Retinal differentiation in an altricial bird species, Taeniopygia guttata: An immunohistochemical study

Highlights

• Altricial and precocial bird species differ in the timing of visual system maturation.

• Visual system development occurs faster in precocial bird species than in altricial bird species.

• Retinal neurogenesis is intense at perinatal stages in altricial bird species.

Abstract

The bird retina offers an excellent model to investigate the mechanisms that coordinate the morphogenesis, histogenesis, and differentiation of neuron and glial cells. Although these developmental features have been intensively studied in the chicken (Gallus gallus, Linnaeus 1758), a precocial bird species, little is known about retinogenesis in altricial birds. The purpose of this study was to examine the differentiation of retinal cells in the altricial zebra finch (Taeniopygia guttata, Vieillot, 1817) and compare the results with those from previous studies in G. gallus. By using immunohistochemical techniques, the first differentiated TUJ1-/Isl1-positive neuroblasts were detected in the vitreal surface of the neuroblastic layer at later incubation times in T. guttata than in G. gallus (108 h vs 55 h). The immunoreactivity of these early differentiation markers coincided temporo-spatially with the appearance of the first PCNA-negative nuclei. Furthermore, the first visinin-positive photoreceptors (132 h vs 120 h) and the first Prox-1-immunoreactive neuroblasts (embryonic day 7.25 (E7.25) vs E6.5) were also detected at later embryonic stages in the retina of T. guttata than in the retina of G. gallus. At E13, one day before hatching, abundant PCNA- and pHisH3-immunoreactivities were detected in the T. guttata retina, while proliferation was almost absent in the G. gallus retina at perinatal stages. Therefore, these results suggest that cell differentiation in the retina is delayed in the altricial bird compared to precocial birds. Furthermore, the T. guttata retina was not completely developed at hatching, and abundant mitotically active precursor cells of retinal neurons were found, suggesting that retinal neurogenesis was intense at perinatal stages.

Introduction

There is extensive information about embryological features in precocial birds that are considered as model organisms, such as the chicken, Gallus gallus (Hamburger and Hamilton, 1951), or the Japanese quail, Coturnix coturnix japonica, (Ainsworth et al., 2010). There have also been studies describing the embryonic ontogeny of altricial bird species such as the society finch, Lonchura striata (Yamasaki and Tonosaki, 1988), the barn owl, Tyto alba (Köppl et al., 2005), or the feral pigeon, Columba livia (Olea and Sandoval, 2012), although only partial information about specific features of their embryonic ontogeny is currently available. A few comparative studies have shown that there are substantial differences in embryological development between precocial and altricial birds. Altricial birds show less advanced growth at the early-to-middle stages of development than the precocial birds (Yamasaki and Tonosaki, 1988; Olea and Sandoval, 2012). The late embryonic stages in precocial bird species are primarily characterized by growth (Hamburger and Hamilton, 1951; Ainsworth et al., 2010) which could explain the increased tissue maturation observed at hatching in precocial bird species in comparison with altricial birds.

Most of the studies describing the morphological and physiological maturation of elements in specific tissues in altricial birds are related to the status of the chicks at hatching. With respect to the degree of maturation of the central nervous system and sensory systems in altricial hatchlings, these bird species delay many aspects of their brain and sensory organ maturation to the post-hatching period, contrary to precocial bird species (Stark and Ricklefs, 1998; Rojas et al., 2007; Charvet and Striedter, 2011). The retina of the altricial hatchlings of the pigeon, Columba livia, has no morphologically differentiated photoreceptors (Rojas et al., 2007). The morphological differentiation of the photoreceptors in this species occurs mainly during the first two weeks after hatching (Bagnoli et al., 1985; Rojas et al., 2007), much later than the other retinal cell types. It has also been shown that maturation of the retinal pigment epithelium (RPE) in newly hatched pigeons courses in parallel with photoreceptor outer segment differentiation (Porciatti et al., 1985). In contrast, photoreceptor differentiation is complete in quail hatchlings (Rojas et al., 2007). Furthermore, at the time of hatching, the precocial bird retina is functional and seems to be postmitotic, with the exception of some proliferating cells located in the extreme periphery of the postnatal retina, in the ciliary marginal zone (CMZ) (Fischer and Reh, 2000; Kubota et al., 2002; Ghai et al., 2008). In conclusion, retinal differentiation occurs later in altricial than in precocial bird species.

During the embryonic period, the chicken retina serves as an excellent model to study vertebrate central nervous system development (Vergara and Canto-Soler, 2012). The retinal development of G. gallus is complete very early in its embryonic life (Kahn, 1973; Prada et al., 1991; Snow and Robson, 1994; Bruhn and Cepko, 1996; McCabe et al., 1999; Drenhaus et al., 2003, 2007; Francisco-Morcillo et al., 2005; Rojas et al., 2007; Bejarano-Escobar et al., 2015). The first differentiating neuroblasts are observed near the posterior pole of the chicken retina from Hamburger and Hamilton (1951) Stage 13, HH13 (48 h), to HH16 (51–56 h) (Prada et al., 1991; McCabe et al., 1999; Francisco-Morcillo et al., 2005). From this point onwards, cell differentiation extends to the rest of the retina following central-to-peripheral, dorsal-to-ventral, and vitreal-to-scleral gradients (Prada et al., 1991; Francisco-Morcillo et al., 2005; Drenhaus et al., 2007). Neurogenesis in the chicken retina ends at E12, when the last bipolar cells exit the cell cycle (Prada et al., 1991). Indeed therefore, the main events during the chicken visual system morphogenesis, histogenesis, and onset of cell differentiation have been well identified histochemically and immunohistochemically.

In recent years, the zebra finch (Taeniopygia guttata, Vieillot 1817), an altricial bird species, has become a choice experimental model bird species to address questions in a wide variety of fields, including memory and learning (Moorman et al., 2011), neuroanatomy (Vargha-Khadem et al., 2005), and postnatal neurogenesis (Dewulf and Bottjer, 2005; Charvet and Striedter, 2008, 2011; Striedter and Charvet, 2008). More recently, Murray et al. (2013) have described the embryological staging of this altricial bird species, finding that it takes more time to reach each developmental stage than G. gallus. Furthermore, in a recent morphological study conducted in our laboratory, we obtained some interesting results concerning the T. guttata embryonic retina (Álvarez-Hernán et al., 2018). First, we detected intense proliferative activity at the hatching stage, whereas in the chicken, only sparse mitotic cells restricted to the CMZ are observed at this stage. Second, various maturational features of the visual system (morphogenesis of the optic vesicle and optic cup, appearance of the first differentiated neurons, and emergence of the plexiform layers) occurred at later stages than in the chicken. Third, at hatching, some features characteristic of immaturity were still detected in the T. guttata retina, such as very thin plexiform layers and poorly developed photoreceptors.

In the present study, we analysed immunohistochemically the retinal maturation of T. guttata in order to detect the onset of three processes: (i) the cessation of proliferation, (ii) neurogenesis, by using early post-mitotic markers that are expressed by neuroblasts shortly after the last mitotic division, and (iii) cell differentiation, by using cell-specific markers. We discuss the results together with those obtained in other bird species, mainly in G. gallus, to compare the variability in the chronology of specific developmental features during bird ontogeny.