
Immediately after fertilization, the fertilized ovum or zygote stays in the Fallopian tube for 3 days, during which time rapid cell division (mitosis) is taking place. The developing cells are now called blastomere and when there are already 16 blastomeres, it is now terms a morula. In this morula form, it will start to travel (by ciliary action and peristaltic contractions of the Fallopian tube) to the uterus where it will stay for another 3-4 days. When there is already a cavity formed in the morula, it is now called blastocyst. Fingerlike projections, called trophoblasts, form around the blastocyst and these trophoblast are the once which will implant high on trophoblasts or posterior surface of the uterus. Thus, implantation, also called nidation, takes place about a week after fertilization.
General Consideration:
A. Once implantation has taken place, the uterine endothelium is now termed decidua
B. Occasionally, a small amount of vaginal spotting appears with implantation because capillaries are ruptured by the implanting throphoblasts = implantation bleeding. Implication: this should not be mistaken for the Last Menstrual Period (LMP).
Table 1. Outline of Trophoblast Differentiation
I. Cytotrophoblast – the inner layer
II. Syncytiotrophoblast – the outer layer containing fingerlike projections called chorionic villi:
A. Langhan’s layer – believed to protect the fetus against Treponema Pallidum (etiologic agent of syphilis). Present only during the second trimester of pregnancy.
B. Syncytial layer – gives rise to the fetal membranes:
1. Amnion – inner layer which gives rise to:
a. Umbilical cord/funis – contains 2 arteries and one vein, which are supported by the Wharton’s jelly
b. Amniotic fluid – clear, albuminous fluid in which the baby floats. Begins to form at 11-15 weeks gestation. Approximates water in specific gravity (1.007-1.025) and is neutral to slightly alkaline (pH=7.0-7.25. Note: the higher the pH, the more alkaline; the lower the pH, the more acidic). Near term, is clear, colorless, containing little white specks of vernix caseosa and other solid particles. Produced at a rate of 500 ml in 24 hours and fetus swallows it at an equally rapid rate. By the 4th lunar month, urine is added to the amount of amniotic fluid. It is, therefore, derived chiefly from maternal serum and fetal urine. (Implication: a case of polyhydramnios = more than 1500 ml of aminiotic fluid, stems from inability of the fetus to swallow amniotic rapidly, as in Tracheoesophageal fistuli; while oligo-hydramnios = amniotic fluid less than 500 ml, results when kidneys are not functioning normally, as in congenital renal anomaly.) Also know as bag of waters (BOW), it serves the following purposes:
• Protection:
* Shields the fetus against blows or pressure on the mother’s abdomen
* Protects the fetus against sudden change sin temperature because liquid changes temperature more slowly than air
* Protects the fetus against certain infections
• Diagnosis:
* As in amniocentesis
* Meconium-stained amniotic fluid means fetal distress
• Aids in descent of the fetus during active labor
2. Chorion – together with the deciduas basalis gives rise to the placenta, which starts to form on the 8th week of gestation. Develops into 15-20 subdivision called cotyledons. Placenta serves the following purposes:
a. Respiratory system – exchange of gases takes place in the placenta, not in the fetal lungs
b. Renal system – waste products are being excreted through the placenta (Note: it is the mother’s liver which detoxifies the fetal waste products).
c. Gastrointestinal system – nutrients pass to the fetus via the placenta by diffusion through the placental tissues.
d. Circulatory system – feto-placental circulation is established by selective osmosis
e. Endocrine system – it produces the following hormones (before 8 weeks gestation, the corpus luteum is the one producing these hormones):
Human chronionic gonadotropin (HCG) – “order’s the corpus luteum to keep on producing estrogen and progesterone that is why menstruation does not ta ke place during pregnancy. It is also the basis for pregnancy test.
Human placental lactogen (HPL)/human chronionic somatomammotropin – promotes growth of the mammary glands necessary for lactation. Also has growth-stimulating properties
Estrogen
Progesterone
f. Protective barrier – inhibit passage of some bacteria and large molecules.
3. Stage of human prenatal development:
a. First 12-14 days = zygote
b. From 15th day up to the 8th week = embryo
c. From 8th week up to the time of birth = fetus
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