Sexual Reproduction, Development and Tissues
Meiosis: gamete formation
-two nuclear divisions, meiosis I and meiosis II, half the number of chromosomes in the four daughter cells
Steps leading up to meiosis 1:
-all organelles are replicated during interphase
chromosomes replicate prior to meiosis I occurs during the S-phase of interphase
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Steps of meiosis 1:
1) During prophase 1 ,
-the nuclear membranes break down
-the spindle forms
-the chromatin condenses
-homologous pairs of chromosomes undergo synapsis and form tetrads with their homologous partners
-crossover, the exchange of genetic material among tetrads, occurs during synapsis
2) Tetrads line up at the spindle equator during metaphase I
3) In anaphase I, homologous chromosomes still composed of joined sister chromatids are distributed to opposite ends of the cell
4) In telophase I :
-The nuclear membranes re-form around the chromosomal masses
- The spindle breaks down
-The chromatin reappears, forming two daughter cells

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At the end of meiosis I each daughter cell has:
-Two copies of either a maternal or paternal chromosome
-A 2 n amount of DNA and haploid number of chromosomes
Meiosis II
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Mirrors mitosis except that chromosomes are not replicated before it begins
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Meiosis accomplishes two tasks:
-It reduces the chromosome number by half (2 n to n )
-It introduces genetic variability
Spermatogenesis
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Cells making up the walls of seminiferous tubules are in various stages of cell division: these spermatogenic cells give rise to sperm in a series of events
1) Mitosis of spermatogonia, forming spermatocytes
2) Meiosis forms spermatids from spermatocytes
3) Spermiogenesis – spermatids form sperm
Mitosis of Spermatogonia
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Spermatogonia – outermost cells in contact with the epithelial basal lamina
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Spermatogenesis begins at puberty as each mitotic division of spermatogonia results in type A or type B daughter cells
-Type A cells remain at the basement membrane and maintain the germ line
-Type B cells move toward the lumen and become primary spermatocytes

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Oogenesis

Production of female sex cells by meiosis
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In the fetal period, oogonia (2 n ovarian stem cells) multiply by mitosis and store nutrients
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Primordial follicles appear as oogonia are transformed into primary oocytes
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Primary oocytes begin meiosis but stall in prophase I
Oogenesis: Puberty
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At puberty, one activated primary oocyte produces two haploid cells
-The first polar body
-The secondary oocyte
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The secondary oocyte arrests in metaphase II and is ovulated
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If penetrated by sperm the second oocyte completes meiosis II, yielding:
-One large ovum (the functional gamete)
-A tiny second polar body
From Egg to Embryo
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Pregnancy – events that occur from fertilization until the infant is born
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Conceptus – the developing offspring

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Gestation period – from the last menstrual period until birth
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Preembryo – conceptus from fertilization until it is two weeks old
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Embryo – conceptus during the third through the eighth week
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Fetus – conceptus from the ninth week through birth
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Accomplishing Fertilization
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The oocyte is viable for 12 to 24 hours
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Sperm is viable 24 to 72 hours
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For fertilization to occur, coitus must occur no more than:
-Three days before ovulation
-24 hours after ovulation
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Fertilization – when a sperm fuses with an egg to form a zygote
Preembryonic Development
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The first cleavage produces two daughter cells called blastomeres
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Morula – the 16 or more cell stage (72 hours old)
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By the fourth or fifth day the preembryo consists of 100 or so cells (blastocyst)
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Blastocyst – a fluid-filled hollow sphere composed of:
-A single layer of trophoblasts
-An inner cell mass
Implantation
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Begins six to seven days after ovulation when the trophoblasts adhere to a properly prepared endometrium
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Implantation is completed by the fourteenth day after ovulation
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Viability of the corpus luteum is maintained by human chorionic gonadotropin (hCG)

secreted by the trophoblasts
 hCG prompts the corpus luteum to continue to secrete progesterone and estrogen
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Chorion – developed from trophoblasts after implantation, continues this hormonal stimulus
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Between the second and third month, the placenta:
-Assumes the role of progesterone and estrogen production
-Is providing nutrients and removing wastes
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Placentation
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Formation of the placenta from:
-Embryonic trophoblastic tissues
-Maternal endometrial tissues
-fully formed and functional by the end of the third month
Germ Layers
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The blastocyst develops into a gastrula with three primary germ layers: ectoderm, endoderm, and mesoderm
-these give rise to the basic tissues of the body
Tissues
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Groups of cells similar in structure and function
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The four types of tissues: Epithelial, Connective, Muscle, Nerve
Epithelial Tissue
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Cellularity – composed almost entirely of cells
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Special contacts – form continuous sheets held together by tight junctions and desmosomes
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Polarity – apical and basal surfaces
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Supported by connective tissue – reticular and basal laminae
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Avascular but innervated – contains no blood vessels but supplied by nerve fibers
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Regenerative – rapidly replaces lost cells by cell division

Classification of Epithelia
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Simple or stratified- layering
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Squamous, cuboidal, or columnar-cell shape
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Epithelia: Simple Squamous
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Single layer of flattened cells with disc-shaped nuclei and sparse cytoplasm

