Prenatal Development

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Chapter: Anatomy and Physiology for Health Professionals: Pregnancy and Development

Prenatal Development : Cleavage: Blastocyst Formation, Implantation, Placentation: Embryonic Development, Gastrulation, Development of the Fetal Circulation


Prenatal Development

The time spent in prenatal development is called gestation . The gestation period occurs from the last menstrual period until birth, which takes approx-imately 280 days. A gestation time of less than 37 weeks is referred to as “premature.” If gestation continues beyond 42 weeks, it is considered “post-mature,” regardless of the size of the fetus or other ­factors. A developing offspring is referred to with a variety of names based on the period of development. The fertilized egg is called a conceptus or, more com-monly, a zygote. It undergoes mitosis about 30 hours after it has formed. The period from fertilization through the Eighth week of gestation is known as the embryonic period, with the zygote now being called an embryo. The period from the Ninth week of ­gestation through birth is known as the fetal period, with the embryo now being called a fetus. At birth, the fetus is then called an infant. During the first period of gestation, all major organ systems appear.

 

Cleavage

Cleavage is the series of repeated mitotic cell divi-sions that occur in an ovum immediately after fertil-ization (FIGURE 26-3). During this period, the tiny cell mass moves through the uterine tube to the uterine cavity. This takes about three days, with the structure consisting of a solid “ball” of 16 or more cells that is called a morula. The tiny dividing cells have a high surface-to-volume ratio, allowing them to uptake oxygen and nutrients while disposing of wastes. The quickly dividing cells begin to build embryonic structures.


Blastocyst Formation

From the initial single fertilized egg cell, there is divi-sion into two blastomeres. These, in turn, mitoti-cally divide into four cells, then eight, and so on. By the Fifth day after fertilization, the embryo is made up of approximately 100 cells. It has begun accumulating fluid within an internal cavity and floats freely inside the uterus. During this time, the zona pellucida degen-erates. The morula becomes a hollow blastocyst, which is basically a ball of cells that attaches to the endometrium. The blastocyst is filled with fluid and made up of one layer of large flat trophoblast cells as well as an inner cell mass of 20–30 rounded, clustered cells. Therefore, the sequence of preembryonic struc-tures is as follows: zygote, morula, and blastocyst.

Soon, the trophoblast cells develop L-selectin adhe-sion surface molecules. They aid in the formation of the placenta and secrete several immunosuppressive fac-tors, which protect the trophoblast and embryo from being attacked by the mother’s cells. The embryo forms from the inner cell mass after it forms the embryonic disc. Three extraembryonic membranes also form from the embryonic disc. The fourth membrane, called the chorion, is formed from the trophoblast.

 

Implantation

After about one week, the developing offspring has become superficially implanted in the endome-trium (FIGURE 26-4). Until this point, the cells that will form the developing offspring, known as plu-ripotent stem cells, can give rise to specialized cells, including ­additional stem cells. Uterine secretions, rich in glycoproteins,­ steroids, vitamins, and other nutrients, nourish the blastocyst. The window of implantation describes how receptive the endo-metrium is to implantation. This occurs because of surging ovarian hormone ­levels—estrogens and progesterone—in theblood.


When the endometrium is receptive, integrin and selectin proteins located on the trophoblast cells bind to extracellular matrix components and selectin-­ binding carbohydrates, respectively, located on the inner uterine wall. The extracellular matrix compo-nents are collagen, fibronectin, laminin, and other components. The blastocyst is implanted in the upper area of the uterus. Shortly after implantation, the tro-phoblast forms two distinct layers.

The placenta produces several hormones during pregnancy. Human chorionic gonadotropin (hCG), estrogen, and progesterone are the main hormones. hCG is secreted by trophoblast cells and maintains via-bility of the corpus luteum. It bypasses the effects of the hypothalamus, pituitary, and ovaries, prompting the corpus luteum to continually secrete progesterone and estrogen. This is controlled by the chorion. Therefore, the developing offspring assumes hormonal control of the uterus in this period. hCG is not only similar to luteinizing hormone in its effects, but also has prote-ase activity, promoting development of the placenta by acting as an autocrine growth factor. In areas where the trophoblast “faces” the endometrium, hCG levels are higher. The placenta also releases human placental lactogen, which is also called human chorionic soma-tomammotropin. This polypeptide hormone functions similarly to growth hormone. It modifies the mother’s metabolism to facilitate the energy supply of the fetus and also has anti-insulin properties.


