Observations on Fur Development in Echidna (Monotremata, Mammalia) Indicate that Spines Precede Hairs in Ontogeny

Abstract

In the primitive mammal echidna, the initial 2-3 generations of skin appendages produced from birth forms spines and only later true hairs appear. Microscopy on preserved museum specimens reveals that the morphogenesis of spines and hairs is similar but that a larger dermal papilla is formed in spines. The growing shaft comprises a medulla surrounded by a cortex and by an external cuticle. A thick inner root sheath made of cornified cells surrounds the growing shaft inside the spine canal that eventually exits with a pointed tip. Hairs develop later with the same modality of spines but have a smaller papilla and give rise to a fur coat among spines. Therefore the integument of developing echidnas initially produces spines from large dermal papillae but the reduction in size of the papillae later determines the formation of hairs. Although the morphogenesis of spines and hairs can represent a case of specialization in this species, the primitive mammalian characteristics of echidnas has also inspired new speculations on the evolution of the mammalian hair from mammalianlike reptiles with a spiny coat. The resemblance in the morphogenesis between spines and hairs has suggested some hypothesis on hair evolution, in particular that hairs might be derived from the reduction of protective large spines present in ancient mammalian-like reptiles possibly derived from the reduction of pre-existing pointed scales. The hypothesis suggests that spines became reduced and internalized in the skin forming hairs. Anat Rec, 298:761-770, 2015. V C 2014 Wiley Periodicals, Inc. Key words: echidna; skin; spine developmentThe echidna (Tachyglossus aculeatus) is a monotreme mammals typically covered by a coat fur made of long spines or quills of variable size mixed to hairs or spinyhairs (Hausman, 1920;Griffith, 1968Griffith, , 1978Rismiller, 1999). While the dorsal and lateral sides of the body contain numerous spines mixed with hairs, spines are reduced in number and disappear in the ventral side where softer hairs are present, especially around the incubatorium, the ventral pouch of females. The size and diameter of spines however varies and also a gradation in size of spines and stiff hairs is present.Spines indicate medium-small (0.5-1.5 mm 3 10-25 mm) pointed thick hairs, while quills indicate larger (3-4 mm 3 40-70 mm) hard, and often curved spines. The microstructure of quills varies in different mammals

Ancient lineages of the keratin-associated protein (KRTAP) genes and their co- option in the evolution of the hair follicle

“…The almost invisible ne hairs on the skin of elephants has been shown to be responsible for up to 23% of the heat dissipation in these animals at low wind speeds, where their thermoregulation needs are greatest 10 . That hair was preceded in evolution by spines was suggested by Alibardi and Rogers (2014) 11 .…”

Comparative genomics of monotremes provides insights into the early evolution of mammalian epidermal differentiation genes

“…The results of the present study provide the basis for investigations of the spatio-temporal expression patterns of EDC genes in monotremes, which will be important to fully evaluate the functions of these genes. Previously, histological and immunohistochemical studies have shown similarities between the epidermis of monotremes and therian mammals 56 – 58 , but comprehensive investigations of the molecular structure of the epidermis and specialized skin appendages of monotremes 37 , 38 , 59 remain to be performed. The design of specific probes and antibodies against EDC proteins (this study), keratins 40 and other epidermal proteins will help to clarify the molecular evolution of the mammalian skin barrier and skin appendages.…”

Ancient lineages of the keratin-associated protein (KRTAP) genes and their co-option in the evolution of the hair follicle

“…The almost invisible fine hairs on the skin of elephants has been shown to be responsible for up to 23% of the heat dissipation in these animals at low wind speeds, where their thermoregulation needs are greatest [ 10 ]. That hair was preceded in evolution by spines was suggested by Alibardi and Rogers [ 11 ].…”