- Also called:
- antenatal development
- Key People:
- Bernard Siegfried Albinus
- Related Topics:
- gestation
- embryo
- fetus
- precocial young
- altricial state
Ectodermal derivatives
Integumentary system
The skin has a double origin. Its superficial layer, or epidermis, develops from ectoderm. The initial single-layered sheet of epithelial cells becomes multilayered by proliferation, and cells nearer the surface differentiate into a horny substance. Pigment granules appear in the basal layer. The epidermis of the palm and sole becomes thicker and more specialized than elsewhere. Cast-off superficial cells and downy hairs mingle with a greasy glandular secretion and smear the skin in the late fetal months; the pasty mass is called vernix caseosa. The deep layer of the skin, or dermis, is a fibrous anchoring bed differentiated from mesoderm. In the later fetal months the plane of union between epidermis and dermis becomes wavy. The permanently ridged patterns are notable at the surface of the palm and sole.
Nails develop in pocketlike folds of the skin near the tips of digits. During the fifth month specialized horny material differentiates into proliferating ectodermal cells. The resulting nail plate is pushed forward as new plate substance is added in the fold. Fingernails reach the fingertips one month before birth. Hairs, produced only by mammals, begin forming in the third month as cylindrical buds that grow downward from the epidermis into the dermis. Cells at the base of the hair bud proliferate and produce a horny, pigmented thread that moves progressively upward in the axis of the original cylinder. This first crop of hairs is a downy coat named lanugo. It is prominent by the fifth month but is mostly cast off before birth. Unlike nails, hairs are shed and replaced periodically throughout life.
Sebaceous glands develop into tiny bags, each growing out from the epithelial sheath that surrounds a hair. Their cells proliferate, disintegrate, and release an oily secretion. Sweat glands at first resemble hair pegs, but the deep end of each soon coils. In the seventh month an axial cavity appears and later is continued through the epidermis. The mammary glands, unique to mammals, are specialized sweat glands. In the sixth week a thickened band of ectoderm extends between the bases of the upper and lower limb buds. In the pectoral (chest) region only, gland buds grow rootlike into the primitive connective tissue beneath. During the fifth month 15 to 20 solid cords foretell the future ducts of each gland. Until late childhood the mammary glands are identical in both sexes.
Mouth and anus
The mouth is a derivative of the stomodaeum, an external pit bounded by the overjutting primitive nasal region and the early upper and lower jaw projections. Its floor is a thin membrane where ectoderm and endoderm fuse (oropharyngeal membrane). Midway in the fourth week this membrane ruptures, making continuous the primitive ectodermal mouth and endodermal pharynx (throat). Lips and cheeks arise when ectodermal bands grow into the mesoderm and then split into two sheets. Teeth have a compound origin: the cap of enamel develops from ectoderm, whereas the main mass of the tooth, the dentin, and the encrusting cementum about the root differentiate from mesoderm. The salivary glands arise as ectodermal buds that branch, bushlike, into the deeper mesoderm. Berrylike endings become the secretory acini (small sacs), while the rest of the canalized system serves as ducts. The palate is described in relation to the nasal passages. A tiny pocket detaches from the ectodermal roof of the stomodaeum and becomes the anterior, or frontward, lobe of the hypophysis, also called the pituitary gland. The anterior lobe fuses with the neural lobe of the gland.
A double-layered oval membrane separates the endodermal hindgut from an ectodermal pit, called the proctodaeum, the site of the future anal canal and its orifice, the anus. Rupture at eight weeks creates a communication between the definitive anus and the rectum.
Central nervous system
Both the brain and the spinal cord arise from an elongated thickening of the ectoderm that occupies the midline region of the embryonic disk. The sides of this neural plate elevate as neural folds, which then bound a gutterlike neural groove. Further growth causes the folds to meet and fuse, thereby creating a neural tube. The many-layered wall of this tube differentiates into three concentric zones, first indicated in embryos of five weeks. The innermost zone, bordering the central canal, becomes a layer composed of long cells called ependymal cells, which are supportive in function. The middle zone becomes the gray substance, a layer characterized by nerve cells. The outermost zone becomes the white substance, a layer packed with nerve fibres. The neural tube is also demarcated internally by a pair of longitudinal grooves into dorsal and ventral halves. The dorsal half is a region associated with sensory functioning and the ventral half with motor functioning.
The gray substance contains primitive stem cells, many of which differentiate into neuroblasts. Each neuroblast becomes a neuron, or a mature nerve cell, with numerous short branching processes, the dendrites, and with a single long process, the axon. The white substance lacks neuroblasts but contains closely packed axons, many with fatty sheaths that produce the whitish appearance. The primitive stem cells of the neural tube also give rise to nonnervous cells called neuroglia cells.
Brain
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.
