Lecture Two: Major Milestones in Human Growth and Development

Lecture Two: Major

Milestones in Human Growth and Development

NURS 2208

Maternal-Newborn Nursing
T. Dennis, RNC, MSN

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.

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.

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).
• 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.

 

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.

Twins

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

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.

 

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.

 

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.

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.

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.

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

Ectoderm

• Epidermis
• Sweat Glands
• Sebaceous glands
• Nails
• Hair follicles
• Lens of eye
• Sensory epithelium of ear and mouth
• nervous systems
• Nasal cavity
• Oral glands and tooth enamel
• Pituitary gland
• Mammary glands

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

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

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

 

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.)

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.

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.

Amniotic Fluid

• Functions as a cushion to protect against injury.
• Helps control embryo’s 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 (< normal )Hydramnios or Polyhydramnios (> normal)

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.

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.

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 placenta’s 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.

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.

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
• Wharton’s jelly surrounds the blood vessels (specialized connective tissue prevents compression)
• Usually appear spiraled or twisted

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.

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.

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.

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.

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 1.2:1
• 36 weeks – skin pink, body rounded, definite sleep cycle, L/S ratio ≥ 2:1.
• 40 weeks – Skin smooth and pink, vernix caseosa small amount, lanugo on shoulders and upper body only, testes in scrotum, labia majora well developed.

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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