Prenatal Development: Zygote Stage

Prenatal Development

• This is the process where a baby grows from a single cell after conception, first into an embryo, and then a fetus.
• The average time for prenatal development is 38 weeks from conception.
• A single-celled zygote goes through a series of stages to become a full-term baby.
• Conception happens when sperm fertilizes the egg.
• One egg is released from a woman's ovary about once a month during ovulation.
• The egg travels to the fallopian tube, which guides it to the uterus.
• For fertilization, a sperm must penetrate the zona pellucida, the egg's outer membrane.
• The genetic material of sperm and egg combine into a single cell called a zygote, marking the start of the germinal stage.

Zygote Stage

• The zygote rapidly divides through cleavage, forming two identical cells called blastomeres, which then divide into four, eight, and so on.
• The dividing cells move along the fallopian tube toward the uterus.
• About 60 hours post-fertilization, it becomes a morula of roughly sixteen cells, still within the zona pellucida.
• Three days after fertilization, the morula enters the uterus.
• The cell division continues, forming a fluid-filled blastocoele cavity in the center, with an outer layer of trophoblasts and an inner cell mass of embryo-blasts.
• The zona pellucida then disappears, and the morula becomes a blastocyst consisting of 200-300 cells.
• The inner cell mass of the blastocyst becomes the embryo, while the trophoblast becomes the support system.
• Development milestones include:
• Day 1: two identical cells (blastomeres)
• Day 2: four identical cells
• Day 3: eight identical cells
• Day 4: sixteen identical cells (morula)
• Day 5: blastocyst

Implantation

• The blastocyst implants in the uterine wall about six days after fertilization.
• Hormones from the mother's ovaries and chemicals from the trophoblasts prepare the uterine wall.
• The blastocyst attaches to the wall and moves into the uterine tissue, which marks the end of the germinal stage and the start of the embryonic stage.

Embryonic Stage

• This stage begins after implantation and lasts until eight weeks after conception.
• Cells rapidly divide and start to specialize in function through differentiation.
• Prenatal growth occurs in two directions:
• Cephalocaudal development (head to tail)
• Proximodistal development (midline outward)
• Gastrulation has the formation of 3 germ layers:
• Ectoderm (outer layer)
• Mesoderm (middle layer)
• Endoderm (inner layer)
• Each layer becomes different tissues and structures as the embryo develops

Germ Layers

• The ectoderm forms skin, nails, hair, brain, nervous tissue, nose, sinuses, mouth, anus, tooth enamel, and other tissues.
• The mesoderm forms muscles, bones, heart, lungs, reproductive organs, lymphatic tissue, and other tissues.
• The endoderm forms the lining of the lungs, bladder, digestive tract, tongue, tonsils, and other organs.
• Differentiation occurs over several weeks, with structures forming concurrently.

Major events in weeks 3-8

• Week 3: Brain, heart, blood cells, circulatory system, spinal cord, and digestive system start developing.
• Week 4: Bones, facial structures, and limbs start developing; the heart (which begins to beat), brain, and nervous tissue continue to develop.
• Week 5: Eyes, nose, kidneys, and lungs start developing; the heart (formation of valves), brain, nervous tissue, and digestive tract continue to develop.
• Week 6: Hands, feet, and digits start developing; brain, heart, and circulation system continue to develop.
• Week 7: Hair follicles, nipples, eyelids, and sex organs (testes or ovaries) start developing; first urine formation in the kidneys and first evidence of brain waves are present.
• Week 8: Facial features become more distinct, internal organs are well developed, the brain can signal for muscles to move, heart development ends, and external sex organs begin to form.
• By the end of the embryonic stage, all essential external and internal structures have formed, and the embryo is now called a fetus.

