Unit 07 · Endocrine & Reproductive Systems

Hormones circulate, but a target response depends on receptors and intracellular machinery. Peptide and many amine hormones commonly act at cell-surface receptors; steroid and thyroid hormones can regulate gene expression through intracellular receptors. Sensitivity, pulsatility, and delays matter. A “master gland” label does not mean the pituitary is independent of hypothalamic input or target-gland feedback.

Hypothalamic releasing signals regulate anterior-pituitary secretion through portal blood. In the thyroid axis, TRH supports TSH secretion, TSH stimulates thyroid hormone output, and thyroid hormone feeds back on hypothalamus and pituitary. ADH and oxytocin are synthesized in the hypothalamus and stored/released by the posterior pituitary, not manufactured by the posterior lobe. Pancreatic insulin/glucagon regulation and parathyroid calcium regulation show that not all feedback runs through a pituitary axis.

Insulin generally supports nutrient uptake/storage and restrains hepatic glucose output; glucagon supports hepatic glucose availability during appropriate conditions. Kidney water handling, bone mineral balance, and reproductive signaling are other endocrine targets. A graph of a hormone alone cannot distinguish secretion, clearance, binding, and target sensitivity. No hormone dose or treatment is calculated here.

Negative feedback reduces a departure from a controlled state; it does not mean every arrow is inhibitory. Our discrete model y_next = y + g(target − y) assumes a fixed target, constant gain, no delay, and a linear response. It represents a generic feedback variable, not a measured hormone axis. Physiological set points and responsiveness can change with time and development. A linear rule can predict impossible values outside its domain.

Use models to locate testes, epididymis, ductus deferens, accessory glands, urethra, and external structures, and ovaries, uterine tubes, uterus/cervix, vagina, and external structures. Sperm production occurs in seminiferous tubules and maturation continues in the epididymis. Oocytes develop in ovarian follicles. Meiosis reduces chromosome number; after fertilization, the typical diploid complement is restored. These descriptions are general biological patterns, not tests of identity, fertility, or anyone’s anatomy.

GnRH, FSH, LH, gonadal hormones, and inhibin form interacting loops. In a simplified testicular pathway, LH supports testosterone production and FSH supports Sertoli-cell functions in sperm production. In an ovarian cycle, follicular growth, ovulation, and luteal activity connect to changes in the uterine lining. Sustained high estradiol can briefly favor positive feedback leading to an LH surge, unlike the usual negative feedback. A 28-day cycle is an illustration, not a universal schedule; timing and pubertal development vary.

Fertilization typically occurs in a uterine tube; implantation is a later interaction with uterine endometrium. They are not synonyms. Early cleavage increases cell number without proportional overall growth in our simplified geometry. Gastrulation establishes ectoderm, mesoderm, and endoderm: nervous tissue and epidermis are largely ectodermal, muscle/vascular tissue and much renal tissue mesodermal, and much gut/airway epithelium endodermal. Whole organs combine several tissue origins.

Placental exchange connects maternal and embryonic/fetal support systems while their blood normally remains in distinct circulations across a barrier. It supports gas, nutrient, and waste exchange rather than making fetal lungs the main prenatal gas exchanger. Organ systems develop over time; adult water-compartment proportions and renal concentrating capacity cannot simply be assigned to an infant. Calendar gestational age and fertilization age use different starting conventions, often roughly two weeks apart in a textbook example, not an exact rule for an individual.

Selected levels must be declared before instruction: Foundation studies curated age-appropriate structure/function, gametes, and broad development; Core adds feedback and ovarian/testicular cycles; Honors adds quantitative model critique and germ-layer/placental connections. No personal, reproductive, or health disclosure is required. All students use supplied educational diagrams and fictional records; no intimate examination, pregnancy tracking, or real patient image is part of the course.

The ovarian time series below are synthetic teaching records, not observations of any person. Day 1 is the stipulated start of uterine shedding in each fictional model; later samples are irregularly spaced. Estradiol, LH, FSH, and progesterone use different assay scales. Compare a hormone with itself across time; do not compare the numerical heights of different hormones as if they were equal concentrations. Model B deliberately stretches the earlier part of the sequence without making a claim about a population range.

Pulsatile GnRH supports anterior-pituitary FSH and LH. FSH supports follicular granulosa-cell functions; LH supports theca-cell androgen production used in estradiol synthesis. Growing follicles increase estradiol, promoting endometrial proliferation. Estradiol and inhibin usually reduce parts of the upstream drive. Sustained high estradiol in the appropriate feedback state can instead support a positive-feedback LH surge, which participates in ovulation and luteinization. The corpus luteum then produces progesterone and estradiol; progesterone supports secretory endometrial changes. Falling ovarian steroids after luteal regression can permit shedding. This causal account comes from broader physiology, not from fitting one invented graph.

An estradiol maximum at sampled day 12 followed by a sampled LH maximum at day 14 does not prove a two-day biological response delay. Sampled peaks are not necessarily true peaks, the actual maximum can lie between samples, and endocrine effects depend on duration and other signals. The dataset alone does not establish the occurrence or exact time of ovulation. An area under a relative assay-time curve summarizes a signal over a stated interval; it is not the amount of hormone secreted, receptor occupancy, or a fertility measure.

The testicular axis is not a monthly ovarian cycle. LH supports Leydig-cell testosterone production; FSH and testosterone support Sertoli-cell functions involved in spermatogenesis. Testosterone feeds back on hypothalamus/pituitary, while Sertoli-derived inhibin B chiefly restrains FSH. Our hypothetical paired states hold LH and testosterone constant while changing the inhibin/FSH relation. They illustrate a selective feedback hypothesis, not the full pulsatile dynamics or proof from a two-point correlation.

Use no personal cycle tracking, no reproductive disclosure, no fertility predictions, and no pregnancy advice or diagnosis. Cycle timing and patterns vary within and between people, especially across development; neither a universal 28-day calendar nor this invented pair describes everyone. Foundation uses instructor-selected factual diagrams and within-column arithmetic; Core connects phase mechanisms and feedback; Honors compares irregular time series and inference limits. Record the chosen level before instruction and use the supplied records only.