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Simon Fraser University
Health Sciences
HSCI 100
Nienke Van Houten

CHAPTER 18 – DEVELOPMENT THROUGHOUT LIFE STAGES OF HUMAN DEVELOPMENT 1. Pre-embryonic development – fertilization through second week 2. Embryonic development – week 3-8 3. Fetal development – week 9-birth (total human gestation = 265-280 days) 4. Development after birth – post natal – infancy – childhood – adolescence – adulthood Main processes of development - Cleavage: cells undergo mitotic division (2,4,8etc) w/o embryo increasing in size - Growth: cells undergo mitotic division & increase in size - Morphogenesis: embryo begins to take shape as cells migrate - Differentiation: when cells take on specific structure & function (nervous system first visible) PRE-EMBRYONIC PERIOD: FERTILIZATION 1. Secretions from female reprod tract alter surface of plasma membrane of sperm 2. Sperm contacts corona radiate & enzymes spill out of acrosome 3. Enzymes disrupt cellular attachments, allowing passage of sperm through corona radiate & then zona pellucida 4. Sperm contacts plasma membrane of secondary oocyte & their plasma membranes fuse 5. Sperm enters cytoplasm of secondary oocyte 6. Granules near plasma membrane of secondary oocyte release enzymes & cause overlying zona pellucida to harden 7. Nucleus of sperm & egg fuse & fertilization is complete WEEK 1 1 - Cleavage: mitotic cell division that increases # of cells, without increase in size - Morula: compact ball of embryonic cells (~32) - Blastula: forms after about 1 week, creation of inner cell mass becomes embryo covered by layer of cells (trophoblast) that becomes chorion (so chorion is really first cell type that differentiates) - Implantation: embryo embeds into uterus around day 6-16 FUNCTIONS OF EXTRAEMBRYONIC MEMBRANES IN HUMANS  Protect and nourish embryo & eventual fetus - Chorion: fetal half of placenta, organ provides embryo with nourishment & gets rid of wastes by exchange with mother’s blood - Allantois: gives rise to bladder & blood vessels of umbilical cord that carry blood to & from fetus - Yolk sac: contains many blood vessels & where blood cells first form, contains primordial germ cells that migrate to gonads (becomes sperm/oocytes) – primary source of nourishment - Amnion: contains amniotic fluid that cushions & protects embryo The placenta: orchestrates all intxns btwn mother & fetus. Forms from chorion of embryo & portion of endometrium of mother (where implantation occurred) Placenta previa: implantation occurs lower in uterus & covers cervix  premature birth, maternal hemorrhage – need C-section - Allows oxygen & nutrients to diffuse from maternal blood to embryonic blood - Produce hormones (HCG, ESTROGEN, PROGESTERONE) Monozygotic twins – split before trophoblast forms, split after trophoblast but before amnion forms, split after amnion forms WEEK 2 - Pregnancy begins after implantation - Human chorionic gonadotropin (HCG) is secreted, maintaining corpus luteum & endometrium - Inner cell mass detaches & becomes embryonic disk that goes through gastrulation to become 3 primary germ layers (endoderm, mesoderm, ectoderm) EMBRYONIC DEVELOPMENT – - Cell division, cell differentiation (process by which cells become specialized – structure and function), morphogenesis (development of overall body org & shape) WEEK 3 GASTRULATION - Inner cell mass moves away from surface of blastocyst & amniotic cavity forms. o Inner cell mass becomes embryonic disc o Cells within differentiate & migrate into primary germ layers PRIMARY GERM LAYERS: 2 1. Ectoderm (outer layer) - Epidermis of skin, epithelial lining of oral cavity & rectum, nervous system 2. Mesoderm (middle layer) - Skeleton, muscular system, dermis of skin, cardio vascular, urinary, reprod, outer layers of respiratory & digestive systems 3. Endoderm (inner layer) - Epithelial lining of digestive tract & resp tract, associated glands of these systems, epithelial lining of urinary bladder - Nervous system begins to develop - Neural tube becomes spinal cord + brain - Development of heart begins Neurulation: process by which neural tube is formed - 1. Notochord induces overlying ectoderm to thicken & form neural plate (where vertebral column