| Human Anatomy, 6/e Kent Van De Graaff,
Weber State University
Developmental Anatomy, Postnatal Growth, and Inheritance
Chapter SummaryInternal Affairs (113.0K) Fertilization - Upon fertilization of
a secondary oocyte by a spermatozoon in the uterine tube, meiotic development
is completed and a diploid zygote is formed.
- Morphogenesis is the
sequential formation of body structures during the prenatal period of human
life. The prenatal period lasts 38 weeks and is divided into a preembryonic,
an embryonic, and a fetal period.
- A capacitated spermatozoon
digests its way through the zona pellucida and corona radiata of the secondary
oocyte to complete the fertilization process and formation of a zygote.
Preembryonic Period - Cleavage of the zygote
is initiated within 30 hours and continues until a morula forms; the morula
enters the uterine cavity on about the third day.
- A hollow, fluid-filled
space forms within the morula, at which point it is called a blastocyst.
- Implantation begins between
the fifth and seventh day and is enabled by the secretion of enzymes that
digest a portion of the endometrium.
- During implantation,
the trophoblast cells secrete human chorionic gonadotropin (hCG), which
prevents the breakdown of the endometrium and menstruation.
- The secretion of
hCG declines by the tenth week as the developed placenta secretes steroids
that maintain the endometrium.
- The embryoblast of the
implanted blastocyst flattens into the embryonic disc, from which the primary
germ layers of the embryo develop.
- Ectoderm gives rise
to the nervous system, the epidermis of the skin and epidermal derivatives,
and to portions of the sensory organs.
- Mesoderm gives rise
to bones, muscles, blood, reproductive organs, the dermis of the skin,
and connective tissue.
- Endoderm gives rise
to linings of the GI tract, digestive organs, the respiratory tract and
lungs, and the urinary bladder and urethra.
Embryonic Period - The events of the 6-week
embryonic period include the differentiation of the germ layers into specific
body organs and the formation of the placenta, the umbilical cord, and the
extraembryonic membranes. These events make it possible for morphogenesis
to continue.
- The extraembryonic membranes
include the amnion, yolk sac, allantois, and chorion.
- The amnion is a thin
membrane surrounding the embryo. It contains amniotic fluid that cushions
and protects the embryo.
- The yolk sac produces
blood for the embryo.
- The allantois also
produces blood for the embryo and gives rise to the umbilical arteries
and vein.
- The chorion participates
in the formation of the placenta.
- The placenta, formed
from both maternal and embryonic tissue, has a transport role in providing
for the metabolic needs of the fetus and in removing its wastes.
- The placenta produces
steroid and polypeptide hormones.
- Nicotine, drugs,
alcohol, and viruses can cross the placenta to the fetus.
- The umbilical cord, containing
two umbilical arteries and one umbilical vein, is formed as the amnion envelops
the tissues on the underside of the embryo.
- From the third to the
eighth week, the structure of all the body organs, except the genitalia, becomes
apparent.
- During the third
week, the primitive node forms from the primitive line, which later gives
rise to the notochord and intraembryonic mesoderm.
- By the end of the
fourth week, the heart is beating; the primordial tissues of the eyes,
brain, spinal cord, lungs, and digestive organs are properly positioned;
and the superior and inferior limb buds are recognizable.
- At the end of the
fifth week, the sense organs are formed in the enlarged head and the appendages
have developed with digital primordia evident.
- During the seventh
and eighth weeks, the body organs are formed, except for the genitalia,
and the embryo appears distinctly human.
Fetal Period - A small amount of tissue
differentiation and organ development occurs during the fetal period, but
for the most part fetal development is primarily limited to body growth.
- Between weeks 9 and 12,
ossification centers appear, the genitalia are formed, and the digestive,
urinary, respiratory, and muscle systems show functional activity.
- Between weeks 13 and
16, facial features are formed and the fetal heartbeat can be detected with
a stethoscope.
- During the 17-to-20-week
period, quickening can be felt by the mother, and vernix caseosa and lanugo
cover the skin of the fetus.
