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Endocrine System


Endocrine System
Time period: day 24 to birth
Introduction
The glands of the endocrine system begin to form during the embryonic period and continue to mature during the foetal period. Functional development can be detected by the presence of the various hormones in the foetal blood, generally in the second trimester of pregnancy.

The development of the gonads, pancreas, kidneys and placenta are covered elsewhere in this book.
Endocrine System, Pituitary gland, Hypothalamus, Pineal body, Adrenal glands, Thyroid gland, Parathyroid glands,

Pituitary gland
Also known as the hypophysis, the pituitary gland develops from two sources. An out pocketing of oral ectoderm appears in week 3 in front of the buccopharyngeal membrane (Figure 39.1). This forms the hypophysial diverticulum (or Rathke’s pouch), which will become the anterior lobe.
The second source is an extension of neuroectoderm from the diencephalon, called the neurohypophysial diverticulum (or infundibulum). The infundibulum grows downwards, developing into the posterior lobe. These two parts grow towards one another and by the second month the hypophysial diverticulum is isolated from its ectodermal origin and lies close to the infundibulum.
Growth hormone secreted by the pituitary gland can be detected from 10 weeks.
Hypothalamus
The hypothalamus begins to form in the walls of the diencephalon (see Chapter 44), with nuclei developing here that will be involved in endocrine activities and homeostasis.
Pineal body
The pineal body first appears as a diverticulum in the caudal part of the roof of the diencephalon. It becomes a solid organ as the cells here proliferate.
Adrenal glands
The adrenal (or suprarenal) glands develop from two cell types. The cells of the cortex differentiate from mesoderm of the posterior abdominal wall near the site of the developing gonad (Figure 39.2). The adrenaline and noradrenaline secreting cells of the medulla are derived from migrating neural crest cells that formed a sympathetic ganglion nearby. These cells become surrounded by the cell mass of the cortex.
The foetal cortex produces a steroid precursor of oestrogen that is converted to oestrogen by the placenta. More mesenchymal cells surround the foetal cortex and will become the layers of the permanent cortex.
The adrenal glands are exceptionally large in the foetus because of the size of the cortex which regresses after birth. Substances secreted from the adrenal glands are involved in the maturation of other systems of the embryo, such as the lungs and reproductive organs.
Thyroid gland
This is the first endocrine gland to develop, beginning at about 24 days between the first and second pharyngeal pouches from a proliferation of endodermal cells of the gut tube. It begins as a hollow thickening of the midline where the future tongue will develop. It eventually becomes solid and then splits into its two lobes.
As the thyroid descends into the neck it remains connected to the tongue via the thyroglossal duct with an opening on the tongue called the foramen cecum. The duct degenerates between weeks 7 and 10 and the thyroid reaches its end location anterior to the trachea by week 7. If parts of the duct remain the person may also have a pyramidal lobe. This is quite common and seen in about 50% of the population.
C cells (or parafollicular cells) are derived from neural crest cells that invade the ultimobranchial body (a fifth pharyngeal pouch derivative; see Chapter 43).
Parathyroid glands
The inferior parathyroid glands develop from epithelium (endoderm) of the dorsal wing of the third pharyngeal pouch. The cells here move with the migration of the thymus gland into the neck (see Chapter 42). When this connection breaks down they become located on the dorsal surface of the thyroid gland.
Endoderm cells of the dorsal wing of the fourth pharyngeal arch begin to collect and differentiate to form the superior parathyroid glands (initially the superior parathyroid glands are inferior to the inferior parathyroid glands). These cells are associated with the developing thyroid gland and migrate with it, but for a shorter distance than the cells of the inferior parathyroid glands (see Chapter 43). They also rest on the dorsal surface of the thyroid, but generally more medially and posteriorly.

Clinical relevance
Pituitary gland
Congenital hypopituitarism is a decrease in the amount of one or more of the hormones secreted by the pituitary gland. Symptoms are wide ranging, depending upon which hormones are affected. The cause is often hypoplasia of the gland or complications with delivery. Treatment is commonly oral or injection replacement of the insufficient hormones.
Adrenal glands
Congenital adrenal hyperplasia is an autosomal recessive disease causing excessive production of steroids, with 95% of patients deficient in the enzyme 21‐hydroxylase (required in the production of adrenal secretions). There are degrees of severity and this can cause ambiguous genitalia and infertility. Various treatment options are available and can include glucocorticoids, sex hormone replacement and genital reconstructive surgery.
Thyroid gland
Congenital hypothyroidism is a deficiency in thyroid hormone production. Symptoms include excessive sleeping and poor feeding. Newborn infants are screened for this and if this deficiency is found treatment is a daily thyroxine tablet.
Ectopic thyroid tissue left behind during migration is relatively common but asymptomatic. Parts of the thyroglossal duct may persist and form a midline, moveable cyst in a child.
Parathyroid glands
Hypoparathyroidism is an absence of parathyroid hormone. Symptoms are wide ranging but often not diagnosed until 2 years of age. They include seizures and poor growth. Treatment includes vitamin D and calcium supplements.
Ectopic parathyroid tissue left behind during migration is relatively common but asymptomatic more common for the inferior parathyroid glands.