Untitled - D Ank Unlimited

thymus 699 thymus
Marrow
stem cell
TdT –
Lyt –
θ –
β 3
FR.5
Prothymocyte FR.5 Thymocyte
TdT +
Lyt –
θ, + θ –
Bone marrow Thymus
The normal thymus of infant showing lobulation and sharp separation of
cortex from the stalk-like medulla (40× magnification).
markers for thymic tissue. The blood supply to the cortex
comes from capillaries that form anastomosing arcades.
Drainage is mainly through veins; the thymus has no
lymphatic vessels. The thymus develops from the branchial
pouches of the pharynx at about 6 weeks of embryonal age.
β 4
α 1
α 7
Thymosin
TdT +
Lyt 1 + 2 + 3 +
θ ++
FR.5
α 1
Peripheral
lymphoid tissue
Proposed site of action of thymosin polypeptides on maturation of T cell subpopulations.
Normal adult thymus. The thymus often shows an X- or H-shaped
configuration.
α 7
?
Hassall’s corpuscle.
T-cells
suppressor
Killer
Helper
TdT –
Lyt 1 + 2 + 3 +
θ +
Hassall’s
corpuscle
In most species, it is fully developed at birth. In humans,
the weight of the thymus at birth is 10 to 15 g. It continues
to increase in size, reaching a maximum (30 to 40 g) at
puberty. It then begins to involute with increasing age, but
remains functional. The medulla involutes first with pyknosis
and beading of the nuclei of small lymphocytes, giving
a false impression of an increased number of Hassall’s
corpuscles. The cortex atrophies progressively. The blood–
thymus barrier protects thymocytes from contact with
antigen. Lymphocytes reaching the thymus are prevented
from contact with antigen by a physical barrier. The first
level is represented by capillary walls with endothelial cells
inside the pericytes outside the lumen. Potential antigenic
molecules that escape the first level of control are taken
over by macrophages present in the pericapillary space.
Further protection is provided by a third level, represented
by a mesh of interconnecting epithelial cells that enclose the
thymocyte population. The effects of thymus and thymic
hormones on the differentiation of T cells are demonstrable
in animals congenitally lacking a thymus (nu/nu animals),
in neonatally or adult thymectomized animals, and in
T