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The placenta (pl.: placentas or placentae) is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation.It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate maternal and fetal circulations, and is an important endocrine organ, producing hormones that regulate both maternal and fetal ...
The placenta can be defined as an organ formed by the sustained apposition or fusion of fetal membranes and parental tissue for physiological exchange. [1] This definition is modified from the original Mossman (1937) [ 2 ] definition, which constrained placentation in animals to only those instances where it occurred in the uterus.
The inner cell mass will give rise to the pre-embryo, [9] the amnion, yolk sac and allantois, while the fetal part of the placenta will form from the outer trophoblast layer. The embryo plus its membranes is called the conceptus , and by this stage the conceptus has reached the uterus .
The process begins with fertilization, where an egg released from the ovary of a female is penetrated by a sperm cell from a male. The resulting zygote develops through mitosis and cell differentiation , and the resulting embryo then implants in the uterus , where the embryo continues development through a fetal stage until birth .
Due to the fact that placental mammals and marsupials nourish their developing embryos via the placenta, the ovum in these species does not contain significant amounts of yolk, and the yolk sac in the embryo is relatively small in size, in comparison with both the size of the embryo itself and the size of yolk sac in embryos of comparable developmental age from lower chordates.
Implantation, also known as nidation, [1] is the stage in the mammalian embryonic development in which the blastocyst hatches, attaches, adheres, and invades into the endometrium of the female's uterus. [2] Implantation is the first stage of gestation, and, when successful, the female is considered to be pregnant. [3]
By the end of the tenth week of gestational age, the embryo has acquired its basic form and is referred to as a fetus. The next period is that of fetal development where many organs become fully developed. This fetal period is described both topically (by organ) and chronologically (by time) with major occurrences being listed by gestational age.
Depending on the animal species, the process can occur within the body of the female in internal fertilization, or outside in the case of external fertilization. The fertilized egg cell is known as the zygote. [2] [5] To prevent more than one sperm fertilizing the egg , fast block and slow block to polyspermy are used. Fast block, the membrane ...