Ciliogenesis and ciliation of the respiratory epithelium in the human fetal cartilaginous trachea

Axonemal dynein expression in human fetal tracheal epithelium

Ciliogenesis in human fetal airway epithelium occurs from 11 to 24 gestational weeks. Using genetic and antigenic markers specific for human axonemal dynein heavy chain 9, we characterized temporal aspects of axonemal dynein expression associated with large airway epithelial ciliogenesis during human fetal development. Late in the first trimester, an undifferentiated columnar epithelium is characteristic of the large airways, and immunocytochemical studies exhibited focal localization of axonemal dynein antigen on luminal epithelial cell borders at sites consistent with emergent ciliary beds. From 12 to 22 wk, immunocytochemical labeling of new ciliary beds was prominent, and localization within the cytoplasm of epithelial cells suggested avid synthesis of axonemal dynein in advance of ciliogenic events. Quantitative RT-PCR of tracheal RNA and in situ hybridization studies compared favorably with immunocytochemical findings with the earliest expression of axonemal dynein at 9-10 wk gestation. These studies have documented that axonemal dynein is expressed early in human fetal life during airway epithelial maturation and well before histological or ultrastructural evidence of ciliogenesis is apparent.

An autoradiographic assessment of epithelial cell proliferation and post-natal maturation of the tracheal epithelium in infant ferrets

The tracheal epithelium of infant ferrets undergoes rapid postnatal maturation over the first month of life to achieve the pseudostratified columnar configuration characteristic of the large airways of other mammals. We have used in vivo pulsing with tritiated thymidine ((3)HT) to elicit autoradiographic labeling of cells synthesizing nucleic acids in order to characterize more fully the contribution to development of different cell types comprising the nascent epithelial layer during this period of rapid growth. These studies indicate that two distinct populations of epithelial cells possess proliferative potential and contribute to the establishment of the mature adult epithelial layer. These investigations further confirm the mitotic potential of basal cells during a period of rapid postnatal growth and development of the tracheal epithelial layer. These studies also document the contribution to early airway development by non-ciliated cells, which predominate on the luminal border of the ferret trachea at birth. The temporal and histologic patterns of airway epithelial maturation during post-natal life in the ferret as contained in this study exhibit similarities to those which occur with recovery from injury by infection and irritant exposure in mature airways. Thus, the characterization of epithelial cell compartments having proliferative potential may provide insights into the mechanisms whereby normal airway epithelial organization is established and maintained during development as well as the possible recapitulation of these mechanisms during times of epithelial regeneration following injury.

Connexin 26 expression in human and ferret airways and lung during development

Coordinated microscopic and molecular biological studies were used to document gap junction expression during postnatal development in ferret tracheal epithelium and lung and in fetal and adult human airway and lung. Expression of connexin 26 (Cx26) in the ferret airways was limited to the epithelial layer and was observed only during the newborn interval. In contrast, we found Cx26 expressed in the alveolar epithelium of the ferret lung by in situ hybridization, Northern blotting, RT-PCR amplification, and immunocytochemical labeling at all ages examined. This finding was further confirmed by documentation of gap junctional plaques upon ultrastructural examination of freeze-fracture replicas of adult ferret lung tissue. Parallel studies of developing human fetal lung and airway suggested connexin expression in the airways only in the first trimester but, as in the ferret, persistent expression was observed in both fetal and adult lung. These studies suggest that the transient expression of Cx26 is a reliable early indicator of airway epithelial development and differentiation in the airways. In contrast, Cx26 expression persists throughout life in the lung, suggesting that gap junctions serve more perennial intercellular communication functions in the peripheral lung.

The glucose-6-phosphatase enzyme in developing human trachea and oesophagus

Glucose-6-phosphatase is an endoplasmic reticulum system which is found primarily in liver and kidney. Recently, it has become clear that it is also present in lower amounts in a variety of other tissues. Previous histochemical studies of glucose-6-phosphate hydrolysis in trachea have given equivocal results and only one study on adult oesophagus has shown glucose-6-phosphatase, enzymatic activity but without cellular localization. We have now shown, using microassay techniques, that microsomes isolated from human foetal trachea and oesophagus both contain low levels of specific glucose-6-phosphatase activity (mean = 0.9 and 1.5 nmol min-1 mg-1 microsomal protein, respectively) which are less than 10% of the levels in microsomes of human foetal liver of similar age. In the developing trachea, glucose-6-phosphatase immunoreactivity has been found, using a monospecific antibody to the catalytic subunit of the glucose-6-phosphatase enzyme, to be first present at 10-11 weeks' gestation, and thereafter in foetal life, predominantly present in ciliated cells, with smaller amounts in non-ciliated secretory cells, duct lining cells, and occasional basal cells. The foetal oesophageal epithelium is transiently ciliated from 10 to 11 weeks' gestation, but ciliated cells are gradually replaced by squamous cells from 14 to 16 weeks onwards. Glucose-6-phosphatase immunoreactivity in human foetal oesophagus is predominantly confined to ciliated cells, but non-ciliated luminal cells are also reactive, as are occasional basal cells. Mucus secretory cells in foetal trachea and oesophagus are immunonegative, as is the entire epithelium of both organs in the embryo (up to 56 postovulatory days.