Analysis of Blastocyst Culture Procedures and Culture Medium Components

I. Full Workflow of Blastocyst Culture

Blastocyst culture refers to the process of culturing fertilized zygotes in vitro up to the blastocyst stage (approximately 5 to 7 days). The system mimics the maternal uterine microenvironment and maximizes embryonic developmental potential via precise culture condition control. The core procedures are detailed as follows:

1. Initial Culture of Fertilized Zygotes (Day 0–Day 3)

After oocyte retrieval and fertilization, zygotes are incubated in a thermostatically controlled incubator with 5% CO₂ at 37°C. The basal medium is prepared with Earle’s or Hank’s balanced salt solution supplemented with protein additives such as human serum albumin (HSA) to maintain osmolarity at 280–300 mOsm/kg. Embryos remain at the cleavage stage during this period; daily microscopic observation is performed to screen 4–8 cell embryos with zero fragmentation.

2. Optimized Culture Environment for Blastocyst Stage (Day 3–Day 5)

Upon reaching the 8-cell stage on Day 3, embryos are transferred to dedicated blastocyst medium via a two-step culture protocol. The first medium contains high concentrations of amino acids and energy substrates (pyruvate, lactate) to facilitate cell cleavage, followed by a medium with low amino acid and high glucose concentrations to mimic the late uterine environment. The incubator maintains a low-oxygen atmosphere of 5% O₂, 5% CO₂ and 90% N₂ to alleviate oxidative stress. Real-time monitoring systems including time-lapse equipment record developmental events such as blastocoel expansion and inner cell mass (ICM) formation.

3. Blastocyst Grading and Evaluation (Day 5–Day 7)

Mature blastocysts are graded in accordance with Gardner’s scoring system. Blastocysts are classified into Stage 1 to Stage 6 based on expansion degree (Stage 3 denotes blastocoel occupying over half of the embryonic volume). Assessments further cover the developmental quality of ICM and trophectoderm (TE): Grade A ICM features compact cell clusters, while Grade B TE presents sparse cell distribution. High-quality blastocysts such as 5AA require intact zona pellucida, fully expanded blastocoel, distinct cell boundaries and fragment-free cytoplasm.

4. Cryopreservation and Pre-transfer Preparation

For deferred embryo transfer, blastocysts are preserved via vitrification. Embryos undergo rapid dehydration with high-concentration cryoprotectants (e.g., DMSO combined with ethylene glycol) and are plunged into liquid nitrogen at -196°C. Thawing is conducted via serial gradient diluents to prevent cellular damage caused by abrupt osmotic shifts. One day prior to transfer, blastocysts are equilibrated in transfer medium to restore optimal developmental status.

II. Core Components and Functions of Culture Media

Blastocyst culture medium is a complex formulation simulating oviductal and uterine fluid, categorized into four major fractions: basal nutrients, energy metabolites, regulatory factors and cytoprotective agents.

1. Basal Electrolytes and Buffering System

- Electrolyte formulation: Composed of NaCl (100–140 mM), KCl (5–7 mM), CaCl₂ (1.5–2.0 mM) and other salts to stabilize osmolarity and membrane potential. Ca²⁺ plays an indispensable role in cleavage-stage cell division, while excessive K⁺ triggers cellular dehydration.

- Buffering system: Dual buffering with HEPES (10–20 mM) and CO₂/HCO₃⁻. HEPES stabilizes pH during open manipulation, whereas CO₂ interacts with medium bicarbonates in closed incubation to sustain physiological pH between 7.2 and 7.4.

2. Energy Metabolic Substrates

- Carbohydrates and organic acids: Cleavage-stage embryos primarily utilize pyruvate (0.2–0.4 mM) as fuel, while blastocysts shift to glucose metabolism (5.5–6.0 mM). The ratio of the two substrates is adjusted dynamically across developmental stages. Lactate (1–4 mM), a glycolytic end product, indirectly modulates embryonic metabolic efficiency by regulating medium pH.

- Amino acid mixture: A complete set of essential and non-essential amino acids. Glutamine (1–3 mM) serves as a primary nitrogen source for cell proliferation; taurine (0.02–0.05 mM) mitigates oxidative stress and alleviates DNA damage induced by reactive oxygen species (ROS).

3. Protein and Growth Regulators

- Protein supplements: Human serum albumin (HSA, 5–10 mg/mL) or fetal bovine serum (FBS) are commonly applied. HSA binds toxic heavy metals, maintains colloid osmotic pressure and transports fatty acids such as linoleic acid for membrane biosynthesis. Clinicians prefer HSA to lower immune risks derived from heterologous protein.

- Growth factors: Selective media incorporate insulin (5–10 μg/mL), transferrin (10–20 μg/mL) and epidermal growth factor (EGF, 1–5 ng/mL). These activate the PI3K-AKT signaling cascade to boost cell proliferation, and EGF specifically drives trophectoderm differentiation.

4. Antioxidants and Cytoprotective Reagents

- Antioxidants: Vitamin E (α-tocopherol, 5–10 μM), vitamin C (10–50 μM) and glutathione (1–3 mM) neutralize ROS in culture media. Under atmospheric oxygen (20% O₂), antioxidants effectively reduce embryonic DNA mutation rates.

- Osmoprotectants: Beyond glucose and amino acids, inositol (1–4 mM) and dextran (5–10 kDa, 1–3 mg/mL) balance intracellular and extracellular osmotic pressure and avoid cell rupture during blastocoel expansion.

III. Clinical Significance of Blastocyst Culture

Extended in vitro incubation enables natural screening of embryos with superior developmental competence, lifting clinical pregnancy rates to 60%–70%, an improvement of approximately 20% compared with cleavage-stage embryo transfer. Sophisticated medium formulation, including dynamic amino acid ratios and low-oxygen incubation systems, has pushed blastulation rates from 50% toward 70%, serving as a core guarantee for improved IVF success rates.

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