aDevelopment and Inheritance Notes

Overview of Development

• Objectives:

  • Describe the sequence of events involved in development.

  • Describe the trimesters of prenatal development.

• Sexual Reproduction:

  • Organisms produce offspring through gametes: male (sperm) and female (secondary oocytes).

  • Gonads: testes (male) and ovaries (female).

  • Fertilization occurs when sperm meets secondary oocyte, initiating development.

• Pregnancy Timeline:

  • Begins with fertilization, followed by implantation, embryonic development, and fetal development.

  • Typically lasts about 38 weeks (40 weeks from last menstrual period).

• Development Biology:

  • Embryonic period: fertilization to 8 weeks (embryo).

  • Fetal period: 9 weeks to birth (fetus).

  • Prenatal development: includes both embryonic and fetal periods, divided into trimesters.

• Trimesters of Prenatal Development:

  • First Trimester

    1. Major organ systems begin to form.

    2. Most vulnerable to drugs, radiation, and microbes.

  • Second Trimester

    1. Nearly complete development of organ systems.

    2. Fetus develops distinct human features.

  • Third Trimester

    1. Rapid fetal growth; weight doubles.

    2. Organ systems become fully functional.

The Embryonic Period (Fertilization and development)

• Introduction:

  • The embryonic period lasts from fertilization to the end of the 8th week.

  • It involves rapid changes and the establishment of the basic body plan and major organ systems.

• Fertilization (Week 1):

  • The Journey:

    1. Mature oocyte travels down the fallopian tube post-ovulation.

    2. Sperm undergo capacitation to fertilize the egg.

  • Penetration of the Zona Pellucida

    1. Zona pellucida is a glycoprotein layer surrounding the oocyte.

    2. Sperm release enzymes to penetrate this barrier.

  • Fusion of Genetic Material

    1. Typically, one sperm penetrates the oocyte membrane.

    2. This triggers the oocyte to complete its second meiotic division.

  • Formation of the Zygote

    1. Male and female pronuclei fuse to form a diploid zygote.

    2. This marks the beginning of a new individual with unique genetics.

• Early Development and Implantation (Week 1-2):

  • Cleavage

    1. Zygote undergoes rapid mitotic divisions without significant growth.

    2. Results in a cluster of smaller cells called blastomeres.

  • Morula Formation

    1. After 3-4 days, a solid ball of 16-32 cells forms, known as the morula.

  • Blastocyst Formation

    1. Morula continues to divide and differentiate, forming a blastocyst with:

       • Trophoblast: Outer layer contributing to the chorion/fetal placenta.

       • Inner Cell Mass (ICM/Embryoblast): Cluster of cells that will develop into the embryo.

  • Implantation

    1. Around days 6-7, the blastocyst implants into the uterine lining (decidua).

    2. Trophoblast cells invade the uterine wall, connecting with maternal blood vessels.

  • Extraembryonic Membranes

    1. Form alongside the embryo for support and nourishment:

    2. Amniotic cavity: Fluid-filled sac providing a protective environment. Amnion is its “roof”

    3. Chorion: Outermost membrane forming the fetal part of the placenta.

    4. Yolk Sac: Contributes to early blood cell formation and primitive gut development.

• Gastrulation (Week 3):

  • Formation of the Primitive Streak

    1. Marks the beginning of gastrulation and establishes the embryo's longitudinal axis.

  • Germ Layer Formation

    1. Cells migrate inward through the primitive streak, forming three germ layers:

       • Ectoderm: Develops into skin, nervous system, sensory organs, and parts of teeth and glands.

       • Mesoderm: Forms muscle tissue, connective tissues, dermis, urogenital system, and body cavity linings.

       • Endoderm: Creates the lining of the digestive and respiratory systems, and several glands.

  • Other events in week 3:

    1. Neurulation: Neural tube appears from ectodermal cells, will become brain and spinal cord

    2. Heart starts to form and beat

• Organogenesis: The Beginnings of Organs (Weeks 4-8):

  • Week 4

    1. Rapid folding of the embryonic disc

    2. Limb buds appear.

  • Week 5

    1. Rapid brain development; facial features begin to emerge.

    2. Limb buds elongate and differentiate.

  • Week 6

    1. Further facial development, including jaw formation.

    2. Digits begin to appear on limb buds; liver starts producing blood cells.

  • Week 7

    1. Major organs begin to form but remain immature.

    2. Distinct fingers and toes become visible; umbilical cord is well- defined.

  • Week 8

    1. Embryo develops recognizably human features; limbs and digits are well-developed.

    2. Major organ systems are established; the embryonic period concludes, and the developing human is referred to as a fetus.

