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Chapter 22: Development and Aging

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Early Developmental Stages

Fertilization occurs when the sperm and egg interact to produce a zygote.

Several sperm may penetrate the corona radiata, several may attempt to penetrate the zona pellucida, but only one sperm enters the egg.

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Fertilization – fusion of sperm and egg nuclei

During fertilization, the acrosome of a sperm releases enzymes that digest a hole in the corona radiata, then in the zona pellucida around the egg.

Depolarization of the egg’s plasma membrane after the sperm touches the egg prevents a second sperm from fertilizing the egg.

Fig. 22.1

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Embryonic Development

Development - all the changes that occur during the life cycle of an organism.

The embryo is the first stage in human development.

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Lancelet early developmentThe zygote undergoes cleavage, a period of cell division without growth.

Cleavage leads to a ball of cells called the morula.

The morula becomes a blastula when an internal cavity, the blastocoel, appears.

Fig. 22.2

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At the gastrula stage, invagination of cells into the blastocoel results in formation of the germ layers: ectoderm, mesoderm, and endoderm.

The three germ layers will have different developmental fates.

Fig. 22.2

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Neurulation and the Nervous System

The notochord forms from mesoderm.

During neurulation, the nervous system develops from midline ectoderm, just above the notochord.

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Development of neural tube and coelom in a frog embryo

A neural plate is seen first, then a neural tube; the anterior neural tube becomes the brain.

Fig. 22.4

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Chordate embryo, cross section

At the neurula stage, cross sections of all chordate embryos appear similar.

Fig. 22.5

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Developmental Processes

Development requires:

1) Growth

2) Cell differentiation (become specialized in structure and function).

3) Morphogenesis (produces shape and form).

**Each cell remains totipotent.

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Human Embryonic and Fetal Development

Human development is divided into the:

embryonic period (first 2 months) early formation of the major organs

fetal period (months 3–9) refinement of these structures.

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Four Extraembryonic membranesFig. 22.11

1) The amnion envelops the embryo/fetus in a protective amniotic fluid.

2) The yolk sac is the first site of red blood cell formation.

3) The blood vessels of the allantois become the umbilical blood vessels.

4) The chorion contributes to the placenta.

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The embryo is a ball of cells called a morula when it reaches the uterus on the third day.

By the fifth day, the morula is transformed into a blastocyst which consists of an outer trophoblast and an inner cell mass.

Fig. 22.12

Fertilization occurs in the upper third of the oviduct; a zygote is produced.

Embryonic Development The First Week

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Gastrulation occurs and the inner cell mass becomes the embryonic disk while the trophoblast becomes the chorion.

The yolk sac and amnion form.

Fig. 22.13The Second Week

Embryo begins to implant at end of first week.

Trophoblast secretes human chorionic gonadotropin (HCG).

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The Third WeekNeurulation occurs and the nervous

system becomes the first visible organ system.

The heart begins to form and pump blood.

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The Fourth and Fifth Weeks

The allantois forms and is contained within the umbilical cord.

Limb buds appear and sense organs develop.

Fig. 22.13

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Fig. 22.14

Human embryo at beginning of fifth week

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The Sixth Through Eighth WeeksBy end of eight weeks, the embryo is

only 38 mm (1.5 inches) long but is easily recognized as human.

All organ systems are established, even though the embryo weighs no more than an aspirin tablet at this point.

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Fetal Development and BirthThe Third and Fourth

MonthsThe body increases in size, and epidermal refinements (eyelashes, nipples) become apparent. Bone is replacing cartilage. Males can be distinguished from females, and the heartbeat is audible.

Fig. 22.15

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The Fifth Through Seventh MonthsThe thin skin is covered with lanugo and

coated with a vernix caseosa.

The eyelids open fully.

At the end of seven months, the fetus can possibly survive if born prematurely.

The fetus is now 300 mm (12 inches) in length and weighs 1,380 grams (3 lb).

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Fetal circulation and the placenta

Blood passes from the right to the left atrium through the foramen ovale Fig. 22.16

One umbilical vein takes nutrients to the systemic system when the umbilical vein joins the vena cava by a venous duct.

Two umbilical arteries that branch off the iliac arteries lead to the placenta.

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The Structure and Function of the Placenta

Chorionic villi project into maternal tissue as the placenta develops.

By the tenth week, the placenta is fully formed and secretes estrogen and progesterone.

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Anatomy of the placenta in a fetus at six to seven months Fig. 22.17

Fetal and maternal blood cells do not mix within the placenta.

Carbon dioxide and wastes diffuse from the fetal to the maternal side, and oxygen and nutrients diffuse from the maternal to the fetal side.

Harmful chemicals can cross the placenta and some alter normal fetal development.

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Three stages of parturition (birth)Fig. 22.18

1

2 3

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The breast contains 15–25 lobules with milk ducts.

No milk is produced during pregnancy, but milk ducts and alveoli proliferate during that time, and breasts enlarge.

Once the baby is delivered, the pituitary secretes prolactin, and milk is produced.

Female Breasts and Lactation

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Suckling of the baby at the breast stimulates the hypothalamus to direct the pituitary to secrete oxytocin that, in turn, causes milk to flow.

Breast milk contains antibodies that supplements the baby’s immature immune system.