periods of development
Here is a pictorial representation of the main
landmarks in human prenatal development:
Day 0 - Development of the new
individual begins when a spermatozoon penetrates the jellylike zona
pellucida and contacts the plasma membrane of the secondary oocyte.
Note the smaller first polar body alongside the secondary oocyte
within the zona pellucida. This was formed during the first meiotic
division. Several follicular cells are shown still attached to the
outer surface of the zona pellucida. After the arrival of the
fertilising sperm, no further spermatozoa
are allowed to enter.
In response to the fertilising sperm,
the oocyte completes its second meiotic division. The resulting cell
division is unequal - the larger daughter cell becomes an ovum, and
the other much smaller daughter cell becomes the second polar body.
While the oocyte is dividing, the first polar body may also divide. The
head of the spermatozoon enters the cytoplasm of the oocyte and
swells, forming the male pronucleus. (The mid-piece and tail of the
spermatozoon do not enter the ovum.)
- After completing its second meiotic division, the nucleus of the
larger ovum becomes the female pronucleus. The male pronucleus
swells, and the two pronuclei approach each
other and merge. This establishes the diploid genome and also the genetic sex of the new
individual. The fertilised ovum is called a zygote.
nuclear DNA is replicated and after several hours the zygote begins
its first mitotic cell division. This illustration shows metaphase,
with the chromosomal pairs arranged at the equator of the mitotic
spindle ready to be separated and moved to opposite ends of the
Day 1 - The
two-cell stage. The cytoplasm of the original zygote has been
subdivided to form two smaller cells - there is little synthesis of
new cytoplasmic material at this time. This also applies to
subsequent cell divisions during the first few days, so they are
often called ‘cleavage divisions’. The cells remain enclosed by the
zona pellucida. The polar bodies are less prominent, and probably
cleavage continues, a closely-packed ball of cells is produced - the
morula. Each one of these cells is still very high in developmental
potential, and could each give rise to a new individual, although
usually they will co-operate in the development of just one baby.
Day 4 - The
morula drifts along the fallopian tube as cleavage continues. When
it reaches the uterus, the zona pellucida dissolves
and fragments, releasing the morula.
Fluid is drawn
into the centre of the morula, creating a hollow blastocyst.
Within the hollow, fluid-filled
blastocyst there is a special groups of cells called the inner cell
mass. The cells forming the outer surface of the blastocyst are
called trophoblast cells.
Day 7 - the blastocyst makes contact with the lining
of the uterus - the endometrium. This has prepared for such a
possibility by thickening, becoming more glandular, and developing a
rich blood supply. The blastocyst begins to implant, a process that
will take several days.
Implantation brings the outer cells of the blastocyst into direct
contact with the maternal cells of the endometrium. The conceptus is
genetically different from the mother since half its chromosomes
have come from the father. Usually, genetically different cells
would be rejected by the mother’s immune system, but this does not
occur during normal pregnancy.
implantation, the trophoblast thickens and forms two layers - an
outer syncytiotrophoblast (grey) and an inner cytotrophoblast. Meanwhile,
the inner cell mass becomes organised into a two-layered plate of
cells called the embryonic disc, with the amniotic cavity above and
the yolk sac below. The two layers are called ectoderm and endoderm.
conceptus implants itself completely within the endometrium. The
enlarging syncytiotrophoblast develops fluid-filled lacunae within
it and comes into contact with the maternal blood vessels.
Extra-embryonic mesodermal cells derived from the cytotrophoblast
form a layer around the external surfaces of the amnion and yolk
embryonic disc will give rise to the baby itself. The first sign of
the midline axis of the baby is the formation of the primitive
streak in the ectodermal layer facing the amniotic cavity.
In this diagram, part of the embryonic disc has been removed to show
the primitive streak in cross-section.
Ectodermal cells migrate towards the primitive streak and then tuck
inwards to form a new layer of cells in between the ectoderm and
endoderm. The new layer is called the mesoderm. These three layers
of cells - ectoderm, mesoderm, and endoderm -then give rise to all parts of the body by a process called
following pictures come from: “Life before birth”, published
in 1979 by the British Museum (Natural History) and Cambridge
University Press. Illustrations by John Bavosi. These will be
replaced by new artwork as it becomes available.
primitive streak is situated in the midline towards the tail end of
the future embryo. Further towards the future head end, the ectoderm
thickens to form the neural plate. This begins to fold, initially
forming a groove and then closing over to form the neural tube. The
neural tube will give rise to the brain and spinal cord.
Closure of the neural tube begins in the future hindbrain region and
then continues for several more days, extending both forwards and
caudally until closure is complete. Clusters of mesodermal cells
produce pairs of somites alongside the developing neural tube. The
number of somites increases steadily as neural closure continues.
In the future brain region, the
neural folds enlarge as they close and begin to overshadow the part
of the embryonic disc that lies further forwards. Within the
mesoderm of this part the heart begins to develop.
heart develops, blood vessels form in the wall the yolk sac, the
embryo itself, and in the stalk that connects the embryo to the
trophoblast. Blood cells are also formed in the yolk sac wall. On
day 23 after fertilisation, the heart begins to beat and the
circulation of blood is established.
embryo develops, the amniotic sac enlarges and gradually the yolk
sac begins to diminish in relative size. The surrounding
trophoblastic tissue develops finger-like processes which extend
outwards into the endometrium. These extensions are best-developed
across the most deeply-implanted part of the conceptus, and will
contribute to formation of the placenta.
buds begin to form alongside the neural tube and somites. The
developing eyes are visible, and the beating heart is now tucked
ventrally in the future thoracic region.
fingers and toes are becoming apparent, and the external ear is
visible on the side of the neck region. The connection between the
embryo and surrounding trophoblast is becoming the umbilical cord,
and passing through this is a narrow duct connecting the yolk sac
with the developing digestive tract in the embryo.
end of the second month, all the different parts of the new
individual have formed. Morphogenesis is complete. However, the
embryo is only about 3 cms long from the top of its head to its
rump. Most of the body organs and systems are only partially
third month, the fetal period begins. It is a time of rapid growth
in size and weight, the further maturation of function in organs and
body systems, and the rehearsal of increasingly complex activities
that must be perfected before birth - breathing movements,
swallowing, production of urine, and digestion, for example.
uterus and placenta continue to enlarge to accommodate the growing
fetus and meet its increasing needs.
Towards the end of pregnancy, space is at a premium, and the
placenta is finding it increasingly difficult to meet the needs of
Approximately 9 months after conception, the process of birth
begins. A difficult transition must be achieved with the baby’s
systems taking over many of the responsibilities that were met
previousy by the placenta and mother.
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