1. The bulk of our body develops in the mesoderm.
2. The mesoderm like all embryological tissue is a matrix to mean an environment or a substance in which a thing develops.
3. At an early stage this matrix is a syncytium which means a tissue with multiple nuclei but without well defined cell membranes.
4. In this tissue is laid a capillary network.
5. This capillary network is common to all tissues that are developed in the mesoderm simultaneously.
6. As differentiation progresses, muscles, tendons, bones, ligaments, capsules, synovial membrane, fascia and skin (dermis) partake of this capillary network.
7. As bulk is added, some parts of this capillary network coalesce to form blood vessels and the rest of the capillary network remains intact.
8. This development happens pari passu that is simultaneously or equally and hand in hand, across the body.
9. In vertebrates, for example, the development of the bony spine is accompanied by laying down of vessels on either side.
10. These form the main vessels of the body (for example the aorta).
11. Parts are developing simultaneously and the vessels that are laid in these parts are called internal vessels for e.g. iliac vessels.
12. The capillary network within individual organs for e.g. bone, muscle and skin also develop into vessels at the same time by a congregation of the capillaries already existent in the developing tissue.
13. When the heart starts to pump, a linked vascular architecture therefore is already present which has by then developed an arterial and a venous side.
14. The muscles and the dermis have by far the richest vascular network, the muscle for its high metabolic rate to support its mechanical actions and in the skin for its thermo-regulatory function.
15. As these two develop hand in hand, their shared capillary network has already resulted in a vessel or vessels which supply the muscle and then are linked to the skin via the fascia. These we call musculo-cutaneous vessels for e.g. an artery supplies the gastrocnemius (medial sural) and because the muscles’ network is connected to the skin, a musculo-cutaneous flap can be raised to include the muscle, the fascia and the skin. The muscle alone can also be lifted as a flap based on this vessel.
16. In the extremities where muscles are longitudinal and are arranged in compartments by way of septa made of fascia, an internal artery can get linked to the skin via this fascial envelope without going through a muscle. This arrangement is called a septo-cutaneous vascular network. This connects to the skin via a network over the surface of the fascia. For example, the saphenous artery emerges from the adductor canal (a septal canal) to run along the fascia on the medial side of the calf. We therefore can harvest a fascio-cutaneous flap and this can be used locally or as a free flap.
Photographs courtesy V. Bhattacharya of Varanasi who has original work on the subject of Fasciocutaneous Flaps to his credit.
Comment: The advantage of a fasciocutaneous flap is that the branches of the perforator travel both superiorly and inferiorly and therefore an inferiorly based flap which is very useful for defects in the lower leg and foot can be harvested. Also as compared to a muscle harvesting fascia is (theoretically) less debilitating.
17. Occasionally, such a septo-cutaneous vessel will preserve its identity and run under the skin to supply a distinct area for e.g. the external superficial circumflex iliac vessel. This then is the basis of the groin flap which includes skin above and below the inguinal ligament.
18. In the leg itself the mesoderm which is to later develop as the fibula is connected to its immediate environment which is to develop into the muscles of the lateral compartment and share their capillary network. Out of this arises the nutrient artery of the fibula, a branch of the peroneal artery and the rest of the peroneal artery supplies the muscles. The fibula is a commonly harvested as a free bone graft with a muscular cuff at the point of entry of the nutrient vessel.
Samir Kumta from Mumbai adds that when the fibula is harvested in the lower two-third the nutrient artery may get sacrificed and the bone survives on the branches of the peroneal artery which constitutes its periosteal blood supply. The peroneal artery here being the main artery anastomosed to the donor vessel.
19. The iliac crest which is a flat bone gets branches from the deep circumflex iliac artery and then also perfuses muscles in the adjoining area as well as the skin. This is a direct branch of an internal artery (the external iliac) which perfuses a segment of the mesoderm which differentiates into bone and muscles and is usually used as a free osteo-musculo-cutaneous flap.
20. An interesting, useful and popular flap in the inferior extremity is the antero-lateral thigh flap. This flap draws its blood supply from a branch of the lateral circumflex femoral artery which first gives branches to the rectus femoris and the vastus lateralis muscles and then emerges from the space between them to perforate the fascia and supply a fairly large portion of the skin and the subcutaneous tissue. This vessel though classically septocutaneous in the end, also supplies muscles prior to its emergence through the fascia and is therefore interesting. This brings to the fore the difficulty of nomenclature such as septocutaneous, myocutaneous and perforator vessel flaps which is convenient up to a point but might be confusing if made too didactic. This flap is useful because it harvests a fairly large area of the skin with substantial subcutaneous tissue which can fill deep defects and is popular in India because the defect left behind can be easily hidden by most clothing that Indians wear. This flap is mainly used as a free flap.
Prabha Yadav and her group from TMH Mumbai add that the anterior-lateral thigh flap can be used with great advantage in defects of the lower abdomen, groin, perineum and the gluteal region as a local flap.
A purely random pattern flap is perfused by unnamed small vessels which are the terminal branches of one of the above systems.
Photographs courtesy: Prabha Yadav and her group at the Tata Memorial Hospital, Mumbai
Addendum: The December 2011 issue of JPRAS has reported a flap of the bare skin with a specific perforator. The skin is thin enough and therefore suitable for reconstruction for microtia and external ear atresia. The same issue also reports a thin osteo-periosteal flap for reconstruction in a finger which in turn has been skin grafted. The osteo-periosteal flap is also perfused by a very minute specific blood vessel. In both cases microvascular technique has been employed.