Lecture Two: Major Milestones in Human Growth and Development

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Lecture Two Major Milestones in Human Growth and
Development

• NURS 2208
• Maternal-Newborn Nursing
• T. Dennis, RNC, MSN

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Objectives

• Explain specific cellular changes that occur in
  the developmental stages of a human embryo.
• Describe significant intrauterine organ systems
  and other significant organ formation and
  development during the embryonic and fetal
  stages.

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The Genetic Process

• Chromosomes are classified according to their
  length and to the position of their centromere.
• The threadlike structures, composed of strands of
  DNA and protein, within the nucleus of a cell
  that carry the genes.
• Each chromosome contains two longitudinal halves
  called chromatids.

• Genes Regions in the DNA strands that contain
  coded information used to determine the unique
  characteristics of an individual.
• Genes are arranged in linear order on the
  chromosomes.
• Referred to as homozygous or heterozygous.

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Cellular Division

• Mitosis process of cell division whereby both
  daughter cells have the same number and pattern
  of chromosomes as the original cell. Results in
  growth and development.

• Meiosis The process of cell division that occurs
  in the maturation of sperm and ova that
  decreases their number of chromosomes by one-half
  (needed to produce a new organism).

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• Gametogenesis The process by which germ cells
  (gametes) are produced.
• Gametes must have a haploid number of chromosomes
  (26).
• The female gamete and male gamete unite to form
  the zygote. The zygote has the normal human
  diploid number (46) of chromosomes.
• Meiosis occurs during gametogenesis.

• Oogenesis process by which female gametes/ova
  are produced.
• All the ova a female will produce in her lifetime
  are present at birth.
• Ova are considered fertile for 24 hours after
  ovulation.
• Spermatogenesis process by which mature sperm
  are formed, and the number of chromosomes is
  halved.
• Sperm can survive in the female reproductive
  tract for 48 to 72 hours.

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The Process of Fertilization

• The process by which a sperm fuses with an ovum
  to form a new diploid cell or zygote.
• Takes place in the ampulla (outer third) of the
  fallopian tube.
• High estrogen levels increase peristalsis within
  the fallopian tube helping to move the ovum
  through the tube to the uterus.
• It is at the moment of fertilization that the sex
  of the zygote is determined.

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Twins

• Occurs in approximately 1 in 80 pregnancies, with
  triplets occurring 1 in 8000 pregnancies.
• Twins may be fraternal (dizygotic) or identical
  (monozygotic).

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Dizygotic Twinning

• Means the twins arise from two separate ova
  fertilized by two separate sperm.
• There are two placentas, two chorions and two
  amnions (may appeared fused to look like one
  placenta).
• Increases with maternal age up until age 35.
• May occur in certain families.

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Monozygotic Twinning

• Develop from a single fertilized ovum.
• They are the same sex and have the same genotype
  (appearance).
• Number of amnions and chorions depend on day of
  cell division. (3, 5 7-13)
• Monozygotic twinning is a random event occurring
  in approximately 3.5 per 1000 live births.

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Intrauterine Development

• The zygote begins cellular multiplication in the
  fallopian tube.
• Changes from blastomeres to the morula.
  Blastocyst and trophoblast are formed.
• Blastocyst and trophoblast are embryonic
  membranes that form at implantation.
• Trophoblast eventually develops into the chorion.
  
• Blastocyst eventually develops into embryo and
  the amnion.

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Implantation (Nidation)

• Trophoblast attaches to the surface of the
  endometrium.
• Most frequent site of implantation is the upper
  part of the posterior uterine wall.
• Cells of the trophoblast grow into the thickened
  lining forming villi.
• Progesterone increases the endometrial thickness
  and vascularity in preparation for implantation.
• Implantation is completed by the 9th day.

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Decidua

• After implantaion, the endometrium is called the
  decidua.
• The decidua can be divided into three different
  layers Decidua capularis, Decidua basalis, and
  decidua vera (parietalis).
• These layers will form the maternal and fetal
  portion of the placenta.

