Early stages: zygote to bilaminar disc
Development begins at fertilisation, when a sperm and an oocyte fuse to form the zygote. The key early terms are:
- Cleavage: rapid mitotic divisions of the zygote without overall growth, producing smaller cells called blastomeres.
- Morula: a solid ball of blastomeres.
- Blastocyst: a hollow sphere with a fluid-filled cavity. Its inner cell mass (embryoblast) forms the embryo; its outer layer (trophoblast) forms much of the placenta.
- Implantation: the blastocyst embeds in the endometrium, beginning at the end of the first week.
- Bilaminar disc: in week 2 the inner cell mass becomes a flat disc of two layers, the epiblast and the hypoblast.
Gastrulation and the germ layers
Gastrulation, in week 3, converts the bilaminar disc into a trilaminar disc with three germ layers. A groove called the primitive streak forms on the epiblast. Epiblast cells migrate through it: some replace the hypoblast as endoderm, some spread between the layers as mesoderm, and those remaining on the surface become ectoderm. The notochord, a rod of cells in the midline, forms at the same time. It induces the overlying ectoderm to form the nervous system, and its remnant in the adult is the nucleus pulposus of the intervertebral discs.
| Germ layer | Main derivatives |
|---|---|
| Ectoderm | Epidermis, hair, nails, lens, enamel, anterior pituitary; the nervous system (via the neural tube); neural crest derivatives |
| Mesoderm | Skeletal, cardiac and smooth muscle; bone and cartilage of the trunk and limbs; dermis; heart, vessels and blood; kidneys and gonads; spleen; adrenal cortex; serous membranes |
| Endoderm | Epithelial lining of the gut and respiratory tract; liver and pancreas; thyroid follicular cells; parathyroid glands; thymic epithelium; most of the bladder epithelium |
Neurulation, somites and folding
Neurulation forms the neural tube, the precursor of the brain and spinal cord. The ectoderm over the notochord thickens as the neural plate, its edges rise as neural folds, and the folds fuse in the midline to form a tube. The open ends, the cranial and caudal neuropores, close during week 4. Neural crest cells break away from the edges of the folds and migrate to form sensory and autonomic ganglia, Schwann cells, melanocytes, the adrenal medulla and much of the skeleton and connective tissue of the face.
The mesoderm on each side of the notochord divides into three parts:
- Paraxial mesoderm forms paired somites. Each somite divides into a sclerotome (vertebrae and ribs), a myotome (segmental muscle) and a dermatome (dermis of the back). The segmental origin of these tissues explains why spinal nerves supply dermatomes and myotomes.
- Intermediate mesoderm forms the kidneys, gonads and their ducts.
- Lateral plate mesoderm splits into a somatic layer (body wall, limb bones and connective tissue) and a splanchnic layer (gut wall muscle and connective tissue, heart). The space between them becomes the body cavities.
Folding in week 4, both head to tail and side to side, turns the flat disc into a cylinder. It draws the endoderm into the gut tube, divided into foregut, midgut and hindgut, and supplied in the adult by the coeliac trunk, superior mesenteric artery and inferior mesenteric artery respectively.
Terms for developmental anomalies
Precise terms describe how development can go wrong.
| Term | Meaning |
|---|---|
| Agenesis | Complete absence of an organ because its primordium never formed |
| Aplasia | Primordium present but the organ fails to develop |
| Hypoplasia | Underdevelopment, leaving the organ small |
| Atresia | Absence or closure of a normal opening or lumen |
| Stenosis | Narrowing of a lumen |
| Ectopia | An organ in an abnormal position |
| Teratogen | An agent that causes a congenital anomaly |
The embryonic period is the most sensitive time for teratogens because the organs are forming. Neural tube defects, such as spina bifida and anencephaly, result from failure of the neural tube to close, and folic acid supplementation before and in early pregnancy reduces their risk.