Embryology Flashcards
Fertilisation
Fertilisation takes place between a male and female
gamete, each containing 23 chromosomes, and their two nuclei coalesce to form a single nucleus containing the usual complement of 46 chromosomes, which then is called the zygote.
A series of mitotic divisions then occurs which, through growth and differentiation, eventually leads to the formation of the embryo.
First week
During the first week, the zygote divides and
becomes the morula. Spaces develop within these cells and forms the blastocyst. This blastocyst continues to divide and develop, and undergoes implantation within the maternal uterine wall. As this occurs, there is differentiation into two distinct embryonic layers: the bilaminar embryonic disc.
The outer layer of one side of this disc forms the amniotic and yolk sacs which connect the embryonic disc to the uterus – and will become
the umbilical cord through which nutrients and oxygen are delivered to the developing embryo.
Six weeks
The next six weeks sees the most rapid period of
development of this embryonic disc into the true
embryo.
The bilaminar disc is converted into a trilaminar disc within the third week by the primitive streak which develops within the embryonic disc and becomes the mesoderm.
Trilaminar disc
The trilaminar disc has the three germ layers of
ectoderm, mesoderm and endoderm – these three
layers giving rise to the tissues and organs of the
embryo.
The embryonic ectoderm gives rise to the epidermis, nervous system, sensory epithelium of eye, ear and nose, and skin.
The embryonic mesoderm becomes muscle, connective tissue, bone and blood vessels. The embryonic endoderm forms the linings of the digestive and respiratory tract.
Primitive streak
From this primitive streak (or mesenchymal or mesodermal area) of the embryonic disc, cells migrate cranially and caudally as the notochord from the mouth to the cloaca.
The notochord provides some rigidity, and the vertebral column develops.
The embryonic ectoderm overlying this notochord thickens to form the neural plate which will subsequently develop into the brain, spinal cord and nerves and the neural crest.
notochord and neural tube formation
As the notochord and neural tube form, the adjacent
mesoderm forms longitudinal columns called
paraxial mesoderm which divide into paired cuboidal
bodies called somites. The first pair of somites develop at the cranial end, and subsequent pairs develop more caudally and develop into the vertebral column, ribs, sternum and skull and associated muscles. Lateral to this paraxial mesoderm is the mesoderm from which
coelomic spaces will develop into the three body cavities: pericardial, pleural and peritoneal cavities.
3rd-4th week
The primitive streak continues to form mesoderm
until the end of the fourth week, when it begins to
shrink and is sited at the sacrococcygeal region and should degenerate and disappear, but it may persist and develop into a sacrococcygeal teratoma – a tumour of neonates which is initially benign, but will become malignant if not removed at birth.
Further development
The embryo folds and converts the flat trilaminar
disc into a c-shaped cylindrical embryo.
The original endoderm has developed into the yolk
sac, and part of this is incorporated into the embryo
as the gut. As the cranial end of the embryo folds, it
takes the mouth and heart ventrally, and incorporates the adjacent yolk sac as the foregut, and the most cranial part of the embryo is then the brain as it develops from the neural plate.
As the caudal end of the embryo folds, the adjacent yolk sac is incorporated as the hindgut, and is carried ventrally as the cloaca, allantois and umbilical cord. The embryo also folds horizontally and incorporates part of the yolk sac as the midgut, which in these early stages is outside the
embryo, within the umbilical cord. The rest of the yolk sac remains attached to the midgut as a stalk.
Branchial arch development I
At the cranial end, six branchial arches develop in
pairs, with the ears developing between the fi rst (i.e. most cranial) and second arches. The limb buds are developing and grow into limbs with hands and feet,and the tail, which was prominent, has gone before the end of the eighth week.
And so, from an ovum and a spermatozoa, a miniature human being has developed. The various important surgical congenital anomalies which can affect the future baby will be discussed below.
Branchial arch development II
During embryological development, the six branchial
arches go through various stages of development and regression. They each originate from embryological mesenchyme and contain a core of cartilage, muscle, an artery and nerve supply from a cranial nerve.
The cartilage of the first arch develops into the malleus and incus (middle ear bones) and an associated ligament, the second into the stapes (third middle ear bone) and styloid and part of the hyoid bone, the third into the rest of the hyoid, and part of the fourth and sixth arch into the larynx, and most of the rest of the cartilage disappears. The muscles of the arches develop into facial muscles, each keeping their original nerve supply.
The arteries become paired aortic arches, but the only ones which remain are the third (carotids), fourth (right subclavian on right and aortic arch on left) and sixth (right pulmonary artey on right and left pulmonary artery plus ductus arteriosus on left – which with its nerve supply from the vagus, the tenth cranial nerve, explains how the recurrent laryngeal nerve loops under the ductus arteriosus on the left and under the subclavian on the right).
Branchial arches and their remnants
During embryological development, the six branchial
arches go through various stages of development and regression. They each originate from embryological mesenchyme and contain a core of cartilage, muscle, an artery and nerve supply from a cranial nerve.
Cartilage of branchial arches
The cartilage of the first arch develops into the malleus and incus (middle ear bones) and an associated ligament, the second into the stapes (third middle ear bone) and styloid and part of the hyoid bone, the third into the rest of the hyoid, and part of the fourth and sixth arch into the larynx, and most of the rest of the cartilage disappears.
Muscles of branchial arches
The muscles of the arches develop into facial muscles, each keeping their original nerve supply.
The arteries become paired aortic arches, but the only ones which remain are the third (carotids), fourth (right subclavian on right and aortic arch on left) and sixth (right pulmonary artey on right and left pulmonary artery plus ductus arteriosus on left – which with its nerve supply from the vagus, the
tenth cranial nerve, explains how the recurrent laryngeal nerve loops under the ductus arteriosus on the left and under the subclavian on the right).
Persistence or failure of complete regression of the
branchial arches gives rise to many congenital abnormalities which are present at birth.
First branchial arch anomalies
The first arch anomalies include cleft lip and palate. Upper preauricular sinuses and skin tags
(which invariably contain cartilage) are usually superficial and can easily be excised, but a low preauricular sinus may have a deep internal connection to the first arch – a surprising finding for the unsuspecting surgeon who follows a track and ends up within the middle ear – and close to the facial nerve, with its consequent
problems if damaged.
Second branchial arch anomalies
A branchial fistula arises from the second arch, from the anterior border of the bottom third of the sternomastoid muscle, and travelling up inside the neck to open in the tonsillar fossa in the pharynx. This fistula may present as a discharging dimple on the neck (as the fi stula is lined by mucus-secreting glands), and it needs to be excised (usually requiring two separate neck incisions) in its entirety from the lower neck up to the pharynx to prevent continuous discharge, infection, or the rare possibility of subsequent malignant transformation.
This second arch remnant may also present as a skin or cartilaginous tag at the site of the dimple.
A branchial cyst also originates from a remnant of
the second arch without external connection, and contains the glairy fl uid containing cholesterol crystals which typifi es the mucus-secreting glands within the cyst. It should be excised before it gets infected.
