7.1 Early embryology

Question 1

Gametogenesis: what happens in the 3rd week?

Answer 1

Formation of primordial germ cells (PGCs) in the epiblast and migration to the wall of the yolk sac

Question 2

Gametogenesis: what happens to the gametes in the 4thweek?

Answer 2

Migration from the yolk sac to the developing (indifferent) gonads

Question 3

Gametogenesis: what happens to the gametes in the 5th week?

Answer 3

Arrival at the genital ridges

Question 4

Gametogenesis: what happens to the gametes in the 6th week?

Answer 4

Entry into the genital ridges → gonads do not develop if PGCs fail to reach ridges

Question 5

Oogenesis (prenatal period)

1. Differentiation into __________ (2n)
2. Multiple ___________ (gives rise to large number → 7000000 by 5th month)
3. Differentiation into _____________ (2n)
4. Enter meiosis I (arrested in ____________)

Answer 5

oogonia;

mitotic divisions;

primary oocytes;

prophase I

Question 6

Oogenesis (post pubertal period)

1. Completes meiosis I → forms _____________________
2. Secondary oocyte enters meiosis II (arrested in _____________)
3. Ovulation
4. Fertilisation → completes meiosis II → forms zygote (2n) and second polar body (1n)
   * If no fertilisation → secondary oocyte degenerates without completing meiosis II

Answer 6

secondary oocyte (1n) and polar body (1n);

metaphase II

Question 7

Spermatogenesis: what happens in the prenatal period?

Answer 7

PGCs migrate into the developing male gonads and become spermatogonia

Question 8

Spermatogenesis (puberty):

1. Spermatogonia undergo mitosis to form _____________
2. Some type A spermatogonia commit to form type B spermatogonia (others remain at the basal lamina)
3. Become ______________
4. Enters meiosis I → forms 2 _________________
5. Immediately enters meiosis II → 4 early spermatids
6. Spermiogenesis → mature into late spermatids then ____________

Answer 8

type A spermatogonia:

primary spermatocytes;

secondary spermatocytes;

spermatozoa

Question 9

Spermiogenesis is when spermatids transform into spermatozoa (last stage):

1. Nucleus condenses into the ____________
2. _________________ forms a large vesicle (acrosomal vesicle) on one aspect of the developing nucleus (acrosomal head cap)
3. ______________ elongate and align themselves opposite the head cap to form the neck, middle piece, and tail (flagellum) → excess cytoplasm phagocytosed by surrounding ____________ as the flagellum elongates
4. Mitochondria arrange themselves into a ____________ around the middle part

Answer 9

head of the sperm ;

Golgi apparatus

Centrioles;

Sertoli cells;

tubular sheath

Question 10

Capacitation is where spermatozoa undergo essential changes allowing them to penetrate the ______________ and fertilise the oocyte in the uterus/Fallopian tubes:
• Involves changes in the acrosomal sperm head membrane
• Removal of the _______________ and seminal proteins
• Influx of calcium increases intracellular [cAMP] → increased _________________

Answer 10

corona radiata;

glycoprotein coat;

sperm motility

Question 11

Ovarian cycle: About 25 – 50 primordial follicles undergo maturation in each cycle, stimulated by FSH and LH from the pituitary gland during each menstrual cycle (after puberty)

Primary stage
- Squamous cells in primordial follicular epithelium convert to cuboidal epithelium (________________ cells) → mitosis → several layers of follicular cells
• Follicular cells secrete a thin layer of glycoprotein (____________ → barrier between the follicular cells and oocyte)

Secondary stage
- Primary follicles → secondary follicles (larger with more follicular cells):
• Spaces filled with follicular fluid appear in the granulosa cells → spaces merge to form ________________ which pushes oocyte aside
• _______________ (surrounding the follicle) become condensed → forms the theca interna (secretes _________________) and theca externa

