The Questions of Developmental Biology

Initially, the morula does not have an internal cavity. However, during a process called
cavitation, the trophoblast cells secrete fluid into the morula to create a blastocoel. The inner cell
mass is positioned on one side of the ring of trophoblast cells (see Figures 11.23 and 11.25).
The resulting structure, called the blastocyst, is another hallmark of mammalian cleavage.
Escape from the Zona Pellucida
While the embryo is moving through the oviduct en route to the uterus, the blastocyst
expands within the zona pellucida (the extracellular matrix of the egg that was essential for
sperm binding during fertilization; see Chapter 7). The plasma membranes of the trophoblast cells
contain a sodium pump (a Na + /K + -ATPase) facing the blastocoel, and these proteins pump
sodium ions into the central cavity. This accumulation of sodium ions draws in water osmotically,
thus enlarging the blastocoel (Borland 1977; Wiley 1984). During this time, the zona pellucida
prevents the blastocyst from adhering to the oviduct walls. When such adherence does take place
in humans, it is called an ectopic or tubal pregnancy. This is a dangerous condition because the
implantation of the embryo into the oviduct can cause a life-threatening hemorrhage. When the
embryo reaches the uterus, however, it must "hatch" from the zona so that it can adhere to the
uterine wall.
The mouse blastocyst hatches from the zona by lysing a small hole in it and squeezing
through that hole as the blastocyst expands (Figure 11.25). A trypsin-like protease, strypsin, is
located on the trophoblast cell membranes and lyses a hole in the fibrillar matrix of the zona
(Perona and Wassarman 1986; Yamazaki and Kato 1989). Once out, the blastocyst can make
direct contact with the uterus. The uterine epithelium (endometrium) "catches" the blastocyst on
an extracellular matrix containing collagen, laminin, fibronectin, hyaluronic acid, and heparan
sulfate receptors. The trophoblast cells contain integrins that will bind to the uterine collagen,
fibronectin, and laminin, and they synthesize heparan sulfate proteoglycan precisely prior to
implantation (see Carson et al. 1993). Once in contact with the endometrium, the trophoblast
secretes another set of proteases, including collagenase, stromelysin, and plasminogen activator.
These protein-digesting enzymes digest the extracellular matrix of the uterine tissue, enabling the
blastocyst to bury itself within the uterine wall (Strickland et al. 1976; Brenner et al. 1989).
Gastrulation in Mammals
Birds and mammals are both descendants of reptilian species. Therefore, it is not
surprising that mammalian development parallels that of reptiles and birds. What is surprising is
that the gastrulation movements of reptilian and avian embryos, which evolved as an adaptation
to yolky eggs, are retained even in the absence of large amounts of yolk in the mammalian
embryo. The mammalian inner cell mass can be envisioned as sitting atop an imaginary ball of
yolk, following instructions that seem more appropriate to its reptilian ancestors.