Fetal Period

• The fetal period is primarily concerned with the growth and differentiation of tissues and organs that developed during the embryonic period

• The rate of body growth is remarkable, especially during the ninth and sixteenth weeks.

• The principal changes associated with embryonic and fetal growth are summarized in Table 29.2.

Maternal Changes During Pregnancy

• Hormones of Pregnancy:

  • Progesterone & Estrogens

    1. Secreted initially by the corpus luteum (sustained by hCG), then predominantly by the placenta from the 3rd month onward.

    2. Maintain uterine lining, prepare mammary glands, ensure myometrial relaxation, and maintain cervical closure. Levels normalize post-delivery.

  • Human Chorionic Gonadotropin (hCG)

    1. Source: Chorion of the placenta.

    2. Maintains corpus luteum activity in early pregnancy, preventing menstruation.

    3. Detectable ~day 8 post-fertilization, peaks ~week 9, then declines significantly.

       • Along with estrogen, is implicated in emesis gravidarum, or morning-sickness.

  • Relaxin

    1. Source: Corpus luteum, later the placenta.

    2. Increases flexibility of pelvic ligaments (pubic symphysis, sacroiliac, sacrococcygeal joints) and aids cervical dilation, facilitating delivery.

  • Human Chorionic Somatomammotropin (hCS)

    1. Source: Chorion; secretion increases with placental mass (max after 32 weeks).

    2. Aids mammary gland preparation, enhances maternal growth (protein synthesis), regulates metabolism (spares glucose for fetus by modifying maternal fuel use).

  • Corticotropin-Releasing Hormone (CRH)

    1. Source: Placenta (secretion starts ~12 weeks, increases near term).

    2. Contributes to the timing of birth ("placental clock"); increases maternal cortisol, crucial for fetal lung maturation/surfactant.

• Changes During Pregnancy:

  • Uterine Growth & Mechanical Effects:

    1. Uterine expansion into abdomen (near xiphoid by term).

    2. Displaces organs, elevates diaphragm.

    3. May cause heartburn and urinary issues.

  • General Systemic Adjustments:

    1. Weight gain (fetus, fluids, tissues).

    2. Increased storage (proteins, triglycerides, minerals).

    3. Breast enlargement.

    4. Lumbar lordosis & back pain possible.

  • Cardiovascular System Modifications:

    1. Supports increased O_2/nutrient needs.

    2. Increased stroke volume, cardiac output, heart rate, blood volume (especially 2nd half).

    3. Supine position may compress vessels, affecting uterine flow or causing leg edema/varicose veins.

  • Respiratory System Adaptations:

    1. Accommodates higher O_2 needs.

    2. Increased tidal volume & minute ventilation.

    3. Decreased expiratory reserve & functional residual capacity.

    4. Reduced airway resistance.

    5. Increased O_2 consumption; dyspnea possible.

  • Digestive System Changes:

    1. Increased appetite, decreased GI motility.

    2. Common: constipation, delayed emptying, nausea, vomiting, heartburn.

  • Urinary System Adjustments:

    1. Uterine pressure: frequency, urgency, incontinence.

    2. Enhanced renal function (plasma flow, GFR): faster waste removal.

  • Integumentary System (Skin) Changes:

    1. Increased pigmentation (chloasma, areolae, linea nigra).

    2. Stretch marks.

    3. Possible increased hair loss.

  • Genital System Modifications:

    1. Increased vascularity, edema, pliability (vulva/vagina).

    2. Significant uterine growth (hyperplasia early, hypertrophy later).

Labor

• Overview of Labor

  • Labor is the process of expelling the fetus from the uterus through the vagina, also known as giving birth.

  • Synonym for labor: parturition.

• Hormonal Regulation of Labor Onset

  • Onset of labor is influenced by placental and fetal hormones.

  • Progesterone inhibits uterine contractions; labor begins when its effects are overcome by high estrogen levels.

  • Rising estrogen levels towards the end of gestation:

    1. Result from increased secretion of corticotropin-releasing hormone by the placenta.

    2. Stimulates fetal anterior pituitary to secrete adrenocorticotropic hormone (ACTH).

    3. ACTH prompts fetal suprarenal gland to release cortisol and dehydroepiandrosterone (DHEA).

    4. DHEA is converted into estrogen by the placenta.

  • High estrogen levels:

    1. Increase oxytocin receptors on uterine muscle fibers.

    2. Promote formation of gap junctions between uterine muscle fibers.

  • Oxytocin from the posterior pituitary stimulates uterine contractions.

