|Human Endocrine - Embryonic Timeline
|Data based on data from O'Rahilly (1983).
- Pineal (Epiphysis) | Pituitary (Hypophysis) | Thyroid | Parathyroid | Thymus | Pancreas | Adrenal (Suprarenal)
|Stage 13 (week 4)
- Pituitary - basement membranes of the craniopharyngeal pouch and the brain are clearly in contact.
- Thymus - Weller (1933) recognized already a thymic primordium "of considerable size" on the ventral part of the third pharyngeal pouch, whereas Norris (1938) considered this stage to be "preprimordial"
- Thyroid - median thyroid is now bilobed and is connected to the pharynx by a hollow pedicle. The telopharyngeal body has been regarded as a "lateral thyroid component" by some workers.
- Pancreas - ventral pancreas may perhaps be distinguishable.
|Stage 14 (week 5)
- Pineal - a slight irregularity in the surface outline of the intact head corresponds to the future pineal body (O'Rahilly et al. 1982).
- Pituitary - craniopharyngeal pouch is prominent and the notochord appears to be inserted into its dorsal wall. The craniopharyngeal pouch has become elongated and blood vessels are beginning to grow in between the basement membranes of the pouch and brain.
- Thyroid - thyroid pedicle shows further elongation but is still connected to the epithelium of the pharynx. Right and left lobes and an isthmus may perhaps be presaged (ibid.).
- Parathyroids - "Parathyrogenic zones"  are recognizable. The parathyroid 4 primordium has been illustrated at this stage by Weller (1933, Fig. 16).
- Thymus - Weller's (1933) "thymus" (the third pharyngeal pouch) becomes elongated.
- Pancreas - ventral pancreas (which may perhaps be distinguishable as early as stage 13) appears as an evagination from the bile duct at stages 14 and 15. It is generally described as unpaired but, at least in some cases, may perhaps be bilobed or even multiple (Delmas 1939).
- Adrenal Cortex - A change in the characteristics of the cells of the coelomic epithelium appears between the mesogastrium and the lateral end of the mesonephros.
- Adrenal Medulla - paravertebral sympathetic ganglia increase in size as a result of cell division and the addition of nerve fibres from the rami communicantes. The ganglia contain three types of cells: MI, M2, and M3. The M3 cells are the" parasympathetic cells" of Zuckerkandl.
- Pineal - pineal body is detectable in the roof of the diencephalon (Stadium I of Turkewitsch)
- Thyroid - thyroid primordium may be detached from the pharyngeal epithelium in some instances. "At about the time" when the thyroglossal duct "becomes broken it loses its lumen".
- Adrenal Cortex - primordium is first recognizable. A new type of cell (C1) from the coelomic epithelium is found in the subjacent mesenchyme. New cells (C2) appear in the medial wall of mesonephric glomeruli and begin to migrate into the suprarenal primordium. denies a mesonephric contribution to the suprarenal.
- Adrenal Medulla - all types of cells (M1, M2, and M3) increase in number. From stage 15 to stage 18, the suprarenal primordium is cigar-shaped and extends from segment T6 to segment L1, lateral to the aorta and mesogastrium.
|Stage 16 (week 6)
- Pituitary - slight indication of the infundibular recess may be seen in some embryos.
- Pineal - cellular migration in an external direction occurs in the pineal body during stages 16 and 17 (Stadium 2 of Turkewitsch)
- Thymus - according to Norris (1938) , "not until the primordium of the parathyroid  has been outlined can the remaining portion of the third pouch be recognized, by exclusion, as the primordium of the endodermal thymus".
- Parathyroids - parathyrogenic zones are closely related to the third and fourth aortic arches at 9 mm unstaged embryo).  Parathyroid 3 is identifiable on the anterior wall of the third pharyngeal pouch (Weller 1933, Fig. 17) and "does not arise from a dorsal lobule" of the pouch. The "sudden appearance of well-differentiated clear chief cells in the early primordia of the parathyroids" at 9 mm.
- Thyroid - has lost its continuity with the pharynx and it consists of two lobes, an isthmus, and a remnant of the pedicle.
- Adrenal Cortex - Another type of cell (C3) arises from the coelomic epithelium. Both C1 and C3 cells enter the suprarenal primordium. An "enormous immigration" of C2 cells occurs.
- Adrenal Medulla - cells of neural origin are migrating into the gland, separating the cortical cells into islands. Nerve fibres from the ganglia accompany the M1 and M3 cells. The M2 cells remain in the ganglia and become sympathetic ganglion cells.
- Pancreas - dorsal pancreas and the ventral pancreas are contiguous.
- Pituitary - juxtacerebral wall of the craniopharyngeal pouch is the thicker. The lateral lobes (future infundibular, or tuberal, part) and the anterior chamber (Vorraum) are clearly visible.The infundibular recess displays a characteristically folded wall, namely the neurohypophysis.
- Thymus - connection of the thymus with the pharynx has been severed (Weller 1933). The thymus is intimately approximated to the cervical duct (ibid.) According to Norris (1937), both third and fourth pouches make contact with the ectoderm, although only the third "receives an increment from the ectoderm".
