Anatomy for Dental Medicine (eBook)

Fig. 1.1 Embryonic development (after Sadler)

Age in postovulatory days.

A-C Posterior (dorsal) view after removal of the amnion.

D-E Schematic cross sections of the corresponding stages at the horizontal planes of section marked in A to C. Gastrulation occurs in week 3 of human embryonic development. It produces three germ layers in the embryonic disk: ectoderm (light grey), mesoderm (red), and endoderm (dark grey).

A, D Day 19, the three layers are visible in the embryonic disk. The amnion forms the amniotic cavity dorsally, and the endoderm encloses the yolk sac. The neural tube is developing in the area of the neural plate.

B, E Day 20, the first somites have formed, and the neural groove is beginning to close to form the neural tube, with initial folding of the embryo.

C, F Day 22, eight pairs of somites flank the partially closed neural tube, which has sunk below the ectoderm. The yolk sac elongates ventrally to form the gut tube and yolk sac. At the sites where the neural folds fuse to close the neural tube, cells form a bilateral neural crest that detaches from the surface and migrates into the mesoderm.

Fig. 1.2 Somatic muscle development

Age in postovulatory days. Each somite divides into a dermatome (cutaneous), myotome (muscular), and sclerotome (vertebral) at around day 22 (see Fig. 1.1).

A Day 28, sclerotomes migrate to form the vertebral column around the notochord (primitive spinal cord).

B Day 30, all 34 or 35 somite pairs have formed. The neural tube differentiates into a primitive spinal cord. Motor and sensory neurons differentiate in the anterior and posterior horns of the spinal cord, respectively.

C By day 40, the posterior and anterior roots form the mixed spinal nerve. The posterior branch supplies the epiaxial muscles (future intrinsic back muscles); the anterior branch supplies the hypaxial muscles (anterior muscles, including all muscles except the intrinsic back muscles).

D Week 8, the epiaxial and hypaxial muscles have differentiated into the skeletal muscles of the trunk. Cells from the sclerotomes also migrate into the limbs. During this migration, the spinal nerves form the plexuses (cervical, branchial, and lumbosacral), which innervate the muscles of the neck, upper limb, and lower limb, respectively.

Fig. 1.3 5-week-old embryo

The human embryo at 5 weeks has a crown-rump length of approximately 5 to 7 mm. The umbilical cord, which attaches the embryo to the mother, is seen. The future cerebral hemispheres form along with the eye, ear, pharyngeal arches (which form a large portion of the structures of the head and neck), heart, neural tube, and limb buds.

Fig. 1.4 Development of the neural tube and neural crest (after Wolpert)

The tissues of the nervous system orginate embryonically from the posterior surface ectoderm. The notochord in the midline of the body induces the formation of the neural plate, which lies dorsal to the notochord, and of the neural crests, which are lateral to the notochord. With further development, the neural plate deepens at the center to form the neural groove, which is flanked on each side by the neural folds. Later the groove deepens and closes to form the neural tube, which sinks below the ectoderm. The neural tube is the structure from which the central nervous system (CNS) – the brain and spinal cord – develops (further development of the spinal cord is shown in Fig. 1.5, further brain development in Fig. 1.7). Failure of the neural folds to fuse completely in the caudal region will leave an anomalous cleft in the vertebral column known as spina bifida. In the cranial region, this will lead to a defect known as anencephaly. The administration of folic acid to potential mothers around the time of conception can significantly reduce the incidence of spina bifida and other neural tube defects. Cells that migrate from the neural crest develop into various structures, including cells of the peripheral nervous system (PNS), such as Schwann cells, and the pseudounipolar cells of the spinal ganglion (see Fig. 1.6).

Fig. 1.5 Differentiation of the neural tube in the spinal cord during development

Cross-section, superior view.

A Early neural tube. B Intermediate Stage. C Adult spinal cord.

The neurons that form the basal plate are efferent (motor neurons), while the neurons that form the alar plate are afferent (sensory neurons). In the future thoracic, lumbar, and sacral spinal cord, there is another zone between them that gives rise to autonomic neurons. The roof plate and the floor plate do not form neurons.

Fig. 1.6 Development of a peripheral nerve

Afferent (sensory) axons (blue) and efferent (motor) axons (red) sprout from the neuronal cell bodies during early embryonic development.

A Primary afferent neurons develop in the spinal ganglion, and alpha motor neurons develop from the basal plate of the spinal cord.

B The interneurons (black), which functionally interconnect the afferent and efferent neurons, develop at a later stage.

Fig. 1.7 Development of the brain

A Embryo with the greatest length (GL) of 10 mm at the beginning of the 2nd month of development. Even at this stage, we can see the differentiation of the neural tube into segments that will generate various brain regions.

• Red: telencephalon (cerebrum)

• Yellow: diencephalon

• Dark blue: mesencephalon (midbrain)

• Light blue: cerebellum

• Gray: pons and medulla oblongata

Note: The telencephalon outgrows all of the other brain structures as development proceeds.

B Embryo with a GL of 27 mm near the end of the 2nd month of development (end of the embryonic period). The telencephalon and the diencephalon have enlarged. The olfactory bulb is developing from the telencephalon, and the primordium of the pituitary gland is developing from the diencephalon.

C Fetus with a GL of 53 mm in approximately the 3rd month of development. By this stage the telencephalon has begun to cover the other brain areas. The insula is still on the brain surface but will subsequently be covered by the hemispheres (compare with D).

D Fetus with GL of 27 cm (270 mm) in approximately the 7th month of development. The cerebrum (telencephalon) has begun to develop well-defined gyri and sulci.

Fig. 1.8 Head and neck region of a 5-week-old embryo, showing the pharyngeal (branchial) arches and clefts

Left lateral view. The pharyngeal arches are instrumental in the development of the face, neck, larynx, and pharynx. Development of the pharyngeal arches begins in the 4th week of embryonic life as cells migrate from the neural crest to the future head and neck region. Within 1 week, a series of four oblique ridges (first through sixth pharyngeal arches, with the fifth arch only rudamentary in humans and the sixth arch not visible on the surface) form that are located at the level of the cranial segment of the foregut and are separated externally by four deep grooves (pharyngeal clefts). The pharyngeal arches and clefts are prominent features of the embryo at this stage.

Fig. 1.9 Cross section through an embryo at the level of the pharyngeal gut (after Drews)

Left superior oblique view. Due to the craniocaudal curvature of the embryo, the cross section passes through the pharyngeal arches and pharyngeal gut as well as the prosencephalon and spinal cord. The pharyngeal gut is bounded on both sides by the pharyngeal arches, which contain the mesodermal core. They are covered externally by ectoderm and internally by endoderm. Ectodermal pharyngeal clefts and endodermal pharyngeal pouches lie directly opposite one another. Because the embryo is curved craniocaudally, the pharyngeal gut and pharyngeal arches overlie the prominence of the rudimentary heart and liver.

Fig. 1.10 Structure of the pharyngeal arches (after Sadler)

A Cross section through a pharyngeal arch and the neural tube, showing the pharyngeal arch cartilage and artery. B Oblique cross section through a pharyngeal arch and the neural tube, showing the pharyngeal arch nerves. C Blow up of section in B, showing the relationship of pharyngeal arch cartilage, artery, and nerve in the pharyngeal arches. The pharyngeal...