  • Relaxin from the placenta:

    1. Increases flexibility of the pubic symphysis.

    2. Aids in cervical dilation.

  • Estrogen stimulates the release of prostaglandins, which soften the cervix.

• Control of Labor Contractions

  • Labor contractions follow a positive feedback cycle:

    1. Uterine contractions push the baby’s head into the cervix, causing it to stretch.

    2. Stretch receptors in the cervix send signals to the hypothalamus, releasing oxytocin.

    3. Oxytocin increases uterine contractions, further stretching the cervix.

  • Cycle continues until the baby is born, breaking the feedback loop.

• Characteristics of True vs. False Labor

  • True labor:

    1. Regular contractions at intervals, causing pain.

    2. Cervical dilation and "show" (blood-containing mucus discharge).

  • False labor:

    1. Irregular abdominal pain that does not intensify.

    2. No cervical dilation or "show."

• Stages of Labor

  • Stage of Dilation

    1. Duration: 6–12 hours.

    2. Features:

       • Regular uterine contractions.

       • Possible rupture of the amniotic sac.

       • Complete cervical dilation (to 10 cm).

  • Stage of Expulsion

    1. Duration: 10 minutes to several hours.

    2. Time from complete cervical dilation to delivery of the baby.

  • Placental Stage

    1. Duration: 3–5 minutes to over an hour.

    2. Time after delivery until the placenta is expelled.

    3. Uterine contractions help constrict blood vessels to reduce hemorrhage risk.

• Duration of Labor

  • First-time mothers: average labor lasts about 14 hours.

  • Women with previous births: average labor lasts about 8 hours.

  • Variability in duration among births.

• Fetal Response to Labor Stress

  • Fetus experiences stress during childbirth due to compression.

  • Fetal adrenal medullae release high levels of epinephrine and norepinephrine.

  • Hormones help:

    1. Clear lungs and prepare for breathing.

    2. Mobilize nutrients for metabolism.

    3. Increase blood flow to the brain and heart.

• Post-Term Deliveries

  • About 7% of pregnant women do not deliver within 2 weeks after the due date.

  • Risks include brain damage and fetal death due to inadequate oxygen and nutrients.

  • Induction of labor may be necessary, using oxytocin or cesarean section.

• Puerperium

  • A 6-week period post-delivery for maternal recovery.

  • Uterus undergoes involution, reducing in size.

  • Cervix regains firmness.

  • Uterine discharge for 2–4 weeks post-delivery, initially blood, then serous fluid.

Inheritance

• Inheritance Overview

  • Inheritance is the passage of hereditary traits from one generation to the next.

  • It involves the genetic material from both parents uniting to form a zygote.

  • The study of inheritance is known as genetics.

  • Genetic counseling provides advice on genetic issues.

• Genotype and Phenotype

  • Human cells (except gametes) contain 23 pairs of chromosomes (diploid number 2n

  • Each chromosome pair consists of one chromosome from each parent.

  • Genes on homologous chromosomes control the same traits.

    1. Genes are hereditary units of nucleotide sequences, arranged along a chromosome.

  • Alleles are alternative forms of a gene located at the same position on homologous chromosomes.

  • A mutation is a permanent change in an allele that results in a different variant of a trait.

  • Example: Phenylketonuria (PKU)

    1. PKU is a disorder caused by the inability to produce the enzyme phenylalanine hydroxylase.

    2. The normal allele is represented as P‍ and the mutated allele as p.

    3. A Punnett square illustrates the possible combinations of alleles from two parents.

    4. Offspring genotypes can be PP, Pp, or pp, with specific probabilities

    5. Individuals with PP or Pp do not have PKU, but those with pp do.

• Dominant and Recessive Alleles

  • A dominant allele masks the presence of a recessive allele.

  • Dominant alleles are represented in capital letters (e.g., P‍), while recessive alleles are in lowercase (e.g.,p‍).

  • Homozygous individuals have identical alleles (e.g., PP‍ or pp).

  • Heterozygous individuals have different alleles (e.g.,Pp‍).

  • Phenotype is the physical expression of a genotype.

  • Variations on Dominant–Recessive Inheritance

    1. Inheritance patterns can be more complex than simple dominant- recessive interactions.

    2. Traits may be influenced by multiple genes and environmental factors.

    3. Incomplete Dominance

       • In incomplete dominance, neither allele is dominant, resulting in an intermediate phenotype.