will form) (Begins form of CNS) - 2. Neural plate folds inwards, forming neural groove - 3. Raised sides of neural groove (neural folds) grow upward - 4. Neural folds fuse to form neural tube = develop into brain (anterior portion)/spinal cord(posterior portion) o Mesoderm cells organize into blocks (somties) to form skeletal muscles of neck/trunk, connective tiss, vertebrate  Anencephaly = neural tube defect = incomplete dev of brain & = stillbirth/death shortly after birth Development of reproductive system - WEEK 4& 5 th - 4 week: embryo slightly larger than height of print in book o Chorionic villi form o Umbilical cord forms from allantois o Heart starts to beat o Limb buds form (later develop into legs/arms) o Hands & feet apparent th - 5 week: head enlarges o Ears, eyes, nose become prominent WEEKS 6-8 WEEK 6: Development of the sex organs - Sex of individual determined at conception (xx=female, xy male) - SRY (sex determining region on the Y chromosome) present at week 6, embryo = male - Anti-mullerian hormone – secreted by testes prevents development of sex organs 3 - @14 weeks: primitive testes & ovaries with gametes are already developing - Development of ext organs dependent on presence/absence of dihydrotestosterone (DHT) produced by testes - Have small tissue btwn legs (below = urogenital groove) - Embryo beings to look like human being - Reflex actions occur - Organ systems have been established - 38mm by end of this period & weight = same as aspirin tablet FETAL DEVELOPMENT - In the heart: - Foramen ovale: left atrium through small hole in wall - Ductus arteriosus: connects pulmonary trunk to aorta & functions to divert blood away from the lungs o Blood flows into right ventricle into pulmonary trunk shunted away from lungs & into aorta o Blood through fetus moderate to low in o2 MONTHS 3 & 4 - Allometric growth = difference in relative rates of growth of various parts of body - Hair develops - Head slows in growth so body size catches up - Cartilage begins to be replaced by bone - Able to distinguish btwn female/male (month 3) - Can hear heart beat with stethoscope – month 4 - End of this period, fetus = 6 inches, 6 ounces MONTHS 5-7 - Fetal movement can be felt by mother - Fetus in fetal position - Eyelids fully open - Fetus size increased to 12 inches, 3 pounds MONTHS 8 & 9 - Weight gain = pound per week - Fetus rotates (most of the time) so head is pointed towards cervix - End of fetal development, fetus = 7.5 pounds, 20.5 inches Changes to mother’s body: - Nausea/vomiting (morning sickness) - Some mothers report overall increase in energy levels & sense of well being - Acid reflux & constipation ( effect of progesterone on smooth muscles) - Increase in vital capacity in lungs - Edema & varicose veins can result 4 - Incontinence not uncommon - Placenta produces peptide hormones = makes cells resistant to insulin o Gestational diabetes - Stretch marks (change in skin pigmentation due to increased melanocyte activity) not actual stretched tissue BIRTH (PARTURITION) – 38 weeks after fertilization - 1: DILATION STAGE o Begins with onset of regular contractions & ends when cervix is dilated ~10cm o Effacement – dilation of cervical canal, 6-7 hrs o Breaking of water = rupture of amniotic sac - 2: EXPULSION STAGE o Begins with full dilation of cervix & ends with delivery of baby o Uterine contractions 1-2 mins & last about 1 min each, baby comes head first out of vagina & exits womb o 1+ hr duration o Episiotomy – facilitates delivery & avoids ragged tearing. Incision to enlarge vaginal opening just before passage of baby’s head - 3. PLACENTAL STAGE o Begins with delivery of baby & ends with expulsion of placenta - Breech birth = difficult labors, umbilical cord accidents, but first BIRTH DEFECTS - Developmental defects present @ birth – genetic or environmental - Environmental: drugs, chemicals, radiation, malnutrition, viruses (herpes/rubella) o E.g. spina bifida o More minor defects during fetal period when most organs growing (not differentiating) o CNS developing = highly sensitive HORMONE REGULATION OF BIRTH: positive feedback! Estrogen – from ovaries, induces oxytocin receptors on uterus Oxytocin – from fetus & mothers posterior pituitary - Stimulate