Endocrine Embryology Flashcards
A few general features
a. Most endocrine organs contain cells that derive from more than one embryonic origin.
b. Quite generally, because endocrine glands secrete into the blood, they are vascularized with capillary networks that arise from infiltration of mesodermal cells around groups/clumps of endocrine cells that often come from different origins.
i. different cell types that were pierced by mesoderm (which became the eventual blood vessels to the endocrine organ)
c. The endocrine cells typically develop from early primordia as solid groups of cells that break up, often initially into cords and then smaller collective groups, or get arranged in layers infiltrated with vasculature from local mesoderm.
d. Consequently, all endocrine organs will have a mesodermal component that gives rise to the vasculature, but not all will arise from endoderm, for example.
Development of the pituitary gland
a. Early in development, the neural tube and, in the head region, the primitive brain, arise from ectoderm through an infolding that pinches off is then referred to as “neural ectoderm”—and will become our brain and spinal cord.
b. Endoderm, a tubular arrangement of cells initially, gives rise to the majority of the digestive system.
c. However, early in development of the mouth, ectoderm comprises the stomodeum (the mouth opening) and gives rise eventually, in the upper part of the mouth, to the tissues as far back as the pharynx (the pharynx is endodermal in origin and fuses with the ectoderm of the stomodeum).
d. In the lower part of the mouth, ectoderm gives rise to tissues about half-way along the length of the tongue.
e. At about 4 weeks, an evagination of the lower part of the neural ectoderm of the primitive diencephalon forms the beginnings of the posterior pituitary and central portion of the infundibular stalk.
Where and from what tissue does the Posterior Pituitary rise from
4th week—> lower portion of neural ectoderm
At about 4 weeks, an evagination of the lower part of the neural ectoderm of the primitive diencephalon forms the beginnings of the posterior pituitary and central portion of the infundibular stalk.
At the 4th week, the neural ectoderm will start to give rise to the Posterior Pituitary… The anterior pituitary will also develop
a. At the same time, oral ectoderm from the upper part of the mouth evaginates (called Rathke’s pouch or pocket) and comes in contact with neural ectoderm,
b. The anterior pituitary develops from the original oral ectoderm, which pinches off from the oral epithelium, with resultant degeneration of the former “stalk” that connected the anterior pituitary with the epithelium of the mouth.
c. During the third month, the pituitary takes on the more typical shape of the pituitary gland in the adult .
d. The pars distalis, pars intermedia and pars tuberalis are all derived from Rathke’s pouch whereas the posterior (pars nervosa and infundibular stalk) are derived from neural ectoderm.
e. Connective tissue develops around the pituitary forming the sella turcica, the C.T. pocket that is part of the sphenoid bone in an adult.
f. Local mesoderm infiltrates the glandular/nervous tissues of the pituitary giving rise to the vasculature of the hypophyseal portal system as well as small vessels in the posterior pituitary.
i. Consequently, the posterior pituitary can really be thought of as an extension of the brain, where neurons are secreting hormones that are utilized systemically
ii. whereas the anterior pituitary is really an organ derived from oral ectoderm that responds, like other organs, to blood-borne releasing factors that arrive via the blood (via the hypophyseal portal system).
g. The different cell types of the anterior pituitary that are hormone-secreting all differentiate from cells derived from Rathke’s pouch
During the third month, the pituitary takes on the more typical shape of the pituitary gland in the adult .
a. The pars distalis, pars intermedia and pars tuberalis are all derived from Rathke’s pouch (anterior pituitary all develops from this pouch)
i. whereas the posterior pituitary (pars nervosa and infundibular stalk) are derived from neural ectoderm.
b. Connective tissue develops around the pituitary forming the sella turcica, the C.T. pocket that is part of the sphenoid bone in an adult.
c. Local mesoderm infiltrates the glandular/nervous tissues of the pituitary giving rise to the vasculature of the hypophyseal portal system as well as small vessels in the posterior pituitary.
d. Consequently, the posterior pituitary can really be thought of as an extension of the brain, where neurons are secreting hormones that are utilized systemically
i. whereas the anterior pituitary is really an organ derived from oral ectoderm that responds, like other organs, to blood-borne releasing factors that arrive via the blood (via the hypophyseal portal system).
e. The different cell types of the anterior pituitary that are hormone-secreting all differentiate from cells derived from Rathke’s pouch
Development of the thyroid/parathyroid glands
Introduction
a. The thyroid contains cellular components that derive from the endoderm (the thyroid follicle epithelial cells), the neural crest (originally ectodermal, the calcitonin-secreting cells) and the vasculature (mesodermal).
b. The parathyroids originate from endoderm (glandular cells) and mesoderm (vasculature), and become embedded in the thyroid, but originate from different locations along the developing pharynx.
c. It is important to point out that early in development, the pharynx develops in a complex way as a set of four bilateral pouches
Pouches in Thyroid and Parathyroid Development
a. There are four recognized pouches (1-4) arranged as indicated in lateral and ventral views of the pharynx (endoderm)
b. The thyroid begins as a medial evagination of the endoderm called the thyroid diverticulum, which begins to extend at about the region between the first and second pharyngeal pouches.
