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A series of grafting experiments was conducted to determine pituitary origins prior to brain tube closure in Xenopus laevis. Extirpation experiments indicated that the ventral neural ridge (VNR) tissue of stage-18+ embryos was essential for pituitary development. Bolton-Hunter reagent was used to label stage-18+ VNR tissue with 125I, and this tissue was then returned to the donor and its subsequent ontogenesis followed. Labelled tissue was ultimately found in the ventralhypothalamus, the ventralretina, and the anterior pituitary. Using immunocytochemical techniques with antisera to adrenocorticotropin (ACTH), it was found that some of the VNR-derived cells were corticotropes. A region of the nucleus infundibularis which was radioactive labelled also gave ACTH-positive immunoreaction. This might indicate that some ACTH-containing neurones of the hypothalamus are VNR in origin. We suggest that stage-18+ VNR is the site of attachment of brain and anterior pituitaryectoderm. Part of this adherence point is eventually incorporated into the anterior pituitary and will form corticotropes. It is concluded that the ventralretina, the preoptic region of the hypothalamus, some hypothalamic ACTH-immunoreactive cells, and the most anterior portion of the adenohypophysis are all ventral neural ridge in origin.
Fig. 1. (A,B) Appearance of donor and host embryos shortly after removing ventral
ridge tissue from donor embryo and grafting it in host fin. (A) Epidermal tissue quickly
grows over the excised region of donor (arrow). (B) The tissue graft survives in host fin
(arrow). (C,D) Appearance of stage-42 larvae of donor and host embryo. The melanin
pigment of dermal melanophores of donor larvae remains in a punctuate condition
indicating a lack of pituitary melanotropin (C) while the host larvae display
hyperpigmentation (D). Arrow indicates position of graft in host larvae.
Fig. 2. (A) Xenopus larvae which had their stage-18+ VNR extirpated, completely
lacked a hypophysis (single arrow). The infundibulum was unaffected by this
operation. The preoptic recess usually exhibited minor breaks and tears (small
arrows), oc, optic chiasma. Bar, 0-1 mm. (B) The grafted stage-18+ VNR was allowed
to develop in hosts. This grafted tissue rolled up and formed a spinal-cord-like
structure. It often induced vacuolation within host spinal tissue, g, graft; m, melanophores;
nc, host neural cord;/, finepidermis of host. Bar, 0-1 mm.
Fig. 3. Schematic diagram of procedure to radiolabel ventral neural ridge tissue with 125I.
Fig. 4. Autoradiographic analysis of stage-42 embryo, the VNR of which was radiolabelled
at stage 18+. Radiolabelled stage-18+ VNR tissue consistently contributed to
the formation of three distinct organs. The edges of the VNR eventually became
localized within ventral retinal tissue. The ventral portion of the VNR was localized
within the anteriorpituitary. The bulk of the VNR tissue was found within the anterior
hypothalamus just anterior to the preoptic recess (por), pa, preoptic area;pg, pituitary
gland; r, retina. Bars, 0-1 mm.
Fig. 5. Morphogenetic movements of the labelled stage-18+ VNR. (A) At stage 30
most of the label is observed within the anteriorforebrain in the region of the
chiasmatic ridge (cr). The outline of the pituitaryprimordium (pp) is indicated with a
dotted line. No label was found within the primordium. (B) At stage 33/34 the
pituitaryprimordium has detached or is torn free from the preoptic area (pa). Thus the
most anterior portion of this pituitary anläge is labelled (arrow). Also indicated is the
preoptic recess (por) and the optic chiasma (oc). (C) By stage 42, the pituitary gland
(pg) is under the infundibulum and label becomes incorporated within the adenohypophysis
(arrow). Bars, 0-1 mm.
Fig. 6. The radiolabelled VNR (A) shows a strong correlation with the ACTH immunoreactive
cells (B). Even the hypothalamic area (nucleus infundibularis
ventralis) that exhibits label corresponded to an ACTH-immunoreactive area
(indicated by arrow), por, preoptic recess; oc, optic chiasma; pd, pars distalis; me,
median eminence; pi, pars intermedia. Bar, 0-1 mm.
Fig. 7. Schematic illustration of the most probable morphogenetic events leading to
the labelling of three distinct brain regions. Stage 18: introduction of the radiolabelled
VNR tissue. Stage 19: internalization of the radiolabelled tissue during epidermal
overgrowth and brain tube fusion. Stage 22: attachment of overlying epidermis occurs
along suture line of involuted VNR tissue. Stage 32: proliferation of prosencephalon
within the radioactive VNR tissue subdivides the label into two areas. Stage 42: further
proliferation results in the final distribution of radiolabelled areas.
