Embryology Flashcards
When does MSK embryology begin?
Near the end of the 4th week
Due to activation f mesenchymal cells in somatic lateral mesoderm
On day 24 Upper limb bud appears
Lower limb bud appears 1-2 days later
What is proimo-distal patterning?
Genes that are expressed in different regions which ultimately decides what skeletal element is formed
Allows for the different bones to be formed in their correct place
HOX expression results from a combination of SHH, FGF and WNT-7a expression
How do limb buds elongate?
By the proliferation of mesenchyme
Where to the limb buds appear?
Upper limb - opposite cuadal cervical segments
Lower limb - opposite lumber/sacral segments
What is the AER (apical ectodermal ridge)?
A thickening of the ectoderm at the apex of each limb bud
A multilayered epithelial structure that exerts influence on the mesenchyme
This mesenchyme is undifferentiated and rapidly proliferating cells
What do the cells proximal to the AER differentiate into?
Mesenchymal cells proximal to it differentiate into blood vessels, cartilage and bone models
How do hand and foot plates form?
Distal ends of the limb buds flatten into paddle like plates for hands, flipper like for feet
By end of 6th week he mesenchymal tissue condeses to form digital rays
7th week for foot plates
How do the limbs develop?
Cartilagionous precurosrs develop through chondrification within mesoderm in 6th week
Primary ossification centres appear in weeks 8-12 in response to growth factors
Endochondrial ossification occurs for all limb bones except for the clavicle
What ossification occurs for limb bones? (except clavicle)
Endochondrial ossification
What ossification occurs for the clavicle?
Membranous ossification
How do the upper limbs rotate?
Upper limbs rotate 90 degrees laterally
Brings extensors to the lateral and posterior surfaces
How do the lower limbs rotate?
Lower limb rotates 90 degrees medially, brings extensors to anterior surface
How are dermatomes formed?
During the 5th week, peripheral nerves grow from developing nerve plexi
Grow into mesochyme of limbs
What are the possible birth defects of limbs?
Amelia Meromelia Phocomelia Micromelia Polydactyly Syndactyly Congenital clubfoot Cingenital dysplasia of hip
What is Amelia?
Amelia is total absence of a limb
What is meromelia?
Partial absence of a limb
What is phocomelia?
Some long bones absent in limb
What is micromelia?
All parts of limb are present, by smaller than normal
What is polydactyly?
Where more than 5 digits are on hands or feet
Extra digit is incompletely formed and lacks muscular development
May be genetic or teratogenic
What is syndactyly?
Webbing of the hand or foot
Fiarly common
What are the types of syndactyly?
Cutaneous - simple webbing between digits, common and more frequent in feet
Osseous - fusion of bones
Due to notches between digital rays fail to develop
What is congenital clubfoot?
(AKA talipes equinovarus)
Sole of fot turned to arus side (medially) and inverted
Relatively common
What are the causes of clubfoot?
Multifactoral (genetic + environment)
Abnormal postioning or restricter movement of foetus’ lower limb in utero
What are the types of developmental dysplasia of the hip?
Abnormal development of acetabulum
Accounts for 15%
Common after breech births
Genralised joint laxity
What is the effect of development dysplasia of the hip?
Increased risk of dislocation
With which system does the reproductive system share embryological links?
Urinary system
What are the three types of kidneys that develop?
Pronephros (cervical region)
Mesonepros (thoracolumbar region)
Metanepros (pelvic region)
What happens to each type of kidney?
Pronephros degenerated
Mesonephros goes on to form reproductive system
Metanepros forms kidneys
When does the gonadal ridge form?
weeks 5-6
How does the genital ridge form?
The mseonephris forms a long ovoid structure, associated with thoracib/lumbar vertebrae
Medial part of this ridge thickens forming the genital ridge
How is the indifferent gonad formed?
When germ cells from the yolk sac migrate towards the genital ridge
Migrate via dorsal mesentery
Embed into Primartive sex cords
Why are they called indifferent gonads?
