Pregnancy, Growth and Development

nuclei of ovum and sperm unite to form a zygote.

Twins \fraternal twins occur when two separate eggs are ovulated. May be of different sexes. 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.

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 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 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 8 th 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.

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

Reflection Teacher modeled reflection: The process of fertilization involving both the male and female reproductive systems are truly a miracle. Everything must function perfectly for the correct instructions to be past on through gametes in order to combine and make a human zygote. It’s amazing that more things do not go wrong seeing how perfect this process must be in order to create an embryo and to keep an embryo within the womb to develop throughout a period of 9 months. Reflection: Why is it critical that both the male gamete and the female gamete must contain 23 chromosomes each to form a zygote? What could happen if there is more chromosomes or less chromosomes within each gamete?