Levels of hCG are often detectable in the mother’s blood just one week after fertilization. They rise until the end of the second month of development. Levels then decline sharply, reaching their lowest point at four months of gestation, and remain low from this point on (FIGURE 26-5). All pregnancy tests are actually antibody tests, detecting hCG in a mother’s blood or urine. At the Eighth week of development, the basic structural form of the human body is recognizable and the embryo is renamed a fetus. Simple versions of all organs are present­. These organs and other structures enlarge and become specialized as the fetus develops. The hormonal changes of pregnancy are summarized in TABLE 26-1.


 

Placentation

Placentation is the formation of the placenta FIGURE 26- 6). This temporary structure takes over ­production of progesterone and estrogen produc-tion for the remainder of the pregnancy, beginning in between the second and third months of gestation. The corpus luteum has degenerated by this time and the ovaries are inactive until birth occurs.


A layer of extraembryonic mesoderm develops from cells derived from the original inner cell mass. They line the inner surface of the trophoblast, forming the chorion, which develops chorionic villi. These finger-like structures are more elaborate in structure where they contact the maternal blood. Blood ves-sels form in their cores, extending to the embryo to form the umbilical arteries and vein. It is important to understand that originally all the blastocyst is sur-rounded by the chorionic villi. As the chorion enlarges, it expands like a balloon inside the endometrium In week four, the embryo, amnion, and yolk sac are suspended inside a growing, fluid-filled chamber.

Blood-filled lacunae are formed in the stratum functionalis of the endometrium. The villi are contin-ually nourished by extravascularized maternal blood. During placentation, the distal parts of the allantois and blood vessels carrying blood in and out of the pla-centa are contained in the body stalk, which connects the embryo and chorion. The yolk stalk is a narrow structure that connects the endoderm of the embryo with the yolk sac.

Beneath the embryo, the present endometrium becomes the decidua basalis, whereas the endome-trium surrounding the uterine cavity in the area of implantation becomes the decidua capsularis. The decidua basalis is located between the developing embryo and the myometrium. The placenta is actually formed by the combination of the decidua basalis and the chorionic villi. The decidua capsularis expands to accommodate the fetus, filling and stretching the uter-ine cavity over time. The villi in the decidua capsu-laris become compressed and degenerate, and the villi in the decidua basalis proliferate and become more branched. The remainder of the uterine endometrium, which has no contact with the chorion, is called the decidua parietialis.

By the third month of gestation, the placenta is usually fully functional. It supplies nutrition, oxygen, hormones, and also removes wastes. Barriers exist that prevent free passage of substances between the two blood supplies: the chorionic villi membranes and the embryonic capillaries of the endothelium.

Normally, the maternal and embryonic blood sup-plies do not mix.

Blood levels of estrogens and progesterone contin-ually increase throughout pregnancy. The uterine wall is maintained during the second and third trimesters by placental estrogens and placental progesterone. The placenta also secretes the hormone known as placen-tal lactogen, which helps to stimulate breast devel-opment and prepares the mammary glands for milk secretion. The relaxin hormone is also produced by the placenta. After birth, the placenta detaches and sloughs off.

1. What is cleavage?

2. Differentiate an embryo from a fetus.

3. What is a morula?

4. Which hormone suppresses uterine contractions until the birth process begins?

5. What are lacunae?

Embryonic Development

The process of embryonic development during and after implantation involves many significant steps. The blastocyst is converted to a gastrula and the three primary germ layers form. The extraembryonic membranes develop. Before developing three layers, the inner cell mass divides into two layers known as the upper epiblast and the lower hypoblast. The inner cell mass is then called the embryonic disc. The extra-embryonic membranes form during weeks two and three of development and are the amnion, yolk sac, allantois, and chorion.

The amnion is a transparent, membranous sac that develops from cells of the epiblast and fills with amniotic fluid. As the embryonic disc eventually curves and forms the tubular body, the amnion also curves. The sac eventually extends entirely around the embryo, with its only break being the umbilical cord. The amnion protects the embryo against trauma and maintains a constant temperature. The amniotic fluid prevents the growing structures of the embryo from sticking together or fusing, while allowing freedom of movement. The amniotic fluid is first formed from maternal blood, but the growing functionality of the embryo’s kidneys means that fetal urine later contrib-utes to the amniotic fluid.

From cells of the primitive gut, the yolk sac forms, which hangs from the embryo’s ventral surface. With the embryonic disc being the point of contact, the amnion and yolk sac appear like two balloons that touch each other. Human eggs contain only small amounts of yolk, with nutritive functions being assumed by the placenta. The yolk sac is important for two main reasons: it forms part of the embryo’s diges-tive tube and it is the source of the first blood cells and vessels that form.

The allantois forms at the caudal end of the yolk sac as a small formation of embryonic tissue. It is the structural basis for the umbilical cord, which links the embryo to the placenta. The allantois eventually becomes part of the urinary bladder. When fully formed, the umbilical cord has a core of embryonic connective tissue that is also called Wharton’s jelly.

