1. Fertilization and Fetal Development
Prepared by :
Ayda khader
Feb.2017

2. Genes and Chromosomes :
The Gens are composed of tiny segments of DNA which enable them to duplicate themselves during cell division.
Each cell contains two sets of genes arranged in a line to form larger structures, the chromosomes within the cell nucleus.
Each cell nucleus contain 2 sets of chromosomes consisting of 2 matching genes, normal human somatic cells contain 46 chromosomes arranged as 23 pairs of matched chromosomes, one set obtained from each parent during the process of fertilization.
When the genes are alike and produce the same effect, they are called homozygous, when they are not alike and produce different effects they are called heterozygous.

3. Cells division
Somatic cells divided by the process of mitosis in which the cell components including genetic material are distributed equally to the new formed cells.
Each new cell contains the same composition and genetic potential as the original cell.
The process of cell division in the reproductive cells is called gametogenesis.
Gametogenesis takes place by meiosis

4. Gametogenesis Spermatogenesis
Meiosis in the male gonad is a contentious process that begins about the time of puberty and last until senescence.
Each normal sperm contains a haploid (23) complement of chromosome (22 autosomes and X or Y chromosome).
The male supplies the genetic material X or Y sex chromosome. Generally the presence of a Y chromosome causes embryo to develop as a male and in absence of a Y chromosome develop as a female fetus.

5. Oogenesis
Unlike spermatogenesis, the process of meiosis in the ovaries is not a contentious process.
Oogenesis begins during intrauterine life and the female gametes have already enlarge and developed into primary oocytes at birth. Approximately 2-4 million in number.
Each normal ovum contains 22 autosomes and X chromosome.
An ovum fertilized by a sperm bearing a Y chromosome, results in male zygote, whereas an ovum fertilized with X-bearing sperm results in a female zygote.

6. Conception
Conception (Fertilization) refers to the fusion of a sperm and an ovum (oocyte) that this process requires about 24 hrs.
At the time of ejaculation, 3-5 ml of semen is released that each ml contains 100 million sperm.
By flagger movement the sperms take their way through the fluid of cervical mucus, across the endometrium and into the uterine tube to meet the descending ovum in the ampulla of the tube.
Sperm remain viable within women’s reproductive system for an average 1-2 days and few remain fertile as long as 5 days.
The ovum may have capacity to be fertilized no longer than 24 hrs

7. Before fertilization the sperm undergo a physiologic change called Capacitation (refers to the removal of a protective coating from the sperm by assisting of enzyme presented in the uterine tube) and structural change called Acrosome reaction (refers to the small perforation that form in the anterior head of the sperm through which a hyaluronidase and acrosin enzymes escape and digest a path through the ovum).
The acrosomal reaction does not begin until the sperm comes into contact with the oocyte's jelly layer.
When a sperm successfully penetrates the membrane surrounding the ovum, both sperm and ovum are enclosed and becomes impenetrable to other sperm within the membrane; this process called Zona reaction.

8. When the sperm and ovum meet and form zygote, the diploid number of chromosome (44+2 sex chromosome) is restored.
If the ovum unfertilized by sperm, it will be degenerated and reabsorbed.
. Implantation (Nidation)
After conception, the zygote propelled by ciliary action and irregular peristaltic contractions, start to move through the uterine tube into the uterine cavity.
During the 3-4 days period it takes to travel down to the uterus, the zygote begins the process of rapid mitotic cell division called Cleavage.

9. The initial division of the zygote results in 2 blastomeres, then 4, then 8 and so which subsequently divide into progressively smaller blastomeres.
At the end of 3-4 days the developing individual comprises about 16-cell of blastomeres arranged in a ball-like structure called a Morula (mulberry).

10. After the morula enters the uterus, a fluid-filled cavity formed within the dividing cells changing morula into a blastocyst The blastocyst remains free in the uterus for 1-2 days before implanted in the endometrium, then develops into two masses:
Trophoblast: outer layer of blastocyst (which develop in to the placenta) usually implemented in anterior or posterior fundal region.
Embryoblast: inner layer of blastocyst ( which develop in to the embryo).

