Neuromuscular embryology Flashcards
Describe the development and differentiation of somites
LO
Somites develop and differentiate via a process called somitogenesis. the somites are derived from the paraxial mesoderm (parallel to the mesoderm) and are seen lateral to the neural tube and notochord forming in pairs. Transient structures are formed from the somites in the cranial to caudal direction. these structures include the:
- skeletal muscles,
- cartilage,
- tendons,
- dermis of the back,
- meninges of the spinal cord and
- intervertebral blood vessels.
Somitogenesis is a regular and coordinated process that occurs in synchrony to neurulation. somites appear at the same time on both sides of the neural tube and the number of somites and timings of their development are species specific. these somites look identical, but will go on to form different structures in the body as the embryo continues to grow and develop. This is because somites contain multipotent cells, giving them the potential to become different tissue types. the location of these cells in relation to other cells determines their ultimate function.
Describe the formation of vertebrae
LO
Bones in the axial skeleton are formed by **somites.** the portion of the somite that gives rise to cells capable of forming bones is called a sclerotome. Sclerotome cells are attached to the notochord and neural tube region and form the vertebral bodies. each vertebral body develops from the caudal part of one sclerotome and cranial portion of the next
As the nervous system develops from the neural tube, the nerves split the sclerotomes, forcing resegmentation to occur.
“Resegmentation” is a process that occurs during the development of the nervous system from the neural tube. As the nerves begin to split the sclerotomes, it causes the neighbouring portions to undergo resegmentation. This process is responsible for the formation of the segmental division of the spinal cord and spinal nerves.
During neuromuscular embryology, the sclerotome cells covering the neural tube form neural arches that fuse with vertebral bodies, while cells passing ventrolaterally develop into costal processes and ribs. The notochord regresses in the region of the vertebral bodies but persists in the intervertebral discs to form the nucleus pulposus. Vertebrae form by chondrification of the sclerotomal mesenchyme, and cartilaginous vertebrae undergo endochondral ossification during the early fetal period.
Describe how limb buds form
LO
Limb buds are the first signs of the future limbs of the embryonic skeleton. They originate in specific positions along the body from the lateral plate mesoderm, which later develops into the limb skeleton. In the beginning, the process is initiated by the rapid increase of skeletal precursors from the lateral plate mesoderm, as well as the muscle precursors from the axial skeleton somites. The precursor cells then accumulate under the ectoderm, resulting in the formation of a bulge referred to as an apical ectodermal ridge. These bulges are the limb buds which eventually develop into the upper and lower limbs, with the formation of the upper limb slightly preceding that of the lower limb.
The thickening of the ectoderm at the tip of the limb bud is induced by mesenchymal cells and becomes a major signalling center that is essential for sustained outgrowth and limb development. This thickening is due to the accumulation of specialized cells that are responsible for the complex process of limb development. As the bud grows, the cells begin to differentiate and form cartilaginous structures proximally to distally. This formation of cartilage is crucial for the proper development of the limb, and it is a delicate process that requires the coordination of many different cells.
A small block of mesoderm referred to as the zone of polarizing activity organizes how digits are formed. This region is responsible for the proper formation of digits, and it is a key component of the limb development process. The signalling molecule, called Sonic Hedgehog, causes a gradient to form which determines digit identity. This gradient is essential for the proper formation of digits, and it is a complex process that involves the interaction of many different cells.
Mutations in the polarizing region cause conditions such as polydactyly, due to the mutation increasing the levels of Sonic Hedgehog.
An important step in the differentiation of the digits is called apoptosis. Apoptosis is referred to as “programmed cell death” and is the process by which joints form and fingers become separate. This process is essential for the proper formation of the digits, and it requires the coordination of many different cells.
Explain the segmental division of the spinal cord and spinal nerves and explain the origins of the neural tube and how it differentiates
LOs
During embryonic development, the neural tube is formed from the ectodermal tissue and gives rise to the central nervous system, including the brain and spinal cord. The neural tube is a complex structure that undergoes various processes to differentiate into different types of neurons. One such process involves the expansion of the neural tube, which occurs mainly laterally, and leads to the formation of thick lateral walls and thin roof and floor plates.
As the neural tube expands, an indentation of the neural canal appears, which delineates dorsal and ventral columns within the intermediate zone. These columns are called the alar and basal plates, respectively, and give rise to dorsal and ventral horns of the spinal cord. The differentiation of these plates is crucial for the proper functioning of the nervous system, as the dorsal horn contains sensory neurons, while the ventral horn contains motor neurons.
The notochord, floor plate, and roof plate are the sources of signals that induce ventral or dorsal cell types. The effect of these signals is dependent on a concentration gradient, which ensures the proper differentiation of the neurons. The notochord is a transient structure that induces the formation of neural tissue, while the floor plate and roof plate are responsible for the induction of ventral and dorsal cell types, respectively.
what are Somites
Somites are precursor populations of cells that give rise to important structures associated with the vertebrate body plan and will eventually differentiate into dermis, skeletal muscle, cartilage, tendons, and vertebrae
what is differentiation
the process by which cells differentiate into different functional groups
define paraxial
situiated alongside or on either side of an axis
what is mesenchyme
embryonic connective tissue of undifferentiated cells that give rise to most tissues, such as skin, blood or bone
what is mesoderm
the middle layer of the 3 germ layers that develops during gastrulation
what is an apical ectodermal ridge
a structure that forms from the ectodermal cells at the distal end of each limb bud and acts as a major signaling center to ensure proper development of a limb.
what is the zone of polarizing activity
a region at the posterior margin of the limb bud that induces mirror-image duplications when grafted to the anterior of a second limb.
what is apoptosis
programmed cellular death
which mesodermal compartments will ultimately develop into the limb bones
lateral plate mesoderm
which mesodermal compartments will ultimately develop into the vertebrae
paraxial mesoderm (somites)
what mesodermal compartments will ultimately develop the urogenital system
intermediate mesoderm
