Human Development

In humans, gestation, or pregnancy, lasts about 266 days, or 9 months. In smaller mammals gestation is shorter—for example, 21 days in mice—and in larger mammals it is longer—for example, 330 days in horses and 600 days in elephants. The events of human gestation can be divided into three periods of roughly 3 months each, called trimesters.

Intrauterine development can be divided into three trimesters

THE FIRST TRIMESTER. Implantation of the human blastocyst begins on about the sixth day after fertilization. After implantation, gastrulation occurs, the placenta forms, tissues differentiate, and organs begin to develop. The heart begins

Chorionic Membrane

Sampled cells can be cultured and analyzed.

Cervical canal Vagina

20.20 Chorionic Villus Sampling Information about genetic defects can be obtained from chorionic tissues.The fetus and placenta are imaged by a sonogram to guide a catheter, which samples a chorionic villus.

Sampled cells can be cultured and analyzed.

Cervical canal Vagina

20.20 Chorionic Villus Sampling Information about genetic defects can be obtained from chorionic tissues.The fetus and placenta are imaged by a sonogram to guide a catheter, which samples a chorionic villus.

to beat in week 4, and limbs form by week 8 (Figure 20.21a). Most organs have started to form by the end of the first trimester. By that time, the embryo looks like a miniature version of the adult, and is called a fetus.

The first trimester is a time of rapid cell division and tissue differentiation. Signal transduction cascades and the resulting branching sequences of developmental processes are in their early stages. Therefore, the first trimester is the period during which the embryo is most sensitive to damage from radiation, drugs, chemicals, and pathogens that can cause birth defects. An embryo can be damaged before the mother even knows she is pregnant. A classic and tragic case is that of thalidomide, a drug widely prescribed in Europe in the late 1950s to treat nausea. Women who took this drug in the fourth and fifth week of pregnancy, when the embryo's limbs are beginning to form, gave birth to children with severely malformed arms and legs.

Hormonal changes cause major and noticeable responses in the mother during the first trimester, even though the fetus at the end of that time is still so small that it would fit into a teaspoon. Soon after the blastocyst implants itself, it begins to secrete human chorionic gonadotropin (hCG). This hormone stimulates the mother's ovary to continue to produce the hormones estrogen and progesterone, which help to maintain the pregnancy. These hormonal changes cause the well-known symptoms of pregnancy: morning sickness, mood swings, changes in the senses of taste and smell, and swelling of the breasts.

the second trimester. During the second trimester the fetus grows rapidly to a weight of about 600 g, and the moth-

20.21 Stages of Human Development (a) The organs and body structures of this 8-week-old embryo are forming rapidly, and it is visibly a human male. The embryo is approximately 4 cm long and weighs less than 10 g.The umbilical cord attaches the embryo to the placenta (upper left). (The dark red structure at the upper right is the remnant of the yolk sac.) (b) At 4 months, the fetus is about 14 cm long and weighs about 200 g. It has fully formed limbs and digits (fingers and toes) and moves freely within its protective amniotic cavity.

Months Human Fetus

er's abdomen enlarges considerably. The limbs of the fetus elongate, and the fingers, toes, and facial features become well formed (Figure 20.21b). Fetal movements are first felt by the mother early in the second trimester, and they become progressively stronger and more coordinated. By the end of the second trimester, the fetus may suck its thumb.

the third trimester. The fetus and the mother continue to grow rapidly during the third trimester. Even though the embryo is most susceptible to adverse effects of drugs, chemicals, and diseases during the first trimester, the potential for serious effects of exposure to many environmental factors continues throughout pregnancy. Severe protein malnutrition, alcohol consumption, and cigarette smoking are examples of factors that can result in low birth weight, mental retardation, and other developmental complications. As the third trimester approaches its end, many internal organs mature. The digestive system begins to function, the liver stores glycogen, the kidneys produce urine, and the brain undergoes cycles of sleep and waking.

Developmental changes continue throughout life

Development does not end with birth. Obviously, growth continues until adult size is reached, and even when growth stops, organs of the body continue to repair and renew themselves through cycles of cell replacement by the progeny of undiffer-entiated stem cells. In humans, in particular, enormous developmental changes occur in the brain in the years between birth and adolescence. Especially in the early years, there is a great deal of plasticity in the organization of the nervous system as patterns of connection between neurons develop.

For example, if a child is born with its eyes misaligned, a condition known as strabismus, he or she will use mostly one eye. The connections to the brain from that eye will become strong, and connections from the other eye will become weak. The child will develop with reduced visual acuity and depth perception. If the eye alignment is corrected in the first 3 years of life, however, the connections between the eyes and the brain will correct themselves, and the child will develop normal vision. If the eye alignment is corrected after 3 years of age, the correct connections between the eyes and the brain will not develop, and the visual impairment will be irreversible. Thus, plasticity in the development of the visual system in humans continues for several years after birth.

A very exciting area of current research is the role of learning in stimulating the production and differentiation of new neurons in the brains of young and even adult animals (see Chapter 46).

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Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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