The Questions of Developmental Biology

The surfactant enables the alveolar cells to touch one another without sticking together.
Thus, infants born prematurely often have difficulty breathing and have to be placed on
respirators until their surfactant-producing cells mature.
As in the digestive tube, the regional specificity of the
mesenchyme determines the differentiation of the developing
respiratory tube. In the developing mammal, the respiratory
epithelium responds in two distinct fashions. In the region of the
neck, it grows straight, forming the trachea. After entering the
thorax, it branches, forming the two bronchi and then the lungs. The
respiratory epithelium can be isolated soon after it has split into two
bronchi, and the two sides can be treated differently. Figure 15.32
shows the result of such an experiment. The right bronchial
epithelium retained its lung mesenchyme, whereas the left bronchus
was surrounded with tracheal mesenchyme (Wessells 1970). The right bronchus proliferated and
branched under the influence of the lung mesenchyme, whereas the left bronchus continued to
grow in an unbranched manner. Moreover, the differentiation of the respiratory epithelia into
trachea or lung cells depends on the mesenchyme it encounters (Shannon et al. 1998). Thus, the
respiratory epithelium is extremely malleable and can differentiate according to its mesenchymal
instructions.
The Extraembryonic Membranes
In reptiles, birds, and mammals, embryonic development has taken a new evolutionary
direction. Reptiles evolved a mechanism for laying eggs on dry land, thus freeing them to explore
niches that were not close to water. To accomplish this, the embryo produces four sets of
extraembryonic membranes to mediate between it and the environment (see Chapter 11). Even
though most mammals have evolved placentas instead of shells, the basic pattern of
extraembryonic membranes remains the same. In developing reptiles, birds, and mammals, there
initially is no distinction between embryonic and extraembryonic domains. However, as the body
of the embryo takes shape, the epithelia at the border between the embryo and the extraembryonic
domain divide unequally to create body folds that isolate the embryo from the yolk and delineate
which areas are to be embryonic and which extraembryonic (Miller et al. 1994, 1999). These
membranous folds are formed by the extension of ectodermal and endodermal epithelium
underlain with lateral plate mesoderm. The combination of ectoderm and mesoderm, often
referred to as the somatopleure, forms the amnion and chorion; the combination of endoderm
and mesoderm the splanchnopleure forms the yolk sac and allantois. The endodermal or
ectodermal tissue supplies functioning epithelial cells, and the mesoderm generates the essential
blood supply to and from this epithelium. The formation of these folds can be followed in Figure
15.33.
The amnion and chorion
The first problem of a land-dwelling egg is desiccation. Embryonic cells would quickly dry out if
they were not in an aqueous environment. This environment is supplied by the amnion. The cells
of this membrane secrete amnionic fluid; thus, embryogenesis still occurs in water. This
evolutionary adaptation is so significant and characteristic that reptiles, birds, and mammals are
grouped together as the amniote vertebrates, or amniotes.