hello everyone and welcome back to my embryology playlist hum system based embryology and today we are going to start embryology your development of Limbs which is chapter number 12. okay so it's pretty straightforward and easy to understand chapter limb growth and development now the limbs including the shoulder and pelvic girdles comprise the appendicular skeleton skeleton one is the appendicular skeleton just my limbs or NK Associated girdles and all that curdle structure pelvis spinal cord around vertebral columns skeleton so the skeleton is in this context divided into axial and a penticular type so limbs basically they belong to the appendicular skeleton at the end of the fourth week of development limb birds become visible as outpark rings that form rages on the literal body wall just embryology the developing embryo suddenly develops some you know out pocket change so bird is usually the first thing to develop and that bird is then developed into you see this structure and it is developing into the full-blown limb so Upper Limb bird and the lower limbird and then these birds are growing into the full-blown limbs okay so lateral body wall per this is the lateral part of the body little body wall the birds appear at around fourth week of development the four limb Birds they appear first followed by the hind limb Birds which are one to two days late okay or embryology now by the end of fifth week uh limbs have attained the bird stage they are properly visible initially the limb Birds consist of a mesenchymal core is derived from where it is dry from the parietal which is the somatic layer of lateral plate mesoderm so many embryology General embryology playlist okay and that will ultimately form all the bones and the connective tissue of the limbs covered by the layer of cubital act so yeah basically um at the distal border of the limb thickens and forms apical ectodermal Ridge now that's an important terminology for you to remember because this Ridge exerts an inductive influence inductive influence foreign foreign grows the cells further from the influence of the ER begin to differentiate into cartilage and muscle yes they will start developing into cartilage and muscle in this manner the development of each limb proceeds proximo distally into its three components one is called stylopod the other one is called zugopod and the last one is called autopods basically adult structures upper first portion that will be known as stylopods is portion which will form radiation in Upper Limb and tibia now in six week old embryo the terminal portion of upper limbers become flattened to form hand and foot plates and is separated from the proximal segment by a circular constriction later a second constriction divides the proximal portion into two segments ultimately cell death compresses so what basically happens this is the embryo body or yeah this starts forming the hand plates so it is start forming the foot plate hand plate or foot plate ultimate structures appearing something like this so you see this is for example a hand plate you know there is Stars apoptosis the parts of the you know cells they start getting dead it is all because of the process of foreign of the upper and lower limb is similar except that morphogenesis of lower limb is approximately one to two days behind that of the Upper Limb in a lower limb is the difference otherwise here lower limit there is a femur here upper limit there is radius also during the seventh week of gestation the limbs rotate now that's an important thing very very important limbs may be rotation there is a process and phenomena 90 degrees literally so that the extension surface lie on the lateral and the posterior side and the lower limb rotates 90 degrees medially so that's what you have to remember upper limit rotation 90 is degree lateral hair this is Upper Limb and lower limb is 90 degrees medial is so for example if this is the Upper Limb Upper Limb flexor compartment so this is the flexor compartment and this is the extensor compartment posterior difference is the extensor compartment or backside portion that is the flexor compartment so it's kind of uh you know all the way turned and this is because of rotation that happens during seventh week of gestation okay so that's an important point which you have to remember okay Upper Limb key lateral rotation here lower limb immediate rotation now while in the external shape is being established mesenchyme within the birds begin to condense and these cells differentiate into chondrocytes societies starts developing well you know into Highland cartilage or put a high line cartilage this is all actually not telling you much actually embryology so for example you see here this is a development stages of how the bones are formed in the upper and lower limbs so this is how the limb bird would be there or limbert case Center means it develops into cartilage for forming cells which are called the chondrocytes they can these are still the mesenchyme but the central mesenchymal cells have now been converted into chondrocytes or your chondrocytes cartilage with the passage of time and for the development what happens okay in cartilaginous cells blood vessels in weight or use your blood vessels or cartilage in a cells they are now limited only at certain ends of the bones and we are not too much but nature is so beautiful it's still happening now ossification of the bones of the extremities which is the endochondral ossification Begins by the end of the embryonic period primary ossification centers are present in all long bones of the limbs by 12th week of development so these are the centers of ossification Jahan cartilage and start converting into bone or intrambryonically ossification centers development they are all called the primary centers of ossification and they are