Graafian stage: FSH/LH stimulates the single secondary follicle to increase in size on the 7th day of the cycle to form the Graafian follicle (~3 – 5mm in size):
• Primary oocyte completes meiosis I to form the secondary oocyte and first polar body → secondary oocyte starts meiosis II
• Granulosa cells protrude into the antrum to form ___________________
• Graafian follicle reaches the surface of the ovary → further increases in size → bursts and expels the ovum (ovulation)

Answer 11

granulosa;

zona pellucida;

follicular antrum;

Stromal cells;

oestrogen;

cumulus oophoricus;

Question 12

After ovulation, the ruptured follicle collapses and fills with blood clots:
• Follicular (thecal) cells rapidly enlarge, and their cytoplasm is filled with yellow pigment → forms lutein cells (corpus luteum) → secrete __________ and _____________ (important for remodelling of bony pelvis for delivery)

Theca lutein cells: Darker-staining cells lacking ______________

Granulosa lutein cells: Lighter-staining cells containing aromatase enzyme; produces ____________ from androgens produced by the theca lutein cells

Answer 12

oestrogen; relaxin;

microvilli and aromatase enzyme;

oestrogen and progesterone

Question 13

Fertilisation: hCG from ______________ binds to luteal cells → synthesise progesterone (increases the secretory activity of the endometrium)
• Function is largely taken over by the developing placenta (trophoblast) by 8 weeks gestation

Answer 13

trophoblast

Question 14

Phases of fertilization

1) Sperm penetrates the ______________:
• Prevention of polyspermy: plasma membrane of the oocyte depolarises once the sperm touches it to prevent binding of other sperm

2) Sperm penetrates the zona pellucida:
• __________________ in the head of the sperm hydrolyses/dissolves the zona pellucida locally
• Prevention of polyspermy: _______________ release enzymes to make the zona pellucida impermeable to other sperm

3) Fusion of oocyte and spermatozoa membranes:
• Contents of the sperm enter the cytoplasm of oocyte; _____________________ of the sperm are left behind and degenerate
• Secondary oocyte completes meiosis II to form immature oocyte and second polar body → male and female pronuclei fuse to form zygote
• Mitochondrial DNA of the zygote is of ________________

Answer 14

corona radiata;

Acrosomal enzymes (acrosin);

cortical granules;

plasma membrane and mitochondria;

maternal origin

Question 15

IMPLANTATION

After fertilisation of the oocyte, the zygote undergoes several important changes before implantation in the ______________ of the endometrium 6 – 7 days later.

The zygote undergoes a series of mitotic divisions to form blastomeres (increasing number):

• Blastomeres form a _____________ (before the 8-cell stage) then change shape and form tight junctions to form a ________________ (compaction; after 8-cell stage)
o Compaction allows for greater cell-to-cell interaction for development
o Blastomeres are _____________ (able to give rise to all cell types including extra-embryonic tissues) up to the 8-cell stage

• 8-cell compacted embryo then divides to form the __________________ → moves down the Fallopian tube and enters the uterine cavity 3 – 4 days afterwards

Answer 15

stratum functionalis ;

loosely arranged clump ;

compact ball;

totipotent ;

16-cell stage (morula)

Question 16

Blastocyst formation: what does the inner cell mass form?

Answer 16

Embryoblast → forms the embryo

Question 17

Blastocyst formation: what does the outer cell mass form?

Answer 17

Trophoblast → forms the embryonic portion of the placenta

Question 18

when does the blastocyst hatch from the zona pellucida?

Answer 18

Blastocyst hatches from the zona pellucida (broken down by uterine secretions and blastula secretions) about 5 days after fertilisation

Question 19

The blastocyst implants into the functional layer of the endometrium 6 – 7 days post-fertilisation, and the trophoblast differentiates into 2 layers:
• Normally implant into the anterior/posterior wall of the uterus.

