Reproduction

Yolk sac

- produces the primordial germ cells (an embryonic membrane).

Allantois

-an embryonic membrane that provides the structural basis for the body stalk or umbilical cord.

Trophoblast

-becomes the chorion and cooperates with the uterine tissue to form the placenta

Inner cell mass

-produces the amnion, yolk sac, and allantois

Inner cell mass

-produces the embryonic body. Trophoblast

Fertilization

Before fertilization can occur, a secondary oocyte must be ovulated and enter the uterine tube where the sperm must meet it. Prostaglandins in the semen stimulate the swimming sperm and encourage muscular contractions in the walls of the uterus and uterine tubes. A secondary oocyte survives only 12 to 24 hours following ovulation. Sperms cells survive 24 to 72 hours in the female reproductive tract. On the surface of the oocyte are a group of follicular cells called the corona radiata. The zona pellucida is located around the oocyte's membrane. The sperm's acrosome helps the sperm penetrate this zona pellucida. At this point the zona pellucida hardens which serves to prevent other sperm cells from entering oocyte. After the sperm cell enters the oocyte, the oocyte completes meiosis II and divides into a large cell (called the ovum) and a tiny polar body. The nuclei of the ovum and sperm unite, forming 23 pairs of chromosomes.

embryonic stage in which the embryo consists of a hollow ball of cells

blastula

process of rapid mitotic cell division without intervening growth periods

cleavage

Pregnancy

consists of three periods called trimesters, each about three months long. The normal period of prenatal development is 38 weeks. Many hormonal changes occur in the mother to assist in the development. The placenta secretes HCG which helps prevent spontaneous abortion and also maintains the corpus luteum so it continues to secrete high enough levels of estrogen and progesterone to prevent release of LH and FSH. The corpus luteum secretes estrogen and progesterone for about the first three months and these high levels are thought to be associated with "morning sickness." The estrogen and progesterone are also secreted by the placenta and the placenta takes over that role at three months. The high levels of these hormones keep the uterine wall thickened and prepares the mammary glands. Birth is initiated by uterine contraction, as a result of declining progesterone levels, thinning of the cervix and stretching of the uterine tissues. The stretching of the tissues stimulates release of oxytocin, which further stimulates powerful uterine contractions. After birth is expulsion of the placenta.

Embryonic stage

extends from the second week through the eighth week and includes the development of the main internal organs and external body structures. The primary germ layers are three layers of the embryonic disc that from through gastrulation. Once three distinct layers are formed, the embryo is called a gastrula. The three layers are ectoderm, mesoderm and endoderm. Refer to table 28.1 and be familiar with the body tissues that are developed from each layer. The fetal period goes from week 9 to 38 is highlighted in table 28.2. Be familiar with each stage. Fetal circulation is separate from maternal circulation. The umbilical arteries and veins are unique in that they close off at birth but during development they serve to exchange nutrients, gases and waste products with the mother's circulation.

fusion of male and female pronuclei

fertilization

process involving cell rearrangement to form the three primary germ layers

gastrulation

Cleavage

is the first period of development and is characterized by rapid cell division and distribution of the cytoplasm of the zygote into smaller cells. Therefore it is a period of growth but not in proportional mass. Blastomeres are cells produced from cleavage. A morula is a solid ball of cells that is formed through cleavage and travels through the fallopian tube. A blastocyst is a hollow mass of cells that implants into the endometrium. The blastocyst consists of two distinct regions: the inner cell mass and a trophoblast. The inner cell mass gives rise to the embryo and the trophoblast creates the supporting structures necessary for the embryo to survive. Review figure 28.4 and be prepared to describe each stage of cleavage

solid ball of embryonic cells

morula

Implantation

of the embryo to the endometrial wall begins around the first week of development. The stages of implantation include attachment of the blastocyst to the endometrium, digestion of some cells of the endometrium, sinking into the wall of the endometrium and the thickening of the endometrium around the blastocyst . The cells surrounding the embryo and a layer of the endometrium form the placenta, a vascular structure that secretes hormones, attaches the embryo to the uterine wall and provide a site of gas, nutrient and waste exchange between the mother and the growing embryo. Refer to figure 28.5.

combination of egg and sperm

zygote