Pregnancy, Growth and Development

Conception A secondary oocyte can be fertilized for about 24 hours after ovulation Sperm remain viable for up to 48 hours within the female reproductive tract This gives a three day “window” for intercourse to result in fertilization: two days before to one day after ovulation

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Fertilization usually takes place in the outer one-third of the uterine tube, but can take place in the abdominal cavity Sperm swim up the female reproductive tract, aided by muscular contractions of the uterus stimulated by prostaglandins in the semen. The oocyte may also secrete a chemical that attracts sperm

Sperm undergo a functional change in the female tract – called capacitation During this process the membrane around the acrosome becomes fragile, and its enzymes are released. It requires the combined action of many sperm to allow one sperm to penetrate the oocyte.

When the first sperm enters the egg, the cell depolarizes causing the release of calcium ions inside the cell. This stimulates the release of granules that cause changes in the zona pellucida to prevent entry of other sperm. Secondary oocyte completes division, and nuclei of ovum and sperm unite to form a zygote.

Twins Dizygotic or fraternal twins occur when two separate eggs are ovulated. May be of different sexes. Monozygotic or identical twins occur when a single egg is fertilized but dividing cells break into two groups and develop into two individuals. Genetically identical (clones)

Zygote undergoes rapid mitotic cell division, but these do not increase the size of the zygote – called cleavage divisions Cleavage produces a solid sphere of cells, still surrounded by zona pellucida – now called a morula. At 4.5 to 5 days, cells have developed into a hollow ball of cells – blastocyst. It is at this stage that it enters the uterus.

Blastocyst has an outer layer of cells called the trophoblast, an inner cell mass, and a fluid filled cavity called the blastocele. The trophoblast and part of the inner cell mass will form the membranes of the fetal portion of the placenta, the rest of the inner mass forms the embryo.

Implantation The blastocyst remains free in the uterus a short time, during which the zona pellucida disintegrates. Blastocyst nourished by glycogen from glands of the endometrium. At about 6 days after ovulation blastocyst implants – orients cell mass toward endometrium, and secretes enzymes which allow it to penetrate (digest) the endometrial wall. This nourishes the blastocyst for about a week after implantation.

Implantation can also occur in uterine tube, cervix, or the abdominal cavity. Implantation anywhere outside the uterus is called an ectopic pregnancy. It is possible for fetus to grow in the abdominal cavity, but growth inside the uterine tube causes the tube to rupture, resulting in severe bleeding.

As early as 8 -12 days after fertilization, the blastocyst begins to secrete human chorionic gonadotropin or hCG. hCG keeps the corpus luteum active until the placenta can produce estrogens and progesterone. The presence of hCG is the basis for pregnancy tests.

Inner cell mass forms two cavities: The yolk sac Amniotic cavity In humans the yolk sac produces blood cells and future sex cells The amniotic cavity becomes the cavity in which the embryo floats. Fluid is produced from fetal urine, and secretions from the skin, respiratory tract, and amniotic membranes.

Primary germ layers Endoderm Mesoderm Ectoderm In between the yolk sac and the amniotic cavity is the embryonic disc, which gives rise to the primary germ layers: Endoderm Mesoderm Ectoderm

Gestation period Divided into three trimesters. During first trimester individual starts out as a zygote, then morula, blastocyst, and after implantation, is called an embryo. Embryonic phase of development lasts from fertilization until the 8th week of gestation, when it becomes a fetus. By day 35 the heart is beating, and eye and limb buds are present.

By month four, the rudiments of all organ systems are formed and functioning, and from then on, fetal development is primarily a matter of growth. By the end of the third month the placenta is functioning.

The placenta The chorion develops into the fetal part of the placenta. The chorionic villi connect the fetal circulation to the placenta Composed of both fetal and maternal tissues

Functions of the placenta: 1 Transfer gasses 2 Transport nutrients 3 Excretion of wastes 4 Hormone production – temporary endocrine organ – estrogen and progesterone 5 Formation of a barrier – incomplete, nonselective – alcohol, steroids, narcotics, anesthetics, some antibiotics and some organisms can cross

Quickening The first movement of the fetus felt by the mother, usually occurring during the fourth or fifth month of pregnancy By month seven the fetus is quite active During the last month the fetus becomes less active (usually due to space considerations.)

At the end of pregnancy both the mother and the uterus become “irritable” The uterus undergoes Braxton-Hicks contractions: intermittent, painless contractions which can come 10 to 20 minutes apart. Become more frequent as gestation progresses, and can be mistaken for onset of labor Cervix begins to thin and dilate

Labor (parturition) Stage one – the period from the onset of true labor contractions until the cervix is completely dilated at 10 cm. The uterine contractions cause the cervix to dilate, and the amniotic sac may rupture. Usually lasts 6 – 24 hours depending on the number of previous deliveries.

Stage 2 Period from maximal cervical dilation until the birth of the baby Lasts minutes to an hour Contractions become more intense and frequent.

Stage 3 The expulsion of the placenta Usually occurs within 15 minutes after the birth of the baby, but can range from 5 to 60 minutes.

The End !! That’s it ! You’ve made it ! Study well ! Good luck !