Prenatal development is an important part of an individual's life. About 7 of every 100 people are born with some type of birth defect.

Prenatal Development

• Prenatal development is divided into the germinal, embryonic, and fetal periods.

• Developmental age is 14 days less than clinical age.

Fertilization

• Fertilization, the union of the oocyte and sperm cell, results in a zygote.

Early Cell Division

• The zygote undergoes divisions until it becomes an embryonic mass of cells.

Blastocyst

• The embryonic mass develops a cavity and is known as the blastocyst.

• The blastocyst consists of a trophoblast and an inner cell mass, where the embryo forms.

Implantation of the Blastocyst and Development of the Placenta

• The blastocyst implants into the uterus about 7 days after fertilization.

• The embryonic portion of the placenta is derived from the trophoblast of the blastocyst.

Formation of the Germ Layers

• The embryo forms around the primitive streak, which forms about 14 days after fertilization.

• All tissues of the body are derived from three primary germ layers: ectoderm, mesoderm, and endoderm.

Neural Tube and Neural Crest Formation

• The nervous system develops from a neural tube that forms in the ectodermal surface of the embryo and from neural crest cells derived from the developing neural tube.

Formation of the General Body Structure

• The limbs develop as outgrowths called limb buds.

• The face develops by the fusion of five tissue masses.

Development of the Organ Systems

• The GI tract develops as the developing embryo closes off part of the yolk sac.

• The heart develops as two tubes fuse into a single tube that develops septa to form four chambers.

• The kidneys and reproductive system are closely related in their development.

Growth of the Fetus

• The fetus increases 15-fold in length and 1400-fold in weight.

Parturition

• Uterine contractions force the baby out of the uterus during labor.

• Increased estrogens, decreasing progesterone levels, and secretions from the fetal adrenal cortex initiate parturition.

• Stretching of the uterus stimulates oxytocin secretion, which stimulates uterine contractions.

The Newborn

Respiratory and Circulatory Changes

• Inflation of the lungs at birth results in closure of the foramen ovale and the ductus arteriosus.

• When the umbilical cord is cut, blood no longer flows through the umbilical vessels.

Digestive Changes

• The newborn digestive system only gradually develops the ability to digest a variety of foods.

Lactation

• Estrogens and progesterone help stimulate the growth of the breasts during pregnancy.

• Suckling stimulates prolactin and oxytocin synthesis. Prolactin stimulates milk production, and oxytocin stimulates milk "letdown."

The First Year Following Birth

• Many important changes occur during the first year after birth. Many of these changes are linked to continued development of the brain.

Life Stages

• The eight stages of life are: germinal period (fertilization to 14 days),embryo (14-56 days after fertilization), fetus (56 days after fertilization to birth), neonate (birth-1 month), infant (1 month-1 or 2 years), child (1 or 2 years-puberty), adolescent (puberty to 20 years), adult (20 years to death).

Aging

• Aging occurs as irreplaceable cells wear out and the tissue becomes more brittle and less able to repair damage.

• Atherosclerosis is the deposit of lipids in the arteries. Arteriosclerosis is hardening of the arteries.

Death

• Death is defined as the absence of brain response to stimulation, the absence of natural respiration and heart function, and a flat electroencephalogram for 30 minutes.

Genetics

Chromosomes

• Humans have 46 chromosomes in 23 pairs.

• Males have the sex chromosomes XY and females have XX.

• During gamete formation, the chromosomes of each pair of chromosomes separate; therefore half of a person's genetic makeup comes from the father and half from the mother.

Genes

• A gene is a portion of a DNA molecule. Genes determine the proteins in a cell.

• Genes are paired (located on the paired chromosomes).

• Dominant genes mask the effects of recessive genes.

• Sex-linked traits result from genes on the sex
  chromosomes.

• In incomplete dominance, the heterozygote expresses a trait that is intermediate between the two homozygous traits.

• In codominance, neither gene is dominant or recessive, but both are fully expressed.

• Polygenic traits result from the expression of multiple
  genes.

Genetic Disorders

• A mutation is a change in the DNA.

• Some genetic disorders result from an abnormal distribution of chromosomes during gamete formation.

• Oncogenes are genes associated with cancer.

• Genetic predisposition makes it more likely a person will develop a disorder.

Genetic Counseling

• A pedigree (family history) can be used to determine the risk of having children with a genetic disorder.

• Specific chemical tests or examination of a person's karyotype can be used to determine a person's genotype.