12. The outer layer of cells surrounding the blastocyst cavity (trophoblast) secretes proteolytic and cytolytic enzymes help them to burrow their way into endometrium.
This burrowing in the endometrium called Implantation or Nidation.
Slight bleeding called implementation bleeding occurs in some women.
After it’s securely implanted (7-10 days), the blastocyst develops elaborate projections called Chorionic villi which cooperate with tissue of mother’s uterus to form placenta

13. Chorionic villi secrete hormones called human chorionic gonadotrophin hormone (HCG) in order to synthesis protein and glucose for about 12th week of gestation and after that the fetal liver can supply its own glucose and insulin.
HCG stimulate continued secretion of progesterone and estrogen by the corpus luteum (develops from rupture of an ovarian follicle during the luteal phase of the menstrual cycle) and thus preventing ovulation and menstruation during pregnancy.

14. The inner cell mass of blastocyst develops into fetus, while the outer layer of a blastocyst (trophoblast) develops into placenta which forms a vascular process to provide nutrients to the embryo.
After completion of the conception, the endometrium is called Decidua (Discard) that the vascularity of the uterine wall increased greatly under the influence of ovarian hormone

15. Placenta The placenta develops after third week of gestation
The growth of the thickness of placenta continues until weeks, while the circumference growth continues until later pregnancy.
The fully developed placenta (after birth) is reddish, cm diameter, 2.5-3cm thickness and weighted 400 – 600 g or 1/6 newborn’ weight.
The optimal circulation to the placenta and fetus is achieved when mother lying on her left side.

16. The placenta functions:
1. Endocrine gland: HCG secreted by cytotrophoblast (inner layer of the trophoblast), estrogen and progesterone secreted by corpus luteum at 7 weeks after fertilization.
2. Metabolic, respiratory, renal and nutrient functions
3. Immunologic function: help in transfer of immunoglobulin and antibodies from mother to the fetus (passive natural immunity).
4. Protective barrier: serve as a protective barrier against the harmful effect of certain drugs and microorganism

17. Amniotic fluid
Amniotic fluid volume increase at an average rate 25 ml /week during first trimester and 50 ml during second trimester. Full fetus is immersed in about 1000 ml ( ) of clear slightly yellowish liquid.
It’s contain albumin, urea, uric acid, creatinine, fat, bilirubin, inorganic salt, few leukocyte, various enzyme and lanugo hair
Amniotic fluid originates from maternal serum then from fetal urine that replaced every 3 hrs in early pregnancy.
PH is neutral to slightly alkaline

18. Serve functions as the next:
1. Protect the fetus from direct trauma.
2. Separate the fetus from the fetal membrane
3. Allow freedom of fetal movement
4. Facilitate symmetric growth and development
5. Maintain constant fetal body temperature.
6. Source of oral fluid for the fetus (swallow up to 400 ml/day)

19. Umbilical Cord
It’s the life line that links between the embryo and the placenta, about cm long and 2 cm in diameter and does not contain pain receptors
Has two umbilical arteries (carry deoxygenated blood) and one umbilical vein (carry oxygenated blood), that 400 ml of blood flow each minute
Supported by loose connective tissue containing Wharton’s jelly to prevent kinking of the cord in the uterus

20. Fetal membrane
The fetus is surrounded by two closely applied but separated membranes
Inner layer called amnion
Outer layer called chorion
Intact, healthy fetal membranes are required for an optimal pregnancy outcome.

22. Fetal Development
The growth and development of the fetus is typically divided into three stages:
Preembryonic stage(Fertilization to 2-3 wks)
Embryonic Stage: 4-8 weeks of gestation
Fetal Stage: 9 weeks after conception until birth

23. Viability
Refers to a point in fetal development at which the fetus may survive outside the womb without mechanical support.
There is no sharp limit of development, age, or weight at which a fetus automatically becomes viable. The lower limit of viability is approximately 20 weeks of gestation in developed country and 28 weeks of gestation in developing country or fetus weighted more than 1000 gram.
When such babies are born, the main causes of perinatal mortality are that the respiratory system and the central nervous system are not completely differentiated.