Embryology Unit 1 – Key Vocabulary

This set of vocabulary flashcards reviews essential terms from Unit 1 on embryology, spermatogenesis, oogenesis, and the menstrual cycle.

Dorsal

The back (posterior) axis of an embryo or body.

Ventral

The front (anterior) axis of an embryo or body.

Cranial

Toward the head or top end of the body.

Caudal

Toward the tail or bottom end of the body.

Medial

Closer to the midline of the body.

Lateral

Farther from the midline of the body.

Frontal (coronal) plane

Vertical plane dividing body into dorsal and ventral portions; shows medial-lateral and cranial-caudal axes.

Sagittal plane

Vertical plane along the midline; shows cranial-caudal and dorsal-ventral axes.

Transverse (cross) plane

Horizontal plane dividing body into cranial and caudal parts; shows dorsal-ventral and medial-lateral axes.

Embryonic period

Weeks 1–8 of development when the body plan forms and all organ systems appear (heart functions first).

Fetal period

Week 9 to birth; characterized by organ maturation and growth.

First trimester

Weeks 1–12; organ systems established.

Second trimester

Weeks 13–26; fetus triples in length & weight; maternal symptoms increase.

Third trimester

Weeks 27–40; fetus doubles in weight; most physically demanding for mother.

Gestational age

Pregnancy dating from last menstrual period (LMP); used clinically.

Fertilization age

Actual age counted from fertilization; used in embryology research.

Primordial germ cell (PGC)

Embryonic ancestor of gametes that migrates from yolk sac to genital ridge.

Teratoma

Tumor arising from misplaced PGCs; may contain multiple tissue types (hair, teeth, brain).

Spermatogenesis

Process that produces sperm in testes from puberty throughout life.

Spermatogonium

Stem cell in seminiferous tubule that undergoes mitosis to give rise to sperm lineage.

Primary spermatocyte

Diploid cell that begins meiosis I during spermatogenesis.

Secondary spermatocyte

Haploid cell produced after meiosis I; enters meiosis II.

Spermatid

Haploid cell after meiosis II; differentiates into sperm.

Spermiogenesis

Maturation step where spermatids shed cytoplasm, condense DNA, and grow tails.

Seminiferous tubule

Coiled tubule in testes where sperm develop; maturation proceeds from basal lamina to lumen.

Leydig cell

Testicular cell stimulated by LH to secrete testosterone.

Sertoli cell

Support cell in seminiferous tubule that nourishes germ cells and forms blood-testis barrier.

GnRH (Gonadotropin-Releasing Hormone)

Hypothalamic hormone that triggers pituitary release of LH and FSH.

LH (Luteinizing Hormone)

Pituitary hormone; stimulates Leydig cells in males and ovulation & corpus luteum formation in females.

FSH (Follicle-Stimulating Hormone)

Pituitary hormone; supports spermatogenesis and drives follicle growth.

Protamines

Proteins replacing histones in sperm, tightly packaging DNA in the head.

Oogenesis

Process that forms ova in ovaries; begins before birth and completes upon fertilization.

Primordial follicle

Resting follicle with a primary oocyte and single layer of flat cells.

Primary follicle

Growing follicle with cuboidal granulosa cells; oocyte still primary.

Secondary follicle

Follicle with multiple granulosa layers and emerging thecal cells.

Antral follicle

Follicle containing a fluid-filled cavity (antrum).

Graafian (mature) follicle

Fully developed follicle that ovulates a secondary oocyte.

Granulosa cell

Follicular cell converting testosterone to estrogen via aromatase.

Thecal cell

Follicular cell producing testosterone precursor for estrogen synthesis.

Corpus luteum

Post-ovulation structure secreting progesterone and estrogen to support early pregnancy.

Corpus albicans

Scar-like fibrous remnant of a regressed corpus luteum.

Polar body

Small cell lacking cytoplasm generated during oocyte meioses; not a functional gamete.

Menstrual phase

Days 0–5; shedding of endometrium and selection of 5–12 primordial follicles.

Proliferative phase

Days 5–14; estrogen-driven endometrial thickening and follicle maturation.

Secretory phase

Days 14–28; progesterone from corpus luteum maintains thick endometrium for implantation.

Estrogen

Hormone from granulosa cells & corpus luteum that proliferates endometrium and gives positive feedback to LH.

Progesterone

Hormone from corpus luteum that stabilizes and further thickens endometrium.

Atresia

Degeneration and loss of follicles over the female lifespan.

Ovulation

LH-triggered release of a secondary oocyte and surrounding cumulus from the ovary.

Endometrium

Inner uterine lining that thickens each cycle to receive an embryo.

Cervical mucus

Secretion that becomes thin during proliferation for sperm entry and thick during secretory phase.

Aromatase

Granulosa-cell enzyme converting testosterone to estrogen.

Polycystic Ovarian Syndrome (PCOS)

Endocrine disorder with excess androgens, irregular cycles, cystic ovaries, and possible infertility.

Primary Ovarian Insufficiency (POI)

Loss of normal ovarian function before age 40 leading to low estrogen and infertility.

Organogenesis

Formation of organs during embryonic weeks 1–8.

Meiosis I

Reductional division halving chromosome number; completes in oocyte just before ovulation.

Meiosis II

Equational division producing haploid gamete; completed only upon fertilization in oocyte.

Mitosis

Cell division that maintains chromosome number; enlarges germ-cell pools.