Fertilisation to Gastrulaton

Question 1

what are the steps of fertilisation? (+where does it occur)

Answer 1

• Fertilisation occurs in the ampulla

1. sperm binds to zona pellucida glycoprotein
2. Acrosomal enzymes released from the sperm head; the sperm digests its way into the egg.
3. Egg and sperm plasma membrane fuse and sperm contents enter the egg
4. Sperm enters and triggers completion of meiosis 2 & release of cortical granules by oocyte

Question 2

describe cleavege

Answer 2

• one cell undergoes a series of cleavage divisions
• rapid mitotic divisions without an increase of size, because cells are confined inside the ZP the embryo will change shape (COMPACTION)
• forms morula (16+ cells) then the blastocyst (200-300 cells)

Question 3

what 2 different cell types make up there blastocyst?

Answer 3

• outer cell mass (TROPHOBLAST): contacts with the endometrium of the uterus to facilitate implantation and the formation of the placenta
• inner cell mass (EMBRYOBLAST): responsible for the formation of the embryo itself

inner cells form cell junctions, out cells form gap junctions

Fluid enters the through ZP producing a blurred cell mass - signals formation of the blastocyst

Question 4

describe implantation

Answer 4

• once entering the uterine cavity, the embryo floats about 72 hours before implantation
• embryo typically implants itself in the endometrial lining of the posterior wall of the uterine cavity

Question 5

what is an ectopic pregnancy?

Answer 5

when implantation occurs outside the uterus

Question 6

describe placenta previa

Answer 6

placenta lies low in uterus and partially/completely covers the cervix

Question 7

what happens on day 7 of development of the bilaminar embryo?

Answer 7

• trophoblast cells penetrate endometrial epithelial cells

trophoblast (outer cell mass of blastocyst) differentiates into:
- Cytotrophoblast
- Syncytiotrophoblast

Embryoblast (inner cell mass of blastocyst) differentiates into:
-Epiblast
- Hypoblast

Question 8

what occurs on day 8 of the development of the bilaminar embryo?

Answer 8

Formation of amniotic cavity

Question 9

what occurs on day 9 of the development of the bilmaniar embryo?

Answer 9

• defect in endometrium closed by coagulation plug
• formation of house membrane
• formation of primary yolk sac

Question 10

what occurs on days 10-12 of the development of the bilmaniar embryo?

Answer 10

• trophoblast lacunae becomes continuous with maternal sinusoids forming uteroplacental circulation
• formation extra-embryonic mesoderm and cavities

Question 11

what occurs on day 13 of the development of bilmaniar embryo?

Answer 11

• formation of secondary yolk sac, connecting stalk and chronic cavity

Question 12

describe Gastrualtion

Answer 12

inward movement of epiblast cells through primitive streak

formation of 3 primary germ layers:
- Endoderm
- Mesoderm
- Ectoderm

Question 13

what does the ectoderm layer become?

Answer 13

• becomes epidermis of skin, hair and nails
• becomes nervous system

Question 14

what does the endoderm layer become?

Answer 14

• lining of gut tube
• lining of respiratory tract
• lining of the bladder and urethra

Question 15

what does the mesoderm layer become?

Answer 15

paraxial mesoderm - axial skeleton and skeletal muscle

intermediate mesoderm - urgenital organs

lateral plate mesoderm:
somatic layer - dermis, lining of body wall, parts of libs

visceral layer - CV system, mesothelial covering of organs, smooth muscle

Question 16

what is the primitive streak?

Answer 16

first evidence of gastrulation - formation of primitive streak

• located in the mid line of caudal end of embryo
• primitive node and pit are located at the cranial (superior) end of the primitive streak