       • Example: Sickle cell disease (SCD) shows varying degrees of anemia in heterozygotes.

    4. Multiple-Allele Inheritance

       • Some genes have more than two alleles, leading to multiple-allele inheritance.

       • Example: ABO Blood Group System

         1. Alleles and Dominance Relationships:

            • Governed by three alleles: A, B, and O.

            • Alleles A and B are codominant (both traits expressed when inherited together).

            • Allele O is recessive to both A and B.

         2. Genotype-Phenotype Correlations:

            • Type AB Blood: Genotype AB.

            • Type O Blood: Genotype OO.

            • Type A Blood: Genotype AA (homozygous) or AO (heterozygous).

            • Type B Blood: Genotype BB (homozygous) or BO (heterozygous).

    5. Complex Inheritance

       • Many traits are influenced by multiple genes and environmental factors (polygenic inheritance).

       • Examples include skin color, height, and metabolism.

  • Autosomes and Sex Chromosomes

    1. Humans have 46 chromosomes, arranged in 23 pairs (karyotype).

    2. 22 pairs are autosomes; the 23rd pair consists of sex chromosomes (XX in females, XY in males).

    3. Sex Determination

       • The sex of an individual is determined by the father's sperm (X or Y chromosome).

       • The SRY gene on the Y chromosome initiates male development.

    4. Sex-Linked Inheritance

       • Sex chromosomes carry genes for nonsexual traits, leading to sex-linked inheritance patterns.

    5. Example: Red-Green Color Blindness

       • Gene Location: The gene responsible is located on the X chromosome.

       • Inheritance Pattern: Transmitted as an X-linked recessive trait.

       • Expression and Prevalence:

         • Males express the trait if they inherit a single recessive allele on their X chromosome (from their mother).

         • Females express the trait only if they inherit the recessive allele on both of their X chromosomes (one from each parent).

         • This pattern results in a higher prevalence of red- green color blindness among males.

Overview of Development

Development involves a sequence of events, including fertilization, implantation, embryonic development, and fetal development. Sexual reproduction occurs through the fusion of male (sperm) and female (secondary oocytes) gametes, produced in the testes and ovaries, respectively. Fertilization initiates development, leading to pregnancy, which typically lasts about 38 weeks from fertilization or 40 weeks from the last menstrual period. Development biology divides prenatal development into the embryonic period (fertilization to 8 weeks) and the fetal period (9 weeks to birth), with prenatal development encompassing both, divided into trimesters. During the first trimester, major organ systems begin to form, making it the most vulnerable period to drugs, radiation, and microbes. The second trimester sees nearly complete development of organ systems, with the fetus developing distinct human features. Finally, the third trimester is marked by rapid fetal growth and the full functionality of organ systems.

The Embryonic Period (Fertilization and development)

The embryonic period, lasting from fertilization to the end of the 8th week, involves rapid changes and the establishment of the basic body plan and major organ systems. Fertilization begins with the mature oocyte traveling down the fallopian tube, where sperm undergo capacitation to fertilize the egg. Sperm penetrate the zona pellucida, a glycoprotein layer surrounding the oocyte, by releasing enzymes. Fusion of genetic material occurs when one sperm penetrates the oocyte membrane, triggering the completion of its second meiotic division, leading to the formation of a diploid zygote with unique genetics. Early development includes cleavage, where the zygote undergoes rapid mitotic divisions without significant growth, resulting in blastomeres. After 3-4 days, a morula, a solid ball of 16-32 cells, forms. The morula differentiates into a blastocyst, composed of the trophoblast (contributing to the chorion/fetal placenta) and the inner cell mass (ICM/embryoblast), which will develop into the embryo. Around days 6-7, the blastocyst implants into the uterine lining (decidua), with trophoblast cells invading the uterine wall and connecting with maternal blood vessels. Extraembryonic membranes, including the amniotic cavity, chorion, and yolk sac, form alongside the embryo for support and nourishment. Gastrulation begins in week 3 with the formation of the primitive streak, establishing the embryo's longitudinal axis. Cells migrate inward, forming three germ layers: the ectoderm (developing into skin, nervous system, and sensory organs), the mesoderm (forming muscle tissue, connective tissues, and the urogenital system), and the endoderm (creating the lining of the digestive and respiratory systems). Neurulation and the heart formation also occurs in week 3. During weeks 4-8, organogenesis occurs, with rapid folding of the embryonic disc and the appearance of limb buds by week 4. By week 5, rapid brain development and initial facial features emerge, followed by further facial development and digit appearance by week 6. Major organs form in week 7, and by week 8, the embryo develops recognizably human features, with major organ systems established, marking the end of the embryonic period.