all appreciated by 12th week from the primary centers in the shaft of the long bones endocondral ossification gradually progress towards the end of the cartilaginous model so again a diagram so Center May here starts becoming the bone because here is the primary Center of ossification cartilage forming cells now they are converted into bone forming cells orzo cartilage forming cells here they are then you know remaining towards the end is now at Birth what happens diaphysis of the bone is usually completely ossified center part of the bone it's usually completely a bony structure but the two ends which are called the epiphysis are still cartilaginous again look at this diagram so that's an ideal diagram to understand this concept okay birth is a central part of the bone here upper and lower parts and there is still cartilaginous okay so that's what is written here shortly thereafter however ossification centers arise in the epiphysis as well foreign foreign start forming the structures which are required for making the joint between them what is knee joint knee joint is the joint between uh femur and Nisha convention there is what is known as the knee joint is capsules so all this is actually happening like a magic origami exactly again now Sanibel joins between the long bones begin to form at the same time that the mesen camel condensation initiate the process of forming cartilage uh covering all this is happening at the same time so that when the limb is developed it not only contains the bones it also contains the joint structures okay so beautiful the body is designed again we don't know exact details then the next thing is how the muscles are developed limb musculature is derived from venterolateral cells of the somides so so mighty formation General embryology so somites actually you know give rise to the muscles of the limb that's the bottom line okay for example um they are in the embryo they lie opposite to the lower five cervical and upper two thoracic segments your name um let me go back to this diagram for example where is this one so this is the Upper Limb bud so Upper Limb but you have a cervical segment or upper two thoracic segments with the passage of time obviously foreign of the Upper Limb so cervical nerves or upper thoracic nerves keep on supplying the Upper Limb it lies opposite the lower four Lumbar and upper two sacral uh you know segments Supply any segments all rest is too much detail which actually I don't recommend for you to study but this is bottom line this is a must know paragraph for your examinations okay right next is the molecular regulation now listen to this carefully guys molecular regulations is not something is still a mystery okay how exactly this happens into cartilages so this is all a lot of question marks linkins what science has beautifully done okay different stages for example so with all the molecular biology experiments we have some information which is very basic information which is very preliminary information you have listed so for example positioning of the limbs along the cranio chordal axis is because of a set of genes which are the Hawks genes okay or the home box genes home your box genes uh particularly Hawks B8 and then specification of the four lamp is regulated by transcription Factor tbx5 yeah important transcription factors specification that is by tbx4 so that is what is known to us okay um there are some other genes which we know growth factor plays an important role this is something known then there are some Genesis these are msx2 Genesis such as msx2 okay and there is a radical Fringe set of genes which are also important now this Gene induces the expression of ser2 homologue of drosophilia solid it's too much of detail for you to remember now formation of the Border itself it is assisted by the expression of engrailed one gene in the ventral Eternal cells because this Gene represses the expression of radical Fringe this is again too much of information I don't recommend reading this just try to remember the names of the factors which are important okay for example the fibroblast grows factor four the fibrobras growth factor age so they are important and they maintain the undifferentiated Zone this undifferentiated zone of the AER is important for elongation of the limbuds now what else retina quality normal concentrations there is another a marker Gene for this particular region when the growth and differentiation is happening means one okay differentiation of zugupord and Autoport regions involve other genes and they are Sonic Hedgehog Genesis region and then there is zugopod and autopod regions which develop into different parts of the limb uh gene expression profile reported here okay so a diagram so for example this is the limbod okay that is the zone of differentiation it remains undifferentiated zones and then there are carpels and metacarpals and digits all that okay genetic expression profile so the one which is making the single uh you know bone is the means one Hawks A1 Hawks a13 or a11 by the way okay so Hawks a 13 so different genetic expression of different areas but the point is that's very little that we know ladies and gentlemen we don't Know full details of how embryology happens okay so isn't that beautiful Pathways such as Sonic Hedgehog now the dorsal ventral axis is also regulated by um bmps or wind Pathways so this is too much for you to remember I don't actually recommend ratifying all that sort of stuff okay because this keeps on changing every couple of years and new genes coming up what are the important players or important players you must remember homeobox genes uh tbx5 for Upper Limb tbx4 for lower limb all these genes which we have listed here fibroblasts growth factor particularly for example and these three genetic expression profiles of the three regions