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Celluar Differentiation

• About 10 to 14 days after conception, the primary
  germ layers form.
• The ectoderm, mesoderm, and endoderm are formed
  at the same time as the embryonic membranes.
• All tissues, organs, and organ systems will
  develop from these primary germ cell layers.

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Ectoderm

• Epidermis
• Sweat Glands
• Sebaceous glands
• Nails
• Hair follicles
• Lens of eye
• Sensory epithelium of ear and mouth

• Central and peripheral nervous systems
• Nasal cavity
• Oral glands and tooth enamel
• Pituitary gland
• Mammary glands

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Mesoderm

• Dermis
• Wall of the digestive tract
• Kidneys and ureter
• Reproductive organs
• Connective tissue (cartilage, bone, joint
  cavities)
• Spleen

• Skeleton
• Muscles (all types)
• Circulatory system (heart,arteries,veins, blood,
  bone marrow).
• Pleura
• Lymphatic tissue and cells

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Endoderm

• Respiratory tract epithelium
• Epithelium (except nasal)-pharynx, tongue,
  tonsils, thyroid
• Lining of digestive tract

• Urethra and associated glands
• Urinary bladder
• Vagina
• Primary tissue of liver and pancreas Primary
  tissue of liver and pancreas

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Embryonic Membranes

• Begin to form at the time of implantation.
• Protect and support the embryo as it grows and
  develops inside the uterus.
• Blastocyst and trophoblast are embryonic
  membranes that form at implantation.
• Trophoblast develops into the chorion and the
  blastocyst develops into the amnion.
• First membrane to develop is the CHORION.
• AMNION is second membrane to develop.

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CHORION

• Outermost embryonic membrane that encircles the
  the amnion, embryo, and yolk sac.
• Thick membrane that develops from the
  trophoblast.
• Has appendages called chorionic villi.
• Can be used for genetic sampling .
• Begins to degenerate after approx 10th week to be
  smooth by the fourth month (except at the point
  of attachment to uterus.)

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AMNION

• Second embryonic membrane to form, originating
  from the ectoderm.
• Thin protective membrane that contains amniotic
  fluid.
• Space between the amniotic membrane and the
  embryo is the embryonic cavity.
• Amnion expands with embryo growth until it comes
  into contact with the chorion.

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BOW (bag of waters)

• Formed by the two adherent membranes (chorion and
  amnion).
• The membrane containing the amniotic fluid and
  the fetus.
• Provides fluid to protect the floating embryo.

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Amniotic Fluid (pg. 160)

• Functions as a cushion to protect against injury.
• Helps control embryos temperature.
• Allows for symmetrical growth, and freedom of
  movement for the embryo/fetus.
• 30ml at 10 wks, 350ml at 20 wks, and 700 to
  1000ml after twenty weeks.
• Amount decreases after 40 weeks.
• Is slightly alkaline. Contains albumin,urea,
  uric acid, creatinine, lecithin, sphingomyelin,
  bilirubin, fat,fructose, leukocytes, proteins,
  epithelial cells, enzymes, and lanugo (fine
  hair).
• Oligohydramnios (lt normal )Hydramnios or
  Polyhydramnios (gt normal)

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Yolk Sac

• Is small.
• Functions only in early embryonic life.
• During the first 6 weeks of development forms
  primitive red blood cells.
• As the embryo develops, incorporated into the
  umbilical cord.

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Placenta

• Means of metabolic and nutrient exchange between
  the embryonic and maternal circulations.
• Expands until 20 weeks.
• Cover half of the uterus.
• Has two parts maternal(decidua basalis) and
  fetal(amnion).
• Cotyledons anchor the placenta (highly vascular)
  to uterine wall.
• Placental blood flow is increased by the client
  lying on her left side preventing compression of
  the inferior vena cava.

