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.
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 the sternum, which arises
independently by the fusion of a pair of bars.
The skull has three components,
different in origin. Its basal region is an ancient heritage whose bones
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).
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.
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.
Much of each somite differentiates into myoblasts
(primitive muscle cells) that
become voluntary muscle fibres. Aggregations of such fibres become muscles
of the neck and trunk. Muscles of the head and some of the neck muscles
originate from mesoderm of branchial arches. Muscles of the limbs seemingly
arise directly from local mesoderm. In general, muscle primordia may fuse
into composites, split into subdivisions, or migrate away from their sites
of origin. During these changes they retain their original nerve supply.
Regardless of differences in source of origin, all voluntary muscle fibres
are of the same striated type (marked by dark and light stripes).
Spontaneous movements begin to occur in embryos about 10 weeks old. In
general, involuntary muscle differentiates from mesoderm surrounding hollow
organs; only the cardiac muscle type is striated.
All hollow organs, including arteries, veins, and
lymphatics, are lined with epithelium—the principal functional tissue—and
are ensheathed with muscular and fibrous coats.
Primitive blood vessels arise in the mesoderm as
tiny clefts bordered by flat endothelial cells. Growth and coalescence
produce networks, out of which favoured channels persist as definite
vessels, while others decline and disappear. A bilaterally symmetrical
system of vessels is well represented in embryos four weeks old. This early
plan is profoundly altered and made somewhat asymmetrical during the second
month by fusions, atrophies, emergence of new vessels, and rerouting of
older ones. The alterations reflect adjustments to changing form and pattern
within the developing organ systems.
Arteries cranial to the heart (headward of the heart) are mostly products of
the paired aortic arches, which course axially within the branchial arches,
thus interconnecting the ventral aorta with paired dorsal aortas. The third
pair of aortic arches becomes the common carotids; the fourth pair, the
aortic arch and brachiocephalic; the fifth pair, the pulmonary arteries and
ductus arteriosus. The dorsal aortas fuse into the single descending aorta,
which bears three sets of paired, segmental branches. The dorsal set becomes
the subclavian, intercostal, and lumbar arteries. The lateral set becomes
arteries to the diaphragm, the adrenal glands, the kidneys, and the sex
glands. The ventral set becomes the celiac, mesenteric, and umbilical
arteries. Axial arteries to both sets of limb buds emerge from an original
plexus, but they undergo drastic alteration and extensive replacement.
The primitive veins are symmetrically bilateral. They consist of vitelline
veins from the yolk sac, umbilical veins from the placenta, and precardinal
and postcardinal veins from the cranial and caudal regions (the regions
toward the head and toward the tail) of the body. Drastic transformations
occur in all of these, and new pairs of veins (subcardinals and
supracardinals) arise also, caudal to the heart. From the vitellines come
chiefly the portal and hepatic veins. The left umbilical becomes the main
return from the placenta by making a diagonal channel, the ductus venosus,
through the liver to the heart. The precardinal veins change their names to
the internal jugulars, but near the heart an interconnection permits both to
drain into a common stem, then called the superior vena cava. Caudal to the
heart, the postcardinals virtually disappear, and all blood return shifts to
the right side as a new compound vessel, the inferior vena cava, becomes
dominant. Pulmonary veins open into the left atrium. Veins from the limb
buds organize from an early peripheral border vein.
The lymph vessels develop independently in close
association with veins. Linkages produce the thoracic duct, which is the
main drainage return for lymph. Masses of lymphocytes accumulate about
lymphatic vessels and organize as lymph nodes. The spleen has somewhat
similar tissue, but its channels are supplied with blood.
Fusion
combines two endothelial tubes, and these are surrounded by a mantle of
mesoderm that will become the muscular and fibrous coats of the heart. At
three weeks the heart is a straight tube that is beginning to beat (Figure
1M). Starting at the head end, four regions can be recognized: bulbus,
ventricle, atrium, and sinus venosus. Since the heart is anchored at both
ends, rapid elongation forces it to bend. In doing this, the sinus–atrium
and bulbus–ventricle reverse their original relations. Further development
concerns the transformation of a single-chambered heart into one with four
chambers.
The atrium becomes subdivided by the growth of two incomplete partitions, or
septa, placed close together and each covering the defect in the other. The
ventricle also subdivides, but by a single, complete partition. A canal,
connecting atria and ventricles, becomes two canals. The bulbus is absorbed
into the right ventricle, and its continuation (the truncus) subdivides
lengthwise, forming the aorta and the pulmonary artery. The right horn of
the sinus venosus is absorbed into the right atrium, together with the
superior and inferior venae cavae, which originally drained into the sinus.
The transverse portion of the sinus persists as the coronary sinus. The
pulmonary veins retain their early drainage into the left atrium. Important
valves develop and ensure flow within the heart from atria to ventricles,
and outward from the ventricles into the aorta and the pulmonary artery.
Birth initiates breathing, and the abandonment of the placental circulation
follows. These changes entail a drastic rerouting of blood through the
heart. As a result, the two atrial septa fuse and no longer permit blood to
pass from the right atrium to the left atrium. Blood in the pulmonary artery
no longer virtually bypasses the lungs; previously it had passed to the
aorta directly through a shunt offered by the ductus arteriosus. As a sequel
to these changes, the abandoned umbilical arteries, umbilical vein, ductus
venosus, and ductus arteriosus all collapse and become fibrous cords.
