Cellular Division: Gametes and Soma

Cellular Processes

• Cells function as the building blocks of all organs.
• Soma cells constitute the organs and tissues in the human body.
• Gametes are sex cells found in the reproductive system.
• Mitosis refers to the process by which somatic cells produce daughter cells.
• Meiosis is the process wherein gametes go through two sequential cellular divisions of the nucleus.
• Spermatogenesis starts in males during puberty.
• A spermatogonium produces four spermatozoa.
• Oogenesis starts before birth but is only fully complete during childbearing years.
• One oogonium can produce a mature ovum and three polar bodies.
• Sperm cells can reach the cervix as soon as 80 seconds after deposition into the vagina.
• Sperm can reach the outer third of the fallopian tube within 5 minutes

Changes in Sperm

• Capacitation involves removal of plasma membrane overlying the acrosomal area of spermatozoa, in addition to the loss of seminal plasma lipids and proteins
• Capacitation occurs in the female reproductive tract, aided by uterine enzymes, and takes about 7 hours.
• The acrosomal caps of sperm surrounding the ovum release their enzymes during the acrosomal reaction.
• Hyaluronic acid that holds the elongated cells of corona radiata together is broken down by these enzymes.
• The penetration of even a single sperm needs rupturing of hundreds of acrosomes to clear enough hyalurodinic acid.
• As the morula enters the uterus, intracellular fluid increases and a central cavity forms within it.
• The cells in the central cavity differentiate into the embryotic disc (embryo and amnion) and the trophoblast, which develops into the chorion.

Implantation

• Implantation is the process where the fertilized egg attaches to the endometrium.
• Implantation typically happens 7-10 days after fertilization.
• The upper portion of the posterior wall of the uterus is the frequent site.
• Blastocyst implants itself by burrowing into the uterine lining.
• After implantation, the endometrium is called the decudia.

Stages of Fetal Development

• Zygote is the union of ovum and sperm.
• Embryo spans from the 2nd to the 8th week.
• Fetus spans from the 8th week to term.
• Ectoderm forms the skin cells of the epidermis, neurons of the brain and, pigment cells.
• Mesoderm forms cardiac muscle, skeletal muscle cells, tubule cells of the kidney, red blood cells and smooth muscle.
• Endoderm forms lung cells, thyroid cells and pancreatic cells.

Fetal Membranes

• The chorion is the outermost membrane and develops from the trophoblast.
• The amnion is the innermost membrane that contains amniotic fluids.
• Amnion helps protect the fetus.
• Amniotic fluid ranges between 800 and 1200 ml.
• Amniotic fluid protects, controls temperature, supports symmetrical growth, and prevents adherence to the amnion.
• Amniotic fluid allows the fetus to move within the amniotic cavity.
• Hydramnios is >2000 ml of amniotic fluid.
• Oligohydramnios is <500 ml of amniotic fluid.
• The umbilical cord contains 2 arteries which carry deoxygenated blood.
• The umbilical cord contains one vein that carries oxygenated blood.
• The umbilical cord has Wharton's jelly, which prevents compression.
• The placenta begins to function by the 4th week, and is complete by week 12-14.
• The placenta transmits oxygen and nutrients, removing waste and carbon dioxide.
• The placenta functions as the endocrine organ during pregnancy.
• Human Chorionic Gonadotropin prolongs the life of corpus luteum.
• Estrogen is responsible for the proliferative function that causes enlargement of the uterus and breasts.
• Progesterone is essential for pregnancy.
• Human Placental Lactogen functions as an insulin antagonist.
• Relaxin inhibits uterine activity.
• Fetal circulation depends on the maternal circulation for oxygenation.
• It delivers oxygenated blood to major organs.
• There are three major shunts: the ductus venosus, foramen ovale, and ductus arteriosus.

