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This is a very quick overview of the bits of first stages of development that are relevant to the abdomen, intended to set you up for later stages. If you wish to know more, I would suggest The Developing Human (Moore and Persaud) or Langman's Medical Embryology (Sadler).

Week 3

The easiest place to pick up our story is the third week of development (measured in time after fertilisation has taken place). The process of gastrulation transforms the embryo into three different tissue layers:

•  Ectoderm – forms the central and peripheral nervous systems, epidermis and sensory epithelia.

•  Mesoderm – forms muscle, bone, connective tissue, blood and blood vessels, serous membranes and the urogenital system.

•  Endoderm – forms the gut tube and the various structures derived from it (glands, lungs, liver, gallbladder and pancreas).

These are arranged around a solid cord of cells called the notochord and form 2 cavities, the amnionic cavity and the yolk sac cavity. The central, three-layered structure is known as the trilaminar germ disc .

In two places, the trilaminar germ disc remains bilaminar, lacking mesoderm:

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Week 4

The neural and gut tubes form in this week.

The neural tube develops from ectoderm in a process of thickening and folding called neurulation . It initially exists as a neural groove flanked by neural folds . These neural folds eventually meet in the middle, forming the neural tube.

Ventral folding occurs along the sides of the embryo – the sides moving downwards are referred to as lateral folds , causing the endoderm to roll into a gut tube and pulling the amnionic cavity to surround the embryo.

 

Head and tail folds also appear, curving the foetus into a ‘foetal position'.

Transverse section:

The head, tail and lateral folds draw the ventral body wall into a narrow region around the connecting stalk – the umbilical ring . Note how the gut tube is pinched off from the yolk sac to form the narrow vitelline duct .

The allantois develops as a blind-ending outgrowth from the caudal end of the yolk sac into the connecting stalk (it is an evolutionary remnant from when we developed inside eggs and needed a waste storage site). Its blood vessels persist as the umbilical vein and arteries and hence the connecting stalk becomes the umbilical cord .

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Week 5

At this stage, the body essentially exists as two tubes (gut and neural) spanning its length. As the heart develops, mesoderm caudal to the heart proliferates, forming the septum transversum – a block of tissue separating thorax and abdomen.

The septum transversum grows from the anterior body wall to meet the gut tube. Two pleuroperitoneal membranes grow from the posterior body wall to meet the septum transversum, closing off the cavity.

Later, the septum transversum thins to form the central tendon of the diaphragm, while the pleuroperitoneal membranes become covered by muscle to form the muscular part of the diaphragm.

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