Onset of Organogenesis: Week 4
The cardiovascular system is the first to develop actively. Initially, two endocardial tubes form and fuse into a single two-chambered heart tube (one atrium and one ventricle). The primordium becomes surrounded by the myoepicardial mantle, and heart contractions begin at this stage. Embryonic blood vessels connect with yolk sac vessels into a unified network.
Development of other systems:
- Nervous system: Closure of the neural tube completes, and its anterior part expands (forming large brain vesicles). The fully formed notochord lies directly beneath it.
- Digestive system: The primitive gut forms beneath the notochord. The foregut gives rise to the thyroid and parathyroid glands, adenohypophysis, and the respiratory system. The midgut gives rise to the liver and pancreas. The hindgut gives rise to the allantois.
- Urogenital system: The pronephros regresses, giving way to the actively developing mesonephros (primary kidneys). Primordial germ cells begin migrating from the yolk sac toward the kidneys.
Differentiation: Weeks 5–6
The embryo's proportions change significantly. The head grows rapidly due to brain development, and the head fold deepens. The upper limbs acquire a paddle-like shape, and by the 6th week, they divide into segments with digital rays. Lower limbs develop similarly but lag slightly behind due to vascular supply patterns.
Internal transformations:
- Nervous system and sense organs: Five brain vesicles definitively form in the cranial part of the neural tube. By the 6th week, the pons and cerebellum develop within the fourth vesicle, and cranial nerves begin to appear. Complex eye formation involves an outpouching of the forebrain vesicle (optic vesicle) meeting an invagination of the surface ectoderm (lens placode).
- Heart: Acquires an S-shape, becomes three-chambered (the interatrial septum forms), and its walls thicken.
- Digestive tract: The foregut divides into the pharynx, esophagus, and stomach. The midgut forms an intestinal loop that herniates into the umbilical cord, forming a physiological umbilical hernia.
- Urogenital system: On the medial side of the mesonephros, the epithelium thickens to form the genital ridges, which are colonized by migrating gonocytes.
End of the Second Month: Weeks 7–8
The embryo's head remains very large (about half of the body length) but acquires a rounded shape. The eyes are fully formed and covered by eyelids by the end of the 8th week. Auricles appear, limbs are clearly demarcated into segments, and the tail completely regresses.
In the abdominal cavity, the liver occupies a significant volume, and the midgut loops remain outside the abdomen in the umbilical cord. The vitelline duct narrows and begins to obliterate.
During the 8th week, an important reorganization of the excretory system occurs, with two generations of kidneys coexisting:
- Mesonephros (primary kidney): Reaches peak development, drained by the mesonephric (Wolffian) duct.
- Metanephros (definitive kidney): Begins to form. Tubules form within the nephrogenic cords, growing toward vessels branching from the aorta (forming renal corpuscles). The distal ends of the tubules connect with collecting ducts growing out from the mesonephric duct.
Concurrently, the paramesonephric (Müllerian) duct—the foundation of the future female reproductive tract—sprouts from the Wolffian duct, and gonadal sex differentiation begins in the genital ridges.