Between fertilization and an established pregnancy lies a short but decisive phase. During the first week of human development, the single-celled zygote divides repeatedly, forms its first distinct cell types, reaches the uterus and then has to attach to and invade the uterine lining at the right moment. Much of what is known about this phase comes from research on mice and farm animals, combined with observations of human embryos produced during in vitro fertilization. The journals once published on this domain covered the subject for six decades, from wild deer to laboratory mice, pigs and human embryos.
From zygote to blastocyst
The earliest divisions, called cleavage, produce smaller and smaller cells within the protective shell of the egg, the zona pellucida. At first the embryo relies on molecules stored in the egg. Its own genome is switched on in stages: in mice the major activation happens at the two-cell stage, whereas in humans the main wave occurs on day 3, at the four- to eight-cell stage. Human embryos also compact and form a blastocyst later than mouse embryos do.
A widely used timeline for human preimplantation development, counted from fertilization, runs as follows:
- Day 3: major activation of the embryo’s genome at the four- to eight-cell stage.
- Around day 4: the cells compact into a tight ball, the morula.
- Days 5–6: a fluid-filled cavity opens and the blastocyst forms.
- Around day 6: the blastocyst hatches from the zona pellucida.
- Around day 7: implantation begins.
Human embryos at these stages frequently carry chromosome errors. Estimates of meiotic error rates in human eggs range from 5 to 20%, and one array-based study found mosaic aneuploidy, a mixture of cells with normal and abnormal chromosome numbers, in more than 80% of cleavage-stage embryos examined. How far such early errors are tolerated, eliminated or fatal to the embryo remains an active research question.
The first cell fate decisions
The blastocyst contains two cell populations. The outer layer, the trophectoderm, will give rise to most of the placenta; the inner cell mass will form the embryo itself and some of the membranes around it. In the mouse, cells compact at the eight-cell stage and are committed to one of these two fates by about the 32-cell stage, three and a half days after fertilization. In humans, compaction usually occurs later, around the 16-cell stage.
Two classic explanations compete for how a cell chooses its fate. The inside–outside model holds that position decides: cells enclosed by neighbours become inner cell mass. The polarity model holds that cells inherit an orientation that determines their fate when they divide. Current evidence suggests that both position and polarity contribute. At the molecular level, the Hippo signalling pathway keeps the co-factor Yap out of the nucleus of inner cells; in outer cells Yap works with the factor Tead4 to switch on Cdx2, a gene required for trophectoderm. Inner cells keep factors such as Oct4, Nanog and Sox2.
A second decision follows inside the inner cell mass, which splits into the epiblast, the source of the fetus, and the primitive endoderm, which contributes to the yolk sac. Cells first express the competing factors Nanog and Gata6 in a scattered “salt and pepper” pattern, FGF signalling pushes cells toward primitive endoderm, and the two layers are then sorted out. Nearly all of this detail comes from the mouse; how closely human embryos follow the same rules is still uncertain.
A narrow window: uterine receptivity
A blastocyst cannot implant whenever it arrives. The uterus is receptive only during a limited period, often called the window of implantation, and success requires a competent embryo to meet a receptive uterus at that time. In mice the window falls on day 4 of pregnancy; in women, receptivity is reached in the mid-luteal phase, about 7 to 10 days after ovulation. Progesterone, produced by the corpus luteum after ovulation, is required for implantation in every species studied, whereas the need for ovarian oestrogen varies from species to species.
Implantation itself is usually described as a sequence of apposition, adhesion or attachment, and penetration. In humans it is invasive: trophoblast cells pass through the uterine surface epithelium and its basement membrane into the underlying tissue, the stroma. Mouse studies have identified essential molecules, such as the cytokine LIF, without which implantation fails in mice, and the gene Hoxa10, needed for the transformation of the uterine stroma into decidua. Whether LIF plays an equally essential role in women has not been established.
Timing matters in human pregnancies too. A study that followed 221 women trying to conceive used the first appearance of chorionic gonadotrophin, the pregnancy hormone made by the embryo, as a marker of implantation. In pregnancies that lasted at least six weeks, implantation occurred 6 to 12 days after ovulation, and in 84% of them on day 8, 9 or 10. The risk of early pregnancy loss rose with later implantation: 13% when implantation took place by day 9, 26% on day 10, 52% on day 11 and 82% after day 11. In mice, deliberately delayed implantation leads to crowded embryos, growth restriction and smaller litters, which has prompted the hypothesis that implantation outside the normal window may contribute to some human miscarriages.
Pausing development: embryonic diapause
Some mammals can postpone implantation. In embryonic diapause, the embryo remains at the blastocyst stage, with minimal cell division and reduced metabolism, until conditions allow pregnancy to continue. The phenomenon was first described in the European roe deer in the 19th century and has since been reported in more than 130 mammal species, from bears and badgers to mice and marsupials. Triggers include seasonal food supply, temperature, day length and lactation, and the pause can last up to a year in some species.
