Mouse Embryo Development Stages

Mouse fetal development significantly matches that of humans. So much so, that researchers study mice development in order to more fully understand the development of human embryos and fetuses. In mice, embryos mature into fetuses after only ten days, the first four of which constitute pre-embryonic development. During embryonic development, the blastocyst transforms from a mass of cells into an underdeveloped fetus. Fetal development lasts another nine to ten days, and then the mother gives birth to a new born mouse.

  1. Pre-Embryonic Development

    • At fertilization, male spermatocytes penetrate the cellular membrane of the female oocyte (egg). The nuclei of both gametocytes, or sex cells, combine and undergo the process of mitosis. The two cells become one fertilized zygote. Cleavage of the zygote continues and it becomes a morula---a growing mass of undifferentiated cells---after the second day. On the third day, the morula converts to a blastocyst, or a mass of 16 to 40 cells surrounding a blastocoele, which is fluid filled cavity. On day four, the blastocyst implants itself into the uterine lining and becomes an egg cylinder within 24 hours. A placenta forms around the egg cylinder, and it has now become an embryo.

    Embryonic Development: Elongation

    • On day five, the blastocyst elongates and forms an egg cylinder. The placenta develops around the egg cylinder and begins to provide maternal blood to the embryo. Inside the placenta, the egg cylinder begins to differentiate into early embryonic and extra-embryonic regions. Late on the sixth day, gastrulation, or true differentiation, begins to occur.

    Embryonic Development: Gastrulation

    • At the tail end, a fold forms on the seventh day. This will become the coelemic fold, and later the end of the alimentary canal. At the head, a neural groove develops and an early head projection is formed. Within the next 24 hours, an early precursor of the stomach, called a foregut, forms. Also, a rudimentary cardiovascular system appears, which includes branchials, or gill-like structures used to breathe while in the uterus. Optic pits form, which is where eyes will later develop.

    Embryonic Development: Organogenesis

    • On the tenth and eleventh days, the embryo really begins to move into the realm of fetus. The early brain continues to develop and grow. Optic pits grow progressively deeper and optic vesicles develop. Hind- and forelimb buds form and, a short while later, hand plates---the precursors of feet---become visible. Mandibles and maxilla begin to develop. Nasal pits form below the optic pits. The tail elongates and the foregut extends into a slightly more developed digestive tract. Also, sex determination occurs during this phase.

    Post-Embryonic Development

    • Over the remaining week of gestation, the fetus develops more and more into a fully formed newborn mouse. This includes development of signs of individual digits on the front and hind limbs. The pinna---cartilaginous portion of the outer ear---becomes discernible and develops at a right angle to the head. The cardiovascular system develops further to include a heart and lungs. The lungs in mammals are not fully functional until after birth, when they are exposed to air for the first time.

      In the few days prior to birth, skin thickens and whiskers mature.