- Also called:
- antenatal development
- Key People:
- Bernard Siegfried Albinus
- Related Topics:
- gestation
- embryo
- fetus
- precocial young
- altricial state
The head end of the neural plate becomes expansive even as it closes into a tube. This brain region continues to surpass the spinal cord region in size. Three enlargements are prominent: the forebrain, midbrain, and hindbrain. The forebrain gives rise to two secondary expansions, the telencephalon and the diencephalon. The midbrain, which remains single, is called the mesencephalon. The hindbrain produces two secondary expansions called the metencephalon and the myelencephalon.
The telencephalon outpouches, right and left, into paired cerebral hemispheres, which overgrow and conceal much of the remainder of the brain before birth. Late in fetal life the surface of the cerebrum becomes covered with folds separated by deep grooves. The superficial gray cortex is acquired by the migration of immature nerve cells, or neuroblasts, from their primary intermediate position in the neural wall. The diencephalon is preponderantly gray substance, but its roof buds off the pineal gland, which is not nervous tissue, and its floor sprouts the stalk and neural (posterior) lobe of the pituitary. The mesencephalon largely retains its early tubular shape. The metencephalon develops dorsally into the imposing cerebellum, with hemispheres that secondarily gain convolutions clothed with a gray cortex. The myelencephalon is transitional into the simpler spinal cord. Roof regions of the telencephalon, diencephalon, and myelencephalon differentiate the vascular choroid plexuses—including portions of the pia mater, or innermost brain covering, that project into the ventricles, or cavities, of the brain. The choroid plexuses secrete cerebrospinal fluid.
Spinal cord
For a time, the spinal cord portion of the neural tube tapers gradually to an ending at the tip of the spine. In the fourth month it thickens at levels where nerve plexuses, or networks, supply the upper and lower limbs; these are called the cervical and lumbosacral enlargements. At this time the spine begins to elongate faster than the spinal cord. As a result, the caudal (hind) end of the anchored cord becomes progressively stretched into a slender, nonnervous strand known as the terminal filament. Midway in the seventh month the functional spinal cord ends at a level corresponding to the midpoint of the kidneys. Both the brain and the spinal cord are covered with a fibrous covering, the dura mater, and a vascular membrane, the pia-arachnoid. These coverings differentiate from local, neighbouring mesoderm.
Peripheral nervous system
In general, each craniospinal nerve has a dorsal (posterior) root that bears a ganglion (mass of nerve tissue) containing sensory nerve cells and their fibres and a ventral (anterior) root that contains motor nerve fibres but no nerve cells. Ganglion cells differentiate from cells of the neural crest, which is at first a cellular band pinched off from the region where each neural fold continues into ordinary ectoderm. Each of these paired bands breaks up into a series of lumps, spaced in agreement with the segmentally arranged mesodermal somites. Neuroblasts within these primordial ganglia develop a single stem and hence are called unipolar. From this common stem, one nerve process, or projection, grows back into the adjacent sensory half of the neural tube. Another projection grows in the opposite direction, helping to complete the dorsal root of a nerve. Neuroblasts of motor neurons arise in the ventral half of the gray substance of the neural tube. They sprout numerous short, freely branching projections, the dendrites, and one long, little-branching projection, the axon. Such a neuron is called multipolar. These motor fibres grow out of the neural tube and constitute a ventral root. As early as the fifth week they are joined by sensory fibres of the dorsal root and continue as a nerve trunk.
Cells of the neural crest differentiate into cells other than sensory neurons. Among these variants are cells that encapsulate ganglion cells and others that become neurolemma cells, which follow the peripherally growing nerve fibres and ensheath them. The neurolemma cells cover some nerve fibres with a fatty substance called myelin.
Spinal nerves
Spinal nerves are sensorimotor nerves with dorsal and ventral roots. A network called a brachial plexus arises in relation to each upper limb and a lumbosacral plexus in relation to each lower limb. The spine, elongating faster than the spinal cord, drags nerve roots downward, since each nerve must continue to emerge between the same two vertebrae. Because of their appearance, the obliquely coursing nerve roots are named the cauda equina, the Latin term for horse’s tail.
Cranial nerves
Cranial nerves V, VII, IX, and X arise in relation to embryonic branchial arches but have origins similar to the spinal nerves. The olfactory nerves (cranial nerve I) are unique in that their cell bodies lie in the olfactory epithelium (the surface membrane lining the upper parts of the nasal passages), each sending a nerve fibre back to the brain. The so-called optic nerves (II) are not true nerves but only tracts that connect the retina (a dislocated portion of the brain) with the brain proper. Nerves III, IV, VI, and XII are pure motor nerves that correspond to the ventral roots of spinal nerves. The acoustic nerves (VIII) are pure sensory nerves, each with a ganglion that subdivides for auditory functions and functions having to do with equilibrium and posture; they correspond to dorsal roots. Nerves X and XI are a composite of which XI is a motor component.