In laboratory mice, a dormant state can be maintained with progesterone, and an oestrogen injection then induces implantation within one to two days. Diapause is often described as a complete arrest, but research on the roe deer suggests that its embryos keep growing and developing slowly rather than stopping altogether. The molecular signals that halt a blastocyst and later reactivate it are still poorly understood in every mammal studied.
What is settled and what is still debated
Several points are well established: the sequence of early cleavage stages, the dependence of implantation on progesterone, the existence of a limited window of receptivity, and, in the mouse, the main genes involved in the first lineage decisions. Other questions remain open. Ethical limits on research with human embryos, and the difficulty of observing implantation inside the human uterus, mean that many conclusions rest on animal models whose relevance is debated. How human embryos make their first fate decisions, which uterine signals are truly essential in women, and why so many early human pregnancies fail are still being investigated, including through embryos produced during IVF and assisted reproduction.
Sources
- Niakan KK et al., Development, 2012 — Human pre-implantation embryo development
- Cockburn K, Rossant J, Journal of Clinical Investigation, 2010 — Making the blastocyst: lessons from the mouse
- Cha J, Sun X, Dey SK, Nature Medicine, 2012 — Mechanisms of implantation: strategies for successful pregnancy
- Wilcox AJ, Baird DD, Weinberg CR, New England Journal of Medicine, 1999 — Time of implantation of the conceptus and loss of pregnancy
- Renfree MB, Fenelon JC, Development, 2017 — The enigma of embryonic diapause
- Rüegg AB, Ulbrich SE, Animal, 2023 — Embryonic diapause in the European roe deer: slowed, but not stopped
References from the journal archive
These articles were published in Reproduction, the Journal of Reproduction and Fertility or Reviews of Reproduction and are still cited at their old addresses on this domain. Each link leads to the publisher’s version through its DOI.
- EXPERIMENTAL ECTOPIC IMPLANTATION OF EGGS AND EARLY EMBRYOS IN GUINEA-PIGS
- MECHANISM OF ANTI-IMPLANTATION ACTION OF CLOMIPHENE
- INVOLVEMENT OF LUTEINIZING HORMONE IN THE IMPLANTATION PROCESS OF THE RAT
- THE UPTAKE OF HEXOSES BY PRE-IMPLANTATION MOUSE EMBRYOS IN VITRO
- EFFECT OF ALTERED PHOTOPERIOD ON DELAYED IMPLANTATION AND MOULTING IN ROE DEER
- EMBRYONIC MORTALITY IN THE POST-PARTURIENT DOMESTIC RABBIT
- CHROMOSOMAL ABNORMALITIES AND SEX RATIO IN RABBIT BLASTOCYSTS
- INDUCTION AND INHIBITION OF IMPLANTATION IN LACTATING RATS
- A review of embryonic mortality in cattle
- Immunocytochemical localization of binding 'sites' for LH and hCG in preimplantation mouse embryos
- Embryonic development of the degu, Octodon degus
- Cleavage beyond the block stage and survival after transfer of early bovine embryos cultured with trophoblastic vesicles
- Pyruvate and glucose uptake by mouse ova and preimplantation embryos
- Activation of nucleolar and extranucleolar RNA synthesis and changes in the ribosomal content of human embryos developing in vitro
- Maternal and embryonic factors associated with prenatal loss in mammals
- The embryo—maternal dialogue during early pregnancy in primates
- Comparison of embryo recovery, embryo quality, oestradiol-17β and progesterone profiles in domestic cats ( Felis catus ) at natural or induced oestrus
- Measurement of the metabolism of energy substrates in individual bovine blastocysts
- Nucleolus organizer regions and nucleoli in preattachment bovine embryos
- Co-culture of early cattle embryos to the blastocyst stage with oviducal tissue or in conditioned medium
- Effect of oxygen concentration on in-vitro development of preimplantation sheep and cattle embryos
- Morphology and proportion of inner cell mass of bovine blastocysts fertilized in vitro and in vivo
- Chorionic gonadotrophin and embryo-maternal recognition during the peri-implantation period in primates
- Optimization of mouse embryo culture media using simplex methods
- Adverse effects of gonadotrophin treatment on pre- and postimplantation development in mice
- Developmentally related changes in the metabolism of glucose and glutamine by cattle embryos produced and co-cultured in vitro
- Pregnancy rates and births after unilateral or bilateral transfer of bovine embryos produced in vitro
- Uptake and incorporation of inositol by preimplantation mouse embryos
- Activity of enzymes of energy metabolism in single human preimplantation embryos
- A quantitative investigation of gonadal feminization by diethylstilboestrol of genetically male embryos of the quail Coturnix coturnix japonica
- Comparison of development of pig zygotes and embryos in simple and complex culture media
- Oxygen uptake and carbohydrate metabolism by in vitro derived bovine embryos
- Analysis of apoptosis in the preimplantation bovine embryo using TUNEL
- Apoptosis in the human embryo
- Effect of time interval from insemination to first cleavage on the developmental characteristics, sex ratio and pregnancy rate after transfer of bovine embryos
- Reactivating tammar wallaby blastocysts oxidize fatty acids and amino acids
- Role of proteases in implantation
