> ## Content Index > Fetch the complete content index at: https://www.theplasticsfella.com/llms.txt > Use this file to discover other available public pages before exploring further. # Upper Limb Embryology: Development Milestones & Mechanisms - URL: https://www.theplasticsfella.com/upper-limb-embryology/ - Published: 2024-04-24T08:55:00.000Z - Updated: 2024-04-25T22:41:11.000Z - Description: Upper Limb development occurs during weeks 4-8 of gestation. This article details the mechanisms of development and critical milestones. - Author: PlasticsFella - Tags: Upper Limb, Paediatrics #### Summary Card [****Overview of Upper Limb Embryology**](https://www.theplasticsfella.com/upper-limb-embryology/#overview-of-upper-limb-embryology) Upper limb development occurs during weeks 4-8 of gestation from the mesoderm and ectoderm. [****Developmental Milestones**](https://www.theplasticsfella.com/upper-limb-embryology/#developmental-milestones) The limb bud appears on day 26 and differentiates into a limb by day 58. [****Mechanisms of Development**](https://www.theplasticsfella.com/upper-limb-embryology/#mechanisms-of-development) Upper limb embryogenesis is based on 3 axes, 3 signalling zones, and 3 proteins. --- ## Overview of Upper Limb Embryology The upper limb develops during weeks 4-8 of gestation from mesoderm and ectoderm. More specifically, - **3 axis:** proximodistal, anteroposterior, dorsoventral - **3 centres:** apical ectodermal ridge, zone of polarizing activity, and dorsal ectoderm. - **3 proteins:** fibroblast growth centre, sonic hedgehog protein, WNT7a. These critical elements of embryological development are illustrated below. ![Upper Limb Embryology](https://storage.ghost.io/c/a0/9f/a09fd7b4-d396-4b28-aed3-b87e20483292/content/images/2022/08/Upper-Limb-Embryology-Overview.png) Upper Limb Embryology 😎 ****Fun Fact:** The lateral plate of the mesoderm forms bone, cartilage and tendon. The somatic mesoderm differentiates into myofibroblasts that form muscle. --- ## Developmental Milestones ****Key Point** These are general guidelines for the development milestones of limb embryogenesis. ### The upper limb differentiates into a 'human-appearing' limb during 4-8 weeks of gestation. Here are some general timelines to consider: - **\~ Day 26:** Limb bud is present. - \~ **Day 30:** Blood vessels. - **\~ Day 33:** Paddle-shaped hand. - **\~ Day 36:** Nerves appear and myofibroblasts become muscle. - **\~ Day 42:** Webbed-shaped hand as digital rays become present. - \~ **Day 45**: muscles, ligmanets and carpals appear. - **\~ Day 48:** Palms move anteriorly and elbows flex. - **\~ Day 50:** Digits separate. - **\~ Day 56:** Digits ossify secondary to apoptosis. - **\~ Day 58**: Human limb. The below image contains a 9-week human embryo and you can observe the upper limb development. ![9-week human embryo illustrated, upper limb development](https://storage.ghost.io/c/a0/9f/a09fd7b4-d396-4b28-aed3-b87e20483292/content/images/2022/08/Upper-Limb-Embryology.png) Upper limb embryology 💡 ****Tip:** **Tetramelia* is the term used to describe the failure of limb induction. ### Formation of a Blood Supply In the 4th week, the limb bud is supplied by a capillary network. This coalesces into a main stem artery that drains into a marginal vein. These structures then allow for the following to occur: - Subclavian-axillary brachial **arterial** axis is formed. - Basilic-axillary-subclavian **venous** axis is formed. The brachial artery branches into interosseous and median arteries. The median artery is the main blood supply to the hand, which is then replaced by the ulnar and radial arteries around day 44\. The median artery eventually supplies blood to the median nerve. - Ulnar and radial arteries replaced median artery (now supplies median nerve). --- ## Mechanisms of Development ****Key Point** Upper limb embryology occurs via 3 axes, 3 signalling centres and 3 proteins. The three axes of of limb development are proximodistal, anteroposterior, and dorsoventral. ### Proximodistal Axis - **Role:** "distalises" the limb by differentiating limb from the shoulder to finger. - **Signalling centre:** Apical Ectodermal Ridge (AER). - **Proteins:** Fibroblast growth factors (FGFR 2, 4, 8) influence SHH expression. 😎 ****Fun Fact:** AER is an ectodermal thickening at the leading bud edge. Its removal will result in a truncated limb. ### Anteroposterior - **Role:** "ulnarises" the limb by controlling ulnar and radial growth. - **Signalling centre:** Zone of Polarising Activity (ZPA). - **Proteins:** Sonic Hedgehog (SHH), which also influences FGFR expression. 😎 ****Fun Fact:** ZPA contains mesenchymal cells in the posterior limb bud margin. Its removal will result in an ulnar longitudinal deficiency. ### Dorsoventral Axis - **Role:** "dorsalises" the limb by differentiating a dorsal and palmar surface. - **Signalling centre:** Dorsal ectoderm. - **Proteins:** WNT7A, which activates LMX1. 😎 ****Fun Fact:** WNT7A is restricted to the dorsum by the volar-acting en-1\. Its removal will result in abnormal dorsalisation, such as nail-patella syndrome. Importantly, AER and ZPA are "symbitoic" which results in many conditions affected by the anterioposterior and proximodistal axes: - SHH signal is required to maintain AER integrity. - FGFs from the AER are required to express SHH. There is more information in this webinar below, by Pulvertarft: Pulvertaft Webinar --- ## Conclusion Upon completing this article, you will have accomplished the following: ****1\. Basics of Upper Limb Development:** You've learned about the embryological processes that occur from weeks 4 to 8 of gestation, involving the mesoderm and ectoderm, leading to the formation of the upper limb. ****2\. Developmental Milestones:** You now know the key developmental milestones, from the initial appearance of the limb bud to the differentiation into a human-appearing limb. ****3\. Mechanisms of Limb Development:** You've explored the three axes of development (proximodistal, anteroposterior, dorsoventral), the three signaling zones (AER, ZPA, dorsal ectoderm), and the three critical proteins (FGFs, SHH, WNT7a) in the process of limb development. ****4\. Clinical Applications:** You have gained a foundational understanding of recognizing developmental anomalies and their potential clinical implications. ---