First week of pregnancy

Question 1

How does implantation happen ?

Answer 1

1. At the 6th day after fertilization the trophoblastic cells over embryonic pole adhere to the uterine wall then erode the endometrium and the whole blastocyst progressively penetrates it.
2. The trophoblast cells rapidly proliferate and become formed of 2 layers as soon as it endometrium ( decidua ) :
   a. an outer ( syncytial trophoblast ) formed of a mass of protoplasm with randomly dispersed nuclei.
   b. an inner cellular trophoblast ( cytoptophoblast ) formed of cells with well-defined walls.
3. At about the 9th day of the blastocyst becomes more deeply embedded in the endometrium and the penetration defect in the surface of the epithelium is closed by coagulation plug.
4. At the 11th and 12th days, the blastocyst becomes completely implanted and the surface of the epithelium is restored.

Question 2

what are the abnormal sites of implantation ?

Answer 2

1. Inside the endometrium :
   a. placenta previa lateralis
   b. placenta previa marginalis
   c. placenta previa centralis
2. Extra-uterine :
   a. in the fallopian tube : tubal pregnancy
   b. in the abdominal cavity : abdominal pregnancy
   c. on the surface of the ovary : ovarian pregnancy

Question 3

What is the decidua ?

Answer 3

Name given to the endometrium of the pregnant uterus or gravid endometrium.

Question 4

What are the parts of the decidua ?

Answer 4

1. Decidua capsularis : the part that lies superficial to the blastocyst ( between blastocyst and uterine cavity ).
2. Decidua basalis : the part that lies deep to the blastocyst ( between the blastocyst and muscle wall of uterus ).
3. Decidua parietalis : the part that lines the remaining parts of the uterus.

Question 5

Describe the changes that happen to the embryonic cells in the second week.

Answer 5

1. a small space appears in the embryoblasts , the primordium of the amniotic cavity.
2. Soon amnioblasts ( amnion - forming cells ) , separate from the epiblasts and form the amnion , which encloses the amniotic cavity.
3. Concurrently , morphological changes occur in the embryoblast that result in formation of a flat , almost circular , transparent bilaminar embryonic disc , consisting of two layers :
4. Epiblast ( future ectoderm ) : consists of high cloumnar cells related to the amniotic cavity.
5. Hypoblast ( future endoderm ) : consists of small cuboidal cells adjacent to the exo-coelomic cavity ( primary yolk sac ).