels were then measured at 0, 30, 60, and 120 min post injection. Male ICSI-treated mice exhibited significantly higher blood glucose levels at 30, 60, and 120 min during IPGTT compared to naturally bred control. Data are presented as mean ± SEM. Analyses were performed using two-way ANOVA. ns, P > 0.05. *P < 0.05. Abbreviations: IPGTT = Intraperitoneal glucose tolerance test; SEM = Standard Error of the Mean; ICSI = intracytoplasmic sperm injection; ns = not significant; NM = natural mating. Please click here to view a larger version of this figure.

Figure 5: Body weight tracking of naturally bred control (n = 5 males, n = 5 females) and ICSI-treated mice (n = 5 males, n = 5 females) from 4 to 20 weeks of age. Weekly measurements revealed that both male and female ICSI-treated mice exhibited significantly increased body weight compared to their gender-matched controls, beginning at 8 weeks of age. Data are presented as mean ± SEM. Analyses were performed using two-way ANOVA. ***P < 0.001. ****P < 0.0001. Abbreviations: SEM = Standard Error of the Mean; ICSI = intracytoplasmic sperm injection; ns = not significant; NM = natural mating. Please click here to view a larger version of this figure.

	Oocytes	Zygotes	Fertilization rate (%)	2-cell	2-cell rate (%)	4-8 cell	4-8 cell rate (%)	Morula	Morula rate	Blastocysts	Blastocysts rate
Cauda sperm ICSI1	90	84	93.33	76	90.48	72	85.71	69	82.14	64	76.19
Cauda sperm ICSI2	80	72	90	62	86.11	59	81.94	53	73.61	46	63.89
Cauda sperm ICSI3	60	50	83.33	42	84	37	74	32	64	28	56
Total	230	206	89.57	180	87.38	168	81.55	154	74.76	138	66.99

Table 1: Developmental outcomes following mouse ICSI. Abbreviation: ICSI = intracytoplasmic sperm injection.

No.	No. of transferred embryos	No. of birth (%)	Male	Female	Birth rate (%)
1	6	3	3	0
2	6	1	1	0
3	5	2	0	2
4	8	4	2	2
5	8	2	1	1
6	7	5	3	2
Total	40	17	10	7	42.5

Table 2: Birth outcomes following uterine transplantation of ICSI mouse embryos. Abbreviation: ICSI = intracytoplasmic sperm injection.

Discussion

This study integrated mouse intracytoplasmic sperm injection (ICSI) and embryo transfer (ET) techniques to comprehensively recapitulate human assisted reproductive technology (ART) and examine the impact of ICSI in conjunction with ET on offspring development. The application of the ICSI technique with normal mouse sperm yielded high fertilization (86.76%) and 2-cell rates (88.48%). Following ET, the birth rate of offspring mice was approximately 42.50%, indicating the robustness of the technical platform. The most noteworthy finding of this study was the age-related fluctuation in glucose metabolism levels in ICSI offspring, which may be attributed to the application of the ICSI technique⁶.

The observed of fluctuations in glucose metabolism levels constitute a remarkable finding in this study. This finding implies potential long-term implications of the ICSI technique on the health of offspring, specifically regarding glucose metabolism⁷. To elucidate the reasons behind this phenomenon, we conducted a comprehensive examination of the pivotal technical steps encompassed within the ICSI procedure. We found that controlling the microinjection temperature (19 °C), ultrasound cutting duration (within 5 s), and transfer fluid volume (<0.5 cm) were important to maintain elevated embryo viability and implantation rates. Controlling the siphoning effect of the transfer pipette emerged as a critical step for precise embryo transfer. When preparing the transfer pipette, ensuring uniformity in the diameter of the tube is key to mitigating the siphoning effect and better controlling the movement of liquid within the tube. This measure is instrumental in enhancing the success rate and accuracy of embryo transfer. Furthermore, avoiding prolonged in vitro culture and overcrowding during embryo transfer also helped improve developmental competence. Employing optimized protocols, we attained a fertilization rate of 86.76% and a birth rate of 42.50% following ET, suggesting that the ICSI procedure itself did not inherently impair viability. Therefore, the fluctuations in glucose metabolism were more plausibly associated with subtle intracellular changes induced by the ART process rather than overt embryo damage.

While ART is widely regarded as safe, mounting evidence indicates that even seemingly normal ART offspring may exhibit various health risks later in life due to the unnatural reproductive characteristics and the critical periods of intervention during gamete and embryo development^{5,8,9,10,11,12,13}. Our findings provide novel insights into the potential metabolic effects of ART and highlight the need for further studies on the long-term impacts of ART on offspring health¹⁴.

