Which of the following components plays the most active role in invading the endometrium during blastocyst implantation? (A) Epiblast (B) Syncytiotrophoblast (C) Hypoblast (D) Extraembryonic somatic mesoderm (E) Extraembryonic visceral mesoderm
B. The syncytiotrophoblast plays the most active role in invading the endometrium of the mother’s uterus. During the invasion, endometrial blood vessels and endometrial glands are eroded and a lacunar network is formed.
Between which two layers is the extraem- bryonic mesoderm located? (A) Epiblast and hypoblast (B) Syncytiotrophoblast and cytotrophoblast (C) Syncytiotrophoblast and endometrium (D) Exocoelomic membrane and syncytiotrophoblast (E) Exocoelomic membrane and cytotrophoblast
E. The extraembryonic mesoderm is derived from the epiblast and is located between the exocoelomic membrane and the cytotrophoblast. The overall effect is to completely sepa- rate the embryoblast from the trophoblast, with the extraembryonic mesoderm serving as a conduit (connection) between them.
During week 2 of development, the embryoblast receives its nutrients via (A) diffusion (B) osmosis (C) reverse osmosis (D) fetal capillaries (E) yolk sac nourishment
A. During week 2 of development, the embryoblast receives its nutrients from endometrial blood vessels, endometrial glands, and decidual cells via diffusion. Diffusion of nutrients does not pose a problem, given the small size of the blastocyst during week 2. Although the beginnings of a uteroplacental circulation are established during week 2, no blood vessels have yet formed in the extraembryonic mesoderm to carry nutrients directly to the embry- oblast (this occurs in week 3).
The prochordal plate marks the site of the future (A) umbilical cord (B) heart (C) mouth (D) anus (E) nose
C. The prochordal plate is a circular, midline thickening of hypoblast cells that are firmly attached to the overlying epiblast cells. The plate will eventually develop into a membrane called the oropharyngeal membrane at the site of the future mouth. It is interesting to note that at this early stage of development the cranial versus caudal region of the embryo is established by the prochordal plate, and since the prochordal plate is located in the midline, bilateral symmetry is also established.
Which of the following are components of the definitive chorion? (A) Extraembryonic somatic mesoderm and epiblast (B) Extraembryonic somatic mesoderm and cytotrophoblast (C) Extraembryonic somatic mesoderm and syncytiotrophoblast (D) Extraembryonic somatic mesoderm, cytotrophoblast, and syncytiotrophoblast (E) Extraembryonic visceral mesoderm, cytotrophoblast, and syncytiotrophoblast
D. The definitive chorion consists of three components: extraembryonic somatic meso- derm, cytotrophoblast, and syncytiotrophoblast. The chorion defines the chorionic cavity in which the embryoblast is suspended and is vital in the formation of the placenta.
A 16-year-old girl presents on May 10 in obvious emotional distress. On questioning, she relates that on May 1 she experienced sexual intercourse for the first time, without using any means of birth control. Most of her anxiety stems from her fear of pregnancy. What should the physician do to alleviate her fear? (A) Prescribe diazepam and wait to see if she misses her next menstrual period (B) Use ultrasonography to document pregnancy (C) Order a laboratory assay for serum hCG (D) Order a laboratory assay for serum progesterone (E) Prescribe diethylstilbestrol (“morning- after pill”)
C. Human chorionic gonadotropin (hCG) can be assayed in maternal serum at day 8 of development and in urine at day 10. If this teenager is pregnant, the blastocyst would be in week 2 of development (day 10). Laboratory assay of hCG in either the serum or urine can be completed; however, serum hCG might be more reliable. It is important to note that if she is pregnant, she will not miss a menstrual period until May 15, at which time the embryo will be entering week 3 of development.
Carcinoembryonic antigen (CEA) is an oncofetal antigen that is generally associated with which one of the following tumors? (A) Hepatoma (B) Germ cell tumor (C) Squamous cell carcinoma (D) Colorectal carcinoma (E) Teratocarcinoma
D. Oncofetal antigens are normally expressed during embryonic development, remain unexpressed in normal adult cells, but are re-expressed on transformation to malignant neoplastic tissue. CEA is associated with colorectal carcinoma.
Future site of the mouth
The prochordal plate indicates the site of the future mouth. At this early stage of devel- opment, the orientation of the embryo in the cranial versus caudal direction is established. The prochordal plate is a thickening of hypoblast cells that are firmly attached to the epiblast cells.
Forms definitive structures found in the adult
The bilaminar embryonic disk develops definitive adult structures after gastrulation occurs, as contrasted with the trophoblast, which is involved in placental formation.
The chorion consists of threel ayers—extraembryonic somatic mesoderm, cytotrophoblast, and syncytiotrophoblast. The chorion is vital in the formation of the placenta.
The chorion forms the walls of the chorionic cavity in which the conceptus is suspended by the connecting stalk. Note that the inner lining of the chorionic cavity is extraembryonic mesoderm.
Primary chorionic villi
The cytotrophoblast is mitotically active, so that local mounds of cells (primary chorionic villi) form that bulge into the surrounding syncytiotrophoblast. As development continues, primary chorionic villi form secondary chorionic villi and finally tertiary chorionic villi as part of placental formation.
The extraembryonic mesoderm can be thought of as initially forming in a continuous layer and then splitting as isolated cavities begin to appear everywhere except dorsally near the amniotic cavity and epiblast. When the isolated cavities coalesce, the extraembry- onic coelom (or chorion cavity) and connecting stalk are formed.
A 42-year-old woman presents with complaints of severe headaches, blurred vision, slurred speech, and loss of muscle coordination. Her last pregnancy 5 years ago resulted in a hydatidiform mole. Laboratory results show a high hCG level. Which of the following conditions is a probable diagnosis?
(A) Vasa previa
(B) Placenta previa
(C) Succenturiate placenta
(E) Membranous placenta
After a hydatidiform mole, it is very important to assure that all the invasive trophoblas- tic tissue is removed. High levels of hCG are a good indicator of retained trophoblastic tis- sue because such tissue produces this hormone. In this case, the trophoblastic tissue has developed into a malignant choriocarcinoma and metastasized to the brain, causing her symptoms of headache, blurred vision, and so on.
At what location does the amniotic cavity develop?
(A) Between the cytotrophoblast and syncytiotrophoblast
(B) Within the extraembryonic mesoderm
(C) Between the endoderm and mesoderm
(D) Within the hypoblast
(E) Within the epiblast
The amniotic cavity develops within the epiblast, and it is a cavity that contains the embryo and amniotic fluid.
At the end of week 2 of development (day 14), what is the composition of the embryonic disk?
(A) Epiblast only
(B) Epiblast and hypoblast
(C) Ectoderm and endoderm
(D) Ectoderm, mesoderm, and endoderm
(E) Epiblast, mesoderm, and hypoblast
The embryoblast consists of the two distinct cell layers (epiblast and hypoblast) at the end of development week 2 (day 14) and forms the bilaminar embryonic disk.