Musculoskeletal - stage 13

Introduction

This page looks at musculoskeletal events that are occuring at early embryonic Carnegie stage 13. The features to observe are mainly the rostrocaudal development of the somites and the early limb buds.

Somites near the head and through the trunk have begun dispersing into the separate dermal, myotome and sclerotomal components. Towards the tail, newly forming somites with central somitoceol (cavity) are visible. Mesenchyme in the sclerotomal regions will begin to differentiate initially as cartilage, to be replaced later by endochondrial ossification.

The upper limb bud is the most obvious lateral expansion on the trunk, with only small lower limbbud bulges towards the tail. Within the upper limb bud undifferentiated mesenchyme (embryonic connective tissue) and early forming blood vessels are visible, and at tip surface a clearly visible apical ectodermal ridge.