It is essential to acknowledge that this study employed a murine model; consequently, the results may not be directly transferable to human subjects. Nevertheless, given the substantial biological parallels between mice and humans, these findings retain considerable referential significance¹⁵. Furthermore, it is important to acknowledge that patients undergoing ICSI in clinical practice often have sperm abnormalities. Conversely, this study utilized normal mice for ICSI procedures, thus lacking data on ICSI offspring from mice with sperm abnormalities. However, it is reasonable to anticipate that sperm abnormalities would have a more profound impact on the long-term effects of the ICSI technique. Hence, the outcomes of this study maintain notable referential merit.

Another limitation of our study is the methodological discrepancy between our mouse model and human clinical practices in sperm immobilization. In the clinical application of ICSI, it is common practice to immobilize human sperm by physically disrupting the tail with a micropipette. However, in our mouse model experiments, we employed ultrasonic tail clipping to prepare the sperm, a method routinely used in mouse embryonic manipulation. We acknowledge that this approach may inadvertently introduce variability and potential bias in the model, especially considering the possible damage to sperm integrity and function that ultrasound may cause. This methodological difference may limit the direct translatability of our findings. In our future work, we plan to explore the use of micropipettes for disrupting the tails of mouse sperm. This adjustment aims to minimize any unintended effects on sperm and better replicate the conditions and outcomes observed in human ICSI procedures. We are committed to continuously improving our research practices to enhance the clinical relevance of our findings.

Based on the aforementioned technological platform, in future research, we will further investigate the impact of the ICSI technique on other aspects of offspring health, such as cardiovascular health and the immune system. Furthermore, longer-term follow-up studies are also imperative to comprehensively understand the long-term effects of ICSI.

Materials

Name	Company	Catalog Number	Comments
1.25% avertin (2,2,2-tribromoethanol)	Nanjing Aibei	M2960	for anesthetization
Bacteriological Petri Dishes 35 x 10 mm style w/tight lid, crystal-grade virgin polystyrene, sterile	BD	353001
Bacteriological Petri Dishes 50 x 9 mm style w/tight lid, crystal-grade virgin polystyrene, sterile	BD	351006
Biosafety Cabinet	ESCO	class BSC	Aseptic operations, making culture dishes, aliquoting reagents, etc.
CO2 Incubator	Thermo	8000DH	Embryo culture
Dissection Microscope	Olympus	SZX16	Use in mouse embryo transfer
Fluorinert Fc-770	SIGMA	F3556	Fluorinert FC-770 is a thermally stable fully fluorinated liquid with high dielectric strength and resistivity, used as operating fluid
handheld glucometer	Roche	AccuChek performa	Blood glucose measurement
HTF	Merck	MR-070-D	The EmbryoMax Human Tubal Fluid (HTF) (1x), liquid designed for use with Mouse IVF is available in a 50 mL format and has been optimized and validated for Embryo Culture.
Hydraulic Microinjector	Eppendorf	CellTram 4r Oil, 5196000030	For sperm injection
Inverted Microscope	Nikon	TI2-U	Micromanipulation observation host
KSOM	Merck	MR-020P-D	(1x), Powder, w/o Phenol Red, 5 x 10 mL
M2	Merck	MR-015-D	EmbryoMax M2 Medium (1x), Liquid, with Phenol Red
Micromanipulator	NARISHIGE	NTX-N4	Micromanipulation arm
mineral oil	SIGMA	M8410	Mineral oil is suitable for use as a cover layer to control evaporation and cross-contamination in various molecular biology applications.
Needle Cutter	Nanjing Aibei	Sutter MF-800	Use for fabricating micromanipulation needles
Needle Puller	Nanjing Aibei	Sutter model p-100	Use for making micromanipulation needles
Piezo Drill Trip Mouce ICSI	Eppendorf	5195000.087	Application of ICSI injection needle.
Piezoelectric Micromanipulator (Membrane Breaker)	Eppendorf	Eppendorf PiezoXpert	Use of micro-pulses to break the zona pellucida and oolemma of the oocyte
Pneumatic Microinjector	Eppendorf	CellTram 4r Air, 5196000013	For fixing the oocyte
Ready-to-use Human Chorionic Gonadotropin (Hcg)	Nanjing Aibei	M2520	Sterilization reagent, intraperitoneal injection. 50 IU/mL
Ready-to-use Pregnant Mare's Serum Gonadotropin (PMSG)	Nanjing Aibei	M2620	Sterilization reagent, intraperitoneal injection. 50 IU/mL
Stereomicroscope	Olympus	SZX7	Oocyte retrieval and observation of embryo development