Fetal Period

The fetal period is primarily concerned with the growth and differentiation of tissues and organs developed during the embryonic period. The rate of body growth is remarkable, especially during the ninth and sixteenth weeks, as detailed in relevant tables.

Maternal Changes During Pregnancy

Hormonal changes during pregnancy involve progesterone and estrogens, secreted initially by the corpus luteum and later predominantly by the placenta, maintaining the uterine lining and preparing mammary glands. Human chorionic gonadotropin (hCG), produced by the chorion, maintains corpus luteum activity in early pregnancy, peaking around week 9. Relaxin, from the corpus luteum and placenta, increases pelvic ligament flexibility and aids cervical dilation. Human chorionic somatomammotropin (hCS), from the chorion, aids mammary gland preparation and regulates metabolism. Corticotropin-releasing hormone (CRH), from the placenta, contributes to the timing of birth and increases maternal cortisol, crucial for fetal lung maturation. Physical changes include uterine growth, displacing organs and causing heartburn and urinary issues. Systemic adjustments involve weight gain, increased storage of nutrients, breast enlargement, and possible lumbar lordosis. Cardiovascular modifications support increased oxygen and nutrient needs, with increased stroke volume and blood volume. Respiratory adaptations accommodate higher oxygen needs, with increased tidal volume and minute ventilation. Digestive system changes include increased appetite and decreased GI motility, leading to constipation and heartburn. Urinary system adjustments involve increased frequency and enhanced renal function. Integumentary system changes include increased pigmentation and stretch marks, while genital system modifications involve increased vascularity and uterine growth.

Labor

Labor, or parturition, is the process of expelling the fetus from the uterus through the vagina. The onset of labor is hormonally regulated, with progesterone's inhibition of uterine contractions being overcome by rising estrogen levels. This increase in estrogen results from increased corticotropin-releasing hormone secretion, stimulating fetal ACTH release and subsequent cortisol and DHEA production, which is converted into estrogen by the placenta. High estrogen levels increase oxytocin receptors and promote gap junction formation in uterine muscle fibers. Oxytocin stimulates uterine contractions, while relaxin increases pubic symphysis flexibility and aids cervical dilation. Estrogen also stimulates prostaglandin release, softening the cervix. Labor contractions follow a positive feedback cycle, where uterine contractions stretch the cervix, triggering oxytocin release, which further increases contractions until the baby is born. True labor involves regular contractions causing pain and cervical dilation, while false labor involves irregular pain without cervical changes. Labor consists of the stage of dilation (6–12 hours), the stage of expulsion (10 minutes to several hours), and the placental stage (3–5 minutes to over an hour). Labor duration varies, with first-time mothers averaging 14 hours and women with previous births averaging 8 hours. The fetus experiences stress during childbirth, releasing epinephrine and norepinephrine to clear lungs, mobilize nutrients, and increase blood flow to the brain and heart. Post-term deliveries, occurring in about 7% of pregnancies, carry risks of brain damage and fetal death, potentially requiring labor induction. The puerperium is a 6-week period post-delivery for maternal recovery, involving uterine involution, cervical firming, and uterine discharge.

Inheritance

Inheritance is the passage of hereditary traits from one generation to the next, involving the union of genetic material from both parents. The study of inheritance is known as genetics, with genetic counseling providing advice on genetic issues. Human cells contain 23 pairs of chromosomes, with genes on homologous chromosomes controlling the same traits. Alleles are alternative forms of a gene, and mutations result in different trait variants. Dominant alleles mask recessive alleles, with homozygous individuals having identical alleles and heterozygous individuals having different alleles. The phenotype is the physical expression of the genotype. Inheritance patterns can be more complex than simple dominant-recessive interactions, including incomplete dominance (e.g., sickle cell disease), multiple-allele inheritance (e.g., ABO blood group system), and complex inheritance (e.g., skin color). Humans have 46 chromosomes, with 22 pairs being autosomes and the 23rd pair being sex chromosomes (XX in females, XY in males). Sex is determined by the father's sperm, with the SRY gene on the Y chromosome initiating male development. Sex-linked inheritance involves genes for nonsexual traits located on sex chromosomes, such as red-green color blindness, which is more prevalent in males due to its X-linked recessive inheritance pattern.