Development of the cloacal bursa in the domestic fowl. II. A quantitative and fine structural analysis of the follicular cortex and medulla.

The estimated volumes of the follicular medulla (x) and cortex (y) from 14-day-old embryos till 28-day-old White Leghorn chicks were associated through the allometric formula y = bxk or log y = log b + k log x. Two successive allometric growth stages (I and II) are discernable, being hatching the transition region between them. The volumetric growth of the cortex is 2.49 times greater than that of the medulla in stage I, whereas cortex and medulla grow isometrically in stage II. The curve fitting procedure analysis of the absolute cortical and medullary growth confirmed these results. The fine structure of the cell types in the follicular medulla revealed that: a) in the allometric stage I pre-existing, bud precursor (Pr) cells appear to give rise to basal (Ba) and medullary epithelial (ME) cells, in both cases showing lucent and dark varieties. A medullary cytoreticulum is established at the onset of this stage. b) The marked lymphocyte proliferation during stage II occurs among the thin and short cytoplasmic processes of BA cells. These processes extend towards the centre of the medulla and also show many lateral interdigitating processes. During this same stage, the cytoplasmic processes of ME cells elongate and become thinner promoting a widening of the cytoreticulum interstices. The fine structural analysis of the cortical cytoarchitectural arrangement showed that: a) before the onset of stage I (14-day-old embryos) the cortex consists mainly of typical fibroblasts (F) and a few blastic (Pr?) cells. Later in this stage I, a poorly defined cortical framework is made up of typical fibroblasts, few cortical branching (CB) cells of the epithelial variety (which seem to be derived from Pr cells) and CB cells of the fibroblast-like variety. These cells are interspersed with mature and immature lymphocytes. b) Allometric stage II of the cortex is characterized by the presence of very thin and long cytoplasmic processes from CB cells of both epithelial and fibroblast-like varieties. The arrangement of CB cell profiles, visualized in electron micrographic montages, is remarkably similar to that of the ME cells profiles which are known to form a cytoreticulum. We thus propose that the mature follicular cortex is endowed with a cellular framework forming wide interstices in which packed mature lymphoid cells are lodged.

Development of the cloacal bursa in the domestic fowl. I. An allometric and morphological study of the interfollicular surface epithelium (ISE).

The allometric relation applied between the body weight and the volume increment of the interfollicular surface epithelium (ISE) showed just one developmental stage during the period analysed (from 16d old embryos through 28 old d chicks). In this single allometric developmental stage the ISE growth is relatively more pronounced than that of the bursa as a whole due to profound cellular modifications which that epithelium undergoes from the primitive undifferentiated epithelial cells through goblet and prismatic cells. The latter could have an endocytic capability due to their primary and secondary lysosomes content. A possible cellular differentiation of the ISE during the histogenesis of the cloacal bursa is also discussed.

An allometric study of the development of the cloacal bursa in the domestic fowl.

The allometric relation between the increment of the body weight and the relative growth of the cloacal bursa shows that the bursal growth is greater than the body growth, during the period analysed (from the 16th d of embryonic development through the 28th post-hatched day). No statistical difference in the mean relative bursal weight between male and female birds was achieved in the above mentioned period analysed. The advantages of using an allometric study for the evaluation of an organ's growth during development are discussed.