Embryonic periods

The embryonic period is the first major stage of gestational development and includes fertilization, blastocyst formation, implantation, and organogenesis. This period is approximately two-thirds the total gestational time in rodents, whereas in monkeys and humans, it comprises a much smaller fraction (approximately one-quarter) of total gestation. 

The blastocyst is a hollow, fluid-filled ball of approximately 1000 cells. The cells that form the outer layer are referred to as trophoblasts and will ultimately develop as extraembryonic tissues (e.g., placenta), while the cells of the inner cell mass are omnipotent (i.e., stem cells) and form the embryo. Depending on the species, the blastocyst arrives at the uterus within 5-10 days of fertilization, whereupon it hatches from the zona pellucida and implants into the uterine wall, which has been preconditioned by ovarian-derived steroid hormones (see Chapter 33). Shortly after implantation, the inner cell mass undergoes gastrulation to form a trilaminar embryo composed of three primary germ layers, the ectoderm, mesoderm, and endoderm. 

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The FGFs stimulate the proliferation of mesodermally and ectodermally-derived cells and play central roles in mammalian development. Members of the FGF family are expressed in the embryonic period and are required for several critical events in neural development and specifically for neural induction. FGF-8 is necessary for positional identity required for axial specification and patterning of limb development. FGF-2 stimulates the proliferation of multipotential stem cells that subsequently give rise to neurons of the cortex and other brain regions. 

Niimi, G., et al. A light and electron microscopic study of the mouse visceral yolk sac endodermal cells in the middle and late embryonic periods, showing the possibility of definitive erythropoiesis, Ann. Anat., 184,425, 2002. 

Longer treatment duration (embryonic and fetal periods versus embryonic period only for pharmaceuticals). 

Kidney development can be used again here to illustrate the kinds of events that occur during organ maturation, which takes place during fetal and postnatal development. While the induction of the organ and formation of its basic structure are initiated in the embryonic period, differentiation of the nephrons is not complete until term in humans, and not until the 2nd week postnatally in rodents. 

Embryonic period The period from fertilization to the end of major organogenesis. 

Animals Implan- tation Period 13 to 20 Somites End of Embryonic Period End of Metamorphosis Fetal Development Parturition... 

Exposure to retinoic acid, methylnitrosourea, and clomiphene during the early embryonic period, prior to the induction of the neural plate (before day 18 in the human), results in an increased incidence of neural tube defects and other malformations in experimental animal models (Bennett Finnell, 1998). In addition, exposure of rodents to teratogens such as retinoic acid, arsenic, and valproic acid during the period of neurulation results in neural tube defects such as spina bifida and encephaloceles (Adams Lammer, 1993 Bennett Finnell, 1998). Of these, therapeutic use of... 

Segment II the embryo-foetal development (embryo during major organ development i.e. organogenesis, the foetus in the post-embryonic period) and... 

Binder, R.L. and JJ. Stegeman. Basal levels and induction of hepatic aryl hydrocarbon hydroxylase activity during the embryonic period of development in brook trout. Biochem. Pharmacol. 32 1324-1327, 1983. 

The embryonic period lasts from approximately 2 weeks after fertilization until 8 weeks after fertilization, when the conceptus is renamed fetus Most body structures are formed during the embryonic period, and they continue to grow and mature during the fetal period. The fetal period continues until the pregnancy reaches term, approximately 40 weeks after the last menstrual period. ... 

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