Exam 25 - Fetal Development

• The lesson objective is to explain fetal development.
• Normal human chromosomes come in pairs with one from each parent.
• Each body cell has 46 chromosomes with 22 pairs of autosomes and 1 pair of sex chromosomes.
• External environmental factors like maternal drug use, malnutrition, or smoking affect biological development.
• Cell division starts in the nucleus, which contains gene-bearing chromosomes.
• Mitosis and meiosis are the two types of cell division.

Mitosis

• A continuous process for body growth, development, and the replacement of dead cells.
• Daughter cells have the same number of chromosomes as the parent cell.
• Diploid number of chromosomes: 46

Meiosis

• Reproductive cells have two sequential divisions.
• Reduces the number of chromosomes by half to 23 in each cell.
• Haploid number of chromosomes: 23
• Meiosis produces gametes.

Gametogenesis

• Development of ova in women and sperm in men
• Oogenesis involves the formation of female gametes.
• One ovum develops with 23 chromosomes, including one X sex chromosome.
• Females have all the ova they will ever have which is around 2 million cells by the 30th week of gestation.
• At puberty, ova begin to mature.
• Spermatogenesis involves the formation of male gametes.
• Four sperm develop from one spermatocyte, each with 23 chromosomes, including one X or Y sex chromosome.
• Spermatogenesis starts during puberty and takes 70 days.
• New spermatogonia can mature into sperm throughout a male's lifetime.
• Gametogenesis was discussed in this section

Human Fertilization

• Ovulation happens about 14 days before the start of the next menstrual cycle.
• Several oocytes mature before ovulation under the influence of FSH and LH, but only one reaches final maturity.
• The maturing ovum and corpus luteum produce estrogen and progesterone to enlarge the endometrium.
• The corpus luteum turns yellow after ovulation and secretes progesterone to prepare the uterine lining for a fertilized ovum.
• Around 12 days after ovulation, the corpus luteum degenerates, lowering hormone levels and causing menstruation.
• More FSH and LH are secreted to begin a new cycle.
• The ovum can survive less than 24 hours after it is released upon ovulation.
• Sperm can reach the fallopian tube within 5 minutes after the ejaculation of semen and remain viable for up to 5 days in the female reproductive tract.
• A few thousand sperm reach each fallopian tube, but only one sperm fertilizes the ovum.
• Sperm use the flagellar activity of their tails to pass through the cervix and uterus into the fallopian tubes within 5 minutes of coitus .
• Fertilization happens when a sperm penetrates the ovum and unites with it, restoring the total number of chromosomes to 46.
• Fertilization normally occurs in the outer third of the fallopian tube.
• A chemical change happens in the membrane protecting the fertilized ovum to prevent penetration by additional sperm, as soon as fertilization occurs.

Sex Determination

• Sperm can carry either an X or Y chromosome and determines the sex of the child.
• The pH of the female reproductive tract and estrogen levels, affect the survival rate and movement speed of X and Y bearing sperm.
• Female physiology influences which sperm fertilizes the mature ovum.
• DNA carries coded information responsible for individual traits.
• Genes carry dominant and recessive traits, with dominant traits overpowering recessive ones and being passed on to offspring.
• A gene defect or alteration can cause a disorder in a developing fetus.
• Knowledge of inheritance helps nurses provide information about reproductive decision-making and suggest genetic testing.

Implantation of the Zygote

• A mature ovum releases from the ovary's surface then is picked up by the fimbriae of the fallopian tube.
• Muscular action and cilia movement within the tube transport the ovum to the uterus.
• A zygote forms when sperm and ovum unite and happens in the distal third of the fallopian tube near the ovary.
• The zygote undergoes rapid mitotic division while being transported, a process known as cleavage.
• The zygote subdivides into two, four, and then eight cells, forming smaller cells called blastomeres.
• The zygote's size does not increase; it becomes a solid ball called the morula, which enters the uterus on the third day and floats there for 2-4 days.
• The cells form a cavity and two distinct layers: the inner layer makes it a blastocyst and the outer layer/trophoblast develops into the chorion.
• The zygote typically implants in the upper section of the posterior uterine wall.
• The cells burrow into the prepared lining of the uterus which is called the endometrium/decidua. The area under the blastocyst is called the decidua basalis.

