Human Life Cycle 2 – Foetal Adaptations and the Endometrial Cervix
Anil Chopra
1 Outline the nature of the neural crest and its contribution to human development.
2 Outline the origin and fates of the pharyngeal arches and pouches.
3 Describe and explain the changes in organisation of the endometrium through the
menstrual cycle making correct use of the terms proliferative, secretory and
menstrual phases
4 Relate these changes to the endocrine changes of the ovarian cycle
5 Draw a simple, labelled sketch of the relationship between the uterine body, the
cervix and the upper vagina
6 Define and explain the two different types of epithelium found within the cervical
canal and on the vaginal aspect of the cervix
7 Explain the importance of cervical mucus, including the changes in its properties
during the menstrual cycle
Development of Systems
Between the end of the 3rd week and the 3rd month, organogenesis occurs. Pattern
development is also occurring so that minor errors in expression of genes controlling
development can lead to maldevelopment ranging from the trivial to errors that are
devastating and may be incompatible with life.
The actual development of systems occurs between the 3rd month and birth.
Making the Head and neck
When the head is folded, the only
recognisable head components are:
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The brain, which overhangs the
Foregut tube, which is separated from
the outside world by the
Buccopharyngeal plate
The Neural Crest
Neural crest folds to form neural tube:
neurulation – induced by the underlying
notochord (which eventually forms the CNS).
It develops from the ectoderm lateral to the
neural plate. It becomes buried during
neurulation but is not incorporated into the
neural tube and migrates to form multiple
neural and non-neural tissues in the body.
Also forms:
- Sensory neurones
- Postganglionic autonomic neurones
- Adrenal medulla
- Schwann Cells
- Enteric nervous system.
- Melanocytes
- Choroid and sclera of the eyes.
- Bulba ridges producing the septa of
the heart.
- Some bones of the skull
- Teeth
- Branchial arch mesenchyme – the
neural crest cells migrate to the
pharyngeal region where they form the jaws, auditory ossicles, the hyoid bone,
the laryngeal cartilages and the muscles and nerves that supply them:
1st Arch:
- jaws
- malleus
- incus
- muscles of mastication
- mandibular division of trigeminal
nerve (V)
nd
2 Arch:
- hyoid
- styloid
- stapes
- facial and hyoid muscles
- facial nerve (VII)
3rd Arch:
- laryngeal cartilage
- carotid artery
- some pharyngeal and laryngeal muscles
- glossopharyngeal nerve (IX)
4th Arch:
- aortic arch
- subclavian arteries
- some pharyngeal and laryngeal muscles
- Vagus nerve (X)
th
5 Arch:
- some parts of vagus nerve (X)
th
6 Arch:
- pulmonary arteries
- ductus arteriosus
- some parts of vagus nerve (X)
The pharyngeal pouches between the arches give
rise to
• The middle ear
• The tonsils
• The parathyroid glands
• The thymus
The pharyngeal floor
buds downward to
produce the thyroid
gland.
Endometrial Organisation through the Menstrual Cycle
The lining of the endometrium is made up of simple (one
cell thick) columnar epithelium that contain the uterine
glands. Between the glands is a stroma of loose
connective tissue that contains the essential service such
as the microcirculation and sensory nerve endings.
The bulk of the uterus is the myometrium, made up of
layers of smooth muscle that form most of the total
thickness of the uterine wall. The myometrium is covered
by visceral peritoneum.
The endometrium is supplied by two main types of artery
Spiral Arteries: supply the parts of the endometrium that
are shed during menstruation – the functional layer – so
the spiral arteries have to regenerate each cycle.
Straight arteries: supply the basal, unshed part of the
endometrium – the basal layer
Hormonal and Endometrial Changes in
the Menstrual Cycle
1. Oestrogen-dominated
Proliferative Phase
- this occurs after menstruation
- the endometrium is shed apart
from a thin layer of stroma
containing the basal parts of the
endometrial glands
- oestrogen levels begin to rise
- The glandular epithelium
proliferates to re-cover the bare
stromal surface
- Stroma and epithelium
proliferate to increase gland length and endometrial thickness, which reaches
about 3 mm
- The glands are long, but straight and regular and the epithelial cells are rich in
stored glycogen.
