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thymectomy 696 thymic hormones and peptides
thymectomy
Surgical removal of the thymus, leading to failure to develop
cell-mediated and humoral immunity to thymus-dependent
antigens in mice thymectomized as neonates. T cells fail to
develop, lymphoid tissues atrophy, and the failure to gain
weight are characteristic of runt or wasting disease.
thymic alymphoplasia
Severe combined immune deficiency (SCID) transmitted as
an X-linked recessive trait.
thymic anlage
The tissue that gives rise to thymic stroma during
embryogenesis.
thymic cortex
Refer to thymus.
Type 2 “pale” epithelial cell in outer cortex (9000× magnification). R =
profiles of RER. G = Golgi complex. Arrow = multivesicular body.
thymic epithelial cells (TECs)
Cells present in the cortex and to a lesser degree in the
medulla of the thymus. They are derived from the third
and fourth pharyngeal pouches. They affect maturation
and differentiation of thymocytes through the secretion of
thymopoietin, thymosins, and serum thymic factors. TECs
express both major histocompatibility complex (MHC) class
I and class II molecules. They secrete interleukin-7 (IL7)
required for early T lymphocyte development during positive
selection. Maturing T lymphocytes must recognize self
peptides bound to TEC surface MHC molecules to avoid
programmed cell death. TECs mediate positive and negative
selection through presentation of self ligands in the thymus.
thymic hormones
Soluble substances synthesized by thymic epithelial cells
that promote thymocyte differentiation. They include
thymopoietin and thymosins—peptides that help regulate
differentiation of T lymphocytes.
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thymic hormones and peptides
Polypeptides extracted from thymus glands include
thymulin, thymopoietin, thymic humoral factor (THF),
and thymosins. Only thymulin has been characterized as
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Two type 6 large medullary epithelial cells (6) adjacent to the Hassall’s
corpuscle and two type 4 epithelial cells (4) in the medulla (magnification,
7000×).
Thymic epithelial cells: cortex. Cortical type epithelial cells have large,
round to oval, clear nuclei and conspicuous nucleoli.
Thymic epithelial cells: cortico-medullary junction. Cortical type
epithelial (arrows) and medullary type (arrowheads) are intermingled.
Thymic epithelial cells: medulla. Fusiform epithelial cells have spindleshaped
nuclei, coarse chromatin structure, and interconnecting cell
processes. A Hassall’s corpuscle can be seen in the lower section of the
photomicrograph.