Fetal Stage

• From the ninth week until birth, the organism is known as a fetus.
• The age of viability (the first chance of survival outside the womb) is attained at approximately 24 weeks.
• Prenatal development is most dramatic during the fetal stage.
• At eight weeks, the fetus is about 3 centimeters (1.2 inches) long and weighs 3 grams (0.1 ounce).
• At 38 weeks, a full-term fetus may be 50 centimeters (20 inches) long and weigh 3.3 kilograms (7.3 pounds).
• All organ systems were formed during embryonic development.

Major Events in weeks 10-20

• Weeks 10-13 (8th-11th week)
• The fetus reaches 8 cm (3.2 inches) in length.
• The head is nearly half of the fetus' size.
• The face is well-formed.
• The eyelids close and will not reopen until the 28th week.
• Tooth buds appear.
• The limbs are long and thin.
• The fetus is able to make a fist with fingers.
• Genitals appear well differentiated.
• Red blood cells are produced in the liver.
• Weeks 14-17 (12th-15th week)
• About 15 cm (6 inches) in length.
• Lanugo, a fine hair, develops on the head.
• Fetal skin is almost transparent.
• Muscle tissue develops and the bones harden.
• The fetus makes active movements.
• Sucking motions with the mouth are present.
• Meconium is created in the intestinal tract.
• The liver and pancreas produce fluid secretions.
• Week 20 (18th week)
• 20 cm (8 inches) in length.
• Lanugo covers the entire body.
• Eyebrows and eyelashes appear.
• Nails appear on fingers and toes.

Major Events in weeks 24-36

• Week 24 (22nd week)
• Fetus reaches 28 cm (11.2 inches).
• Fetus weighs about 725 g (1 lb 10 oz).
• Eyebrows and eyelashes are well formed.
• All eye components are developed.
• A hand and startle reflex are present.
• Footprints and fingerprints continue forming.
• Alveoli (air sacs) form in the lungs.
• Week 28 (26th week)
• Fetus reaches 38 cm (15 inches).
• Fetus weighs about 1.2 kg (2 lb 11 oz).
• The brain develops rapidly.
• The nervous system develops enough to control some body functions.
• Eyelids open and close
• Cochleae are now developed, though the myelin sheaths in the neural portion of the auditory system will continue to develop until 18 months after birth.
• The respiratory system, while immature, has developed to the point where gas exchange is possible.
• A premature baby may survive, but the possibilities for complications and death remain high.
• Weeks 32 (30th week)
• Fetus reaches 38-43 cm (15-17 inches).
• Fetus weighs about 2 kg (4 lb 6 oz).
• Body fat rapidly increases.
• Rhythmic breathing movements occur, but the lungs are not fully mature.
• Thalamic brain connections, which mediate sensory input, form.
• Bones are fully developed, but are still soft and pliable.
• The fetus begins storing iron, calcium, and phosphorus.
• Week 36 (34th week)
• Fetus reaches 40-48 cm (16-19 inches).
• Fetus weighs about 2.5 to 3 kg (5 lb 12 oz to 6 lb 12 oz).
• Lanugo begins to disappear.
• Body fat increases.
• Fingernails reach the end of the fingertips.
• A baby born at 36 weeks has a high chance of survival but may require medical interventions.

Major Events in weeks 37-40

• Weeks 37-40 (35th-38th week)
• The fetus is considered full-term at the 37th week.
• The fetus may be 48 to 53 cm (19 to 21 inches) in length.
• Lanugo is mostly gone except on the upper arms and shoulders.
• Fingernails extend beyond fingertips.
• Small breast buds are present on both sexes.
• Head hair is coarse and thickest.
• Regions of the brain that contain the cell bodies are referred to as the gray matter.
• The axons that form the neural pathways make up the white matter because they are covered in myelin, a fatty substance that is white in appearance.
• Teratology is the study of factors that contribute to birth defects.
• Teratogens are risk factors such as maternal diseases, pollutants, drugs and alcohol.
• Ectopic pregnancy (fertilized egg implants itself outside of the uterus).
• Placenta previa (placenta lies low in the uterus and covers all or part of the cervix).
• Placenta abruption (placenta separates prematurely from the uterine wall).

