After fertilization, there are changes in the mother and changes in the zygote.  We will look at both of these here.

Changes in the zygote

    The zygote has a tremendous challenge ahead of it.  It must change from being just one cell, to be trillions of cells of various types (epithelium, muscle, connective tissue cells, muscle), and these cells must organize themselves into a human form.  This task may seem quite daunting to us, but the zygote can accomplish it.  Exactly how all the cells know what to do is unknown, but we have gotten a lot of insight into how this happens by studying other organisms.  It turns out that there are many signals that occur during development... for example, any cell might be able to send out a chemical cue, but only one cell does this first; whichever cell sent out the cue ends up having a different fate than the ones that recognized the cue.   Anyway, that is all beyond the scope of this class.  I just thought you may want a little clue into it.

    After a day, the zygote is ready to begin to divide.  It divides in two, and then is considered to be at the 2-cell stage of development.   Each of these cells divide, and it is then at the 4-cell stage of development.   These divisions and subsequent ones that double cell number again continue until a ball of 16 cells is found about 3 days later.  At this point, we no longer have a zygote (which was only the single fertilized egg)... instead we have a structure called a morula.  By the time the morula exists, it has made it from the uterine tube to the uterus.  The cells within the morula continue to divide and they rearrange themselves from a solid ball of 16 cells to a hollow ball of cells called a blastocyst.  It is the blastocyst that implants into the wall of the uterus (into the endometrium).

    The blastocyst starts off as a hollow ball.  But then, some cells begin to grow as a clump within that ball.  At this point, the growing cell mass can be officially called an embryo.  The outer cells of the ball are called the trophoblast, and these cells are important for hormone production, implantation, and development of embryonic membranes (as you will see in the embryonic membranes web page).  The cells that form a clump within the blastocyst, the inner cell mass, are the cells that will give rise to the new human organism.  These cells are also called the embryo proper.   Note that the inner cell mass doesn't form until about 5 or 6 days after fertilization!

    Note that there are more images of these embryonic and pre-embryonic structures in Figures 23.3, 23.4, and 23.5 (on pages 895 - 896).

What happens next to the embryo?

    The embryo cells (of the inner mass) grow into a flattened disk, called the embryonic disk.  From there, they fold in a way called gastrulation, so that cells of the disk suddenly find themselves against other cells.  This new juxtaposition of the embryonic cells causes some cells to develop into pre-epithelial or pre-nervous system cells (called ectodermal cells), pre-connective tissue or pre-muscle cells (called mesodermal cells), or pre-internal organ cells (called endodermal cells).   Thus, the plan for the major cell types is laid out by the end of the 3rd week.

    The rest of the development of the embryo is a bit too detailed for us.  Keep in mind that after 8 weeks of embryonic development, the embryo is referred to as a fetus rather than an embryo.  The term fetus is then used for the remainder of the pregnancy.

Changes in the mother (a.k.a., changes during pregnancy)

    When a woman becomes pregnant, meaning that the blastocyst implanted, there are many changes that occur in her body.  Those changes can be linked to hormonal changes caused by the pregnancy.  How do these hormonal changes occur?

    First of all, think back to Figure 22.29, which shows the non-pregnancy changes in hormone levels.  About a week after ovulation, progesterone levels normally begin to decline, causing the endometrium to degenerate and slough off.   But, about a week after ovulation is typically the time a blastocyst would be implanting in the endometrium.  So, the first important hormonal change to consider is the one that prevents the sloughing off of the endometrium.

    The trophoblast of the blastocyst secretes hCG (human chorionic gonadotropin).  hCG prevents the corpus luteum from turning into the corpus albicans... in the presence of hCG, the corpus luteum remains and continues to secrete high levels of progesterone (and estrogen).  hCG levels remain high for the first couple of months, and during this time it also prevents the normal rhythmic cycling of LH and FSH that would cause more ovulation.  The corpus luteum is maintained throughout the pregnancy, but after the first couple of months, it is not really important as a source of hormones; it is important, by the end of pregnancy, for secreting another hormone called relaxin.

    The next big hormonal change is that along with the development of the embryo and fetus comes the development of the placenta.  The placenta is not just an area for exchange of blood materials between mother and embryo/fetus, but it is also a source of hormones.  The placenta secretes placental estrogen and placental progesterone.  The secretion of these hormones by the placenta maintains high estrogen and progesterone levels throughout pregnancy.  Also, the loss of the placenta during childbirth removes this extra hormonal source.  Your book notes that the placenta also secretes another hormone, placental lactogen, that helps prepare the mammary glands for milk production.

    By the end of pregnancy, relaxin has worked on the symphysis pubis, to make this joint more flexible.  That way, the pelvic girdle can move a bit to enable the fetus to leave the birth canal.

    The other changes during pregnancy are not ones that I will focus on.  They are also pretty straightforward and understandable from your book.

© 2006 STCC Foundation Press
written by Dawn A. Tamarkin, Ph.D.