Section One

2. The primary oocytes remain dormant in the ovarian follicles until ___puberty____. ​ ​

3. There are approximately _2 million_ primary oocytes in the ovaries of a newborn female, but ​ ​ most regress during childhood so that by adolescence no more than _40,000_ remain. Of these, ​ ​ ​ ​ _only approximately 400_ become secondary oocytes and are expelled at ovulation during the ​ ​ reproductive period.

4. What is the average size of the uterus? ● The average size of the uterus is 7 to 8 cm in length, 5 to 7 cm in width at its superior part, and 2 to 3 cm in wall thickness.

Section Two

1. When does implantation of the blastocyst occur? ● Implantation of the blastocyst occurs during a restricted time period 6 to 10 days after ovulation. As the blastocyst implants, more trophoblast contacts the endometrium and differentiates into two layers: The inner cytotrophoblast, which forms the , and the syncytiotrophoblast, which develops into the outer layer of the . 2. The syncytiotrophoblast produces which pregnancy sustaining hormone? ● The syncytiotrophoblast produces hCG in pregnancy. Enough hCG is produced by the syncytiotrophoblast at the end of the second week to give a positive pregnancy test, even though the woman is probably unaware that she is pregnant. 5. Why does placenta previa occur? ● Placenta previa occurs because the blastocyst implants in the inferior segment of the uterus near the internal os of the cervix. Placenta previa may cause bleeding because of premature separation of the placenta during pregnancy or at the time of delivery of the fetus. As the cervix opens during labor, it can cause blood vessels that connect the placenta to the uterus to tear.

Section Three

4. What malformations can occur during this stage of development? ● Remnants of the primitive streak may persist and give rise to a sacrococcygeal teratoma. A teratoma is a type of germ cell tumor. Because they are derived from pluripotent primitive streak cells, these tumors contain tissues derived from all three germ layers in incomplete stages of differentiation. Sacrococcygeal teratomas are the most common tumor in newborns and have an incidence of approximately one in 35,000; most affected infants (80%) are female. A presacral teratoma may cause bowel or urinary obstruction in the newborn. Although the tumors can grow very large, they are usually not malignant (that is, cancerous). ● Both benign and malignant tumors (chordomas) may form from vestigial remnants of notochordal tissue. Approximately one third of chordomas occur at the base of the cranium and extend to the nasopharynx(the upper part of the throat behind the nose). Chordomas grow slowly and malignant forms infiltrate bone. ● Allantoic cysts, remnants of the extraembryonic portion of the allantois, are usually found between the fetal umbilical vessels and can be detected by ultrasonography. They are most commonly detected in the proximal part of the , near its attachment to the anterior abdominal wall. The cysts are generally asymptomatic until childhood or adolescence, when they can become infected and inflamed. Allantoic cyst is a rare swelling formed at the base of umbilicus associated with a patent urachus which results from an allantoic remnant. ● Neural tube defects are among the most common congenital anomalies. Meroencephaly (partial absence of the brain) is the most severe neural tube defect and is also the most common anomaly affecting the CNS (brain and spinal cord). ● Sometimes the embryo dies and the chorionic villi do not complete their development; that is, they do not become vascularized to form tertiary villi. These degenerating villi form cystic swellings—hydatidiform moles—which resemble a bunch of grapes. The moles exhibit variable degrees of trophoblastic proliferation and produce excessive amounts of human chorionic gonadotropin. Some moles develop after spontaneous abortions, and others occur after normal deliveries. Three percent to 5% of moles develop into malignant trophoblastic lesions—choriocarcinomas. Choriocarcinomas invariably metastasize (spread) through the bloodstream to various sites, such as the lungs, vagina, liver, bone, intestine, and brain. Dude!!!! I spent so much time just looking up these abnormalities, and then it just spiderwebbed to a whole bunch of other abnormalities. It’s crazy!! Wow dude.

5. During which week does cardiovascular activity begin, and on approximately which day? ● Paired, longitudinal endothelial-lined channels—the endocardial tubes— develop during the third week and fuse to form a primordial heart tube. The tubular heart joins with blood vessels in the embryo, , chorion, and umbilical vesicle to form a primordial cardiovascular system. By the end of the third week, the blood is circulating and the heart begins to beat on the 21st or 22nd day. The cardiovascular system is the first organ system to reach a functional state.

6. When can the heartbeat be detected via ultrasound? ● The embryonic heartbeat can be detected using Doppler ultrasonography during the fifth week, approximately 7 weeks after the last normal menstrual period.