- Parathyroids -parathyroid 4 is attached to the lateral surface of what Weller (1933) termed the "lateral thyroid component"
- Thyroid. The lobes of the thyroid curve around the carotid arteries and are connected by a delicate isthmus. Lacunae "should not be confused with lumina of follicles".
- Adrenal Cortex - dorsal part of the whole suprarenal primordium is disorganized by the invasion of sympathetic nerves and cells, while the band of C2 cells and the coelomic epithelium remain intact (Crowder 1957).
- Adrenal Medulla - first neural migration is at its height. Growth of the para-aortic complex is extensive. The plexiform complex is derived from paravertebral sympathetic ganglia T6-12 and usually L 1. Included in it are the primordia of the suprarenal medulla and of the celiac, superior mesenteric, and renal plexuses. Nerve fibres and "paraganglion" (M3) cells enter.
- Pancreas - ventral pancreas has now fused with dorsal. Perhaps the ventral and dorsal ducts have begun to blend (Russu and Vaida 1959).
|Stage 18 (week 7)
- Pineal - cellular migration in the pineal body forms a distinct "anterior lobe" in which follicles appear (Stadium 3 of Turkewitsch))
- Thymus- thymus makes contact with the thyroid gland and contains a series of canals internally (Weller 1933).
- Thyroid - median thyroid is in contact with "lateral thyroid components"but others have maintained that the telopharyngeal body should not be regarded as a thyroid component (Bejdl and Politzer 1953). The lobes of the thyroid are "composed of series of continuously communicating solid annectent bars" this is "the earliest stage of the definitive thyroid". First differentiation occurs in Weller's (1933) "lateral thyroid component," which is beginning to "blend into uniformly constituted thyroid tissue". Weller (1933) illustrated (Fig. 11) a thyroid gland that still showed continuity between its pedicle and the epithelium of the pharynx.
- Adrenal Cortex - gland becomes reorganized. The C1, 2, and 3 cells form cords as sinusoids develop. Cells divide at or near the surface, where new cells are added.
- Pituitary - the caudal part of the craniopharyngeal pouch is reduced to a closed epithelial stem.
- Epiphysis - the "anterior lobe" of the pineal body shows a characteristic step and wedge appearance (Stadium 4 of Turkewitsch))
- Parathyroids - 3 become detached from the pharyngeal endoderm.
- Adrenal Cortex - C2 cells lie on the surface of the gland and form a "capsule".
- Adrenal Medulla - Sympathicoblasts penetrate the cortex at stages 19 and 20, and form scattered islets of medullary tissue throughout the cortex.
|Stage 20 (week 8)
- Pituitary - the adenohypophysial epithelium adjacent to the neurohy- pophysis constitutes the beginning pars intermedia. The walls of the craniopharyngeal pouch bud into the mesenchyme.
- Thymus - the right and left components are in contact with each other but are "never completely fused" ). Thymic cortex appears (in stages 20-22) as a result, according to Norris
(1938), of migration of and covering by "cells derived from the cervical sinus".
- Parathyroids - the parathyroid glands are attached to the lateral lobes of the thyroid (Weller 1933).
Weller (1933, Fig. 23) showed parathyroid 3 still rostral to parathyroid 4 at 23 mm, whereas (presumably due to variation in the "descent" of the thymus) (Norris 1937, Fig. 4) showed parathyroid 3 rostral to, level with, and caudal to parathyroid 4 in embryos of 16-17 mm.
- Thyroid - the "annectent bars" of the thyroid are more compact then previously. The thyroid now exhibits its definitive external form.
- Hypophysis - the pharyngeal stalk becomes fragmented.
- Adrenal Cortex - the cellular "capsule" becomes covered by a layer of fibrous tissue.
- Parathyroids - Parathyroids 4 become detached from the pharyngeal endoderm.
- Adrenal Cortex - the C2 cells have changed and resemble fibrocytes.
- Pituitary - adenohypophysis loss of the stalk and lobules of epithelium project into the mesodermal component of the gland, and oriented epithelial follicles are present (Streeter, 1951, plate 2). Abundant angioblasts and capillaries are found.
- Epiphysis - The pineal body has reached Stadium 5 of Turkewitsch
- Thymus - The cortex is well-developed, "true lobulation" has begun with the appearance of" fine superficial scallops," lymphocytes are present sparsely in the subcortical zone, and vessels are found within the thymus.
- Adrenal Cortex - It appears that C2 cells first enter the body of the gland at this stage. The pattern of the arterial supply is established. The cellular "capsule" is penetrated by arterial capillaries which join the sinusoids. Their points of entry give the surface of the gland an appearance of cobblestones. The zona glomerulosa is formed of CI and C3 cells. Cells from this zone and from the "capsule" migrate centrally into the cords.
- Adrenal Medulla - Nerve fibres and neuroblasts are first seen in the body of the gland. The paragangtion (M3) cells are beginning to multiply rapidly and, from 30 mm (stage 23) until birth, some are differentiating into chromaffin cells.