which are known as different areas which develops into Upper Limb bones such as the first part Stylo pod which is the first part which either develops into humerus or femur and then we have zugo pod which either develops into radius or Allah in case of Upper Limb and chibi and fibula in case of lower limb and then we have um you know the Autoport region which develops into couples metacarpals and tarsals metatarsals okay so genetic profile hey this is all what you should remember okay so rest is I mean you can search on Google genes associated with live development and there will be so many genes okay but I don't recommend reading them all basic understanding and focus on the points which are important and then we move on to a very important stuff which is the clinical correlate limited normalities are very very common actually or limb abnormalities many clinicians actually use the you know body scans x-rays to determine the bone age or the appropriate age of the children okay so primary centers of ossification and secondary centers of ossification by ultrasonography for example they give a very good idea about fetal growth and gestational ages okay now coming to the limb defects they have I said they are very common so this is pretty common okay and they can be of different categories so for example abnormalities of the limbs vary greatly they may be represented by partial absence or there can be a complete absence of flame so that is called email yeah okay so yeah you don't have so much then sometimes the long bones are absent and rudimentary hand and feet are attached to the trunk by small irregular bones this is known as focamelia so all these terminologies are important examples clearly they're very easy to understand is partial absence of foreign and there is a short bone attachment with the trunk so that's probably for Familia okay now next terminology sometimes the digits are shortened short digits brachy ductilely and sometimes there are few digits for example this is called syndic tile sometimes there are more than five digits that is called Polydactyly up name uh you know there are very famous stars for example uh Bollywood star Heritage so there is an extra digit as well political so there is synthetically as well so you should know these terminologies then abnormalities involving polydactyl here is a multiple fingers are usually bilateral whereas absence of a digit which is called ectroductile is usually unilateral so there can be short digits bracketectary there can be fused digit synthetically there can be multiple digits Polydactyly there can be absence of a digit Char fingers so that is called extroductively there can be cleft in either hand or foot and that can be associated with a lot of different genetic mutations such as Hawks a 13 mutation Hawks D13 mutation tbx5 mutation tbx5 was the gene for Upper Limb okay so most of these are associated with Upper Limb disorders and as part of other genetic problems as well for example the halter Ram syndrome which is characterized by the Upper Limb upper limit tbx5 involved virtually all types of limb defects affecting the upper link have been observed including apps and digits we are talking about this syndrome Haltom syndrome tbx5 mutation is equal to Halt Aram syndrome okay now osteogenesis imperfecta is a condition which is characterized by shortening Boeing hypomineralization of the long bones of the limbs and blues sclera so whenever there are limb abnormalities with blue sclera osteogenesis imperfecta which is usually because of mutations in collagen genes so Collision uh call one a one Gene and call one A2 Gene type 1 Collision basically a uh translate so these genes are defected then there is another very commonly used terminology called Marfan syndrome which is due to the mutation of fibrillin Gene which is present on chromosome 15 and characteristics include long thin limb and long thin face other characteristics include external defects so external problem joint hyper flexibility validation and dissection of aortama problem so because then there is a condition called arthrogryptosis and this is congenital joint contractures okay so what happens is more than one joint and may be caused by neurological defects and you see the joints have contractual formations so they are not normally moving joints they have contracture formations right then club foot is is this is the representation of a club foot and we also had I had to show you the amniotic bands sometimes what happens okay this is what happens during embryogenous is part of the limb digit is okay so that's also not uncommon amniotic bands congenital absence or deficiency of the radius this is also commonly reported then syndrome so what basically happens is okay there is a congenital absence of radius bone usually a genetic abnormality observed with malformation in other structures such as cranial scentosis radial aplasia syndrome so yeah sutures or radius absence so they are talking about this Baylor syndrome okay that's the same thing then transverse limb deficiencies see the upper part of the limb is already there but suddenly it is missing from the lower part this is what is known as transverse limb deficits okay then congenital hip dislocation consists of under development of the acetabulum hip joint establum development therefore there is congenital dislocation because many babies with Continental hip dislocation are breach deliveries they are not normally positions uh cephalic and neota they are breach Ulta baby it has been thought that bridge posture May interfere with the development of the hip joint so that's just a hypothesis okay but the point is the babies are born with congenitally dislocated hips okay so that's basically all about limb development which is too much okay so that's the beauty take care of yourself