development of musculoskeletal system

when does gastrulation occur

week 3

gastroderm layers

endoderm

mesoderm

ectoderm

tissues and organs derived from

3 germ layers

ectodermal derivatives

CNS

PNS

epidermis (inc. skin, nails)

endodermal derivatives

epithelial lining of digestive + respiratory tracts

mesodermal derivatives

CT

cartilage + bone

dermis

striated + smooth muscles

what do -tomes come from

somite

sclerotome

bones + joints supplied by a single dorsal root + its ganglion

forms bone + cartilage

form vertebrae + ribs

cells of ventromedial part of somite

myotome

group of muscles supplied by a single ventral root

forms skeletal muscles

(wk 6) separate

• dorsal part: epaxial muscles

• ventral part: hypaxial muscles

epaxial muscles

intrinsic back muscles

Dorsal rami (epimere)

hypaxial muscles

anterolateral muscles of chest/abdominal wall

limb musculature

ventral rami (hypomere)

dermatome

patch of skin, sensory nerves come from single dorsal root + its ganglion

dermis of skin of the back + subcutaneous

last to form

-tomes

all have own segmental nerve

each somite forms own -tomes (all 31)

as many -tomes as there are spinal segments (NO C1)

cartilage formation

mesenchymal cells → chondroblasts → matrix

matrix can form hyaline, elastic, or fibrocartilage

precartilage

chondroblasts proliferate into round, compact cell rich tissue

bone formation types

intramemembranous

endochondral

inter membranous ossification

mesenchymal cells → osteoblasts → bone

skull bones (calvaria), facial bones, mandible, clavicle formed this way

osteoprogenitor cells diff. into osteoblasts

osteoid

osteoblasts proliferate into round shape + secrete organic matrix called osteoid

endochondral ossifcation

mesenchymal cells → chondroblasts → hyaline cartilage → bone

forms base of skull bones, axial skeleton, extremities (hyaline → bone)

steps of endochondral bone formation

mesenchyme cells condense + diff. into chondrocytes

chondrocytes form cartilaginous model of bone

primary center of ossification in long bone begins in shaft

blood vessels enter + bring osteoblasts

chondrocytes at ends mineralize matrix

osteoblasts bind to matrix + deposit bone

blood vessels invade epiphyses + secondary ossification center forms

when does secondary centers of ossification appear in epiphyses

shortly after birth

where does all growth in long bone happen

epiphyseal plate

when growth finishes, plates disappear + reunite w/ shaft

ventral trunk muscles (striated) derived from

myotomes

epimere

dorsal portion of body wall muscles

epaxial

form deep muscles of back

innervated: dorsal rami of spinal nerves

hypomere

ventral part of body wall muscles

hypaxial muscles

muscles of ventrolateral body wall

innervated: ventral rami of spinal nerves

mm’s split into 3 layers

3 layers of thoracic wall mm’s

external intercostals

internal intercostals

innermost innercostal muscles

(maintain segmental character)

3 layers of abdominal wall mm’s

external oblique

internal oblieque

transverse abdominis

(lose segmental character)

limb buds appear

end of 4th week

on ventrolateral surface of embryo

upper appear before lower

upper limb buds lie

oppo. C4-T2 segments

lower limb buds lie

oppo. L1-S3 segments

buds contain

core of mesenchyme covered by ectoderm

mesenchyme condenses + produces?

skeleton of limb

chondrification + ossification as limb grows

8th wk: hyaline cartilage models of bone

12th wk: primary ossification centers present in all long bones

development of muscles of limbs

from hypomere of somite

invade limb bud

CT from somatic mesoderm + form intrinsic limb bud mesenchyme

diff. muscular tiss. splits into ventral flexor + dorsal extensor group

innervation of mesenchyme of limb buds

ventral rami of adjacent spinal nerves

C4-T2 upper limb

L1-S3 lower limb

each muscle gets N supply from 2/+ SC segments

rotation of limbs

6 wk: up+low limbs project outwards

7/8 wk

• up limb rotate laterally (ext.) 90, elbows face post.

• low limb rotate medially (int.) 90, knees face ant.

when are limbs in fetal position

end of 8th wk