Vertebrates have made three experiments in
kidney production: the
pronephros, or earliest type; the mesonephros, or intermediate kidney; and
the metanephros, or permanent kidney. All arise from the cellular plates
called nephrotomes that connect somites with the mesodermal sheets that bound the body cavity. The vestigial
pronephros is represented solely by several pairs of tubules; they join
separately formed excretory ducts that grow downward and enter the cloaca,
the common outlet for urine, genital products, and for intestinal wastes.
Next tailward arise some 40 pairs of nephric (kidney) tubules that
constitute the mesonephros; these tubules join the same excretory ducts,
hereafter called the mesonephric ducts. The two sets of mesonephric tubules
serve as functioning kidneys until the 10th week.
Each permanent kidney, or metanephros, develops still farther tailward. A
so-called ureteric primordium buds off each mesonephric duct, near its hind
end. The ureteric stem elongates and expands terminally, thereby forming the
renal pelvis and calices; continued bushlike branching produces collecting
ducts. The early ureteric bud invades a mass of nephrotome tissue. The
branching collecting ducts progressively break this tissue up into tiny
lumps, each of which becomes a long secretory tubule, or
nephron, and joins a nearby
terminal twig of the duct system. Continued proliferation of ducts and
nephric tissue produces over a million urine-producing tubules in each
kidney.
The blind caudal end of the endodermal hindgut absorbs the stem of each
mesonephric duct, whereupon the remainder of the duct and the ureter acquire
separate openings into the hindgut. This expanded region of the gut, now a
potential receptacle for feces, urine, and reproductive products, is known
as a cloaca. It next subdivides into a rectum behind and a urogenital sinus
in front. The sinus, in turn, will specialize into the urinary bladder and
the urethra. The prostate gland develops as
multiple buds from the urethra, close to the
bladder.
The genital organs begin to develop in the second
month, but for a time sex is not grossly distinguishable. Also, a double set
of male and female ducts arise, and not until later does the unneeded set
decline. Hence, this period is commonly called the indifferent stage.
Sex glands develop in a pair of longitudinal ridges
located alongside the mesentery, the anchoring fold of membrane to the gut.
The primordial sex cells appear first in the cloacal wall, from which they
migrate upward in the gut, pass through its mesentery, and finally invade
the genital ridges, where they proliferate. The
testes are the earliest type of gonad to organize. They begin by
developing testis cords and a testis capsule. The cords radiate from one
focal point at the periphery, and thin fibrous partitions segregate groups
of the cords within wedge-shaped compartments. These cords do not gain
channels and become semen-producing tubules until near the time of puberty.
The ovaries organize somewhat tardily by
differentiating an outer portion, the cortex, and a central portion, the
medulla. The cortex contains the primordial sex cells; these become
surrounded by a layer of ordinary cells, thereby forming primary ovarian
follicles. Both the testes and the ovaries undergo relative shifts from
their early sites to lower positions in the body. But only the testes make a
bodily descent; this is into the scrotum.
In the male, a few mesonephric tubules on each side
do not degenerate but link up with the neighbouring testis tubules. The
converted mesonephric tubules and the retained mesonephric ducts become the
male sex ducts. Near their terminations they outpouch seminal vesicles and
then open into the urethra. In the female, a pair of ducts develops from the
epithelium clothing the mesonephric ridges. These ducts, known as the
uterine tubes, mostly parallel the courses of the mesonephric ducts, but at
their lower ends they unite into a common tube that becomes the uterus and
vagina.
Both sexes develop a genital tubercle (i.e., a knob)
and a pair of urogenital folds flanked by a pair of genital swellings. At
three months these rudiments begin to assume male or female characteristics.
In the male, the tubercle and the united urogenital folds combine as the
penis, thereby continuing the urethra to its end; the genital swellings
shift toward the anus, fuse, and become the scrotum. In the female, the
tubercle remains small, as the clitoris; it does not contain the urethra.
The urogenital folds remain unclosed as the lesser vulvar lips and are
flanked by the unshifted and unfused genital swellings, or greater lips.
The lateral mesoderm, beyond the somites and
nephrotomes, splits into two layers: the somatic layer and, underlying the
somatic layer, the splanchnic layer. The intervening space is the coelom. As
the embryo's body folds off, its coelom becomes a single closed cavity. In
it can be recognized, regionally, a provisional pericardial cavity (cavity
for the heart), two pleural canals (for the lungs), and a peritoneal cavity
(for the abdominal contents). A thick plate of mesoderm, the transverse
septum, constitutes a partial partition just ahead of the developing liver.
Two pairs of membranes grow out from the septum. One set separates the
pericardial cavity from the two pleural cavities; these membranes later
expand into the pericardium and enclose the heart. The other pair of
membranes separates the pleural cavities from the peritoneal cavity of the
abdomen. The definitive diaphragm is a composite partition, much of which is
furnished by the transverse septum; lesser contributions are from the
lateral body walls and the paired membranes that separated the pleural and
peritoneal cavities.
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