Chapter 27: Development and Heredity – Study Notes

Development and Heredity

Overview

This chapter covers the processes of human development from fertilization through birth, including embryonic and fetal development, placentation, parturition, lactation, congenital defects, and the principles of heredity. Understanding these processes is essential for comprehending how genetic and environmental factors influence human growth and inheritance.

Fertilization and Early Development

Fertilization

• Fertilization is the union of a sperm and an oocyte, resulting in a zygote (fertilized egg).

• Capacitation: Sperm undergoes changes in the female reproductive tract, making its membrane fragile and enabling the release of acrosomal enzymes (takes 6–8 hours).

• The oocyte is surrounded by two layers: corona radiata and zona pellucida.

• Hundreds of sperm release acrosomal enzymes to penetrate the zona pellucida (acrosomal reaction).

• Once a sperm fuses with the oocyte, the oocyte membrane depolarizes, blocking polyspermy (entry of additional sperm).

• The secondary oocyte completes meiosis II after fertilization.

• The zygote undergoes DNA replication and the first mitotic division, forming two cells.

Cleavage and Blastocyst Formation

• Cleavage: Rapid mitotic divisions without growth, producing smaller cells called blastomeres.

• Morula: A solid ball of 16 cells, formed about 72 hours after fertilization.

• Blastocyst: A hollow sphere of about 100 cells (day 5), consisting of an outer trophoblast (forms placenta) and an inner cell mass (ICM, forms embryo).

Implantation

• Occurs around day 7 post-fertilization, usually in the superior uterus.

• The trophoblast differentiates into cytotrophoblast (inner) and syncytiotrophoblast (outer), which invades the endometrium.

• The trophoblast secretes hCG (human chorionic gonadotropin), maintaining the corpus luteum and progesterone production for the first trimester.

• Immune suppression occurs to prevent rejection of the embryo.

Embryonic Development

Gastrulation

Gastrulation is the process by which the blastocyst forms three primary germ layers, which give rise to all tissues and organs.

• The inner cell mass splits into an upper embryonic disc and a lower layer.

• A groove called the primitive streak forms; cells migrate to form:

  • Endoderm (innermost layer)

  • Mesoderm (middle layer)

  • Ectoderm (outer layer)

• Neural tube formation (from ectoderm) gives rise to the brain and spinal cord.

• Embryonic folding forms the ventral body surface and gut.

Fate of Embryonic Tissues

Embryonic Tissue	Final Fate
Surface ectoderm	Skin, nervous system
Neural tube	Brain, spinal cord
Neural crest	Nerves, ganglia, adrenal medulla
Notochord	Axial skeleton
Somite	Segments: dermatome, myotome, sclerotome
Dermatome	Dermis of skin
Myotome	Skeletal muscles
Sclerotome	Vertebrae, ribs
Splanchnic mesoderm	Heart, blood vessels, connective tissue, serosa
Somatic mesoderm	Bones
Endoderm	Digestive tract, digestive/endocrine glands
Yolk sac	Gut
Intermediate mesoderm	Kidneys, gonads
Coelom	Ventral body cavity

Placentation and Fetal Development

Placentation

• The placenta forms from chorionic villi (from syncytiotrophoblast) and maternal endometrium.

• Maternal and fetal blood supplies remain separate; exchange occurs via diffusion.

• The placenta provides nutrients, gas exchange, and waste removal for the fetus.

• After birth, the placenta is expelled from the uterus.

Fetal Period Milestones

Weeks	Developmental Milestones
8	Brain development, brain waves, liver, limbs, heart pumps (by 4 weeks), 1.2 inches
12	Blood cells in bone marrow, ossification, skin, gender detectable
16	Digestive glands, kidneys, bones/joints, blinking, sucking
20 (5 months)	Fetal position, lanugo (fine hair), limb development, quickening (movement), 8 inches
28 (7 months)	Weight gain, body proportions, toenails, tooth enamel, lungs lack surfactant
36–40 (8–9 months)	Fat under skin, 16 inches, 6–10 pounds

Teratogens

• Teratogens are substances that cause birth defects (e.g., alcohol, nicotine, thalidomide).

• Effects depend on timing and dose of exposure.

Maternal Changes During Pregnancy

• Increased metabolism, vascularization, weight gain (~30 lbs), nausea, acid reflux, frequent urination, difficulty breathing.

• Pelvic ligaments relax; Chadwick’s sign (purple appearance of vagina due to vascularization).

Fetal Circulatory Adaptations

• Fetal lungs and liver are nonfunctional; maternal organs perform these roles.

• Ductus arteriosus: Shunts blood from pulmonary trunk to aorta, bypassing lungs.

• Foramen ovale: Opening between right and left atria, allowing blood mixing.

• Ductus venosus: Bypasses fetal liver.

• These shunts close after birth; foramen ovale becomes fossa ovalis.

