Development and Pathology of the Equine Mammary Gland

Abstract

An understanding of the anatomy, histology, and development of the equine mammary gland underpins study of the pathology of diseases including galactorrhoea, agalactia, mastitis, and mammary tumour development. This review examines the prenatal development of the equine mammary gland and the striking degree to which the tissue undergoes postnatal development associated with the reproductive cycle. The gland is characterised by epithelial structures arranged in terminal duct lobular units, similar to those of the human breast, supported by distinct zones of intra- and interlobular collagenous stroma. Mastitis and mammary carcinomas are two of the most frequently described equine mammary pathologies and have an overlap in associated clinical signs. Mastitis is most frequently associated with bacterial aetiologies, particularly Streptococcus spp., and knowledge of the process of post-lactational regression can be applied to preventative husbandry strategies. Equine mammary tumours are rare and carry a poor prognosis in many cases. Recent studies have used mammosphere assays to reveal novel insights into the identification and potential behaviour of mammary stem/progenitor cell populations. These suggest that mammospheres derived from equine cells have different growth dynamics compared to those from other species. In parallel with studying the equine mammary gland in order to advance knowledge of equine mammary disease at the interface of basic and clinical science, there is a need to better understand equine lactational biology. This is driven in part by the recognition of the potential value of horse and donkey milk for human consumption, particularly donkey milk in children with ‘Cow Milk Protein Allergy’.

Mammary gland physiology and farm management of dairy mares and jennies

“…Moreover, the wide knowledge on dairy ruminants cannot be translated to equids because of differences in morphological, physiological, and behavioral patterns. The equine udder is subjected to unusually high cellular replication and increase in gland volume in postnatal life, particularly after puberty and pregnancy (Hughes, 2021), as in other mammals. The equine udder is characterized by one pair of mammae, each with a teat.…”

“…The equine mammary gland is characterized by a fibrous stroma in which the epithelial structures are arranged in terminal duct lobular units similar to those of the human breast (Howard and Gusterson, 2000). A lobular unit is composed of a group, or lobule, of blind-ending mammary acini and both intralobular and extralobular portions of the subtending terminal duct, which together comprise the functional unit of the mammary gland (Hughes, 2021). The "milk letdown" process is led by prolactin activity.…”

“…After the high levels of estrogens and progestogens that characterize pregnancy (Chavatte-Palmer, 2002), prolactin rises in the last 7 d before foaling and remains high during the first 2 mo of lactation (Worthy et al, 1986). Prolactin activates the transcription factor signal transducer and activator of transcription 5 (STAT5), which initiates expression of milk protein genes in mammary epithelial cells (Hughes, 2021). When foals suckle, oxytocin binds the recep-tor expressed on basal mammary cells, stimulating intracellular calcium signaling, which results in contraction of the myoepithelial cells and milk expulsion (Stevenson et al, 2020).…”

“…The equid udder has a lower storage capacity (<2.5 L) than ruminants and is more adapted to frequent milk removal (D'Alessandro et al, 2015). This is tightly linked to the high suckling frequency of foals compared with other domestic ruminants (De Palo et al, 2018a) and could explain the lower SCC and the rare occurrence of mastitis in equines compared with ruminants (Hughes, 2021). All of these physiological peculiarities have encouraged researchers and farmers to introduce and study new techniques of artificial suckling in foal breeding.…”

The Indicators of Clinical and Subclinical Mastitis in Equine Milk

“…Mastitis, mammary gland inflammation, is rarely observed in dairy equine farms [ 23 , 24 , 25 ], whereas its occurrence is suspected more often concerning subclinical states [ 26 , 27 ]. Among the main causes of mastitis in dairy equine farms, injuries or improper milking procedures reported for more intensive farming systems are mentioned [ 5 , 24 ].…”