Page 111 - 86 human physiology part-2

Chapter 17

17 PREGNANCY AND BIRTH

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Introduction

I n this chapter we will discuss the topics covering pregnancy, from conception to birth. The chapter

will cover fertilization, implantation of the zygote, to becoming a fetus, the three trimesters, and the
progressive development of the fetus through the weeks of pregnancy. It will cover the topic of birth
and different birthing methods.

Fertilization

Fertilization is the joining of a sperm and an egg. A sperm is a male gamete that is released into the
vagina of a female during intercourse. In order for fertilization to occur there must be a mature ovum
present. Every month one of the ovaries releases an egg which will meet one of the A 4 million sperm
the male ejaculates into the vagina. The sperm swim through the cervix and into the uterus which lead
to the fallopian tubes. This is where fertilization is most likely to take place. The high amount of sperm
in the ejaculate is needed because only around 100 survive to enter reach the fertilization site. In order
to penetrate the egg the sperm must first break through 2 barriers surrounding the ovum. The
acromsome of sperm comes in contact with the corona radiata and releases digestive enzymes that
break down a gelatinous layer around the egg called, the zona pellucida. Once a sperm reaches the
plasma membrane of the egg it sets of a reaction that spreads across the membrane of the egg
preventing other sperm from breaking through the egg membrane. Once the sperm reaches the inside of
the egg it sheds its tail and the two nuclei fuse and now the 23 chromosomes from the egg and the 23
chromosomes of the sperm join and they become a zygote. Chromosomes contain all the information
needed to determine the genetic structure of the new baby. Normally all human beings have two
chromosomes that determine sex: A combination of X and Y makes a male or a combination of X and
X makes a female. All ovum have X sex chromosomes where as sperm have both X or Y sex
chromosomes. Therefore, the male gametes determine the sex of the baby.

Pre-embryonic Period

After fertilization, the zygote begins a process of dividing by mitosis in a process called cleavage.
It divides until it reaches 16 cells. It is now referred to as a morula. As the morula floats freely within
the uterus, it starts to bring nutrients into the cells. The morula fills with fluid and the cells inside start
to form two separate groups. At this stage it is now a blastocyte. The inner layer of cells is called the
embryoblast, and will become the fetus. The outer layer is called a trophoblast which will develop into
part of the placenta. At this point the zona pellucida is disintegrating. The trophoblast contains
specialized cells that become extensions, like fingers, that grow into the endometrium once in contact
with the well thickened endometrium.

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