Because at this point it is impossible to tell which gonad they will become
What cells becomes the paramesonephric duct?
Epithelium invaginates to form a tube, which becomes the paramesonephric duct
Where does the paramesonephric ducts open into and develop?
Paramesonephricdevelops laterally to developing gonads
Cranial end opens into peritoneal cavity
Caudal end crosses centrally to mesonephric ducts into dorsal wall of urogenital sinus
What do the primordial reproductive structures form in males?
Gonad forms testis
Mesonephric duct into epipidymis, vas defernes + seminal vesicle
Paramesonephric duct degenerates
Urogenital sinus forms ballder, prostate and urethra
What do the primordial reproductive structures form in females?
Gonad forms ovary
Mesoneprhic duct degenerates
Paramesonephric duct forms fallopian tube, uterus, cervix part of vagina
Urogenital sinus forms bladder and lower part of vagina
What part of the X chromosome causes the changes in development?
SRY encoding form TDF (testis determining factor)
Expressed by cells within sex cords
What do the sertoli cells develop from?
Primordial supporting cells in promitive sex cords
What do the leydig cells develp from?
Mesenchymal cells between the primitive sex cords
How do the tesits develop?
Wk 5 - primary sex cords, medulla and cortex formed
Wk 6 - primordial germ cells incorporated
wk 7 - Seminiferous form + ends anastomose to make rete testis
Surface epithelium lost when capsule forms
Leydig cells form
Wk 8 - leydig cells produce tesosterone
How do the seminiferous tubules develop?
TDF stimulates sex cords to eleongate forming seminiferous tubules
Extend into medulla
What forms the seminal vesicles?
Outgrowth from caudal end of mesoneprhic duct
What forms the prostate?
Outgrowths from the urethra (urogenital sinus)
How does the ovary develop?
Primary sex cords degenerate leaving germ cells
at 10 wks secondary sex cords develop from epithelium incoroprating the germ cells into them
Germ cells bud off, surrounded by layer of cortical cord cells
Active mitosis of these primordial follicles to form additional follicles
7th month all have formed
Arrested in phrophase 1 until puberty
What brings rise to the female reproductive tract?
Paramesonephric ducts
The abscence of tesosterone causes mesonperos/duct to degenerate
How does the female genital tract develop?
Cranial end of paramesonephric duct becomes associated with ovary (catches)
Caudal end becomes uterus + superior part of vagina
Inferior part formed from urogenital sinus
Mesonephric duct degenerates
How do the gonads descend?
Both in week 7-10 descend to T10
In testis week 12 they are level of deep inguinal ring and remains until 7th month
Enters scrotum just prior to birth
How do the ovaries descend?
Both in week 7-10 descend to T10
Ovary at week 12 the muscle degenerates to form broad liagment, which is drawn through inguinal canal to attach to labrum majorum
What is cryptorchidism?
Absence of testes in scrotum: Undescended testes Ectopic Retraticle testes Absent testes
What are the complications of the undescended testis?
Infertility if left
Malignant transformation
Testicular torsion
What tissue forms the genital system (and urinary) in the developing embryo?
Intermediate plate mesoderm
What does the gubernaculum form in females?
Round ligament
Ovarian ligament
What are the common birth defects in relation to th reproductive system?
Cryotirchidism
Remants of mesonephric duct (gartners cysts)
Anomalies of female reproductive tract
What are some common anomalies of the female reproductive tract?
Complete duplication of uterus
Septate uterus (has septum)
Bicornate uterus
Arcutate uterus
What is mesenchyme?
> Embryonic connective tissue
Same properties as connective tissue and develops in to bone, cartilage blood vessels etc
Once mesoderm merges with neural crest cells considered mesenchyme
What is special about the areas that become the future mouth and anus?
There is no mesoderm present
How is the neural tube formed?
Some of the ectoderm in the middle fold down to form neural tube
>Goes on to form brain + spinal cord
> Forms axis of embryo
How do neural crest cells develop?
Some cells at the crest separate to form neural crest cells
>Migrate to form variety of structures
Mix with mesoderm to form >neural crest cells
How do somites develop?