Milestones of Fetal Growth and Development

• All systems are in rudimentary form by the end of the 4th week.
• The heart chambers form, and the heart beats.
• The embryo length is about 0.4cm, and the weight is about 0.4g.
• Arms and legs are bud-like structures.
• Lungs, stomach, and liver begin to develop.
• Eyes and ears begin to form.
• The pregnancy test is positive.
• End of the 8th week shows the gestational sac on sonogram.
• Organogenesis is now complete.
• The heart is functioning, and facial features continue to develop.
• Fingers and toes have grown longer and more distinct.
• The external genitalia are beginning to form.
• The end of the 12th week shows a fetus that has developed a recognizable form.
• Kidney secretion has begun.
• Heartbeat is audible through Doppler.
• The placenta is complete, and the division of the brain begins to develop.
• Spontaneous movements occur, and the Babinski reflex is present.
• Sex organs start to become clear.
• End of 16th week, the fetus develops reflexes like sucking and swallowing.
• The sex can be identified.
• Heart sounds can be heard through a fetoscope.
• Lanugo is well-formed.
• End of 20th week, the mother begins to feel fetal movement or quickening.
• Brown fat begins to form.
• Distinguishable activity and sleeping patterns can be seen.
• End of 24th week shows active production of surfactant has begun.
• Passive antibodies can be transferred from the mother.
• Hearing can be demonstrated.
• End of week 28 shows surfactant in the amniotic fluid.
• A lethicin/sphingomyelin ratio can be detected.
• The testes begin to descend.
• End of week 32 marks the forming of fat layers.
• The baby responds to sounds outside of the mother's body.
• Iron stores begin to develop.
• End of week 36 shows that overall growth is rapid.
• Movements/kicks are strong, and the kidneys are mature.
• End of the 40th week shows that the lungs are mature.
• The baby is fully developed and can survive outside the mother's body.

Common Teratogens

• Medications like Dilantin are linked to cleft palate.
• Chemotherapy results in major congenital malformations.
• Tetracycline causes damage to dental and osseous tissue.
• Alcohol can induce FAS (Fetal Alcohol Syndrome), low birth weight and affects the brain.
• Varicella as an infectious agent results in Fetal Varicella Syndrome.
• Rubella is linked to cataracts, deafness, and cardiac malformations.
• Cytomegalovirus can cause hearing loss and microcephaly.
• Mercury as an environmental agent is linked to neurologic damage and blindness.
• Radiation causes congenital malformations and mental retardation.
• Week 3 of development being exposed to teratogens commonly affects the heart and the CNS.
• Week 4 can affect the lip, heart, arm, leg, and CNS.
• Week 5 can affect the heart, eye, arm, leg, and CNS.
• Week 6 can affect the CNS, heart, ear and teeth.
• Week 7 can affect the CNS, ear, teeth, palate and the heart.
• Week 8 can affect the CNS, ear, palate, heart, and external genitalia.

Estimating Fetal Growth

• To estimate fetal growth, measure fundic height from symphysis pubis to fundus.
• McDonald's Rule states the number for fundal height in cm. correlates well with AOG between 20 and 34 weeks.
• At 12 weeks, fundal height is over the symphysis pubis.
• At 20 weeks, fundal height is at the level of the umbilicus.
• At 36 weeks, fundal height is at the level of the xyphoid process.
• Fetal movement peaks at 28-38 weeks.
• Assessment requires at least 10 times a day.
• Assessment techniques include the Sandovsky Method and the Count-to-Ten Method.
• An ultrasound measurement of embryo/fetus during the 1st trimester (up to & including 13 6/7 weeks gestation) is the most accurate for establishing/confirming gestational age.
• Phosphatidyl glycerol, L/S ratio and Bilirubin Determination are measured during amniocentesis.
• Chromosome analysis can be performed to assess Down's syndrome and other genetic disorders.
• Alpha-fetoprotein is measured to detect open spinal or abdominal defects.
• Indications for Amniocentesis is advanced maternal age, previous offspring with chromosomal anomalies, recurrent pregnancy loss and ultrasound diagnosis of fetal anomalies.
• Indications for Amniocentesis also include abnormal MSAFP, triple marker screen results, or both parents are known carriers of recessive genetic traits.
• Maternal Serum Alpha-fetoprotein Screening is measured during 16-18 weeks AOG.
• Maternal Serum Alpha-fetoprotein Screening is elevated in neural tube defects.
• Maternal Serum Alpha-fetoprotein Screening is decreased in Down Syndrome.
• Trisomy 18 shows low levels detected for the three markers on the Triple Marker Screening test.