Autonomic nervous system
The autonomic nervous system is made up of two divisions, the sympathetic and the parasympathetic nervous systems. It controls involuntary actions, such as the constriction of blood vessels. Some cells of the neural crests migrate and form paired segmental masses alongside the aorta, a principal blood vessel. Part of the cells become efferent multipolar ganglion cells (cells whose fibres carry impulses outward from ganglions, or aggregates of nerve cells), and others merely encapsulate the ganglion cells. These autonomic ganglia link into longitudinal sympathetic trunks. Some of the neuroblasts migrate farther and assemble as collateral ganglia—ganglia not linked into longitudinal trunks. Still others migrate near, or within, the visceral organs that they will innervate and produce terminal ganglia. These ganglia are characteristic of the parasympathetic system.
Some cells of certain primitive collateral ganglia leave and invade the amassing mesodermal cortex of each adrenal gland. Consolidating in the centre, they become the endocrine cells of the medulla.
Sense organs
Olfactory organ
Paired thickenings of ectoderm near the tip of the head infold and produce olfactory pits. These expand into sacs in which only a relatively small area becomes olfactory in function. Some epithelial cells in these regions remain as inert supporting elements. Others become spindle-shaped olfactory cells. One end of each olfactory cell projects receptive olfactory hairs beyond the free surface of the epithelium. From the other end a nerve fibre grows back and makes a connection within the brain.
Gustatory organ
Most taste buds arise on the tongue. Each bud, a barrel-shaped specialization within the epithelium that clothes certain lingual papillae (small projections on the tongue), is a cluster of tall cells, some of which have differentiated into taste cells whose free ends bear receptive gustatory hairs. Sensory nerve fibres end at the surface of such cells. Other tall cells are presumably inertly supportive in function.
Eye
The earliest indication of the eyes is a pair of shallow grooves on the sides of the forebrain. The grooves quickly become indented optic cups, each connected to the brain by a slender optic stalk. Most of the cup will become the retina, but its rim represents the epithelial part of the insensitive ciliary body and iris. The thicker inner layer of the cup becomes the neural layer of the retina, and by the sixth month three strata of neurons are recognizable in it: (1) visual cells, each bearing either a photoreceptive rod or a cone at one end, (2) bipolar cells, intermediate in position, and (3) ganglion cells, which sprout axons that grow back through the optic stalk and make connections within the brain. The thin outer layer of the cup remains a simple epithelium whose cells gain pigment and make up the pigment epithelium of the retina.
The lens arises as a thickening of the ectoderm adjacent to the optic cup. It inpockets to form a lens vesicle and then detaches. The cells of its back wall become tall, transparent lens fibres. Mesoderm surrounding the optic cup specializes into two accessory coats. The outer coat, the tough, white sclera, is continuous with the transparent cornea. The inner coat, the vascular choroid, continues as the vascular and muscular ciliary body and the vascularized tissue of the iris. The eyelids are folds of adjacent skin, and from the inside of each upper lid several lacrimal glands bud out.
Ear
The projecting part (auricle) of the external ear develops from hillocks on the first and second branchial arches. The ectodermal groove between those arches deepens and becomes the external auditory canal. The auditory tube and tympanic cavity—the cavity at the inner side of the eardrum—are expansions of the endodermal pouch located between the first and second branchial arches. The area where ectodermal groove and endodermal pouch come in contact is the site of the future eardrum. The chain of three auditory ossicles (small bones) that stretches across the tympanic cavity is a derivative of the first and second arches.
The epithelium of the internal ear is at first a thickening of ectoderm at a level midway of the hindbrain. This plate inpockets and pinches off as a closed sac, the otocyst. Its ventral part elongates and coils to resemble a snail’s shell, thereby forming the cochlear duct, or seat of the organ of hearing. A middle region of the otocyst becomes chambers known as the utricle and saccule, related to the sense of balance. The dorsal part of the otocyst remodels drastically into three semicircular ducts, related to the sense of movement. Fibres of the acoustic nerve grow among specialized receptive cells differentiated in certain regions of these three divisions.
Mesodermal derivatives
Skeletal system
Except for part of the skull, all bones pass through three stages of development: membranous, cartilaginous, and osseous. The earliest ossification centres appear in the eighth week, but some do not arise until childhood years and even into adolescence.
Axial skeleton
The ventromedial walls (the walls toward the front and the midline) of the paired somites break down, and their cells migrate toward the axial notochord and surround it. Differentiation and growth of these segmental masses produce the jointed vertebrae. Ribs also grow out of each primitive vertebral mass, but they become long only in the thoracic region. Here their ventral ends join to form sternal bars, which fuse to form the sternum.
The skull has three components, different in origin. Its basal region consists of bones that pass through the three typical stages of development. By contrast, the sides and roof of the skull develop directly from membranous primordia, or rudiments. The jaws are derivatives of the first pair of cartilaginous branchial arches but develop as membrane bone. Ventral ends of the second to fifth arches contribute the cartilages of the larynx and the hyoid bone (a bone of horseshoe shape at the base of the tongue). Dorsal ends of the first and second arches become the three auditory ossicles (the small bones in the middle ear).
Appendicular skeleton
The limb bones develop in three stages from axial condensations in the local mesoderm. The shoulder and pelvic supports are comparable sets, as are the bones of the arms and legs.
Articulations
Some type of joint exists wherever bones meet. Joints that allow little or no movement consist of connective tissue, cartilage, or bone. Movable joints arise as fluid-filled clefts in mesoderm, which condenses peripherally into a fibrous capsule.
