Implantation

Embryonic implantation

The process where the blastocyst attaches to the uterine endometrium

Pre-implantation development

Early stages before implantation

Cleavage

Mitotic division of the zygote into smaller blastomeres

Morula

Solid ball of cells formed through cleavage

Blastocyst

Hollow structure with an inner cell mass (ICM) and trophoblast

Blastocoel

Fluid-filled cavity inside the blastocyst

Trophoblast

Outer layer of the blastocyst contributing to placenta formation

Inner cell mass (ICM)

Cluster of cells that form the embryo proper

Zona pellucida

Glycoprotein shell surrounding the embryo

protects against premature adhesion

Blastocyst hatching

Process where the blastocyst escapes the zona pellucida before implantation

Histotrophe

Uterine secretions providing nutrients to the pre-implantation embryo

Progesterone

Hormone promoting uterine receptivity and endometrial thickening

Apposition

Initial loose contact between the blastocyst and endometrium

Adhesion

Firm attachment of the blastocyst to the uterine epithelium

Invasion

Trophoblast cells penetrate the endometrium

Cytotrophoblasts

Proliferative trophoblast cells anchoring the embryo

Syncytiotrophoblasts

Multinucleated cells secreting enzymes to invade the endometrium

Decidualization

Transformation of endometrial stromal cells into decidual cells

Vascular endothelial growth factor (VEGF)

Promotes angiogenesis during implantation

Maternal-fetal interface

Connection between trophoblast and maternal tissues forming the placenta

Immune adaptation

Maternal immune modulation to tolerate the semi-allogeneic embryo

Leukemia inhibitory factor (LIF)

Endometrial signal promoting blastocyst attachment

Interleukin-1 beta (IL-1β)

Cytokine attracting the blastocyst to the endometrium

Cell adhesion molecules (CAMs)

Mediate blastocyst adhesion to the endometrium

Integrins

CAMs on the trophoblast aiding firm attachment

Superficial (centric) implantation

Blastocyst attaches to the uterine surface without invasion

Eccentric implantation

Partial embedding of the blastocyst into the endometrial crypt

Invasive (interstitial) implantation

Blastocyst fully penetrates the endometrial stroma

Delayed implantation (diapause)

Temporary suspension of blastocyst implantation

Horses’ implantation

Delayed with extensive intrauterine migration before attachment

Pigs’ implantation

Superficial with elongated blastocysts for even spacing

Dogs and cats’ implantation

Superficial with intrauterine migration for optimal spacing

Primates’ implantation

Highly invasive with direct connection to maternal blood vessels

Rodents’ implantation

Eccentric with partial embedding in uterine crypts

Seasonal breeders (mink

seals)

Maternal recognition of pregnancy

Process preventing luteolysis and supporting progesterone production

Interferon-tau (IFN-τ)

Ruminant signal inhibiting prostaglandin release to maintain pregnancy

Conceptus-derived estrogen

Pig embryo signal redirecting prostaglandin secretion to sustain the corpus luteum

Equine conceptus migration

Embryo movement signaling maternal recognition of pregnancy

Human chorionic gonadotropin (hCG)

Hormone secreted by the human embryo to sustain the corpus luteum

Endometrial pinopodes

Uterine projections aiding blastocyst attachment

Placenta accreta

Excessive trophoblast invasion causing abnormal placental attachment

Preeclampsia

Insufficient trophoblast invasion leading to maternal hypertension

In vitro fertilization (IVF)

Assisted reproductive technology (ART) optimizing implantation conditions

Uterine receptivity

Endometrial state conducive to blastocyst attachment

Chorioallantoic placenta

Formed by the fusion of chorion and allantois for fetal-maternal exchange

Choriovitelline placenta

Temporary placenta using the yolk sac for early embryonic nutrition

Species-specific adaptations

Variations in implantation mechanisms across mammals

Folic acid

Nutrient essential for preventing neural tube defects during early development