Novel Features of the Prenatal Horn Bud Development in Cattle (Bos taurus)

Abstract

Whereas the genetic background of horn growth in cattle has been studied extensively, little is known about the morphological changes in the developing fetal horn bud. In this study we histologically analyzed the development of horn buds of bovine fetuses between ~70 and ~268 days of pregnancy and compared them with biopsies taken from the frontal skin of the same fetuses. In addition we compared the samples from the wild type (horned) fetuses with samples taken from the horn bud region of age-matched genetically hornless (polled) fetuses. In summary, the horn bud with multiple layers of vacuolated keratinocytes is histologically visible early in fetal life already at around day 70 of gestation and can be easily differentiated from the much thinner epidermis of the frontal skin. However, at the gestation day (gd) 212 the epidermis above the horn bud shows a similar morphology to the epidermis of the frontal skin and the outstanding layers of vacuolated keratinocytes have disappeared. Immature hair follicles are seen in the frontal skin at gd 115 whereas hair follicles below the horn bud are not present until gd 155. Interestingly, thick nerve bundles appear in the dermis below the horn bud at gd 115. These nerve fibers grow in size over time and are prominent shortly before birth. Prominent nerve bundles are not present in the frontal skin of wild type or in polled fetuses at any time, indicating that the horn bud is a very sensitive area. The samples from the horn bud region from polled fetuses are histologically equivalent to samples taken from the frontal skin in horned species. This is the first study that presents unique histological data on bovine prenatal horn bud differentiation at different developmental stages which creates knowledge for a better understanding of recent molecular findings.

Understanding the effects of the bovine POLLED variants

“…Similar observations have been made for yak fetuses (Li et al ). There is no evidence of ossification in the fetal horn bud (Wiener et al ), and horn growth and ossification occur approximately 1 month after birth (Dove ). Thus, the horn bud differentiates during early fetal development but horn growth does not occur until after the calf is born.…”

“…Polled fetuses carrying the Celtic variant do not develop horn buds, forming only smooth tissue that is histologically indistinguishable from frontal skin tissue (Allais‐Bonnet et al ; Wiener et al ). Horn bud development is also absent in yak fetuses carrying the Mongolian variant (Li et al ), but has not been investigated in fetuses homozygous for the Friesian and Guarani variants.…”

“…There are several histological differences between the horn bud and nearby frontal skin throughout bovine fetal development (Table 1) (Capitan et al 2012;Allais-Bonnet et al 2013;Wiener et al 2015). Firstly, the epidermis of the horn bud is thicker than the epidermis of the frontal skin (Wiener et al 2015).…”

Simultaneous introgression of three POLLED mutations into a synthetic breed of Chinese cattle

“…However, only limited evidence has been effectively elucidated possibly because of low detection power of microarray-based gene expression profiling platform [ 20 ] or insufficient biological samples subjected to high-throughput mRNA sequencing [ 12 ]. Additionally, it would be very difficult to determine appropriate sampling time points due to the complexity of prenatal horn bud dynamic development [ 13 , 23 ].…”

Genome-wide Association Study Identifies Loci for the Polled Phenotype in Yak

“…SYNJ1 encodes synaptojanin 1, a key neural protein highly expressed in nerve terminals with essential roles in the regulation of synaptic vesicles in conventional synapses and hair cells [ 30 , 31 ]. Recently, a histological analysis revealed that nervous tissue and hair follicle development have different features in horn buds and polled frontal skin during the development of the horn buds of bovine fetuses, implying that SYNJ1 maybe have an important role in horn differentiation [ 32 ]. PAXBP1 is an essential binding protein that regulates the proliferation of muscle precursor cells, which in turn, are involved in the development of normal craniofacial features and spine morphogenesis [ 33 ].…”