Genes

• Genes are recipes for making proteins, consisting of the specific sequence of nucleotides.
• They are located on chromosomes with about 20,500 genes for humans
• Normal human cells contain 46 chromosomes (23 pairs; one from each parent) in the nucleus of the cells.

Mitosis and Meiosis

• After conception, most cells are created by mitosis
• Mitosis defined as the cell’s nucleus making
• an exact copy of all the chromosomes and splitting into two new cells
• However, the cells used in sexual reproduction, called the gametes (sperm or ova), are formed in a process called meiosis.
• Meiosis the gamete’s chromosomes duplicate, and then divide twice resulting in four cells containing only half the genetic material of the original gamete.
• Thus, each sperm and egg possesses only 23 chromosomes and combine to produce the normal 46.
• Of the 23 pairs of chromosomes created at conception, 22 pairs are similar in length and are called autosomes. The remaining pair, or sex chromosomes, may differ in length. If a child receives the combination of XY the child will be genetically male, or XX the child will be genetically female.

Genotypes and Phenotypes

• The word genotype refers to the sum total of all the genes a person inherits.
• The word phenotype refers to the features that are actually expressed.
• We may receive either the same version of a gene from our mother and father; that is, be homozygous for that characteristic the gene influences.
• If we receive a different version of the gene from each parent, that is referred to as heterozygous.
• Some genes are dominant, meaning they express themselves in the phenotype even when paired with a different version of the gene, while their silent partner is called recessive. Some dominant traits include having facial dimples, curly hair, normal vision, and dark hair.
• Sometimes the dominant gene does not completely suppress the recessive gene; this is called incomplete dominance
• Recessive genes express themselves only when paired with a similar version gene.
• Some recessive traits include red hair, being nearsighted, and straight hair.
• To inherit the disorder a person must receive the recessive gene from both parents. Those who have inherited only one recessive-gene are called carriers and should be unaffected by this recessive trait.
• Geneticists refer to different versions of a gene as alleles.
• Most characteristics are not the result of a single gene; they are polygenic, meaning they are the result of several genes.

Monozygotic and Dizygotic Twins

• Monozygotic or identical twins occur when a fertilized egg splits apart in the first two weeks of development which results in the creation of two separate, genetically identical offspring that possess the same genotype and often the same phenotype.
• Sometimes, however, two eggs or ova are released and fertilized by two separate sperm. The result is dizygotic or fraternal twins. These two individuals share the same amount of genetic material as would any two children from the same mother and father. In other words, they possess a different genotype and phenotype.
• Older mothers are more likely to have dizygotic twins than are younger mothers, and couples who use fertility drugs are also more likely to give birth to dizygotic twins.

Genetic Disorders

• Recessive gene disorders, such as cystic fibrosis and sickle-cell anemia, are less common, but may actually claim more lives because they are less likely to be detected as people are unaware that they are carriers of the disease.
• Some genetic disorders are sex-linked which means the defective gene is found on the X-chromosome. Males have only one X chromosome so are at greater risk for sex-linked disorders due to a recessive gene, such as hemophilia, color-blindness, and baldness.
• For females to be affected by the genetic defects, they need to inherit the recessive gene on both X-chromosomes, but if the defective gene is dominant, females can be equally at risk.