Cellular Development

• Cells within a zygote are identical until implantation, when they begin to differentiate.
• Cells develop into the chorion, amnion, yolk sac, and primary germ layers.
• The chorion forms from the trophoblast and envelops the amnion, embryo, and yolk sac.
• The chorion has villi on its outermost surface: villi below the embryo extend into the decidua basalis which is the maternal wall and will form the fetal portion of the placenta.
• The amnion is the second membrane, that is thin and protects the embryo.
• Amniotic sac is formed by both the chorion and amnion, it filled with fluid and allows the embryo to float freely.
• Amniotic fluid is clear with a mild odor, containing vernix or lanugo.
• Fluid volume increases from 30 mL at 10 weeks to 350 mL at 20 weeks until it it is around 1000 mL at 37 weeks.
• Fetus may swallow 400 mL of amniotic fluid a day and excretes urine into the fluid.

Amniotic Fluid Functions

• Maintains an even temperature.
• Prevents the amniotic sac from adhering to the fetal skin.
• Allows symmetrical growth.
• Allows buoyancy and fetal movement.
• Cushions and protects from injury.
• The yolk sac forms in the blastocyst around the ninth day after fertilization to produced red blood cells.
• Production of Red Blood Cells continues until the embryonic liver takes over(6 weeks).
• The umbilical cord then encompasses the yolk sac, and the yolk sac degenerates.
• After implantation, the zygote transforms into three germ layers which occurs during the blastocyst stage.
• Ectoderm (outer skin layer): oil glands and hair follicles of skin, nails and hair, external sense organs, mucous membrane of mouth and anus.
• Mesoderm (true skin): skeleton, bone and cartilage, connective tissue, muscles, blood and blood vessels, kidneys and gonads.
• Endoderm: lining of trachea, pharynx, and bronchi, lining of digestive tract, lining of bladder and urethra.
• Embryonic cell differentiation was studied.

Prenatal Development

• Zygote phase is the initial cell formed by uniting sperm and ovum and grows/develops while passing through the fallopian tube.
• Implantation in the uterine wall happens within the first 2 weeks of development.
• The embryo phase is defined as the 2nd - 8th week of development when basic structures form.
• Ectoderm, endoderm, and amnion develop in week 2 after fertilization and the mesoderm and neural tube form.
• Singular-tube primitive heart begins to pump, the neural tube forms (spinal cord and brain) in week 3.
• Women realize they may have missed a period and the embryo can be affected by maternal issues in the first few weeks.
• Esophagus and trachea separate/stomach forms, neural tube closes, limb buds appear, and the ears and eyes begin to form in week 4.
• Auditory canal forms, eye is obvious, 4 heart chambers form, the nasal cavity and upper lip form in week 6.
• Ovaries/testes are present, body systems start to develop, first extremity movements, first internal and external structures appear in week 8.
• The fetal phase 9th week- until birth is mostly about growth.
• External genitalia visible by ultrasound examination in week 10.
• Maternal-fetal circulation establishes, placenta connects to uterine wall in week 12’s first trimester.
• Fetus responds to external stimuli in week 14.
• Genitalia and leg movement visible by ultrasound, mother feels quickening, bones ossify, eye movements occur, ovaries contain ova, and fetus sucks/swallows amniotic fluid in week 17.
• Lungs mature enough for survival outside the uterus (age of viability) but intensive care is needed in week 20.
• Wrinkled skin and lean body from lack of subcutaneous fat, now viable, lungs secrete surfactant, stronger movements, schedule of sleep/moving, vernix caseosa and lanugo on skin, brown fat and fingernails are formed in week 25.
• The eyes open and the fetal position becomes more stable in week 28.
• A cephalic position occurs, central nervous system functions, skin less wrinkled, spleen stops forming blood cells, bone marrow forms blood cells, surfactant increases in week 29.
• Subcutaneous fat presents, skin is pink/smooth, grasp reflex presents, head/abdominal circumference equal, surfactant surge in week 36.
• Considered full term between weeks 39-40 with crown to heel around 18-21 inches length and the weight is around 6-8 pounds.

