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When fetal kidneys begin to function around week 16, fetal urine also contributes to the fluid. [1] In earlier times, it was believed that the amniotic fluid was composed entirely of excreted fetal urine. The fluid is absorbed through the fetal tissue and skin. [2] After 22 to 25 week of pregnancy, keratinization of an embryo's
At the end of the second week of development, some cells of the trophoblast penetrate and form rounded columns into the syncytiotrophoblast. These columns are known as primary villi. At the same time, other migrating cells form into the exocoelomic cavity a new cavity named the secondary or definitive yolk sac, smaller than the primitive yolk sac.
In human pregnancy, prenatal development is also called antenatal development. The development of the human embryo follows fertilization, and continues as fetal development. 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 ...
Organogenesis is the phase of embryonic development that starts at the end of gastrulation and continues until birth. During organogenesis, the three germ layers formed from gastrulation (the ectoderm, endoderm, and mesoderm) form the internal organs of the organism. [1] The endoderm of vertebrates produces tissue within the lungs, thyroid, and ...
The ears begin to form as otic pits. Arm buds and a tail are visible. Lung bud, the first traits of the lung appear. [1] Hepatic plate, the first traits of the liver appear. [1] Buccopharyngeal membrane ruptures. This is the future mouth. [1] Cystic diverticulum, which will become the gallbladder, and dorsal pancreatic bud, which will become ...
Keratinization begins in the stratum spinosum, [5] although the actual keratinocytes begin in the stratum basale. [4] They have large pale-staining nuclei as they are active in synthesizing fibrillar proteins, known as cytokeratin, which build up within the cells aggregating together forming tonofibrils. The tonofibrils go on to form the ...
The gestational sac is spherical in shape, and is usually located in the upper part (fundus) of the uterus.By approximately nine weeks of gestational age, due to folding of the trilaminar germ disc, the amniotic sac expands and occupy the majority of the volume of the gestational sac, eventually reducing the extraembryonic coelom (the gestational sac or the chorionic cavity) to a thin layer ...
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