Fetal Development

The fetus is the final stage of prenatal development, extending from the end of the eighth week until birth. The organs that formed during the embryonic stage now undergo growth

Table 29.2

Derivatives of the Three Primary Germ Layers

Layer

Major Derivatives

Ectoderm

Epidermis; hair follicles and piloerector muscles; cutaneous glands; nervous system; adrenal medulla; pineal and pituitary glands; lens, cornea, and intrinsic muscles of the eye; internal and external ear; salivary glands; epithelia of the nasal cavity, oral cavity, and anal canal

Mesoderm

Skeleton; skeletal, cardiac, and most smooth muscle; cartilage; adrenal cortex; middle ear; dermis; blood; blood and lymphatic vessels; bone marrow; lymphoid tissue; epithelium of kidneys, ureters, gonads, and genital ducts; mesothelium of ventral body cavity

Endoderm

Most of the mucosal epithelium of the digestive and respiratory tracts; mucosal epithelium of urinary bladder and parts of urethra; epithelial components of accessory reproductive and digestive glands (except salivary glands); thyroid and parathyroid glands; thymus

and cellular differentiation and acquire the functional capability to support life outside the mother.

The circulatory system shows the most conspicuous anatomical changes from a prenatal state, dependent on the placenta, to the independent neonatal (newborn) state. The first trace of its development is the appearance of small spaces in the mesoderm before the third week. These become lined with endothelium and merge with each other to form the future blood vessels, lymphatic vessels, and heart. Two side-by-side endothelial tubes fuse to form a heart tube, which folds into the four-chambered heart.

The unique aspects of fetal circulation (fig. 29.11) are the umbilical-placental circuit and the presence of three circulatory shortcuts called shunts. The internal iliac arteries give rise to a pair of umbilical arteries, which pass on either side of the bladder into the umbilical cord. The blood in these arteries is low in oxygen and high in carbon dioxide and other fetal wastes. It discharges these wastes in the placenta, loads oxygen and nutrients, and returns to the fetus by way of a single umbilical vein, which leads toward the liver. Some of this blood filters through the liver to nourish it, but most of it bypasses the liver by way of a shunt called the ductus venosus, which leads directly to the inferior vena cava. The immature liver is not capable of performing many of its postpartum functions; many of these are performed by the placenta.

In the inferior vena cava, placental blood mixes with venous blood from the fetus's body and flows to the right atrium of the heart. While the right heart normally pumps all of its blood into the lungs after birth, there is little need for this in the fetus because the lungs are not yet functional. Most of the fetal blood therefore bypasses the pulmonary circuit. Some passes directly from the right atrium to the left through the foramen ovale, a hole in the intera-trial septum. Some goes into the right ventricle and is

Table 29.3

The Stages of Prenatal Development

Stage

Age*

Major Developments and Defining Characteristics

Zygote

24-30 hours

A single diploid cell formed by the union of egg and sperm

Cleavage

30-72 hours

Mitotic division of the zygote into smaller, identical blastomeres

Morula

3-4 days

A hollow, spherical stage consisting of 16 or more blastomeres

Blastocyst

4-14 days

A fluid-filled, spherical stage with an outer mass of trophoblast cells and inner mass of embryoblast cells; becomes

implanted in the endometrium; inner cell mass forms an embryonic disc and differentiates into the three primary

germ layers

Embryo

2-8 weeks

A stage in which the primary germ layers differentiate into organs and organ systems; ends when all organ systems

are present

Fetus

8-40 weeks

A stage in which organs grow and mature at a cellular level to the point of being capable of supporting life

independently of the mother

*From the time of ovulation

*From the time of ovulation

Saladin: Anatomy & I 29. Human Development I Text I © The McGraw-Hill

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1100 Part Five Reproduction and Development

Fetus

Newborn

Fetus

Fetal Circulation Human

Figure 29.11 Blood Circulation Before and After Birth (a) Fetal circulation. (b) Neonatal circulation.

Aortic arch-Superior vena cava Ductus arteriosus Ligamentum arteriosum Pulmonary artery Pulmonary veins Foramen ovale Fossa ovalis Heart Lung Liver Ductus venosus Ligamentum venosum Hepatic portal vein

Umbilical vein Ligamentum teres

Umbilicus Inferior vena cava Abdominal aorta Common iliac artery Urinary bladder

Umbilical arteries

Median umbilical ligaments Umbilical cord Placenta

Newborn

Umbilical Vein Inferior Vena Cava

Oxygen content of blood Low High

Figure 29.11 Blood Circulation Before and After Birth (a) Fetal circulation. (b) Neonatal circulation.

pumped into the pulmonary trunk, but most of this is shunted directly into the aorta by way of a short passage called the ductus arteriosus. This occurs because the collapsed state of the fetal lungs causes resistance and pressure in the pulmonary circuit to be high, so blood in the pulmonary trunk flows through the ductus into the aorta, where the blood pressure is lower. The lungs receive only a trickle of blood, sufficient to meet their metabolic needs during development. Blood leaving the left ventricle enters the general systemic circulation, and some of this returns to the placenta.

