Light micrograph of a human cleavage embryo (arrow) implanting in the endometrium (18x).

Embryonic Stage

The embryonic stage extends from the beginning of the second week through the eighth week of development. During this time, the placenta forms, the main internal organs develop, and the major external body structures appear.

During the second week of prenatal development, the blastocyst completes implantation, and the inner cell mass changes. A space, called the amniotic cavity, forms between the inner cell mass and the portion of the tro-phoblast that "invades" the endometrium. The inner cell mass then flattens and is called the embryonic disk. By the end of the second week, layers form.

The embryonic disk initially consists of two distinct layers: an outer ectoderm and an inner endoderm. A short time later, through a process called gastrulation, a third layer of cells, the mesoderm, forms between the ectoderm and endoderm. These three layers of cells are called the primary germ layers (pri'mer-e jerm la'erz) of the primordial embryo. All organs form from the primary germ layers. At this point, the embryo is termed a gas-trula. Also during this time, a structure called a connecting stalk appears. It attaches the embryo to the developing placenta (fig. 23.7).

As the embryo implants in the uterus, proteolytic enzymes from the trophoblast break down endometrial tissue, providing nutrients for the developing embryo. A second layer of cells begins to line the trophoblast, and together these two layers form a structure called the chorion (ko're-on). Soon, slender projections grow out from the trophoblast, including the new cell layer, eroding their way into the surrounding endometrium by continuing to secrete proteolytic enzymes. These projections become increasingly complex, and form the highly

Germ layers of embryonic disk


Extraembryonic cavity —t

Yolk sac of embryo

Figure 23.7

Early in the embryonic stage of development, the three primary germ layers form.


Extraembryonic cavity —t

Yolk sac of embryo

Chronic Villi Same Sac


Amnion Amniotic cavity Connecting stalk Chorionic villi


Amnion Amniotic cavity Connecting stalk Chorionic villi branched chorionic villi, which are well established by the end of the fourth week.

Continued secretion of proteolytic enzymes forms irregular spaces called lacunae in the endometrium around and between the chorionic villi. These spaces fill with maternal blood that escapes from endometrial blood vessels eroded by the enzyme action. At the same time, embryonic blood vessels carrying blood to and from the embryo extend through the connecting stalk and establish capillary networks in the developing chorionic villi. These embryonic vessels allow nutrient exchange with blood in the lacunae and provide for the increased nutrient needs of the growing embryo.

Gastrulation is an important process in prenatal development because a cell's fate is determined by which layer it is in. Ectodermal cells give rise to the nervous system, portions of special sensory organs, the epidermis, hair, nails, glands of the skin, and linings of the mouth and anal canal. Mesodermal cells form all types of muscle tissue, bone tissue, bone marrow, blood, blood vessels, lymphatic vessels, connective tissues, internal reproductive organs, kidneys, and the epithelial linings of the body cavities. Endodermal cells produce the epithelial linings of the digestive tract, respiratory tract, urinary bladder, and urethra (fig. 23.8).

During the fourth week of development, the flat embryonic disk becomes a cylindrical structure. By the end of week four, the head and jaws appear, the heart beats and forces blood through blood vessels, and tiny buds form, which will give rise to the upper and lower limbs (fig. 23.9).

During the fifth through the seventh weeks, as figure 23.10 shows, the head grows rapidly and becomes rounded and erect. The face, which is developing the eyes, nose, and mouth, appears more humanlike. The upper and lower limbs elongate, and fingers and toes form (fig. 23.11). By the end of the seventh week, all the main internal organs are established, and as these structures enlarge, the body takes on a humanlike appearance.

D Which major events occur during the embryonic stage of development?

^9 Which tissues and structures develop from ectoderm? From mesoderm? From endoderm?

^9 Describe the structure of a chorionic villus.

D What is the function of the placental membrane?

B How are substances exchanged between the embryo's blood and the maternal blood?

Until about the end of the eighth week, the chorionic villi cover the entire surface of the former trophoblast. However, as the embryo and the chorion surrounding it enlarge, only those villi that remain in contact with the endometrium endure. The others degenerate, and the portions of the chorion to which they were attached become smooth. Thus, the region of the chorion still in contact with the uterine wall is restricted to a disk-shaped area that becomes the placenta (plah-sen'tah) (fig 23.12).

A thin membrane separates embryonic blood within the capillary of a chorionic villus from maternal blood in a lacuna. This membrane, called the placental membrane, is composed of the epithelium of the villus and the endothelium of the capillary (fig. 23.13). Through this membrane, substances are exchanged between the

Figure 23.8

Each of the primary germ layers forms a particular set of organs.

Figure 23.8

Each of the primary germ layers forms a particular set of organs.

Figure 23.9

(a) A human embryo at three weeks, dorsal view; (b) at three and one-half weeks, lateral view; (c) at about four weeks, lateral view.

maternal blood and the embryo's blood. Oxygen and nutrients diffuse from the maternal blood into the embryo's blood, and carbon dioxide and other wastes diffuse from the embryo's blood into the maternal blood. Active transport and pinocytosis also move various substances through the placental membrane.

The embryonic portion of the placenta is composed of parts of the chorion and its villi; the maternal portion is composed of the area of the uterine wall (decidua basalis) to which the villi are attached. When it is fully formed, the placenta appears as a reddish brown disk, about 20 centimeters long and 2.5 centimeters thick. It usually weighs about 0.5 kilogram. Figure 23.14 shows the structure of the placenta.

While the placenta is forming from the chorion, a second membrane, called the amnion (am'ne-on), develops around the embryo. This membrane began to appear during the second week. Its margin is attached around the edge of the embryonic disk, and fluid called amniotic fluid fills the space between the amnion and the embryonic disk.

The developing placenta synthesizes progesterone from cholesterol in the maternal blood. Cells associated with the developing fetal adrenal glands use the placen-tal progesterone to synthesize estrogens. The estrogens, in turn, promote changes in the maternal uterus and breasts and influence the metabolism and development of various fetal organs.

As the embryo becomes more cylindrical, the margins of the amnion fold, enclosing the embryo in the am-nion and amniotic fluid. The amnion envelops the tissues on the underside of the embryo, particularly the connecting stalk, by which it is attached to the chorion and the developing placenta. In this manner, the umbilical cord (um-bil'i-kal kord) forms (see fig. 23.12).

If a pregnant woman repeatedly ingests an addictive substance, her newborn may suffer from withdrawal symptoms when amounts of the chemical it is accustomed to receiving suddenly plummet. Newborn addiction occurs with certain addictive drugs of abuse, such as heroin; with certain prescription drugs used to treat anxiety; and even with very large doses of vitamin C. Although vitamin C is not addictive, if a fetus is accustomed to mega-doses, after birth the sudden drop in vitamin C level may bring on symptoms of vitamin C deficiency.

The fully developed umbilical cord is about 1 centimeter in diameter and about 55 centimeters in length. It begins at the umbilicus of the embryo and inserts into the center of the placenta. The cord contains three blood vessels—two umbilical arteries and one umbilical vein— that transport blood between the embryo and the placenta (fig. 23.15).

Actual Blood Vessels

Actual length

4 weeks

5 weeks
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