Start of organogenesis: Week 4
The cardiovascular system is the first to begin active development. Initially, two endocardial tubes form, which then fuse into a single two-chambered cardiac anlage (one atrium and one ventricle). The anlage becomes surrounded by the myoepicardial plate, and heart contractions begin at this stage. The embryo's blood vessels connect with the vessels of the yolk sac into a single network.
Development of other systems:
- Nervous system: closure of the neural tube is completed, and its anterior part expands (forming the major brain vesicles). The fully formed notochord lies beneath it.
- Digestive system: the primitive gut forms beneath the notochord. The thyroid and parathyroid glands, adenohypophysis, and respiratory system develop from its foregut. The liver and pancreas develop from the midgut. The hindgut gives rise to the allantois.
- Urogenital system: the pronephros regresses, giving way to the actively developing primary kidneys (mesonephroi). Primordial germ cells begin migrating from the yolk sac toward the kidneys.
Differentiation: Weeks 5–6
The embryo changes proportions significantly. The head grows rapidly due to brain development, and the cephalic flexure increases. The upper limbs become paddle-shaped, and by the 6th week, they are divided into segments with digital rays. The lower limbs develop similarly but lag slightly behind due to vascular supply characteristics.
Internal transformations:
- Nervous system and sense organs: the five brain vesicles finally form in the cranial part of the neural tube. In the 6th week, the pons and cerebellum develop in the fourth vesicle, and cranial nerves appear. The eye forms via a complex process: an outpocketing of the forebrain vesicle (optic vesicle) meets 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: the epithelium thickens on the medial side of the primary kidney, forming genital ridges into which migrating gonocytes settle.
End of the second month: Weeks 7–8
The embryo's head is still enormous (about half 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, the segments of the upper and lower limbs are clearly contoured, and the tail regresses completely.
In the abdominal cavity, the liver occupies a significant volume, and the loops of the midgut are still outside it in the umbilical cord. The vitelline stalk narrows and begins to obliterate.
In the 8th week, an important reorganization of the excretory system takes place—two generations of kidneys coexist:
- Mesonephros (primary kidney): reaches the peak of its development; the mesonephric (Wolffian) duct serves as its excretory pathway.
- Metanephros (secondary kidney): begins to form. Tubules form within the nephrogenic cords, growing toward blood vessels from the aorta (thus forming renal corpuscles). The distal ends of the tubules connect with collecting ducts growing from the mesonephric duct.
Concurrently, the paramesonephric (Müllerian) duct—the basis of the future female reproductive tract—detaches from the Wolffian duct, and sexual differentiation of the gonads begins in the genital ridges.
Briefly: answers to topical questions
What derives from the hindgut?
The allantois originates from the hindgut; later, the hindgut divides into the rectum and the urogenital sinus.
- Allantois — a blind/sac-like outpocketing of the hindgut (cloaca) that grows into the connecting stalk.
- Rectum — formed by the division of the hindgut.
- Urogenital sinus — the future urinary bladder and prostate.
Which cranial nerves appear in the 6th week of development?
The provided sources mention that cranial nerves begin to form generally in the 6th week of development, but specific names or numbers of these nerves for this period are not indicated.
To remember the derivatives of the primitive gut regions (Week 4): Foregut — Respiratory system and Head/neck glands; Midgut — Digestion (liver, pancreas); Hindgut — Allantois.
Class and exam questions
When does the embryonic heart start beating?
Heart contractions begin as early as the 4th week of development when the endocardial tubes fuse into a single two-chambered anlage.
What is a physiological umbilical hernia?
This is a normal phenomenon during weeks 5–8 of development, when the midgut loop temporarily herniates outside the abdominal cavity into the umbilical cord.
How are the gonads laid down?
They develop from the genital ridges (epithelial thickenings on the primary kidney) where primordial germ cells migrate from the yolk sac wall.
What forms the lens of the eye?
The lens forms from the lens placode—an invagination of the surface ectoderm that grows toward the optic vesicle (an outpocketing of the brain).
What else to study on this topic
- Mechanisms of secondary kidney (metanephros) formation
- Development of male and female reproductive systems from the Müllerian and Wolffian ducts
- Complex organogenesis of the eye: interaction between the neural tube and ectoderm
- Structural reorganization of the heart from two-chambered to three-chambered
- Differentiation of the foregut
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