Inner cell layer forms: _______________

Outer cell layer forms: ________________

Answer 19

Cytotrophoblast;

Syncytiotrophoblast (forms finger-like projections to implant in the endometrial wall)

Question 20

Abnormal implantation: known as an ectopic pregnancy if it occurs outside the uterus

• 95% of ectopic pregnancies occur in the Fallopian tubes ; may occur anywhere in the ovaries or abdominal cavity (e.g. pouch of Douglas)
• If implantation occurs at the internal os of the cervix , it causes ______________ (placenta forms over the internal os –> bleeding and miscarriage)

Answer 20

placenta praevia

Question 21

Complete molar pregnancy is pregnancy ______________ (only placenta present) → abnormal placenta forms many cysts (cluster of grapes appearance):
• Moles secrete _______________ and may produce benign/malignant tumours
• All 46 chromosomes are from paternal origin → maternal chromosomes regulate _________ development & paternal chromosomes regulate _______________ development → only trophoblast develops:

Mechanism

1) 1 oocyte inseminated by 2 sperms → ______________ → ________________
2) 1 oocyte inseminated by 1 sperm → _____________ → _______________ → _________________

Answer 21

without an embryo;

high levels of hCG;

embryoblast;

trophoblast;

female pronucleus is lost; 2 male pronuclei combine to form a diploid nucleus

female pronucleus is lost; single male pronucleus divides to form 2 haploid nuclei; fuse to form diploid nucleus

Question 22

The partial mole is triploid in origin (2 sets of paternal haploid genes and 1 set of maternal haploid genes):
• Forms following _____________________
• Due to the presence of maternal chromosomes, a foetus develops → grossly abnormal and cannot survive (triploid genome)

Answer 22

dispermic fertilisation of the haploid ovum (e.g. 69XXY)

Question 23

In the 7.5-day blastocyst, the embryoblast differentiates into the bilaminar germ disc:

• epiblast consists of ________________ cells
• hypoblasts consist of _______ cells
• amniotic cavity: develops between the ___________ and _________ → lined by amnioblasts (derived from epiblast cells)

Answer 23

high columnar;

cuboidal

epiblast and trophoblast

Question 24

The 9-day blastocyst is more deeply embedded in the endometrium:
• Vacuoles appear in the syncytiotrophoblast and fuse to form __________________
• Cells from the hypoblast migrate along the cytotrophoblast to line its inner surface (forming the __________________)
• Blastocyst cavity is transformed into the ______________

Answer 24

lacunae (lacunar stage);

exocoelomic membrane;

primitive yolk sac (exocoelomic cavity)

Question 25

[12 day blastocyst]

The further growth of the syncytiotrophoblast erodes endometrial capillaries to form sinusoids (irregularly-shaped blood vessels):
• Maternal blood flows into the lacunar network → early uteroplacental circulation • ___________________ forms and lies between the exocoelomic membrane and cytotrophoblast → extraembryonic coelom (cavity) dividing it into:
- Splanchnopleuric: Covers the ____________ (inner layer)
- Somatopleuric: Lines the _____________ (outer layer) and forms the connecting stalk (eventually becomes the umbilical cord)

By the end of the 2nd week, the lower half of the primary yolk sac pinches off and leaves behind the secondary/definitive yolk sac:
• Part which is pinched off forms the __________________ which completely degenerates
• Second wave of hypoblast cells (differentiated from the embryoblast) forms the ___________________

Answer 25

Extraembryonic mesoderm (EEM);

yolk sac ;

cytotrophoblast;

exocoelomic cyst;

inner lining of the definitive yolk sac (yolk sac endoderm)

Question 26

[Formation of chorionic villi]
At the 2nd month of development, the villi are numerous and well-formed at the _______________, while they are few in number and poorly developed at the ____________________:
• Intervillous spaces are filled with endometrial blood (from spiral arteries)

Primary stage: ______________ form finger-like projections to syncytiotrophoblast

Secondary stage: ______________ grows into the primary villi

Tertiary stage: Mesenchyme develops into blood vessels and cells → forms arteriocapillary network → fuse with placental vessels developing in the connecting stalk

Answer 26

embryonic pole;

abembryonic pole;

Primary cytotrophoblasts ;