c. Note in the figure at the right that components of the parathyroid develop from cells in clefts between the 3rd and 4th pouches (inferior parathyroids) and in a cleft after the 4th pouch (superior parathyroids).
d. Another relevant group of cells early in development just after the 4th pouch (shown in the figure at the right) is the ultimobranchial body.
i. It gets populated by cells derived from the neural crest (ectodermal in origin) and those cells give rise to the calcitonin-secreting or parafollicular cells of the thyroid.
e. Other groups of cells among the pouch clefts, although not related to the endocrine system, give rise to the thymus, the tonsils, and the auditory tube, so this region of the pharyngeal pouches is relevant to development of a number of important organs.
thyroid diverticulum
a. The thyroid begins as a medial evagination of the endoderm called the thyroid diverticulum, which begins to extend at about the region between the first and second pharyngeal pouches.
b. The thyroid diverticulum enlarges and descends along the pharynx with the developing thyroid initially attached via the thyroglossal duct
c. The thyroid gland descends to a region under the larynx, just in front of the trachea.
d. Normally, the thyroglossal duct degenerates but pediatric cases of thyroglossal cysts occur, where portions of the thyroglossal duct may remain and become cystic, or rarely there may be a failure of the thyroid to descend properly (ectopic thyroid).
i. In about half of individuals, a lower portion of the thyroglossal duct develops into a medial pyramidal lobe of the thyroid gland.
Inferior Parathyroids and Superior Parathyroids
Components of the parathyroid develop from cells in clefts between the 3rd and 4th pouches (inferior parathyroids) and in a cleft after the 4th pouch (superior parathyroids)
The Descent of the Thyroid
a. The thyroid diverticulum enlarges and descends along the pharynx with the developing thyroid initially attached via the thyroglossal duct
b. The thyroid gland descends to a region under the larynx, just in front of the trachea.
c. Normally, the thyroglossal duct degenerates but pediatric cases of thyroglossal cysts occur, where portions of the thyroglossal duct may remain and become cystic, or rarely there may be a failure of the thyroid to descend properly (ectopic thyroid).
d. Note that during the descent of the thyroid, it comes in contact with the primordia of the parathyroids that arise from the clefts between the 3rd and 4th pharyngeal pouches as well as the ultimobranchial body.
i. These become embedded in the thyroid normally and result in the parathyroids and the calcitonin-releasing cells, respectively.
The Thyroid Gland and Mesoderm
a. The thyroid becomes infiltrated with mesoderm that gives rise to the vasculature.
b. Initially, the thyroid is a solid mass of endodermal cells but they get interspersed by the infiltrating mesoderm, first to cords of cells that will become the follicular cells, then to small clumps, and then the clumps vesiculate to form the epithelia of the follicles around the follicular colloid.
c. Meanwhile, the mesoderm differentiates into the vasculature that encircles each follicle.
Development of the adrenal glands.
a. The adrenal glands are comprised of a cortex and medulla.
i. The cortex originates from mesoderm whereas the medulla is of ectodermal origin.
b. The initial primordium of the cortex originates from a group of cells of the coelomic epithelium (mesothelium) in a cleft between the region of the gut (actually the dorsal mesentery, a tissue that connects the gut to the dorsal portion of the embryo) and the urogenital ridge (also called the Wolffian body),
i. recall your kidney embryology of the formation of the mesonephric duct from initial nephrotomes.
Development of the Adrenal Gland
a. The initial primordium of the cortex originates from a group of cells of the coelomic epithelium (mesothelium) in a cleft between the region of the gut and the urogenital ridge (also called the Wolffian body),
b. At about 4 weeks, under induction by the mesonephric (or Wolffian duct), cells in the cleft proliferate and migrate into the mesenchyme just dorsal to it.
i. They multiply and organize into a cup-like structure in roughly the same shape as the cortex of the adrenal.
c. Between 2-3 months, another wave of cells from the coelomic epithelium at the same location enters the mesenchyme and surrounds the outer part of the cup, in another layer.
d. The first early set of cells will become cells of the reticularis of the cortex, whereas the second group of cells that layer on the outside will differentiate into the fasciculata and the glomerulosa.
e. Therefore, the cortical region of the adrenal is entirely mesodermal in origin.
Development of Medullary cells in the Adrenal Gland
a. Cells in the medullary region come from an entirely different origin.
b. Neural crest cells (ectoderm) migrate to a region that will become the sympathetic ganglia.
c. Early on, these cells are called sympathogonia and some of them continue to migrate into the center of the cup that will become the medullary region of the adrenal
d. They happen to stain yellow- brown with chrome salts hence are called chromaffin cells, and are the progenitors of the epinephrine- and norepinephrine-producing cells of the medulla.
Development of the adrenal continues after birth
a. Development of the adrenal continues after birth, with the layers of the fasciculata and glomerulosa becoming more distinct, the reticularis regressing somewhat and the medulla increasing in relative volume.
b. Vascularization occurs through infiltration of small vessels of mesodermal origin during development with blood flow from vessels entering the outer cortex and blood collecting toward and leaving via the central vein of the medulla.
