Test 1: Embryology of the lower extremity Flashcards
Myogenic cells
These cells originate from the dermomyotome region of the somites and migrate into the mesenchymal cells of the limb buds where they eventually differentiate into myoblasts. The myoblasts will give rise to the muscle cells of the limbs. The cervical myotomes will give rise to the pectoral girdle muscles and the lumbosacral myotomes provide precursor cells for the pelvic girdle
This easy to understand pre and postaxial arrangement, however, changes during the early part of the _____ when the limbs undergo a rotation along their long axis.
7th week
By the end of the 6th week the hand paddles show presence of
digital rays which will eventually give rise to individual fingers. Eventually five rays develop, one for each finger.
Both limbs go through
a 90 degree rotation but in the opposite direction.
The preaxial border is along the same line as
the thumb or the great toe
During the later part of the 4th week (as soon as the trilaminar embryo completes its lateral folding) limb buds start to develop along
the ventrolateral side of the cylindrical embryonic body. These limb buds, one pair for the upper limbs and one pair for the lower limbs will undergo further development and eventually give rise to the adult limbs.
The limb buds originally contains a core of mesenchymal cells that are derived from the
the somatic component of the lateral mesoderm layer of the embryo. Further development and enlargement of the limb buds initially begins by a very fast division of the mesenchymal cells. Experimental evidence has shown that this cell division is under direct control of the HOX (homeobox- containing) gene.
Dermatomal cells
These cells derived from the somites provide cells that stay deep to the ectoderm and give rise to the dermis of the skin.
Axonal processes
Axonal processes of neurons of spinal cord become established as nerves of the limbs
During early part of the 7th week
toe rays can be seen in the foot plates. In a manner similar to that seen in the hand they will give rise to individual toes.
Under the influence of the _______patterning of the limbs also takes place where the distal ends of the buds flatten into paddle-like hands and feet.
HOX gene
The upper limb buds (for the upper limbs) appear first as two elevations one on each side of the body of the embryo. These limb buds can be seen by day
day 26-27 at the inferior segment of the developing cervical region (C5-T1).
The long bones also develop
secondary ossification centers in the epiphysis. So each long bone will have one primary and two secondary ossification centers.
the postaxial border corresponds to the location of
the 5th toe or the little finger.
The entire skeleton of the limbs at one time consists of cartilage models of bones. The cells of the cartilage models are derived from
the original mesenchymal cells of the embryo beginning by the 5th week of development.
There is much experimental evidence which indicates that the limbs will not develop in the absence of
AER. There is also evidence showing that transplantation of this ridge at a different location will induce the development of a limb at that location.
The inductive influence of AER is known to be as a result of
the expression of fibroblast growth factor.
Initially the digits are attached to each other by loose connective tissue. Separation of individual digits takes place during
during the 8th week by the process of programmed cell death (apoptosis) in the mesenchymal tissue between digital rays. This process gives rise to individual digits. If apoptosis is interrupted or does not happen, for some reason, it will result in one or more digits that remain attached (syndactyly).
Apical Ectodermal Ridge (AER) The proliferating mesenchyme at the site of limb development is covered externally by
cells of the ectoderm layer similar to other parts of the body. However, at the distal segment of the limb bud (apex) the ectodermal cells undergo cell proliferation and shows considerable thickening known as the apical ectodermal ridge (AER).
Neural crest cells
The supporting cells of the nervous system (Schwann cells) are derived from the neural crest cells There is evidence which indicates that the mesenchymal cells of the limbs organize the migrant cells and control their expression.
The limb buds extend from the trunk so that
that the longitudinal axis (or the axial line) of the limb bud is at about 90 degrees to the long axis of the trunk.
the postaxial compartments contain
the extensor muscles
Limb rotation Both the upper and lower limb buds at one time protrude
protrude anteriorly from the embryonic body
. During the _______ the cartilage models start to undergo endochondral ossification whereby the chondrocytes are replaced by bone producing cells. The endochondral bone formation begins in the diaphysis of long bones (e.g. phalanx, metatarsals) as the primary center of ossifications.
7th week