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Placental Circulation

• By the 4rth week , placenta begins to function as
  means of metabolic exchange.
• By 14 weeks, the placenta is a discreet organ.
• Fully developed placentas umbilical cord, fetal
  blood flows through the two umbilical arteries to
  the capillaries of the villi, and oxygen rich
  blood flows back through the umbilical vein to
  the fetus.

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Placental Functions

• Fetal respiration, nutrition, and excretion
• Produces hormones that are vital to the survival
  of the fetus including human chorionic
  gonadotropin (HCG), human placental lactogen
  (hPL), Estrogen and Progesterone (two steroid
  hormones).
• Human placental lactogen (HPL) is responsible for
  stimulating maternal metabolism supplying
  nutrients to the fetus.
• HCG (human chorionic gonadotropin) is the
  placental hormone responsible for maintaining the
  pregnancy during the first two months and the
  basis for the pregnancy test.

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Umbilical Cord

• Formed from the amnion
• Body stalk elongates, contains blood vessels,
  attaches the embryo to the yolk sac
• Vessels decrease to three One large vein and two
  smaller arteries
• Whartons jelly surrounds the blood vessels
  (specialized connective tissue prevents
  compression)
• Usually appear spiraled or twisted

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Fetal Circulatory System (pg. 166)

• Most of the blood supply bypasses the fetal lungs
• Placenta assumes the role of gas exchange
• Key structures umbilical vein, ductus venosus,
  foramen ovale, and ductus arteriosus
• Ductus venosus, foramen ovale, and ductus
  arteriosus allow the blood to bypass the fetal
  liver and lungs.
• Delivers the highest available oxygen
  concentration to the head, neck, brain and heart.

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Fetal Circulatory System (pg. 166)

• Blood from the placenta flows through the
  umbilical vein to the umbilicus. Divides into two
  branches. One to the liver and the second and
  larger, ductus venosis, empties directly into the
  fetal vena cava.
• The blood enters the right atrium, passes through
  the foramen ovale into the left atrium. Flowing
  into the left ventricle which pumps it into the
  aorta.
• A small amount of blood from the upper
  extremities and head flows into the right atrium
  through the tricuspid valve into the right
  ventricle which then is pumped into the pulmonary
  artery and a small portion to the lungs.
• The larger amount of blood from the pulmonary
  artery flows into the descending aorta through
  the ductus arteriosis.
• Blood returns to the placenta through the
  umbilical arteries.

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Embryonic Stage (pg. 169-171)

• Embryonic stage starts on day 15 or the beginning
  of the third week after fertilization.
• Embryo most vulnerable to teratogens during this
  time.
• 4 to 5 weeks leg and arm buds are well
  developed and the brain is well differentiated.
• 8 weeks the basic structure of all body organs
  is completed. Embryo clearly resembles a human
  being.

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Fetal Stage (pg. 171-174)

• 12 weeks face is well formed, fetal heart tones
  
• ( the sound of the heart beat) can be detected by
  electronic devices. Kidney is able to excrete
  urine.
• 16 weeks sex recognizable, scalp hair appears.
• 20 weeks vernix appears, fetal movements strong
  enough for mother to feel.
• 24 weeks skin red and wrinkled, can hear, vernix
  caseosa present. Alveolar ducts and sacs present,
  lecithin begins to appear in amniotic fluid.

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Fetal Stage (pg. 171-174)

• 28 weeks lean body, nails appear, brain is
  developing rapidly, nervous system is complete
  enough to provide some regulation of body
  function.
• 30 weeks - subcutaneous fat is beginning to
  collect, lungs not fully mature, L/S ratio 1.21
• 36 weeks skin pink, body rounded, definite
  sleep cycle, L/S ratio 21.
• 40 weeks Skin smooth and pink, vernix caseosa
  small amount, lanugo on shoulders and upper body
  only, testes in scrotum, labia majora well
  developed.

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Questions?

• Mary Rogers comes into the clinic for her 12 week
  check up. She asks you when she will be able to
  feel the baby move. What is the best answer?
• a) 14 weeks
• b) 18 weeks
• c) 20 weeks
• d) 24 weeks

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