The skin and hair of the southern elephant seal, (Mirounga Leonina (L.) II. pre-natal and early post-natal development and moulting

Abstract

The embryological development of hair follicles and the emergence and growth of the foetal pelage of the southern elephant seal, Mirounga leonina (L.), are described in terms of 10 recognizable stages. Follicle primordia appear at a post-implantation age of about 8 weeks and growth of the first definitive hairs takes approximately 14 weeks. Sebaceous glands develop after the sweat gland anlagen, but their ontogeny is more rapid and they function earlier in the formation of the hair canals. The latter are formed by epidermal cellular accretion from without and sebaceous cellular activity within the developing pilosebaceous unit. Development of three epidermal layers is also described and the absence of a stratum granulosum and stratum lucidum noted. The formation of muscle, collagen, and elastic fibres, and blood vessels in the dermis has been observed and described. The histological development of the first adult-type hair generation has been followed in near-term foetuses and young pups until the latter go to sea. The post-natal moult usually commences about 1 week after birth and takes 3 weeks for its completion. Whereas the foetal hairs are not connected to the stratum corneum lining the pilary canal, with a result that they are shed individually, the epidermis and all subsequent hair generations are united. This union is established after the post-natal moult at the end of the active follicular phase by a process of epidermal and external root sheath cellular proliferation around the bottom of the hair canal.