EXTERNAL ANATOMY OF THE MAMMARY GLAND

The mammary glands, or mammae, are specialized organs derived from sweat glands that play a crucial role in nur­turing offspring through the production of milk and colos­trum. These glands are present in both sexes, but their functionality is primarily evident in females, where hor­monal influences during pregnancy and lactation stimulate the development and activity of the glandular structures.

Hormonal regulation is key to the gland’s function; it dictates the balance between the secretory tissue and con­nective tissue, with the former expanding significantly during lactation. Post-lactation, the gland undergoes invo­lution, where the secretory cells diminish, and the connec­tive tissue becomes more prominent. Evolutionarily, the distribution and number of mammary glands vary across species, often correlating with litter size. For instance, car­nivores and sows, which typically have multiple offspring, possess a more extensive range of mammary glands, while ungulates like horses and cows have fewer glands, reflect­ing their tendency to bear single offspring (Table 22.1). This variation underscores the mammary gland’s adaptability and its integral role in mammalian reproductive strategies.

The mammary gland anatomy in ruminants and horses exhibits a complex yet systematic structure where the indi­vidual glands are closely associated, often collectively referred to as an udder. This association is evident through the presence of a single teat per gland, which facilitates milk discharge via a distinct duct system. The development of these mammary glands is an intricate embryological pro­cess, originating from bilateral thickenings known as mam­mary ridges on the ventrolateral ectoderm of the embryo.

Interestingly, during the developmental phase, multi­ple mammary buds may form, with the surplus typically regressing quickly. However, some may persist, leading to the formation of supernumerary teats that are gener­ally non-functional and can hinder milking practices; thus, they are often removed in domesticated animals. The dairy cow’s udder has undergone significant selective breeding, enhancing its capacity to produce volumes of milk well beyond the basic nutritional needs of its offspring, a tes­tament to both natural and artificial selection pressures in agricultural practices.

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Marine mammals have developed remarkable adap­tations to thrive in the aquatic environment, par­ticularly in the way they nurse their young. The Carnivora order, which includes seals and sea lions, features retractable nipples that allow for efficient feeding without compromising the streamlined body necessary for swimming. Cetaceans, such as dol­phins and whales, possess mammary glands situated in the inguinal region, concealed by mammary slits to maintain hydrodynamic form. It is hypothesized that calves may utilize their tongue’s dexterity or that the mother’s specialized muscles contract to facilitate

DOI: 10.1201/9781003426851-22

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TABLE 22.1

Locations of Mammary Glands and Number of Openings in Teats Across Different Species

Modified and inspired from Akers RM (2002), Nickerson and Akers (2011) & Mukherjee et al.

milk transfer directly into the offspring’s mouth. The mammary glands of these creatures are substantial, weighing approximately 113 kilograms and extend­ing nearly 5 feet, with the capacity to produce about 200 liters of milk daily, rich in fat content ranging from 35-50%. This high-fat milk is crucial for rapid growth and insulation in the cold marine environ­ment. These evolutionary innovations underscore the diverse strategies marine mammals employ to nur­ture their young while navigating the challenges of their aquatic habitats (Mukherjee et al., 2023).

22.2