They are derived from paraxial mesoderm
Form structures like axial skeleton and muscles
33 develop
What do the somites do?
In future head region:
>Help cranial skeleton development
>Bulbous somites at top form brain + two eyes
>Folds embryo
What are the pharyngeal arches?
(AKA branchial arches because gills in a fish)
>Series of arches that develop around future mouth and pharynx
>Support primative pharynx
How many pharyngeal arches are there? (total + at end?)
6 in total formed
5th Arch degenerates
What makes up a branchial arch?
Ectoderm makes outside >Forms clefts between atches Mesochyme forms cartilage and muscles Endoderm on inside >Forms pouches between arches
Clefts + pouches go on to form structures
What nerve arises from the 1st branchial arch?
CN V3
>Mandibular branch of the trigeminal nerve
What nerve arises from the 2nd branchial arch?
CN VII
>Facial nerve
What nerve arises from the 3rd branchial arch?
CN IX
>Glossopharyngeal
What nerve arises from the 4th branchial arch?
CN X
>Specifically superior laryngeal branch
>Vagus
What nerve arises from the 6th branchial arch?
CN X
>Specifically recurrent laryngeal branch
>Vagus
How can you tell what arch forms what muscle?
All muscles innervated by nerves from branchial arches have migrated upwards
>During migration take nerve with them
>Same origin as nerve
What arch forms the facial muscles?
2nd brachial arch
>Supplied by facial
What arch forms the anterior belly of the diagastric muscle?
1st branchial arch
>Supplied by mandibular nerve of trigeminal
What arch forms the posterior belly of the diagastric muscle?
2nd brachial arch
>Supplied by facial
What cartilage arises from the 1st pharyngeal arch?
Mandible
Hammer/anvil of ear (Malleuls, incus)
What cartilage, bone and ligament arise from the 2nd pharyngeal arch?
Upper part of hyoid bone
Styloid process
Stylohyoid ligament
What bone arises from the 3rd pharyngeal arch?
Lower part of hyoid bone
What cartilage arises from the 4th pharyngeal arch?
Upper thyroid cartilage
What cartilage arises from the 6th pharyngeal arch?
Lower thyroid cartilage
Cricoid cartilage
What glands arise from the branchial arches?
Thyroid gland
Parathyroid gland
Part of the pituitary gland
What processes does the face develop from?
1 frontonasal process
2 Maxillary processes
2 mandibular processes
Important for understanding how clefts form
What does the frontonasal process develop from?
Mesenchyme superior to future mouth
What do the maxillary processes develop from?
Mesoderm of 1st branchial arch
What do the mandibularprocesses develop from?
Mesoderm of 1st branchial arch
What are the steps in the face’s development?
The two mandibular processes fuse in midline
Ectoderm thickening of frontonasal placade
>Splits frontnasal process into 3
>Lateral 2 parts from nostrils
Medial nasal process grows further downwards and laterally forming philtrum
How does the palate develop?
Stage 1: Medial nasal process grows downwards and forms philtrum.
>forms the primary palate
Stage 2:From the maxillary processes, two palatine shelves grow inwards
Stage 3: The palatine shelves meet in the midline to form the secondary palate.
Now the oral cavity and nasal cavity are separate.
What are the three sections of the skull?
Cranial vault
Base of skull
Viscerocranium
How is the cranial vault formed?
Membranous bones -> formed by membranous ossification calcaria (ossify directly)
Mesenchym dervied mesoderm that lies above the surface of developing brain
Not fully formed at birth - soft sutures between to allow passage though birth canal
What condition occurs if cranial bones ossify at the wrong time?
craniossynostosis
How is the base of the skull formed?
Formed by endochondral ossification
Mesenchyme around notochord is derived predominantly from neural crest cells
Preform in cartilage first
Then later ossify
How is the viscerocranium formed?
Partly from branchial arches, partly from sensory capsules
Derived from somites in head and neck region
Preform in cartilage and ossify to form bones around sense organs