Recessive and Autosomal Disorders

• Recessive Disorders (Homozygous): The individual inherits a gene change from both parents. If the gene is inherited from just one parent, the person is a carrier and does not have the condition.
• Sickle Cell Disease (SCD): is a condition in which the red blood cells in the body are shaped like a sickle (like the letter C) and affect the ability of the blood to transport oxygen. Carriers may experience some effects, but do not have the full condition.
• Cystic Fibrosis (CF): is a condition that affects breathing and digestion due to thick mucus building up in the body, especially the lungs and digestive system. In CF, the mucus is thicker than normal and sticky.
• Phenylketonuria (PKU): is a metabolic disorder in which the individual cannot metabolize phenylalanine, an amino acid. Left untreated intellectual deficits occur. PKU is easily detected and is treated with a special diet.
• Tay Sachs Disease: is caused by enzyme deficiency resulting in the accumulation of lipids in the nerve cells of the brain. This accumulation results in progressive damage to the cells and a decrease in cognitive and physical development. Death typically occurs by age five.
• Albinism: is when the individual lacks melanin and possesses little to no pigment in the skin, hair, and eyes. Vision problems can also occur.
• Autosomal Dominant Disorders (Heterozygous): In order to have the disorder, the individual only needs to inherit the gene change from one parent.

Autosomal and Sex linked Disorders

• Huntington’s Disease: is a condition that affects the individual’s nervous system. Nerve cells become damaged, causing various parts of the brain to deteriorate. The disease affects movement, behavior and cognition. It is fatal, and occurs at midlife.
• Tourette Syndrome: is a tic disorder which results in uncontrollable motor and vocal tics as well as body jerking.
• Achondroplasia: is the most common form of disproportionate short stature, where the individual has abnormal bone growth resulting in short stature, disproportionately short arms and legs, short fingers, a large head, and specific facial features.
• Sex-Linked Disorders: When the X chromosome carries the mutated gene, the disorder is referred to as an X-linked disorder. Males are more affected than females because they possess only one X chromosome without an additional X chromosome to counter the harmful gene.
• Fragile X Syndrome occurs when the body cannot make enough of a protein it needs for the brain to grow and problems with learning and behavior can occur, caused from an abnormality in the X chromosome, which then breaks.
• if a female has fragile X, her second X chromosome usually is healthy, but males with fragile X don’t have a second healthy X chromosome, which is why symptoms of fragile X syndrome usually are more serious in males.
• Hemophilia: occurs when there are problems in blood clotting causing both internal and external bleeding.
• Duchenne Muscular Dystrophy: is a weakening of the muscles resulting in an inability to move, wasting away, and possible death.

Chromosomal Abnormalities and Other disorders

• Chromosomal Abnormalities occur when a child inherits too many or two few chromosomes.
• One of the most common chromosomal abnormalities is on pair 21, is Trisomy 21 or Down syndrome where there are three rather than two 21st chromosomes. A person with Down syndrome typically exhibits an intellectual disability and possesses certain physical features, such as short fingers and toes, folds of skin over the eyes, and a protruding tongue.
• When the abnormality is on the 23rd pair the result is a sex-linked chromosomal abnormality. A person might have XXY, XYY, XXX, XO. Two of the more common sex-linked chromosomal disorders are Turner syndrome and Klinefelter syndrome.
• Turner Syndrome- occurs when part or all of one of the X chromosomes is lost and the resulting zygote has an XO composition. The external genitalia appear normal, but breasts and ovaries do not develop fully and the woman does not menstruate.
• Klinefelter Syndrome (XXY)- results when an extra X chromosome is present in the cells of a male and occurs in 1 out of 650 live male births. An individual with Klinefelter syndrome typically has small testes, some breast development, infertility, and low levels of testosterone.
• Passive genotype-environment correlation occurs when children passively inherit the genes and the environments their family provides
• Evocative genotype-environment correlation refers to how the social environment reacts to individuals based on their inherited characteristics
  • for example, whether one has a more outgoing or shy temperament will affect how he or she is treated by others.
• Active genotype-environment correlation occurs when individuals seek out environments that support their genetic tendencies; this is also referred to as niche picking.
  • for example, children who are musically inclined seek out music instruction and opportunities that facilitate their natural musical ability.
• Genotype-Environment Interactions involve genetic susceptibility to the environment.
• Epigenetics studies modifications in DNA that affect gene expression and are passed on when the cells divide.