uterus to contract, stimulates placenta to make… - PROSTAGLANDINS – stimulate more contractions of uterus PREVENTING BIRTH DEFECTS - Get physical exams by trained doc - Have good health habits: proper nutrition & adequate sleep and exercise - Avoid smoking, alcohol, drug abuse - Avoid x rays - Avoid certain medications & supplements - Avoid STD + know if you have one 5 DEVELOPMENT: - Continues after birth, through infancy, childhood, adolescence and adulthood - Birth = 5 years – acquire gross & fine motor skills, develop language & form emotional ties - 6-12 – rapid growth and learning, develop separate identities and emphasize relationship w/ peers - Puberty (10-14 for girls, 12-16 for boys) – development of secondary sex characteristics + transition to adulthood Aging & longevity - Aging: stochastic process that primarily relates to intrinsic processes leading to death (e.g. damage to DNA/proteins) - Longevity: length of time an individual remains alive in the absence of death from external causes (peace at which aging occurs) Postnatal period - Aging: normal & progressive decline in structure/function of bodies of adults - Changes to organ systems – gradual - Aging population = decreasing quality of life, burden on health care systems, social safety nets AGING - Stages of life: infancy, childhood, adolescence, adulthood - 1. Genetic origin: suggests that there are genes that can regulate longevity - 2. Whole body processes: suggests aging may be a result from body systems/types of tissue (e.g. loss of hormonal activities or cross-linking support tissues) - 3. Extrinsic factors: aging may be due to years of poor health habits (poor diet/lack of exercise) EFFECTS OF AGE ON BODY SYSTEMS - Skin – thinner, less elastic, dry - Less adipose in skin so you feel colder more easily - Decrease in melanocyte activity leading to gray hair while remaining cells are larger leaving “age spots” - Heart shrinks & arteries become more rigid - Rxn time slower - Senses muted - Lens in eyes lose ability to accommodate - Bp increase - Bone density, lung elasticity, muscle mass, immune function DECREASE - Weight gain from decrease in metabolism + inactivity - Females undergo menopause & males andropause 6 Impact of aging on immune system - Thymus starts to shrink after adolescence - Decrease in T-cell function - Immunizations don’t work as well, loss tolerance (Autoimmunity) - Medications such as those for arthritis may slow wound healing o Infection risk increase, decreased ability to fight diseases, slow wound healing, autoimmune disorders, cancers Senescence – phenomenon where cell/organism has reached limit for cell division - Normal cells have finite lifespan - # of cell divisions appears to obey an internal control/clock as well as extrinsic cues - Cells divide until cell fate adopted (differentiation) - Cells that fail to stop dividing may go on to form cancers 7 Telomere-maintenance model for cell senescence - Germ cells: telomerase present = enzyme that keeps telomeres intact, normal telomere length - Somatic cells: telomerase absent = telomere length shortens with each replication o Telomere absent = Free DNA = DNA damage, checkpoint activated, cell division arrested  senescence - Tumour cells: telomerase present, restored telomere length, senescence averted Other possible causes of cellular aging - Loss of genome integrity: can no longer repair accumulating defects - Accumulation of oxidative damage on cellular macromolecules from reactive oxygen species (ROS) linked to metabolism - Caloric restriction: extend lifespan in several organisms: rodents, worms, flies, perhaps human. May reduce level of oxidative damage REVIEW: Periods of Development in Human Life (p. 362) • The period of development before birth is the prenatal period. The period of development after birth is the postnatal period. Prenatal Period (pp. 362–373) • The prenatal period can be subdivided into the pre-embryonic period (from fertilization through the second week), embryonic period (from week 3 through week 8), and fetal period (from the ninth week until birth). • The pre-embryonic period is characterized by formation and implantation of the blastocyst. The ext
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