Functions:
-Diffusion and filtration
-Provide a slick, friction-reducing lining in lymphatic and cardiovascular systems
Epithelia: Simple Cuboidal
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Single layer of cubelike cells with large, spherical central nuclei
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Function in secretion and absorption
Epithelia: Simple Columnar
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Single layer of tall cells with oval nuclei; many contain cilia
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Goblet cells are often found in this layer
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Function in absorption and secretion
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Nonciliated type line digestive tract and gallbladder
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Ciliated type line small bronchi, uterine tubes, and some regions of the uterus
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Cilia help move substances through internal passageways
Epithelia: Pseudostratified Columnar
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Single layer of cells with different heights; some do not reach the free surface
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Nuclei are seen at different layers
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Function in secretion and propulsion of mucus
Epithelia: Stratified Squamous
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Thick membrane composed of several layers of cells
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Function in protection of underlying areas subjected to abrasion
 Forms the external part of the skin’s epidermis (keratinized cells), and linings of the esophagus, mouth, and vagina (nonkeratinized cells)
Stratified cuboidal
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Quite rare in the body
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Found in some sweat and mammary glands
Stratified columnar
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Limited distribution in the body
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Found in the pharynx, male urethra, and lining some glandular ducts
Transitional
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Several cell layers, basal cells are cuboidal, surface cells are dome shaped
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Stretches to permit the distension of the urinary bladder
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Lines the urinary bladder, ureters, and part of the urethra
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Epithelia: Glandular
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A gland is one or more cells that makes and secretes an aqueous fluid

Classified by:
-Site of product release – endocrine or exocrine
-Relative number of cells forming the gland – unicellular or multicellular
Endocrine Glands
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Ductless glands that produce hormones
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Secretions include amino acids, proteins, glycoproteins, and steroids
Exocrine Glands
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More numerous than endocrine glands
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Secrete their products onto body surfaces (skin) or into body cavities
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Examples include mucous, sweat, oil, and salivary glands
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The only important unicellular gland is the goblet cell
Connective Tissue
Found throughout the body; most abundant and widely distributed in primary tissues
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Connective tissue proper, cartilage, bone, and blood
Functions of Connective Tissue
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Binding and support, protection, insulation, and transportation
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Characteristics of Connective Tissue:

Mesenchyme as their common tissue of origin


Varying degrees of vascularity
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Nonliving extracellular matrix, consisting of ground substance and fibers
Structural Elements of Connective Tissue
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Ground substance – unstructured material that fills the space between cells
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Fibers – collagen, elastic, or reticular
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Cells – fibroblasts, chondroblasts, osteoblasts, and hematopoietic stem cells
Connective Tissue Proper: Loose
Areolar connective tissue
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Gel-like matrix with all three connective tissue fibers
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Fibroblasts, macrophages, mast cells, and some white blood cells
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Wraps and cushions organs
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Widely distributed throughout the body
Adipose connective tissue
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Matrix similar to areolar connective tissue with closely packed adipocytes
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Reserves food stores, insulates against heat loss, and supports and protects
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Found under skin, around kidneys, within abdomen, and in breasts
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Local fat deposits serve nutrient needs of highly active organs
Connective Tissue Proper: Dense
Regular
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Parallel collagen fibers with a few elastic fibers
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Major cell type is fibroblasts
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Attaches muscles to bone or to other muscles, and bone to bone
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Found in tendons, ligaments, and aponeuroses
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Irregular
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Irregularly arranged collagen fibers with some elastic fibers
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Major cell type is fibroblasts
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Withstands tension in many directions providing structural strength
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Found in the dermis, submucosa of the digestive tract, and fibrous organ capsules
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Connective Tissue: Cartilage
Hyaline Cartilage
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Amorphous, firm matrix with imperceptible network of collagen fibers
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Chondrocytes lie in lacunae
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Supports, reinforces, cushions, and resists compression
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Forms the costal cartilage
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Found in embryonic skeleton, the end of long bones, nose, trachea, and larynx
Elastic Cartilage
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Similar to hyaline cartilage but with more elastic fibers


Maintains shape and structure while allowing flexibility
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Supports external ear (pinna) and the epiglottis
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Fibrocartilage Cartilage
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Matrix similar to hyaline cartilage but less firm with thick collagen fibers

Provides tensile strength and absorbs compression shock
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Found in intervertebral discs, the pubic symphysis, and in discs of the knee joint
Connective Tissue: Bone (Osseous Tissue)

Hard, calcified matrix with collagen fibers found in bone
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Osteocytes are found in lacunae and are well vascularized
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Supports, protects, and provides levers for muscular action
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Stores calcium, minerals, and fat
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Marrow inside bones is the site of hematopoiesis

Connective Tissue: Blood

Red and white cells in a fluid matrix (plasma)

Contained within blood vessels
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Functions in the transport of respiratory gases, nutrients, and wastes
Nervous Tissue
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Branched neurons with long cellular processes and support cells

Transmits electrical signals from sensory receptors to effectors
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Found in the brain, spinal cord, and peripheral nerves
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Muscle Tissue
Skeletal

Long, cylindrical, multinucleate cells with obvious striations

Initiates and controls voluntary movement

Found in skeletal muscles that attach to bones or skin
Cardiac
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Branching, striated, uninucleate cells interlocking at intercalated discs
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Propels blood into the circulation

Found in the walls of the heart
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Smooth

Sheets of spindle-shaped cells with central nuclei that have no striations

Propels substances along internal passageways (i.e., peristalsis)

Found in the walls of hollow organs
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