- Spiral arteries lengthen and extend into the stroma
- Ovulation occurs at the end of the proliferative phase
2. Progesterone-dominated Secretory Phase
- Increasing oedema in the endometrium, which now reaches a thickness of 5-6
mm. Further gland lengthening causes the glands to take on a corkscrew
shape.
- The epithelium begins to secrete a carbohydrate-rich fluid that will provide
nutrition should a conceptus arrive.
- The spiral arteries extend further into the endometrial stroma and become
more coiled.
- The stromal cells become capable of developing into decidual cells should
implantation occur
3. Menstrual Phase - triggered by decline in estrogen and progesterone support if
fertilisation and implantation do not occur  decline of corpus luteum  fall in
steroid hormone levels  spiral arteries constrict for period of several hours with
increasing frequency, leading to:
 Cessation of uterine glandular secretion
 Waning of oedema resulting in shrinking of the endometrium
 Ischaemic breakdown of the outer 2/3 of the endometrium (functional layer)
and its blood vessels, with bleeding and desquamation of the blood vessels
 Only the basal part of the endometrium is left (1mm thick), containing stroma,
gland bases and the torn ends of arteries and veins. Clotting of blood is
inhibited for several days, but blood loss is restricted by contraction of the
arteries with only brief periods of relaxation.
 With rising oestrogen secretion from the ovaries, menstrual bleeding ends, and
endometrial repair and growth begin.
Structure and Function of the Cervix
The uterus consists of a body (or corpus) and a cervix (= neck). The part of the body
lying superior to the entries of the uterine tubes is termed the fundus (= deep). The
actual cervix is only 25mm long and projects onto the most superior part of the vagina
– the vaginal and cervical axes are almost at a right angle.
Posterior Fornix: the deepest region of the vaginal wall passing into a crevice behind
the opening of the cervix.
The uterine interior is guarded by a musculo fibrous valve which opens to allow
menstrual flow and entry of sperm around the time of ovulation. The valve also needs
to me appropriately dilated during parturition. It contains fibrous cervical ligaments
attached to the pelvis preventing prolapse.
Cervical Canal: the lumen of the cervix. It opens onto the vagina via the “external
os” and into the uterus via the “internal os”. The smooth muscle coat of the cervix is
thick and sphincter-like and has very limited elasticity because of bundles of collagen
fibres inter-woven with the muscle. During the latter part of pregnancy, this collagen
is progressively broken down do that the cervix can be dilated during birth.
Cervical Mucosa: the lining of the cervical canal is around 2-3mm thick. Whilst it
has mucous secreting glands, it does not have spiral arteries and is not shed during the
menstrual cycle.
Transition Zone: the sharp change between the mucous secreteing cells of the
cervical lining and the opening of the cervix onto the vagina, lined by nonkeratinising stratified squamous epithelium. The position of the transition zone is
further toward the vagina during reproductive age and is further back in to the cervix
during early adolescence and post-menopause. It is thought that the metaplastic
changes that occur in the cervix are the basis of tumour growth.
1. Endocolumnar cervical epithelium entirely within cervical canal
2. expansion of columnar epithelium to area surrounding the external os.
3. transformation of exocervical columnar epithelium to a stratified squamous type.
Functions of the Cervix
• Remains closed during pregnancy but functions as part of birth canal. During the
latter part of pregnancy the collagen in the smooth muscle of the cervical wall is
progressively broken down so that the cervix can be dilated during birth. This
occurs as a result of the hormone changes.
• Microbial barrier but can be passed by sperm and menstrual flow. Canal secretes
mucus that varies in consistency throughout the menstrual cycle: it easily
penetrated by spermatozoa around the time of ovulation, whereas for the rest of
the cycle the mucus is very viscous and effectively forms a plug in the cervical
canal.
• Needs to be adequately dilated for parturition to proceed
• Fibrous cervical ligaments attach to pelvis preventing prolapse.