- During the 21-to-25-week
period, substantial weight gain occurs and the fetal skin becomes wrinkled
and pinkish.
- Toward the end of the
26-to-29-week period, the eyes have opened, the gonads have descended in a
male, and the fetus is developed to the extent that it might survive if born
prematurely.
- At 38 weeks, the fetus
is full-term; the normal gestation is 266 days.
Labor and Parturition
- Labor and parturition
are the culmination of gestation and require the action of oxytocin, secreted
by the posterior pituitary, and prostaglandins, produced in the uterus.
- Labor is divided into
dilation, expulsion, and placental stages.
Periods of Postnatal
Growth - The course of human life
after birth is seen in terms of physical and physiological changes and the
attainment of maturity in the neonatal period, infancy, childhood, adolescence,
and adulthood.
- The neonatal period,
extending from birth to the end of the fourth week, is characterized by major
physiological changes.
- The most critical
need of the newborn is to establish adequate respiratory and heart rates.
A normal respiratory rate is 30 to 40 respirations per minute, and a normal
heart rate ranges from 120 to 160 beats per minute.
- The four reflexes
in the newborn critical to survival are the suckling reflex, the rooting
reflex, the crying reflex, and the breathing reflex.
- Infancy, extending from
4 weeks through the second year, is characterized by tremendous growth, increased
coordination, and mental development.
- By 2 years, most
infants weigh about four times their birth weight and average between
32 and 36 inches in length.
- Growth is a differential
process resulting in gradual changes from infant to adult body proportions.
- Childhood, extending
from the end of infancy to adolescence, is characterized by steady growth
until preadolescence, at which time there is a marked growth spurt.
- During childhood,
the average child becomes thinner and stronger each year as he or she
grows taller.
- The fact that disease
and death are relatively rare during childhood may be due to the fact
that lymphoid tissue is at its peak of development at this time; it is
also present in greater amounts in children than in adults.
- Adolescence is the period
of growth and development between childhood and adulthood.
- Puberty is the stage
of early adolescence when the secondary sex characteristics are expressed
and the sex organs become functional.
- The end result of
puberty is the structural expression of gender, or sexual dimorphism.
- Menarche generally
occurs in adolescent girls at the age of 13, but it may range from 9 to
17 years. At this time, vaginal secretions change from alkaline to acid.
- The first physical
indications of puberty are the appearance of breast buds in females and
the growth of the testes and the appearance of sparse pubic hair in males.
- Although semen may
be ejaculated at age 13, sufficient mature spermatozoa for fertility are
not produced until 14 to 16 years of age.
- Adulthood, the final
period of human physical change, is characterized by gradual senescence as
a person ages.
- Although skeletal
maturity is reached in early adulthood, anatomical and physiological changes
continue throughout adulthood and are part of the aging process.
- Sexual dimorphism
in human adults is evident anatomically, physiologically, metabolically,
and behaviorally.
- Male and female differences
in the stature, proportions, and composition of the body may become more
apparent with age.
Inheritance - Inheritance is the passage
of hereditary traits carried on the genes of chromosomes from one generation
to another.
- Each zygote contains
22 pairs of autosomal chromosomes and 1 pair of sex chromosomes: XX in a female
and XY in a male.
- A gene is the portion
of a DNA molecule that contains information for the production of one kind
of protein molecule. Alleles are different forms of genes that occupy corresponding
positions on homologous chromosomes.
- The combination of genes
in an individual's cells constitutes his or her genotype; the observable expression
of the genotype is the person's phenotype.
- Dominant alleles
are symbolized by uppercase letters and recessive alleles are symbolized
by lowercase letters.
- The three possible
genotypes are homozygous dominant, heterozygous, and homozygous recessive.
- A Punnett square is a
convenient means for expressing probability.
- The probability of
a particular genotype is 1 in 4 (.25) for homozygous dominant and homozygous
recessive, and 1 in 2 (.50) for heterozygous.
- A single trait is
studied in a monohybrid cross; two traits are studied in a dihybrid cross.
- Sex-linked traits such
as color blindness and hemophilia are carried on the sex-determining chromosome.
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