Development of Systems and Organs
Development of
Systems and Organs
During vertebrate gastrulation the three
germ layers are formed. These differentiate,
as we have seen, first into primordial
cell masses and then into specific
organs and tissues. During this process,
cells become increasingly committed to
specific directions of differentiation.
Derivatives of the three germ layers are
diagrammed in Figure 8-24.
The assignment of early embryonic layers to specific “germ layers” (not to be confused with “germ cells,” which are the eggs and sperm) is for the convenience of embryologists and is of no concern to the embryo.Whereas the three germ layers normally differentiate into the tissue and organs described here, it is not the germ layer itself that determines differentiation, but rather the precise position of an embryonic cell with relation to other cells.
Since the mammalian embryo is protected and nourished through the placenta rather than with stored yolk, what becomes of the four extraembryonic membranes it has inherited from the early amniotes? The amnion remains unchanged, a protective water jacket in which the embryo floats. A fluid-filled yolk sac is also retained, although it contains no yolk. It has acquired a new function: during early development it is the source of stem cells that give rise to blood and lymphoid cells. These stem cells later migrate into the developing embryo. The two remaining extraembryonic membranes, the allantois and the chorion, are recommitted to new functions. The allantois is no longer needed for the storage of metabolic wastes. Instead it contributes to the umbilical cord, which links the embryo physically and functionally with the placenta. The chorion, the outermost membrane, forms most of the placenta itself. The rest of the placenta is formed by the adjacent uterine endometrium. The embryo grows rapidly, and all of the major organs of the body have begun their formation by the end of the fourth week of development. The embryo is now about 5 mm in length and weighs approximately 0.02 g. During the first two weeks of development (the germinal period) the embryo is quite resistant to outside influences. However, during the next eight weeks, when all of the major organs are being established and body shape is forming (the embryonic period), the embryo is more sensitive to disturbances that might cause malformations (such as exposure to alcohol or drugs taken by the mother) than at any other time in its development. The embryo becomes a fetus at approximately two months after fertilization. This ushers in the fetal period, which is primarily a growth phase, although some of the organ systems (especially the nervous and endocrine systems) will continue to develop. The fetus grows from approximately 28 mm and 2.7 g at 60 days to approximately 350 mm and 3000 g at term (nine months). Development of Systems and Organs During vertebrate gastrulation the three germ layers are formed. These differentiate, as we have seen, first into primordial cell masses and then into specific organs and tissues. During this process, cells become increasingly committed to specific directions of differentiation. Derivatives of the three germ layers are diagrammed in Figure 8-24. The assignment of early embryonic layers to specific “germ layers” (not to be confused with “germ cells,” which are the eggs and sperm) is for the convenience of embryologists and is of no concern to the embryo.Whereas the three germ layers normally differentiate into the tissue and organs described here, it is not the germ layer itself that determines differentiation, but rather the precise position of an embryonic cell with relation to other cells.
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| Figure 8-24 Derivatives of the primary germ layers in mammals. |
The assignment of early embryonic layers to specific “germ layers” (not to be confused with “germ cells,” which are the eggs and sperm) is for the convenience of embryologists and is of no concern to the embryo.Whereas the three germ layers normally differentiate into the tissue and organs described here, it is not the germ layer itself that determines differentiation, but rather the precise position of an embryonic cell with relation to other cells.
Since the mammalian embryo is protected and nourished through the placenta rather than with stored yolk, what becomes of the four extraembryonic membranes it has inherited from the early amniotes? The amnion remains unchanged, a protective water jacket in which the embryo floats. A fluid-filled yolk sac is also retained, although it contains no yolk. It has acquired a new function: during early development it is the source of stem cells that give rise to blood and lymphoid cells. These stem cells later migrate into the developing embryo. The two remaining extraembryonic membranes, the allantois and the chorion, are recommitted to new functions. The allantois is no longer needed for the storage of metabolic wastes. Instead it contributes to the umbilical cord, which links the embryo physically and functionally with the placenta. The chorion, the outermost membrane, forms most of the placenta itself. The rest of the placenta is formed by the adjacent uterine endometrium. The embryo grows rapidly, and all of the major organs of the body have begun their formation by the end of the fourth week of development. The embryo is now about 5 mm in length and weighs approximately 0.02 g. During the first two weeks of development (the germinal period) the embryo is quite resistant to outside influences. However, during the next eight weeks, when all of the major organs are being established and body shape is forming (the embryonic period), the embryo is more sensitive to disturbances that might cause malformations (such as exposure to alcohol or drugs taken by the mother) than at any other time in its development. The embryo becomes a fetus at approximately two months after fertilization. This ushers in the fetal period, which is primarily a growth phase, although some of the organ systems (especially the nervous and endocrine systems) will continue to develop. The fetus grows from approximately 28 mm and 2.7 g at 60 days to approximately 350 mm and 3000 g at term (nine months). Development of Systems and Organs During vertebrate gastrulation the three germ layers are formed. These differentiate, as we have seen, first into primordial cell masses and then into specific organs and tissues. During this process, cells become increasingly committed to specific directions of differentiation. Derivatives of the three germ layers are diagrammed in Figure 8-24. The assignment of early embryonic layers to specific “germ layers” (not to be confused with “germ cells,” which are the eggs and sperm) is for the convenience of embryologists and is of no concern to the embryo.Whereas the three germ layers normally differentiate into the tissue and organs described here, it is not the germ layer itself that determines differentiation, but rather the precise position of an embryonic cell with relation to other cells.
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