Concept 38.5: Extraembryonic Membranes Protect and Nourish the Embryo

A major step in the evolution of tetrapods (four-limbed vertebrates) was the formation of the amniotic egg, seen today in mammals and reptiles, including birds. We saw in Concept 23.6 the significance of this egg. It provides the embryo with a contained aqueous environment, freeing the processes of reproduction and development from dependence on an external water supply. The three germ layers that form the embryo also create the membranes that provide this protective environment.

Extraembryonic membranes form with contributions from all germ layers

Whether they develop inside or outside the mother’s body, embryos of amniotes are surrounded by four extraembryonic membranes that function in protection, nutrition, gas exchange, and waste removal. The chicken embryo provides a good example, but the process is similar in other reptiles and in egg-laying mammals. (We will discuss placental mammals in the next section.) The four membranes are the yolk sac, allantois, amnion, and chorion (FIGURE 38.18).

Figure 38.18: The Extraembryonic Membranes of Amniotes In birds, other reptiles, and mammals, the embryo constructs four extraembryonic membranes. (A,B) Membrane outgrowth and fusion in 5- and 9-day embryos. (A) Developing amnion and chorion form from ectoderm and endoderm as seen in this 5-day chick embryo. The membranes will fuse when they meet dorsally (arrows), enclosing the embryo in 2 envelopes. The yolk sac and allantois form from endoderm and mesoderm. The yolk sac grows around the yolk (arrows) and the allantois forms a pouch. (B) Membranes surround the embryo in this 9-day embryo. Some will continue to expand as the embryo grows, but the yolk sac shrinks as yolk is used by the embryo. Fluids secreted by the amnion fill the amniotic cavity, providing an aqueous environment for the embryo. The allantois stores the embryo’s waste products. The allantoic membrane will become pressed against the chorion and shell and functions in gas exchange between the embryo and its environment. (C) Developing chick in shell showing development of the circulatory system, which is well underway by day 7. Only the yolk sac and the allantois are capable of making blood vessels. As a result, they are involved in nutrient transport and gas exchange.

• The yolk sac forms first, by extension of the endoderm and the inner layer of lateral plate mesoderm (see Figure 38.18A,B). These two layers grow around the yolk and enclose it. The inner layer of lateral plate mesoderm is special because it can form blood cells and blood vessels. The first blood cells of the embryo and the vessels that carry them—the vitelline artery and its branches—form in the mesoderm of the yolk sac. Stored yolk is not transferred directly to the gut of the embryo through the yolk stalk. Rather, it is digested by the endoderm of the yolk sac, and the products are transported to the embryo by the developing blood and blood vessels in the yolk sac wall (Figure 38.18C).
• The allantois, a sac for storage of metabolic wastes, is also an outgrowth of endoderm and the inner layer of lateral plate mesoderm (Figure 38.18A–C). Because the yolk sac and the allantois are the extraembyronic membranes that contain lateral plate mesoderm, they are the only two that develop blood vessels (Figure 38.18C).
• Ectoderm and the outer layer of the lateral plate mesoderm also grow out from the embryo and surround it (Figure 38.18A). Where they meet, they fuse, forming two membranes, an inner amnion and outer chorion (Figure 38.18B). The amnion surrounds the embryo and secretes fluid into the enclosed cavity, providing a protective, shock-absorbing environment.
• The chorion, located just beneath the egg’s shell, forms a continuous membrane that limits water loss. The chorion cannot make blood vessels, but it fuses with the blood vessel–forming allantoic membrane, forming the chorioallantoic membrane that provides for exchange of O₂ and CO₂ between the embryo and the outside world (Figure 38.18C).

Extraembryonic membranes in mammals form the placenta

In placental mammals, the entire trophoblast becomes embedded in the endometrium of the uterus (see Figure 38.8). Hypoblast cells proliferate to form what in birds would be the yolk sac, even though there is little yolk in the eggs of placental mammals. In eutherian placental mammals, the allantois and chorion combine, forming the chorioallantoic placenta. These embryonic tissues, along with maternal tissue of the uterine wall (the endometrium), produce the placenta. Note that the placenta is a unique organ because it is composed of tissues from two organisms—the mother and her offspring. Interestingly, the yolk sac in mammals continues to be an early site of blood cell production as it is in birds, even though no or very little yolk is present. Meanwhile, the amnion grows around the embryo, enclosing it in a fluid-filled amniotic cavity (FIGURE 38.19). (A pregnant woman’s “water breaks” when the amnion bursts during labor and releases the amniotic fluid.)

Figure 38.19: The Mammalian Placenta In humans and other placental mammals, nutrients and wastes are exchanged between maternal and fetal blood in the placenta. This organ forms from both fetal tissue and tissues of the mother’s uterine wall. The embryo is attached to the placenta by the umbilical cord. Embryonic blood vessels invade the placental tissue to form fingerlike chorionic villi. Maternal blood flows into the spaces surrounding the villi, allowing nutrients and respiratory gases to be exchanged between the maternal and fetal blood.

Human gestation is traditionally divided into three periods of roughly 12 weeks each, called trimesters. The first trimester is a time of rapid cell division and tissue differentiation, and the embryo is very sensitive during this time to damage from radiation, drugs, chemicals, and pathogens that can cause birth defects such as those referred to in the opening story of the chapter. By the end of the first trimester, most organs have started to form, and the embryo is about 8 centimeters long. At about this time, the human embryo is medically and legally referred to as a fetus.

Fish also make yolk sacs

Many fish also have a very yolky egg (see Figure 38.7B). They are not amniotes, however, so how do they obtain nutrients from this stored material? Embryonic fish produce a yolk sac, but it differs from that of amniotes. As the fish embryo forms, all three germ layers—ectoderm, mesoderm and endoderm—grow around the yolk. The yolk sac becomes vascularized as in amniotes, and materials are carried in the blood vessels to the embryo (FIGURE 38.20).

Figure 38.20: Fish Yolk Sac The yolk sac in this developing shark is trilaminar because it is composed of all three tissue layers. Contrast this with the yolk sac of mammals, which forms from just two layers and thus is bilaminar.

We have now seen how a single-celled zygote becomes a complex organism, but development does not stop at birth or hatching. Animals must grow. For some, growth stops when they reach adulthood. Others grow throughout their lives. In either case, this growth is part of development. We will consider post-embryonic growth next.