Chapter 23. Weeks 4 to 6 of Development: The Embryonic Period - Review of Medical Embryology Book

Introduction: during this relatively short embryonic period (weeks 4 to 8), one sees the beginnings of all major internal and external structural (organ and organ systems) develop during which time the 3 germ layers give rise to specific tissues and organs - the period of organogenesis. The shape of the embryo changes, and major features of the external body form (morphogenesis) become recognizable by the end of month 2. In addition, major congenital malformations can occur due to exposure of the embryo to teratogens during this developmental period
Week 4 of development

ABOUT DAYS 22 TO 23: embryo is almost straight or slightly curved, and somites create conspicuous surface elevations. The neural tube is closed opposite the somites but is open at its caudal and rostral neuropores
ABOUT DAY 24: the first (mandibular) and second (hyoid) branchial arches become distinct

Most of the mandibular process of the first arch gives rise to the lower jaw and a rostral extension of it; the maxillary process helps form the upper jaw
Head- and tailfolds cause a slight curvature of the embryo
The heart produces a large ventral prominence

ABOUT DAY 26: 3 pairs of branchial arches are seen and the rostral neuropore closes

The forebrain creates a distinct elevation on the head and, with longitudinal folding, the embryo now has a distinct C-shaped curvature to it
Transverse folding causes a narrowing of the connection between yolk sac and embryo
The arm buds are now recognizable as small swellings on the body wall's ventral surface
The otic pits, the primordia for the inner ears, are clearly seen

ABOUT DAY 28 (END OF WEEK 4): the fourth pair of branchial arches and the leg buds are seen

The lens placodes (ectodermal thickenings) represent the future lenses on the side of the head

The face contacts the heart prominence
The second (hyoid) branchial arch overgrows arches 3 and 4 to form an ectodermal depression, the cervical sinus
The forelimbs show some regional differentiation as the hand plates develop

LIMB BUDS, especially the forelimbs, show regional differentiation. Hind limbs develop later

Elbow and wrist areas are identifiable
Paddle-shaped hand plates develop digital ridges, the finger rays, for future fingers

SOME SMALL SWELLINGS appear around the groove between the first branchial arches. The groove becomes the external auditory meatus, the swellings the ear auricle
THE EYE becomes obvious due to appearance of retinal pigment
THE HEAD appears larger relative to the trunk and bends even farther over the heart prominence

Bending is due to the cervical flexure as a result of bending of the brain in the cervical region
The trunk and neck begin to straighten out