Student-Doctor Podcast

At fertilization, the sperm nucleus enters the oocyte, the oocyte completes meiosis, and the pronuclei of the two mature gamates fuse. Fertilization is a complex interaction between sperm and oocyte. If viable sperm encounter an oocyte in the ampulla of the oviduct, they surround it and force their way through the cumulus mass.

When the spermatozoa reaches the tough zona pellucida surrounding the oocyte, it binds with a glycoprotein sperm receptor in the zona (ZP3) and then the acrosome is induced to release degenerative enzymes that allow penetration of the zona pellucida. After penetration, the cell membranes fuse, which causes thousands of cortical granules located just beneath the cell membranes to release into the perivitelline space between the oocyte and the zona pellucida. The substance release by the granules causes a chemical change in the zona pellucida which alters the sperm receptor molecules, thus causing an impenetrable zona, preventing polyspermy.

Fusion of the two membranes also causes the oocyte to complete the second meiotic metaphase and also goes rapidly through anaphase to produce another polar body.  The oocyte is now considered to be a definitive oocyte. The chromosomes of the oocyte and sperm are then respectively enclosed within the male and female pronuclei. The pronuclei then fuse with one another to produce the single, diploid, 2N nucleus of the fertilized zygote.  This is taken to be the Beginning or zero time of embryonic development.

First few days of embryonic development:

Within 24 hours a series of Cleavages begins that subdivides the zygote without increasing its size. The subdivisions in the zygote are called blastomeres.  We are still inside the zona. At 40 hours, the second division is complete which has produces four equal blastomeres.

3 days—- 6-12 cells;   4 days—- 16-32 cells

By the 32 cell stage, the embryo is termed the morula (from the Latin for mulberry)

The cells of the morula will give rise to the embryo proper and attached membranes, and also to the placenta and related structures.

At the  8 cell stage, the blastomeres begin to flatten, developing an inside-out polarity that maximizes cell-to-cell contact among the blastomeres at the center of the mass.

As differential adhesion develops, the outer surfaces of the cells become convex and the inner surfaces become concave.  This is termed compaction. This development of differential adhesion causes segregation of some of the blastomeres to the center of the morula and others to the outside.

There is now an inner cell mass and the outer cell mass. There is some exchange between the two groups, however the inner group generally gives rise to the embryo proper and is thus termed embryoblast, and the outer cell mass gives rise to the membranes of the placenta and is thus termed the trophoblast.

By 4 days, the morula begins to absorb fluid which collects between the cells.

Also tight junctions and gap junctions begin to develop between blastomeres, especially between those of the outer cell mass.  As a result of the tight junctions forming between cells of the outer cell mass, the fluid collects between the cells of the inner cell mass. As the hydrostatic pressure increases because of the increase in fluid, a large cavity called the blastocyst cavity forms. The embryoblast then form a compact mass at one side of this cavity, and the outer cells or trophoblast is organized into a thin single-layered epithelium. The embryo is now termed a blastocyst.   The side of the blastocyst containing the inner cell mass is termed the embryonic pole and the side containing the outer cell mass is termed the abembryonic pole.

Beginning of implantation:

by day 5 the blastocyst hatches from the zona pellucida through the use of enzymes.

After about 6.5 days the blastocyst becomes very tightly adherent to the uterine lining. The adjacent cells of the endometrial stroma respond to its presence and to the progesterone secreted by the corpus leutem by differentiating into metabolically active, secretory cells called decidual cells.  This response is called the decidual reaction.

The endometrial glands in the surrounding area also enlarge and become highly vascularized and edematous.  (Sort of like an inflammation reaction) The uterine lining is maintained in a favorable state and kept from sloughing partly by the increased secretion of progesterone.  (In the absence of an implanted embryo, the corpus luteum normally degenerates after about 13 days) If an embryo implants, the cells of the trophoblast produce the hormone human chorionic gonadotropin (hCG) which supports the corpus luteum and thus maintains the supply of progesterone (Maternal recognition  of  pregnancy).

Ectopic Pregnancies

When a blastocyst implants in the peritoneal cavity, on the surface of the ovary, within the oviduct, or at an abnormal site of the uterus.

Symptoms:

abdominal pain

vaginal bleeding

Surgical intervention required

Applications to clinical practice:

Down Syndrome - trisomy-21; results from nondisjuncton

Amniocentesis- removal and examination of sloughed off cells contained in the amniotic fluid to determine karyotype

Turner syndrome- XO

Kleinfelter syndrome- XXY

Birth control methods

barrier contraceptives

birth control pills

depot preparations

nonmedicated intrauterine devices

RU-486

Sterilization

Assisted reproduction

in vitro fertilization and embryo transfer

gamate intrafallopian transfer

This entry was posted on Sunday, September 20th, 2009 at 5:00 am and is filed under Reproductive Science. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.