Integument : TOPIC 5 | PART TWO |

Introduction and Context

  • The speaker discusses the approach to handling exams in terms of returning them to students. Exams will be returned within one week following completion, regardless of whether they are lab or lecture exams.

Integument and Evolutionary History

  • Integument: This refers to the outer protective layer of an organism.

  • The focus is currently on the integumentary systems of vertebrates, particularly moving from aquatic to terrestrial forms.

Aquatic vs. Terrestrial Vertebrates

  • Primarily Aquatic Vertebrates:

    • Definition: Organisms whose entire evolutionary lineage has been aquatic.

    • Example: Most fish species, where their ancestry is completely tied to aquatic environments.

  • Secondarily Aquatic Vertebrates:

    • Definition: Organisms that originated from aquatic ancestors, moved to land, and then returned to aquatic environments.

  • Current focus is on the transition to terrestrial vertebrates including:

    • Lissamphibia: Taxonomic group encompassing amphibians: frogs, toads, and salamanders, as well as Gymnophiona (caecilians).

Skin and Scales in Amphibians

  • Most Lissamphibia lack scales, except for a group called Gymnophiona.

  • Characteristics of Amphibian Skin:

    • Scaleless; when scales are present, they are considered unmineralized.

    • Skin contains integumentary scales that are hardened from keratin similar to human fingernails.

Cutaneous Respiration

  • Definition: A method of respiration that relies on gas exchange through the skin.

  • Mechanism:

    • Requires blood capillaries close to the surface of the skin to facilitate gas exchange.

    • The technique needs minimal anatomical specialization, but few layers of dead skin are beneficial to reduce diffusion distance.

  • Obligate Cutaneous Respiration:

    • Defined as organisms that exclusively breathe through their skin.

    • Example: A lungless salamander that has descended from lunged ancestors and performs respiration solely via cutaneous exchange.

Amphibian Glands

  • Gland Types in Amphibians:

    • Two types of multicellular glands in the dermis: mucous glands and poison glands.

    • One type of single cell gland in the epidermis: Leidy cells or glands.

  • Mucous Glands:

    • Located deeper in the dermis; require ducts to release secretions to the skin surface.

  • Leidy Cells:

    • Positioned in the epidermis, secrete chemicals to combat pathogens at the skin surface.

    • Organized as single large cells that discharge their secretions directly onto the skin.

Transition to Reptiles

  • Systematic Classification:

    • Reptiles are the first clade of amniote vertebrates discussed.

  • Molting in Reptiles (Ecdysis):

    • Definition: The process of shedding part or all of the epidermis in large sections rather than cell-by-cell.

    • Layers of the Reptilian Epidermis:

    • Stratum Basale: Deepest layer from which new generations of cells arise.

    • Stratum Granulosum: Transitional layer that produces keratinized cells.

    • Stratum Corneum: Outer layer of dead cells enhanced with keratin.

  • Ecdysis involves shedding the outermost layers, as a new set of strata forms internally prior to shedding.

Observations on Ecdysis

  • Rubbing against a surface can help facilitate the process by loosening dead skin.

  • Lack of Limbs in Snakes: Snakes utilize their bodies to support the shedding process instead of limbs traditionally used by other species.

Glandular Structures in Reptiles

  • Types of Glands:

    • Crocodiles and turtles possess scent glands for mate recognition.

    • Femoral Glands: Located on the ventral thigh in lizards, contributing to mate recognition.

    • Birds have a Uropigial Gland, used in preening and waterproofing feathers.

Feathers: An Evolutionary Aspect

  • Origin of Feathers: Present in all modern birds (the clade Aves), but their history extends to non-avian dinosaurs.

  • Functional Advantages of Feathers:

    • Insulation to retain body heat, which may have been critical long before flight evolved.

    • Potential for mate recognition through visual cues and displays.

Fossil Evidence of Feathers

  • Bytes of non-avian theropods in the fossil record show evidence of feathers or feather-like structures.

  • Types of Feathers:

    • Recognition of primitive feather structures that imply a wide-ranging function beyond flight.

Follicle Development of Feathers

  • Follicle Composition: A structure comprised of epidermal (ectoderm) and dermal (mesoderm) components required for feather growth.

    • The follicle is formed as an invagination of epidermis into the dermis.

  • During growth, feathers develop from follicles covered in a sheathed keratinized epidermis that is later shed to reveal mature feathers.

Anatomy of a Feather

  • Parts of a Feather:

    • Rachis: The central shaft of the feather.

    • Barbs: Branch off from the rachis to form the overall vein of the feather.

    • Asymmetry of Barbs: Can create either a symmetrical or asymmetrical appearance.

    • Velcro-like Structure: Barbs are held together using microscopic hooks, a mechanism for maintaining feather integrity.

Conclusion

  • Recap on the interrelations among amphibians, reptiles, and birds in terms of integumentary structures, featuring the evolution and importance of skin adaptations, respiration methods, glands involved, and the development of feathers.

  • Significance of understanding these evolutionary traits to comprehend modern vertebrate biology and their evolutionary history.