Parturition (Childbirth)

Preparation for Childbirth

• High maternal estrogen levels; increased oxytocin receptors in endometrium.

• Fetus and placenta produce oxytocin and prostaglandins, stimulating uterine contractions.

• Positive feedback increases oxytocin release.

• Braxton-Hicks contractions (false labor) precede true labor.

Stages of Labor

1. Uterine contractions (oxytocin & prostaglandins); initially 20 min apart, 30 sec each.

2. Cervix effaces and dilates to 10 cm (6–12 hours).

3. Head enters pelvis (engagement), rotates for delivery.

4. Expulsion contractions: 2 min apart, 1 min each (20 min–2 hours).

5. Crowning: top of head appears; suction mucus from infant’s mouth.

6. Umbilical cord is clamped and cut.

7. Placental stage: placenta and membranes expelled.

• Episiotomy: Surgical cut to prevent tissue tearing.

• Breech presentation: May require forceps or cesarean section.

• Apgar score: Assesses newborn’s heart rate, respiration, color, muscle tone, reflexes (8–10 is good).

• Meconium: First black feces, contains bilirubin and dead cells.

• Physiological jaundice: Due to immature liver processing bilirubin slowly.

Lactation

• Mammary glands: Modified sweat glands, develop under estrogen control; contain alveolar glands and lactiferous ducts.

• Prolactin (from anterior pituitary) stimulates milk production.

• Oxytocin triggers milk let-down reflex.

• First milk is colostrum: yellow, protein- and IgA-rich.

• Nursing stimulates further milk production and uterine contractions (involution).

Congenital and Birth Defects

• Cleft palate: Most common congenital defect.

• Cystic fibrosis (CF): Defective chloride channel, autosomal recessive; thick mucus in lungs and pancreas.

• Neural tube defects (NTD): Spina bifida, hydrocephaly, anencephaly; often preventable with folic acid.

• Heart defects: Septal defects, Tetralogy of Fallot (four heart defects).

• Cerebral palsy: Brain damage from difficult labor, infections, or prematurity.

• Rh disease: Rh- mother, Rh+ baby; maternal antibodies attack fetal RBCs in subsequent pregnancies.

• Other defects: GI, limb abnormalities.

Principles of Heredity

Basic Genetics

• Inheritance: Transmission of traits from one generation to the next.

• Genotype: Genetic makeup; Phenotype: Observable traits.

• Chromosome segregation and independent assortment during meiosis create genetic diversity.

• Recombination: Crossing over during prophase I of meiosis increases diversity.

• Karyotype: Systematic arrangement of chromosomes for analysis.

Patterns of Inheritance

• Dominant vs. recessive traits: Dominant alleles mask recessive ones.

• Homozygous: Two identical alleles; heterozygous: Two different alleles.

• Incomplete dominance: Heterozygotes show intermediate phenotype (e.g., sickle cell anemia).

• Codominance: Both alleles expressed equally (e.g., AB blood type).

• Multiple allele inheritance: More than two alleles exist (e.g., ABO blood group).

• Polygenic inheritance: Multiple genes influence a trait (e.g., skin color, height).

Sex Chromosomes and Sex-Linked Traits

• Humans: 44 autosomes + 2 sex chromosomes (XX female, XY male).

• Sex-linked diseases (often X-linked): More common in males (e.g., hemophilia, red-green color blindness).

• Females can be carriers if they have one normal and one defective X chromosome.

Chromosomal Abnormalities

• Nondisjunction: Failure of chromosomes to separate properly during meiosis or mitosis, leading to abnormal chromosome numbers.

• Klinefelter’s syndrome: XXY trisomy (male, often sterile).

• Down’s syndrome: Trisomy 21; mental retardation, characteristic facial features, organ malformations.

• Turner’s syndrome: XO (single X chromosome, female, sterile, variable severity).

Fetal Testing

• Amniocentesis and chorionic villus sampling (CVS) are used to detect chromosomal abnormalities prenatally.

Key Terms

• Zygote: Fertilized egg

• Gestation period: Pregnancy, 280 days from last menstrual period

• Preembryo: Fertilization to just before 3 weeks

• Embryo: 3–8 weeks

• Fetus: 9–40 weeks

• Infant: At birth

Summary Table: Hormonal Events in Pregnancy

Event	If No Fertilization	If Fertilization Occurs
Corpus luteum	Degenerates	Maintained by hCG
Progesterone & Estrogen	Decline	Remain high
FSH & LH	Rise	Decline
Endometrium	Sloughs off (menstruation)	Remains intact (part of placenta)

Additional info:

• Breast cancer is most common in epithelial ducts of mammary glands; early detection via self-exam and mammography is crucial.

• Birth control pills maintain high estrogen and progestin levels, inhibiting ovulation by negative feedback on the hypothalamus and pituitary.