Embryology Week 3 Flashcards
Development of the lungs
- Respiratory diverticulum develops into 2 primary bronchial buds
- Lobar bronchi develop from primary buds (2 on left, 3 on right)
- Lobar bronchi divide continuously until terminal bronchioles are formed (week 28)
- Respiratory bronchioles branch off terminal bronchioles
- Terminal sacs at ends of respiratory bronchioles are primitive alveoli
What are the 2 horseshoe shapes of island cells to become blood vessels and where are they found?
- Endocardial tubes (outside)
- Dorsal aortae (inside)
- Both found in visceral lateral plate mesoderm
What happens to the endocardial tubes on day 22?
- They have moved to the midline with embryonic folding, ventral to the gut tube
- They fuse
Structures of the heart at day 23
- Sinus venosus
- Atrium
- Ventricle
- Bulbus cordis
- Aortic roots
- Aortic arches
How does the heart twist in week 4?
- Bulbus cordis moves ventrally and right
- Atrium tucks up behind ventricle and left
- Called cardiac loop
How many pairs of aortic arches are there when the heart twists and which will remain as the aorta eventually?
- Pair 1, 2, 3, 4, 6
- Left 4th will become aorta
3 sections of divided bulbus cordis
- Truncus arteriosus (aorta + pulmonary trunk precursor)
- Conus cordis (outflow tract of both ventricles precursor)
- Primitive right ventricle
What does the ventricle of the original heart tube become after twisting?
Primitive left ventricle
What does the atrium of the original heart tube become after twisting?
Primitive right and left atria
What brings blood from the veins into the primitive right atrium?
Sinoatrial orifice
What grows upwards to begin dividing the 2 ventricles?
Muscular interventricular septum
What carries blood from the atria to the ventricles?
Atrioventricular canal
What growths occur in the atrioventricular canal and what do they do?
- Endocardial cushions
- Separate it into left and right canals
How do the atria begin to divide?
- Septum primum grows down from left atrium ceiling (ostium primum under it allows blood flow)
- Holes begin to appear at top and connect to form ostium secundum when septum primum reaches the endocardial cushions
- Septum secundum grows down from right atrium ceiling
- Gap under it is foramen ovale to allow blood flow
What growths separate the truncus arteriosus and conus cordis in 2?
Conotruncal ridges (become conotruncal septum)
How are the ventricles finally separated?
Conotruncal septum fuses with muscular interventricular septum
What 2 channels are formed by the conotruncal septum and where do they receive blood from?
- Pulmonary channel (right ventricle)
- Aortic channel (left ventricle)
What veins does sinus venosus originally receive blood from?
- Common cardinal vein
- Umbilical vein
- Vitelline vein
Development of the circulatory system
- Vitelline veins forms network called hepatic sinusoids
- Proximal parts of umbilical veins disappear and they drain into hepatic sinusoids
- Distal right umbilical vein disappears
- Ductus venosus carries blood from left umbilical past liver to right hepatocardiac channel
- Distal left vitelline vein disappears
- Right vitelline now called superior mesenteric vein
- Portal vein drains blood from liver to IVC
Development of nephric tubules and glomeruli
- Blind-ended tubules develop in the intermediate mesoderm
- Branches of dorsal aorta form capillary knots at the nephric tubules
Parts of the intermediate mesoderm (cranial to caudal)
- Pronephros
- Mesonephros
- Metanephros
What connects the mesonephros to the cloaca?
Mesonephric duct
What connects the metanephros to the mesonephric duct?
Ureteric bud
Development of a kidney
- Ureteric bud connects to metanephros at renal pelvis
- Ureteric bud branches into metanephros forming major calyxes
- Minor calyxes branch off major calyxes
- Collecting tubules branch off minor calyxes
- Metanephric tissue condenses to form nephrons with a Bowman’s capsule at the end opposite the collecting tubule
- Tissue layers fuse between nephron and collecting tubule
Movement of ureteric bud
- Eventually opens directly into urogenital sinus
- Switches place with mesonephric duct to be above it
What is the gonadal ridge and what happens to it?
- Block of tissue formed on mesonephros
- Epithelium of it grows inwards, making sex cords
- Primordial germ cells migrate into the sex cords
How is sex determined?
- Sex-determining region of Y gene codes for the SRY protein
- This wakes up SOX9 gene on chromosome 17
> Turned on in both males and females, but silenced in females - WNT4 gene silenced in male gonads but unleashed in female gonads
Female gonadal development
- Sex cords replaced by cortical cords
- Cortical cords form primordial follicles around germ cells
- Germ cells start meiosis but don’t complete process until they begin to form ova during ovulation
Male gonadal development
- SRY expressed in testis cords
- Cords remain solid until puberty, where they become hollow and form seminiferous tubules
- Sertoli cells produce growth factor causing Leydig cells to differentiate
What are paramesonephric ducts?
- Run next to mesonephric ducts
- Open at top end into intraembryonic cavity
Fate of (para)mesonephric ducts in females
- Mesonephric ducts regress
- Paramesonephric ducts fuse at caudal end to form uterovaginal canal (forms upper vagina and uterus - eventually fuses with outgrowths of urogenital sinus which forms lower vagina)
- Open ends of paramesonephric ducts become oviducts
Fate of (para)mesonephric ducts in males
- Paramesonephric ducts regress
- Mesonephric duct forms ductus deferens
- Seminal vesicles bud of ductus deferens
- Prostate and bulbourethral glands bud of urethra