Other major aspects of embryonic and fetal development are listed in table 29.4 and depicted in figure 29.12.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

5. Distinguish between trophoblastic and placental nutrition.

6. Identify the two sources of blood to the placenta. Where do these two bloodstreams come closest to each other? What keeps them separated?

Saladin: Anatomy & I 29. Human Development I Text I © The McGraw-Hill

Physiology: The Unity of Companies, 2003 Form and Function, Third Edition

Chapter 29 Human Development 1101

Table 29.4

Major Events of Prenatal Development, with Emphasis on the Fetal Stage

Calendar

Length* and Weight

Month

at End of Month

Developmental Events

1

0.6 cm

Spinal column and central nervous system begin to form; appendages represented by small limb buds; heart begins beating around day 22; no visible eyes, nose, or ears

2

3 cm; 1 g

Eyes form, eyelids fused shut; nose flat, nostrils evident but plugged with mucus; head nearly as large as the rest of the body; brain waves detectable; ossification begins; limb buds form paddlelike hands and feet with ridges called digital rays, which then separate into distinct fingers and toes; blood cells and major blood vessels form; genitals present but sexes not yet distinguishable

3

9 cm; 30 g

Eyes well developed, eyelids still fused; nose develops bridge; external ears present; limbs well formed, digits exhibit nails; fetus swallows amniotic fluid and produces urine; fetus moves but too weakly for mother to feel it; liver is prominent and produces bile; palate is fusing; sexes can be distinguished visually

4

14 cm; 100g

Face looks more distinctly human; body larger in proportion to head; skin is bright pink, scalp has hair; joints forming; lips exhibit sucking movements; kidneys well formed; digestive glands forming and meconium9 (fetal feces) accumulating in intestinal lumen; heartbeat can be heard with a stethoscope

5

19 cm; 200-450 g

Body covered with fine hair called lanugo;10 skin has cheeselike sebaceous secretion called vernix caseosa,11 which protects it from amniotic fluid; skin bright pink; brown fat forms and will be used for postpartum heat production; fetus is now bent forward into "fetal position" because of crowding; quickening occurs—mother can feel fetal movements

6

27-35 cm; 550-800 g

Eyes open, eyelashes form; skin wrinkled, pink, and translucent; lungs begin producing surfactant; rapid weight gain

7

32-42 cm; 1,100-1,350 g

Skin wrinkled and red; fetus turns into upside-down vertex position; bone marrow is now the sole site of hemopoiesis; testes descend into scrotum; can usually survive if born after 25 weeks

8

41-45 cm; 2,000-2,300 g

Subcutaneous fat deposition gives fetus a more plump, babyish appearance, with lighter, less wrinkled skin; twins usually born at this stage

9

50 cm; 3,200-3,400 g

More subcutaneous fat deposited; lanugo is shed; nails extend to or beyond fingertips

*Crown-to-rump (CR) length, measured from the top of the head to the bottom of the buttocks mecon = poppy juice, opium 10lan = down, wool 11vernix = varnish + caseo = cheese

*Crown-to-rump (CR) length, measured from the top of the head to the bottom of the buttocks mecon = poppy juice, opium 10lan = down, wool 11vernix = varnish + caseo = cheese

State the functions of the placenta, amnion, chorion, yolk sac, and allantois.

What developmental characteristic distinguishes a fetus from an embryo? At what gestational age is this attained? Identify the three circulatory shunts of the fetus. Why does the blood take these "shortcuts" before birth?

Development is by no means complete at birth. For example, the liver and kidneys still are not fully functional, most joints are not yet ossified, and myelination of the nervous system is not completed until adolescence. Indeed, humans are born in a very immature state compared to other mammals.

Pregnancy Diet Plan

Pregnancy Diet Plan

The first trimester is very important for the mother and the baby. For most women it is common to find out about their pregnancy after they have missed their menstrual cycle. Since, not all women note their menstrual cycle and dates of intercourse, it may cause slight confusion about the exact date of conception. That is why most women find out that they are pregnant only after one month of pregnancy.

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