Lecture Exam 2 Ultimate Study Guide

The Integumentary System

• Skin Structure: Consists of two primary regions: the epidermis (avascular stratified squamous epithelium) and the dermis (fibrous connective tissue). The deep hypodermis (superficial fascia) consists of adipose and areolar tissue to anchor skin to muscles and insulate the body.

• Epidermal Cell Types:

  • Keratinocytes: Most abundant cell type; produces tough keratin protein, antibiotics, and detoxifying enzymes.

  • Melanocytes: Located in the stratum basale; synthesizes melanin from tyrosine via tyrosinase to protect against UV radiation.

  • Merkel cells: Located in the stratum basale; serves as a touch receptor associated with a sensory nerve ending.

  • Langerhans' cells: Macrophage-like cells in the stratum spinosum that perform receptor-mediated endocytosis to present antigens to T lymphocytes in lymph nodes.

• Epidermal Layers (Deep to Superficial):

  • Stratum basale: Single layer of rapidly dividing stem cells attached to the underlying dermis.

  • Stratum spinosum: Several layers thick containing tonofilaments.

  • Stratum granulosum: 1 to 5 layers of flattened keratinocytes containing keratohyaline and waterproofing lamellated granules.

  • Stratum lucidum: Clear layer present only in thick skin.

  • Stratum corneum: Superficial horny layer of flat, dead keratinocytes packed with keratin that protects against abrasion; an individual sheds approximately $40\,lbs$ of skin in a lifetime.

• Dermis Layers & Vascular Supply:

  • Papillary layer: Superficial $20\%$ composed of areolar connective tissue; features dermal papillae and dermal ridges (fingerprints).

  • Reticular layer: Deep $80\%$ composed of dense irregular connective tissue with collagen and elastic fibers; extreme stretching can tear collagen to form striae.

  • Vascular Supply: Cutaneous plexus (deep) and Subpapillary plexus (superficial) assist in thermoregulation.

• Skin Pigments: Melanin (yellow to black), Carotene (yellow-orange from plants), and Hemoglobin (crimson color from oxygenated blood). UV exposure stimulates synthesis of vitamin D in the deep epidermis.

• Skin Appendages:

  • Sebaceous Glands: Simple alveolar glands that produce oily sebum via holocrine secretion to lubricate skin and hair; activated by androgens at puberty.

  • Sweat Glands: Sudoriferous glands releasing blood filtrate via exocytosis ($500\,mL$ daily baseline; up to $12\,L$ during exercise). Eccrine glands produce true sweat ($99\%$ water, $1\%$ salts/wastes) for thermoregulation; Apocrine glands in axillary and genital areas produce fatty/proteinaceous sweat broken down by bacteria to produce body odor.

  • Hair & Nails: Modified hard keratin structures. Hair consists of a medulla, cortex, and cuticle contained in a hair follicle with an arrector pili muscle. Nails consist of a free edge, body, root, nail bed, nail matrix, eponychium (cuticle), and lunula.

• Burns: Classified into partial-thickness (first-degree: epidermis only; second-degree: epidermis and upper dermis) and full-thickness (third-degree: entire skin thickness, requiring grafting). Dehydration/circulatory shock is the primary immediate threat, followed by infection.

Meiosis and Sexual Reproduction

• Life Cycle: Sexual reproduction alternates between diploid ($2n = 46$) multicellular/zygotic stages and haploid ($n = 23$) gametes.

• Meiosis Division Stages:

  • Meiosis I: Homologous chromosomes form tetrads and undergo crossing over in Prophase I, align at Metaphase I, and separate during Anaphase I to yield two haploid cells with double chromatids.

  • Meiosis II: Sister chromatids separate during Anaphase II, producing four haploid daughter cells with single chromosomes.

Mendelian Genetics and Patterns of Inheritance

• Mendel's Principles: Established by Gregor Mendel using garden peas (Pisum sativum):

  • Principle of Segregation: Two alleles for a gene segregate during gamete formation, so each gamete carries only one allele.

  • Principle of Independent Assortment: Alleles of different gene pairs segregate independently during gamete formation.

  • Principle of Dominance: The dominant allele masks the phenotype of the recessive allele in a heterozygote.

• Ratios and Cross Types:

  • Monohybrid Cross ($Aa \times Aa$): Results in an F2 phenotypic ratio of $3:1$ and a genotypic ratio of $1:2:1$ ($1\text{ AA} : 2\text{ Aa} : 1\text{ aa}$).

  • Dihybrid Cross ($AaBb \times AaBb$): Results in an F2 phenotypic ratio of $9:3:3:1$.

  • Testcross: Mates an unknown dominant phenotype ($A\_$) with a homozygous recessive individual ($aa$) to reveal genotype ($1:1$ phenotypic ratio if heterozygous).

• Non-Mendelian Inheritance Patterns:

  • Incomplete Dominance: Heterozygotes exhibit an intermediate phenotype (e.g., snapdragons, Hypercholesterolemia).

  • Codominance and Multiple Alleles: More than two alleles exist in a population, with both expressed equally in heterozygotes (e.g., ABO blood groups).

  • Pleiotropy: A single gene impacts multiple phenotypic traits (e.g., Sickle-cell disease).

  • Polygenic Inheritance: Additive influence of two or more genes on a single phenotype (e.g., human skin tone).

  • Sex-Linked Traits: Genes located on sex chromosomes (X or Y); recessive X-linked conditions (e.g., Red-green color blindness, Hemophilia) appear far more frequently in males.