Embryology 1

Question 1

Female reproductive organs

Answer 1

• vagina
• cervix
• uterus
• oviduct: isthmus, ampulla, infundibulum, fimbrae
• ovary

Question 2

Gametogenesis

Answer 2

Germline cells (spermatogonia/oogonia) in the gonads undergo meiosis to produce haploid gametes.

Question 3

Spermatogenesis

Answer 3

In males, production of mature spermatozoa occurs in the seminiferous tubules of the testes, beginning at puberty.

Question 4

Oogenesis

Answer 4

In females, primary oocytes are produced during fetal development. After puberty, some follicles and their oocytes resume development each month, and one is released during ovulation. Meiosis is only completed if an oocyte is fertilised by a spermatozoon.

Question 5

Ovulation

Answer 5

An ovum is released from the ovarian follicle and swept up into the oviduct by cilia and muscle contraction.

Question 6

Features of a secondary oocyte

Answer 6

• cumulus oophorus
• corona radiata
• zona pellucida
• membrane
• cytoplasm

Question 7

Fertilisation

Answer 7

Usually occurs in the wider ampulla of the oviduct. Only one sperm can penetrate the outer cell layers to fuse with the oocyte membrane, forming a zygote. This causes the hardening of the sugary, jelly-like zona pellucida, preventing more sperm from entering. The ovum completes meiosis and the male and femal pronuclei fuse before mitosis begins.

Question 8

Events of week 1

Answer 8

Day 1- fertilisation and first division
Day 2- second division
Day 3- formation of the morula
Day 4/5- formation of the blastocyst
Day 6- blastocyst hatches
Day 6/7- implantation

Question 9

Formation of the morula

Answer 9

By day 3, mitosis has formed a 6-12 cell morula. Compaction occurs during day 4, forming tight junctions between cells. Two sets of cells become distinct:

• embryoblast (inner cell mass); will form embryo
• trophoblast (outer cell mass); will form placenta

Question 10

Formation of the blastocyst

Answer 10

During day 4/5, fluid enters the ball of cells, forming a hollow blastocyst. The inner cavity is called the blastocoele.

Question 11

Implantation

Answer 11

On day 6, the blastocyst hatches from the zona pellucida. During day 6/7, it implants into the endometrium, which is thickened by progesterone.

Question 12

Events of week 2

Answer 12

Day 8- division of cell masses and formation of the amniotic cavity
Day 9- formation of the primary yolk sac and lacunae
Day 10/11- formation of the extraembryonic mesoderm
and beginning of uteroplacental circulation
Day 12- formation of the secondary yolk sac
Day 13- reduction of the primary yolk sac
Day 14- attachment of theembryo

Question 13

Division of the embryoblast

Answer 13

On day 8, the embryonic disc becomes bilaminar, dividing into two layers; the epiblast and the hypoblast. The amniotic cavity forms between the epiblast and the trophoblast, which then quickly becomes lined by a thin layer of epiblast cells, forming the amnion.

Question 14

Division of the trophoblast

Answer 14

On day 8, the trophoblast divides into the cytotrophoblast and the syncytiotrophoblast. On day 9, the syncytiotrophoblast completely surrounds the embryo, and lacunae form within it. On day 10/11, these anastomose with maternal sinusoids in the endometrium, starting uteroplacental circulation.

Question 15

Formation of the yolk sac

Answer 15

On day 9, hypoblast cells line the blastocyst cavity, forming the primary yolk sac. On day 12, a second wave forms the secondary yolk sac, and the primary yolk sac degenerates.

Question 16

Formation of the extraembryonic mesoderm

Answer 16

On day 10/11, the inner surface of the trophoblast and outer surface of the yolk sac and amniotic cavity become lined by a new tissue. This extraembryonic mesoderm splits into two layers, creating the chorionic cavity (extraembryonic coelom).

Question 17

Attachment

Answer 17

On day 14, the embryo proper is attached to the chorion by a connecting stalk of extraembryonic mesoderm, which is a precursor of the umbilical cord.

Question 18

Events of week 3

Answer 18

Day 15- primitive streak formation
Day 16- epiblast cells form endoderm, mesoderm and ectoderm
Day 17- notochord formation, differentiation of mesoderm
Day 18- further differentiation of mesoderm
Day 19/20- formation of the neural plate
Day 20/21- notochordal process fuses with endoderm to form notochordal plate

Question 19

Primitive streak formation

Answer 19

During week 3, a primitive streak appears along the midline of the embryonic disc, establishing a longitudinal axis of bilateral symmetry. It consists of a primitive node containing the primitive pit at the cranial end, which is continuous caudally with the primitive groove. Epiblast cells proliferate and migrate through the primitive streak.

Question 20

Gastrulation

Answer 20

Formation of the trilaminar embryonic disc, comprised of 3 distinct germ layers:

• ectoderm; forms epidermis and nervous tissue
• mesoderm; forms muscular, skeletal and circulatory systems and connective tissues
• endoderm; forms digestive and respiratory tracts

Question 21

Differentiation of mesoderm

Answer 21

On day 17, the intraembryonic mesoderm differentiates into paraxial, intermediate and lateral plate mesoderm, which joins up with the extraembryonic layer. The lateral plate mesoderm begins to split into two layers.

Question 22

Notochord formation

Answer 22

On day 17, some mesoderm cells migrate through the primitive pit along the midline to form a thick-walled tube called the notochord (later replaced by the vertebral column).

Question 23

Further differentiation of mesoderm

Answer 23

On day 18, the paraxial mesoderm forms the head mesoderm in the future head, and somites in the future trunk. The two layers of the lateral plate mesoderm, parietal and visceral mesoderm, are separated by the chorionic cavity.

Question 24

Formation of the neural plate

Answer 24

On day 19, the notochord secretes substances including noggin and chordin, stimulating differentiation of the overlying ectoderm into neurectoderm, forming the neural plate.

Question 25

Events of week 4

Answer 25

Ectoderm- neurulation
Mesoderm- somite formation and differentiation
Endoderm- gut tube formation

Question 26

Neurulation

Answer 26

During week 4, the flat neural plate begins to fold, resulting in a neural groove. This deepens and eventually detaches from the overlying ectoderm (which becomes the epidermis) forming the neural tube (which becomes the brain and spinal cord). Neural crest cells migrate to numerous locations and differentiate into various cell types.

Question 27

Mesoderm differentiation

Answer 27

Paraxial mesoderm forms paired segments called somites in the region of the future trunk. Each somite divides into scleratome, myotome and dermatome. Intermediate mesoderm forms urogential structures. Lateral plate mesoderm pulls apart to form the visceral lining of organs and parietal lining of the body wall, within which the chorionic cavity (intraembryonic coelem) is enclosed.

Question 28

Gut tube formation

Answer 28

Longitudinal folding of the endoderm forms a primitive gut tube, consisting of the foregut and hindgut, separated by the midgut, which is attached to the yolk sac. As the edges join along the midline, the neck of the yolk sac is reduced to a slender vitelline duct.