Stages of Fetal Development
June 22, 2008 · 2 Comments
- The ovum is the female sex cell.
- It is regularly released by the ovary through the process of ovulation.
- It has two layers of protective covering, the outer layer is the corona radiata and the inner layer is the zona pellucida.
- The egg cell has a lifespan of 24 hours, thus, it can only be fertilized within this period. After 24 hours, it regresses and is resorbed.
- Sperm cell is present in the fallopian tube only in 3 out of 5 ovulations of married women.
The sperm cell has three parts: a head that contain the chromatin materials, a neck or mid-piece that provides energy for movement, and a tail that is responsible for it’s mobility. The sperm cell has a lifespan of 48 to 72 hours or 3 to 4 days after ejaculation. The sperm must be in the genital tract 4-6 hours before they are able to fertilize an ovum to give time for the enzyme hyaluronidase to be activated. There are two kinds of sperm cell:
- Gynosperm – This is the X carrying sperm cell. It has a large oval head, are lesser in number than androsperms and thrive better in acidic environment.
- Androsperm – the sperm cell which carries the Y chromosome, with a small head, and thrive better in alkaline environment.
- Per ejaculation the average 2.5ml of seminal fluid contains 50 to 200 million spermatozoa per ml or 400 million per ejaculation.
- Fertilization occurs in the outer third (ampullar portion) of a fallopian tube.
- Hyaluronidase released by the spermatozoa dissolves the layer of cells protecting the ovum, facilitating the penetration of the spermatozoon.
- Upon fertilization, the resulting structure is called zygote.
- Only the father can determine the gender of the child – X – carrying spermatozoon leads to XX combination for a female offspring; Y-carrying spermatozoon leads to XY combination for a male offspring. The ovum carries only X chromosome.
Within a few hours after fertilization, after the nucleus of the sperm has united with the nucleus of the egg, the result of their union, the zygote, begins a process of internal division. First, it divides into two cells, then four, eight, sixteen, and so on, doubling the number with each new division. This process of cell division or cleavage in the zygote is called segmentation. It transforms the zygote into a cluster of cells called morula which, seen through a microscope, resembles a mulberry. The morula slowly moves down the Fallopian tubes toward the uterus, where it arrives after about three days. By this time, it has developed into a hollow ball of cells called blastocyst.
- It takes 3 to 4 days for the zygote to journey to the uterus (where implantation will take place), and during such journey mitotic cell division happens. Floating freely in the uterus for the next 3 to 4 days, the morula (16 to 50 cell bumpy appearance resulting from mitotic cell division) grows to become a blastocyst with tropoblast cells cells (forming placenta and membrane in later development). Therefore, it takes 7 to 8 days from fertilization to implantation.
- Implantation occurs at high and posterior portion of the uterus.
- On implantation, the structure is called embryo until 5-8 weeks when it begin to be referred to as fetus.
- Implantation bleeding (mistaken as menstrual period) results from capillary rupture on implantation.
- Endometrium (the inner lining of the uterus) is termed decidua on conception.
Day 1 – conception takes place
7 days – tiny human implants in mother’s uterus
10 days – mother’s menses stop
18 days – heart begins to beat
21 days – pumps own blood through separate close circulatory system with own blood type
28 days – eye, ear, and respiratory system begin to form
42 days – brain waves recorded skeleton complete, reflexes present
7 weeks – capable of thumb sucking
8 weeks – all body systems present
9 weeks – squints, swallows, moves tongue, makes fist
11 weeks – spontaneous breathing movements, has fingernails, all body systems working
12 weeks – weighs one ounce
16 weeks – genital organs clearly differentiated, grasps with hands, swims, kicks, turns, somersaults, ( still not felt by the mother)
18 weeks – vocal cords work
20 weeks – has hair on head, weighs one pound, 12 inches long
23 weeks – 15% chance of viability outside of womb if birth premature
24 weeks – 56% of babies survive premature birth
25 weeks – 79% of babies survive premature birth
Source: Outline in Obstetrics, 2004 edition by Maria Loreto J. Evangelista-Sia