lecture 1

muscle cells are unique - many precursor cells fuse together into 1

1. Mitosis - division of nuclear matter (not whole cell)

2. G1 (gap) - high level of biosynthesis activity (prep for DNA replication)

3. S - synthesis of DNA (replication)

4. G2 (prep for mitosis, cytokinesis) cytokinesis = division of whole cell

interphase = #2,3,4

phases of mitosis

muscle tissue development

zygote (sperm and egg fuse)

| cleavage (the process of cell division that occurs after fertilization of an egg)

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blastula (just cell replication) (until it reaches a critical amount of cells)

| gastrulation - differentiation starts here

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germ layers (stem cells)

morphogenesis (from ball of cells → development of features? like head or feet

organogenesis (development of organs)

organogensis is the phase of embryonic development that starts at the end of gastrulation and continues until birth

during organogensis, three germ layers form called ectoderm, endoderm, and mesoderm

1. ectoderm (outer layer) → epidermis, nervous system

2. endoderm (innermost) → lining of digestive tubes, associated with organs (pancreas, liver, ect.)

3. mesodorm (middle) → heart, kidneys, gonads, blood cells, connective tissues (bone, tendons, cartilege, skeletal muscle)

**^everything is developing at the same time

spinal cord = bone

spinal collumn = neural tisssue

5 regions of mesoderm

1. chordamesoderm → notochord (becomes spine) formation of body structure and neural tube

2. intermediate mesoderm → urinary system, genital ducts

3. lateral plate mesoderm → heart, vasculature, blood cells

4. head mesoderm → muscles of the face

5. dorsal (paraxial) mesoderm → both side of neural tube produce connctive tissue including skeletal muscle

**when you reach critial number of cells, replication slows down -→ let cells develop

mesoderm develop into blocks of “sticky cells” called somites (cells cannot just be a jar of marbles; need to work together)

• ?? basal lamina (collagen, laminin, fibronectin)

  • join together with somites to form tissues

**Cells situated ventromedially in a somite differentiate into the sclerotome, which gives rise to cartilage, while the other part of the somite differentiates into dermomyotome which gives rise to muscle and dermis.

dorsal - back of body, ventral - front of body

ventral cells of somite (scelerotone) migrate ??(laterally?) - becomes chondrocytes (axiel skeleton - bones in head, neck, back, and chest)

remaining cells become bilayered, solid tube called dermomyotome

1. dermatome - dorsal layer, becomes dermis (skin)

2. myotome - ventral layer, becomes striated muscles of back and limbs (limb buds)

basal lamina - exterior of cells; allow cells to join together

• like velcro

1 type of cells → tissue (no specific function)

(multiple) different cells → form organs (have a certain function)

Myogenesis - is the process of muscle tissue formation and development

• myotome (from before) → premyoblasts (precursor to myoblasts that form skeletal muscle cells)

• myoblasts charcteristics

  • bipolar spindle shaped cells

  • single large nucleus

  • many ribosomes → produce proteins (muscles need alot of proteins)

  • diffuse chromatin → can transcript and translate proteins (cannot with chromosomes)

• myoblasts fuse together

• process is calcium dependent and involves several steps

  1. migration (to look for other myoblasts)

  2. recognition (only other myoblasts) ****withdraws from cell cycle

  3. alignment (guided by cell membrane glycoprotein - sugar protein base)

     • find bone to line up properly

     • as bone lengthens, muscle lengthens with it

  4. fusion (reorganization of membrane components leading to tight junctions - make muscle cells stay together)

• fused myoblasts (single nucleus) form nucleated myotubes (multinucleated) (immature myofibers)

• different genes activated (muscle specific proteins i.e. creatine kinase, MHC, Mb, AChR)

• increase in transcription and translation

  • fusion and muscle specific protein synthesis are separate but simultaneous events

• nucleus migrate to perimeter of myotubes

Myotubes are long, cylindrical, multinucleated (syncytial) cells formed from the fusion of myoblasts. When their central nuclei are shifted to a subsarcolemmial position in the later stages of development, they are called myofibers.

• not all myoblasts fuse, some form “satellite cells” - stem cells that help skeletal muscles grow, repair, and maintain themselves

**muscle developing with bones - 2 nervous systems

Muscle Cell Arrangment

• early development of muscles related to skeletal and renal development

• first myotubes formed (foundation) determine arrangment of all fibers in a muscle (collagen)

• muscle development and arrangement of fetal animals dictated by skeletal growth and stretching

  • nervous system has no role in myotube alignment

• proper alignment of myofibrils (actin and myosin) depends on proper attachment of whole cell

• clusters of thick (myosin) and thin (actin) filaments first appear at the periphery and develop inward if sustained by stretching

  • longitudinal orientation of filaments follow membrane stretching

*muscle elongate with bone

**fibrils can still develop tension

********************** is this gonna be a short answer question

Primary, or 1 degree, myotubes (10%, form foundation)

• formed and aligned above

• show contractile activity before innervation

• fiber type determined before innervation

secondary, or 2 degree myotubes (90-95%)

• majority of final muscle mass

• fiber type determined by innervation

• innervation precedes contractile activity (don’t self contract)

process

• strings of myoblasts adhere to exterior of primary/1degree myotube

• contraction (fasciculations) of 1° myotube assists fusion of 2° myoblasts together

• 1° myotube provides architectural framework for proper alignment of 2° myofibrils

• as 2° myotube matures, continued contraction of underlying myotube creates shearing force between associated myotubes

• 2° myotube freed from 1° myotube

• newly developing 2° myotube pushes away older 2° myotube (may support other developing 2 ° myotubes)

**self contractile ability is important function of 1° and 2° myotubes

TERMINOLOGY

• premyoblasts - cells capable of mitosis but not producing muscle proteins

• myoblasts - cells no longer capable of mitosis but now starting to produce muscle proteins

• myotube - a multinuclear myofibre produced by the fusion of myoblasts

• secondary fibers - amultinuclear myofibre produced by the fusion of myoblasts on the surfaces of a myotube

• myofiber (myocyte) - a muscle fiber matured from either a myotube or a secondary fiber

**myotube is not the same as a myofiber

**as fetal development completed, 2° fiber production decreased and distinction between 1° and 2° fiber obscured (myofibrils become centrally located and nuclei become peripheraly located)

Myostatin acts as a negative regulator of muscle growth by inhibiting the proliferation and differentiation of muscle cells (myoblasts)

Belgian blue cows → myostatin deletion

• a lot more muscle but weaker specific force

• not popular because causes a lot of damage to mothers birth canal