Extraembryonic mesoderm;

Question 27

The decidua forms from the endometrium in a pregnant woman, and is divided into three regions based on their relation to the implantation site:
• Decidual reaction: vascular & cellular changes in endometrium as blastocyst implants

Decidua basalis: Deep to the conceptus; forms ________________

________________: Superficial part overlying the conceptus

___________________: Remaining parts of the decidua

Answer 27

maternal part of the placenta;

Decidua capsularis;

Decidua parietalis

Question 28

The branches of the chorionic villi provide a large surface area for oxygen and nutrient exchange across the membrane → no intermingling of foetal and maternal blood

Maternal side: Blood enters via ________________ → flow around intervillous spaces → oxygen/nutrient exchange
• Pushes deoxygenated blood into ________________ → uterine veins

Foetal side: _____________ carry deoxygenated or nutrient-depleted blood away from the foetus → blood vessels in chorionic villi
• Oxygen/nutrient exchange occurs in the _________ → umbilical vein carries oxygenated/nutrient-rich blood to the foetus

Answer 28

decidual spiral arteries;

endometrial veins;

Umbilical arteries;

terminal villi

Question 29

what is the appearance of the placenta from the foetal side?

Answer 29

Smooth and covered by amnion; umbilical cord attaches to the centre

Question 30

what is the appearance of the placenta from the maternal side?

Answer 30

Rough; divided into cotyledons (lobules; contains 2 – 4 tertiary/terminal villi)

Question 31

The primitive streak is formed by the migration of epiblast cells to the midline on the dorsal aspect of embryonic disc:
• Consists of the primitive groove and primitive node (containing primitive pit)
• Can identify cranial and caudal ends, dorsal and ventral surfaces of the embryo

Endoderm: Epiblast cells migrate through
_______________ in response to embryonic growth factors → displace _______________ (lost in process) → endoderm

Mesoderm: __________________

Ecoderm: ______________

Answer 31

primitive groove ; hypoblast cells;

Other epiblast cells lying between the epiblast and endoderm;

Cells remaining in the epiblast

Question 32

[Development of notochord]
After the development of the primitive streak, the bilaminar oropharyngeal membrane (future oral orifice) and cloacal membrane (future anal orifice) develop:
• Both membranes only contain _______________)
• Notochord is formed by the ______________ in the primitive pit → elongates cranially (between the ectoderm and endoderm) towards oropharyngeal membrane plate
o Induces overlying ectoderm to differentiate into ______________ →
neural plate (gives rise to brain and spinal cord)
• Primitive streak and notochord disappear except for the ___________________

Answer 32

ectoderm and endoderm (no mesoderm;

mesodermal cells;

neuroectoderm;

nucleus pulposus (intervertebral discs)

Question 33

What are the derivatives of the neural tube?

Answer 33

CNS (brain, spinal cord)

Question 34

What are the derivatives of the crest cells?

Answer 34

Schwann cells, sensory & autonomic ganglia, adrenal medulla, melanocytes

Question 35

what are the derivatives of surface ectoderm?

Answer 35

Epidermis, hair, nails, cutaneous and mammary glands, anterior pituitary gland, enamel of teeth, inner ear, lens

Question 36

What are the derivatives of paraxial mesoderm?

Answer 36

organised into somites (skeleton except skull, connective tissue, skeletal muscles, dermis of skin)

Question 37

What are the derivatives of intermediate mesoderm?

Answer 37

kidneys & gonads

Question 38

what are the derivatives of lateral plate mesoderm?

Answer 38

connective tissue of viscera and limbs, serous membranes of pleura, pericardium, peritoneum, blood and lymph cells, cardiovascular and lymphatic systems

Question 39

what are the derivatives of intraembryonic coelon?

Answer 39

body cavities (pleural, pericardial, peritoneal cavities)

Question 40

what are the derivatives of endoderm?

Answer 40

Epithelial lining of GI tract, epithelium of liver, pancreas, urinary tract, epithelium of respiratory tract