Embryology Study Guide: Basics, Cell Division, Gametogenesis, and Reproductive Systems

Vocabulary flashcards covering embryological terminology, embryonic stages, egg classifications, mitosis, meiosis, gametogenesis, and the anatomy and endocrinology of the female and male reproductive systems.

Embryogenesis

Prenatal development that begins at fertilisation and ends at birth.

Embryo

The developmental stage following the germ, spanning from weeks $4$ to $8$ until the end of the second month.

Foetus

The developmental stage beginning from the second month (weeks $9$ to $38$) until birth, during which the organism resembles a human being.

Zygote

A unicellular diploid ($2n$) cell produced by the fusion of a male gamete and a female gamete during fertilisation.

Segmentation

A series of cleavage divisions that breaks the zygote into blastomeres with low organisation and high multiplication, yielding a morula.

Gastrulation

A series of cellular movements that organises the embryonic layers and tissues, producing a gastrula.

Neurulation

The morphogenetic phase involving the formation of the primitive nervous system as a neural tube, producing a neurula.

Organogenesis

The embryological phase characterized by the appearance of organs and limb buds, forming an embryo.

Telolecithal egg

An egg type with very abundant vitellus reserves measuring $1\text{ to several cm}$, found in birds.

Centrolecithal egg

An egg type containing abundant vitellus with cytoplasm pushed to the periphery, measuring $1\text{ to several mm}$, found in insects.

Heterolecithal (mesolecithal) egg

An egg type featuring a vitelline gradient and measuring about $10^{-3}\text{ m}$ ($1\text{ mm}$), found in amphibians.

Oligolecithal egg

An egg type low in vitellus measuring about $1\text{ mm}$, found in sea urchins.

Alecithal egg

A tiny egg measuring $0.1\text{ mm}$ that lacks vitelline reserves, characteristic of mammals including humans.

Holoblastic segmentation

Total and equal cleavage segmentation characteristic of alecithal human eggs.

Karyokinesis

The division of the cell nucleus during mitosis or meiosis.

Cytokinesis

The division of the cytoplasm following karyokinesis to form daughter cells.

Leptotene

The first stage of prophase I in meiosis, characterized by chromatin condensation.

Zygotene

The second stage of prophase I in meiosis during which the synaptonemal complex forms between homologous chromosomes (synapsis).

Pachytene

The third stage of prophase I during which crossing-over occurs between non-sister chromatids of homologous chromosomes, generating recombinant chromosomes.

Diplotene

The fourth stage of prophase I where homologous chromosomes begin to separate while remaining attached at contact points called chiasmata.

Diakinesis

The final stage of prophase I characterized by terminal condensation of the chromosomes.

Meiosis I (Reductive division)

The first meiotic division that reduces ploidy from $2n$ to $n$ by separating homologous chromosomes.

Meiosis II (Equational division)

The second meiotic division that separates sister chromatids without prior DNA replication, maintaining ploidy at $n$.

Gametogenesis

The biological process in the gonads transforming diploid germ cells ($2n$) into haploid functional gametes ($n$).

Oogenesis

The discontinuous formation of female oocytes from germ cells, taking place during foetal life.

Folliculogenesis

The selective and cyclical growth and maturation of ovarian follicles occurring from puberty to menopause.

Atresia

The massive destruction and involution of germ cells or ovarian follicles through atrophy followed by degeneration (apoptosis).

Internal theca

A vascular somatic layer of the ovarian follicle that responds to LH stimulation by converting cholesterol into androgens.

Granulosa

An avascular follicular somatic cell layer stimulated by FSH that converts thecal androgens into oestrogens using aromatase.

Slavjanski membrane

The basement membrane separating the vascular internal theca from the avascular granulosa in the ovarian follicle, which ruptures during luteinisation.

Corpus luteum

A vascularised, yellow endocrine structure formed from luteinised granulosa and thecal cells that synthesises progesterone.

Spermatogenesis

The continuous, $74\text{-day}$ process occurring from puberty until death that produces mature spermatozoa from male germ cells.

Spermatocytogenesis

The $51\text{-day}$ phase of spermatogenesis covering the development of spermatogonia through primary and secondary spermatocytes into round spermatids.

Spermiogenesis

The $23\text{-day}$ differentiation phase of spermatogenesis where round spermatids transform into functional spermatozoa without cell division.

Spermiation

The final detachment and release of spermatozoa from Sertoli cells into the lumen of the seminiferous tubule.

Sertoli cells

Somatic supporting cells of the seminiferous tubules that form the blood-testis barrier, nourish germ cells, and secrete seminiferous fluid, inhibin, and ABP.

Blood-testis barrier (BTB)

A virtual barrier formed by Sertoli cell cytoplasmic expansions that divides the seminiferous epithelium into basal and adluminal compartments.

Leydig cells

Endocrine somatic cells of the testicular interstitial compartment that synthesize androgens, primarily testosterone.

Acrosome

An enzymatic organelle on the anterior head of the spermatozoon, formed from Golgi vesicles and containing enzymes such as acrosin.

Decapacitation

An epididymal process where acidic pH ($6.5$), acrostatin, and surface cholesterol temporarily block sperm motility and fertilising capacity.