Accessory Structures of Pregnancy

• The placenta/afterbirth is a temporary organ to provide the baby with respiration, nutrition, and excretion.
• It also functions as an endocrine gland.
• The placenta forms when the embryo's chorionic villi extend into the mother's decidua basalis.
• Maternal component comes from the decidua basalis and has a beefy red appearance. Maternal/fetal blood does not mix in the placenta.
• Fetal component comes from the chorionic vili and chorionic vessels.
• The umbilical cord and fetal side covered by the amnion which gives them a grey appearance.
• Fetal deoxygenated blood/waste leave fetus via 2 umbilical arteries and and enter the placenta main stem villus.
• Mother gives oxygenated, nutrient rich blood from spinal arteries in the decidua and fetus takes in O2/nutrients through the umbilical vein.
• The placental membrane are somewhat protected but will not hold back most substances ingested by the mother which will effect the baby.
• Nicotine, drugs, and viral infection travel to the fetus and can cause congenital anomalies, drug addiction, or fetal infections.
• Progesterone produced by the corpus luteum then by placenta: maintains the uterine lining, reducing uterine contractions, prepares the lactation glands, stimulates testes to produce testosterone to helps the male reproductive tract.
• Estrogen: stimulates uterine growth, increases blood flow to uterine vessels, helps to grows the breast.
• Human chorionic gonadotropin(hCG): indicates the conception has occurred by the corpus luteum and producing progesterone/estrogen
• Can be detected 7-9 days after fertilization and used in most pregnancy tests.
• Human placental lactogen: decreases maternal insulin to create more glucose for fetal nutrition.
• Umbilical cord developes and functions as a lifeline with the placenta and fetal blood vessels.
• Contains two arteries (deoxygenated blood/waste) and one vein (oxygen and nutrients).
• The coiling of the vessles allows for movement and prevents Restricted circulation.
• Wharton's jelly cushions the entire cord and can obstruct pressure.
• The normal cord is around 22 inches long and prodrudes from center of the placenta.
• Structures that maintain pregnancy are maintained as discussed in this section.

Fetal Circulatory Circuit

• After the 4th week of gestation, the feta circulatory system develops.
• The fetus does not breathe, and the liver doesn't handle wastes so there are several diversions needed.
• There are 3 shunts that allow blood with the highest O2 to send to the fetal heart/brain.
• Ductus venosus: separates some blood as it goes away from the liver after circulating the placenta
• Foramen ovale: separates blood from right atrium to the left atrium instead of using the lungs
• Ductus arteriosus: separates blood from the pulmonary artery to the aorta
• After oxygenated blood enters, about half will go to the liver.
• Rest of the blood will bypass the liver and enter through the ductus venosus.
• Blood will then enter atrium.
• Body is nourished when blood is pumped in by the ventricle.
• Oxygenated blood will join in the right vetnricle through the ductus arteriosus, and will return with waste to enter thr placenta
• Fetal circulatory shunts are unessesary when birth happens because the infant starts to oxegenate on their own.

Changes at Birth

• Blood flow increases to the lung after the infant breaths, and closes the foremen ovale.
• After artery oxygen levels rise, ductus arteriosus will constatict
• Umbilical cord blood flow will constrict the ductus venosus.
• Foremen ovale functionally closes within 2 hours of birth and permanently in 3 months.
• Ductus arterious closes within 15 hours and permanently withing 3 weeks.
• The ductus venosus after the cutting of the cord will close in 1 week.
• Then they will become ligaments.
• Fetal lung preperation to aid the labour of the lungs will decrease to to provide for the transition from exrauterine.
• Hormones in the fetus during the labour process will decrease fluid production to except air.
• Explained and compared fetal circulatory before and after birth.

Multifetal Pregnancy

• Twins occur in 33.5/1000 pregnancies in the US.
• Twins or multifetal birth are more likely to occur with hormonally assisted ovulation.
• Twins are conceived by on sperm/ovum and called identical twins
• Identical gens and same gender are from Monozygotic twins.
• These twins will have different blood and placenta.
• With Monozygotic, they will have two inner membrance and a single outer membrane that shares a placenta
• Dizygotic called fraternal will come two separate and will have different genes
• Differeent genders and gens are possible with dizygotix.
• They both will have both types of placenta, but can sometimess fuse.
• These are more possible with females that have inherited their tendency.
• Multiple birth cause premature from distended utereus or lack of nutrients.