The core also contains the umbilical arteries and vein and is covered by amniotic membrane externally. As described earlier, the chorion helps to form the pla-centa. It is the outermost membrane, enclosing the embryonic body and each of the other membranes.

 

Gastrulation

The processes of gastrulation begin with a primitive­ streak appearing on the embryonic disc’s dorsal surface. On day 12 of embryonic develop-ment, a new layer forms through gastrulation. This groove with raised edges creates the longitudinal axis of the embryo. Epiblast cells on the surface of the embryonic disc move medially over other cells to enter the primitive streak. The first cells enter-ing the groove displace the yolk sac’s hypoblast cells to form the inferior germ layer known as the ­endoderm (FIGURE 26-7). The cells that follow move laterally between the cells at the upper and lower surfaces to form the mesoderm . Mesodermal cells just beneath aggregate to form a rod of cells known as the notochord. This is the first axial support of the embryo. Cells that remain on the dorsal surface of the embryo make up the ectoderm. The embryo is now about 2 mm long.


Organogenesis

In organogenesis, the primary germ layers of the embryo begin to develop the organs. Organogene-sis is the primary event of embryonic development (TABLE 26 -2). The formation of the spinal cord and brain are among the first events of organogenesis. These structures rise from the ectoderm of the inner cell mass. FIGURE 26 -8 shows how the spinal cord forms during this period. The ectoderm folds inward, creat-ing the neural groove along the embryo’s entire pos-terior surface. The neural groove deepens and then closes over in a few weeks, creating the neural tube. This structure’s walls thicken to form the spinal cord and expand to form the brain. The spinal and cranial nerves develop from the ectodermal cells that form the neural crest. These cells form axons that attach to other organs of the body as well as the bones, muscles, and skin.


The mesoderm forms deeper muscles, bones, car-tilages, and other structures. Much of it first forms the somites, which then form the backbone and the head and trunk muscles. Mesoderm that is lateral to the somites becomes the dermis of the skin, the con-nective tissues, and the limb bones and muscles. The endoderm of the inner cell mass forms the yolk sac, with the upper part of the yolk sac forming the intes-tinal tract lining. The yolk sac also forms the blood cells and primitive germ cells. The germ cells move from the yolk sac wall to the developing ovaries and testes. These cells become the oogonia in females and the spermatogonia in males.


Cellular Formation of the Fetus

The following cells produce different parts of the developing fetus:

Ectodermal cells: Nervous system, parts of special sensory organs, epidermis, hair, nails, skin glands,linings of mouth and anal canal

Mesodermal cells: Muscle tissues, bone tissue,bone marrow, blood, blood vessels, lymphatic vessels, connective tissue, internal reproductive organs, kidneys, epithelial linings of body cavities

Endodermal cells: Digestive tract epithelium, respiratory tract, urinary bladder, urethra.

The flat embryonic disc becomes cylindrical, with the head and jaws developing by the end of the fourth week. The heart is now beating, forcing blood through the blood vessels, and tiny buds form, which will become the upper and lower limbs.

The head grows quickly and becomes rounded and erect between the fifth and seventh weeks, with the facial features developing. The limbs elongate and the fingers and toes appear. Programmed cell death or apoptosis forms them from the ­preexisting “webbing­.” At the end of the seventh week, all of the most critical internal organs are present.

Past the eighth week, only the villi that remain in contact with the endometrium endure. The others degenerate, with their former locations smoothing. The part of the chorion still contacting the uterine wall becomes the placenta. A thin placental membrane separates embryonic blood inside the capillary of a chorionic villus from the maternal blood in a lacuna. Maternal and embryonic blood exchange substances across this membrane (FIGURE 26-9). Oxygen and nutrients diffuse from the maternal blood into the embryo’s blood. Carbon dioxide and other wastes dif-fuse from the embryo’s blood into the maternal blood. Using active transport and pinocytosis, various sub-stances also cross the placental membrane.


1. Describe how the placenta forms.

2. Explain the various developmental milestones of the embryo.

3. How are substances exchanged between the embryo’s blood and the maternal blood?

4. What is a gastrula?

5. Define the terms “umbilical cord” and “yolk sac.”

 

Development of the Fetal Circulation

Maternal blood supplies oxygen and nutrients while car-rying away wastes, diffusing these substances through the placental membrane. The first blood cells develop in the yolk sac. Before week three, spaces appear in the splanchnic mesoderm that are soon lined by endothe-lial cells and covered with mesenchyme. They are linked together with quickly growing vascular networks that will form the heart, blood vessels, and lymphatics.