- Developmental consequences of embryo and cell manipulation in mice and farm animals
- Insulin-like growth factor axis during embryonic development
- Expression of prolactin-related protein I at the fetomaternal interface during the implantation period in cows
- A ‘minimum dose’ of lipopolysaccharide required for implantation failure: assessment of its effect on the maternal reproductive organs and interleukin-1α expression in the mouse
- Animal models of implantation
- DNA methylation pattern in human zygotes and developing embryos
- Embryonic diapause and its regulation
- Implantation mechanisms: insights from the sheep
- Insulin acts via mitogen-activated protein kinase phosphorylation in rabbit blastocysts
- PKC signalling regulates tight junction membrane assembly in the pre-implantation mouse embryo
- Two insulin-responsive glucose transporter isoforms and the insulin receptor are developmentally expressed in rabbit preimplantation embryos
- Expression profile of protein kinase C isozymes in preimplantation mouse development
- Peroxynitrites and impaired modulation of nitric oxide concentrations in embryos from diabetic rats during early organogenesis
- Role of gap junctions during early embryo development
- Subcellular localization of protein kinase C δ and ε affects transcriptional and post-transcriptional processes in four-cell mouse embryos
- Glucose affects monocarboxylate cotransporter (MCT) 1 expression during mouse preimplantation development
- Imprinted gene expression in the rat embryo–fetal axis is altered in response to periconceptional maternal low protein diet
- Expression of extracellular matrix metalloproteinase inducer and matrix metalloproteinases during mouse embryonic development
- Foetal fibroblasts introduced to cleaving mouse embryos contribute to full-term development
- Advancing the understanding of the embryo transcriptome co-regulation using meta-, functional, and gene network analysis tools
- Blastocyst elongation, trophoblastic differentiation, and embryonic pattern formation
- Crosstalk between the AMP-activated kinase and insulin signaling pathways rescues murine blastocyst cells from insulin resistance
- Expression of genes involved in early cell fate decisions in human embryos and their regulation by growth factors
- Monoallelic expression of nine imprinted genes in the sheep embryo occurs after the blastocyst stage
- Serotonin in pre-implantation mouse embryos is localized to the mitochondria and can modulate mitochondrial potential
- Biological differences between in vitro produced bovine embryos and parthenotes
- Comparative aspects of implantation
- Delayed implantation in giant pandas: the first comprehensive empirical evidence
- Manipulation of SMARCA2 and SMARCA4 transcript levels in porcine embryos differentially alters development and expression of SMARCA1 , SOX2 , NANOG , and EIF1
- Melatonin regulates delayed embryonic development in the short-nosed fruit bat, Cynopterus sphinx
- Potential immunomodulatory role of VIP in the implantation sites of prediabetic nonobese diabetic mice
- Revealing the bovine embryo transcript profiles during early in vivo embryonic development
- Spindle assembly checkpoint-related failure perturbs early embryonic divisions and reduces reproductive performance of LT/Sv mice
- Early stages of implantation as revealed by an in vitro model
- Seasonal effects on gene expression, cleavage timing, and developmental competence of bovine preimplantation embryos
- Successful transfer of day 10 horse embryos: influence of donor–recipient asynchrony on embryo development
- The adverse effects of obesity on conception and implantation
- Transforming growth factor beta (TGFB) signaling is activated during porcine implantation: proposed role for latency-associated peptide interactions with integrins at the conceptus–maternal interface
- A differential autophagic response to hyperglycemia in the developing murine embryo
- Evolution of the germ line–soma relationship in vertebrate embryos
- Increased apoptosis in bovine blastocysts exposed to high levels of IGF1 is not associated with downregulation of the IGF1 receptor
- Transcriptional sexual dimorphism during preimplantation embryo development and its consequences for developmental competence and adult health and disease
- Metabolism of the preimplantation embryo: 40 years on
- Puerarin, a selective oestrogen receptor modulator, disrupts pregnancy in rats at pre-implantation stage
- Early cell fate decisions in the mouse embryo
- Prediction of embryo implantation potential by mass spectrometry fingerprinting of the culture medium
- Histone methyltransferase Smyd3 regulates early embryonic lineage commitment in mice
- Low female birth weight and advanced maternal age programme alterations in next-generation blastocyst development
- Epigenetic reprogramming of the zygote in mice and men: on your marks, get set, go!
- Morphologic and transcriptomic assessment of bovine embryos exposed to dietary long-chain fatty acids
- Prostaglandin F2α promotes angiogenesis and embryo–maternal interactions during implantation
- Spatiotemporal dynamics of OCT4 protein localization during preimplantation development in mice
- Embryo cell allocation patterns are not altered by biopsy but can be linked with further development