By the end of the third week, the embryo has a paired blood vessel system. The two vessels that form the heart have fused and are not bent to form an “S” shape. Just 3–4 days later, the heart is already pump-ing blood, although the embryo is less than one-fourth inch in length. The umbilical arteries and vein as well as three vascular shunts form. They are all occluded when birth eventually occurs. The umbilical vein car-ries freshly oxygenated blood from the placenta into the embryo’s body, bringing it to the developing liver. After birth, the umbilical vein becomes the ligamentum teres.

Fetal blood contains about 50% more oxygen-­ carrying hemoglobin than maternal blood. Fetal hemoglobin­ can carry up to 30% more oxygen than adult hemoglobin. The path of blood in the fetal cardio-vascular system is shown in FIGURE 26-10.


Nearly half the blood carried to the fetus via the umbilical vein passes into the liver, with the rest enter-ing the ductus venosus, which bypasses the liver. This vessel extends to join the inferior vena cava, where oxygenated blood from the placenta mixes with deoxygenated blood from the lower areas of the fetal body. This blood then continues on to the right atrium. Because fetal lungs are nonfunctional, blood largely bypasses them. Much of the blood entering the fetal right atrium is moved directly into the left atrium through an opening in the atrial septum called the foramen ovale. Blood pressure is slightly greater in the right atrium than the left atrium. A small valve helps prevent blood flow from reversing. In the liver, blood flow is important to ensure the health of the liver cells. After birth, the infant’s liver assumes the functions of nutrient processing that are handled by the mother’s liver during gestation.

The rest of the right atrium blood passes into the right ventricle and out through the pulmonary trunk.The pulmonary blood vessels have a high resistance to blood flow during this period of development, but enough blood reaches them to sustain them. Most of the pulmonary trunk blood enters a fetal vessel called the ductus arteriosus, connecting to the descending portion of the aortic arch. Blood low in oxygen is pre-vented from entering the portion of the aorta branch-ing to the heart and brain.

A mixture of highly oxygenated blood entering the left atrium and a small amount of deoxygenated blood from the pulmonary veins moves into the left ventricle and is pumped into the aorta; some reach the myocardium and some reach the brain. Blood from the descending aorta moves to the lower regions of the body, with the rest passing into the umbilical arteries leading to the placenta. There, it is reoxygen-ated. TABLE 26-3 summarizes fetal circulation. At birth, the fetal cardiovascular system must adjust when the placenta stops functioning and the newborn begins to breathe.


Growth During the Fetal Period

Teratogens are environmental factors that cause con-genital malformations by interfering with prenatal growth or development. Among the known teratogens are chemical agents, including drugs such as thalido-mide and alcohol; infectious agents, especially ­German measles; and ionizing radiation (x-rays). The fetal period begins at the end of the eighth week and lasts until birth. Growth occurs rapidly during this period, with body proportions beginning to appear more like those of a normal infant. Growth of the head begins to slow as growth of the body increases. By the 12th week, the external reproductive organs may be distinguished as either male or female. FIGURE 26-11 shows fetal devel-opment at 5–6 weeks, four months, and five months.


The fetus grows rapidly during the fourth month, reaching up to 20 cm in length as the limbs lengthen and the skeleton continues to ossify. Ultrasound may be used to assess a fetus in utero during pregnancy (­FIGURE 26-12). Tests show that four months old fetuses turn away from bright lights if flashed on the moth-er’s belly and show reactions to loud noises. Growth slows during the fifth month and the lower limbs have reached their final relative proportions. The mother may feel movement beginning around this time. The fetus begins to grow hair on its head, and the skin of the body is covered in fine hair and a mixture of dead epidermal cells and sebum from the sebaceous glands.


During the sixth month, the fetus gains substan-tial weight and the eyebrows and eyelashes grow. The skin is wrinkled, translucent, and reddish in appearance because of the many blood vessels. In the seventh­ month, fat is deposited in subcutaneous tissues, smoothing the skin. The eyelids, which were fused during the third month, reopen. By the end of the seventh month, the fetus is about 40 cm long.

During the final trimester, brain cells form net-works, organs specialize and grow, and fat continues to develop beneath the skin. The testes of the male fetus descend into the scrotum. The final systems to mature are the digestive and respiratory systems; hence, many babies have difficulty breathing and digesting milk from the mother. At the end of the ninth month or, more accurately, after 266 days, the fetus is considered full term. By now, it is nearly 50 cm long and weighs between 2.7 and 3.6 kg. The skin has lost its fine hair but is still coated with sebum and dead epidermal cells. The scalp is usually covered with hair. Nails have developed on the fingers and toes and the skull bones are largely ossified. The fetus is normally positioned upside down with the head toward the mother’s cervix (­FIGURE 26-13). During pregnancy, the respiratory rate and tidal vol-ume increase. Maternal blood volume, the glomerular filtration rate, and nutrient requirements also increase.



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